Carbonaceous Aerosol Microphysics

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Atmos. Chem. Phys. Discuss., 10, 4543–4592, 2010 Atmospheric www.atmos-chem-phys-discuss.net/10/4543/2010/ Chemistry ACPD © Author(s) 2010. This work is distributed under and Physics 10, 4543–4592, 2010 the Creative Commons Attribution 3.0 License. Discussions

This discussion paper is/has been under review for the journal Atmospheric Chemistry Carbonaceous and Physics (ACP). Please refer to the corresponding ﬁnal paper in ACP if available. aerosol microphysics S. E. Bauer et al.

A global modeling study on Title Page carbonaceous aerosol microphysical Abstract Introduction Conclusions References characteristics and radiative forcing Tables Figures

S. E. Bauer1,2, S. Menon3, D. Koch1,2, T. C. Bond4, and K. Tsigaridis1 J I 1NASA Goddard Institute for Space Studies, New York, NY, USA J I 2The Earth Institute, Columbia University, New York, NY, USA 3Lawrence Berkeley National Laboratory, Berkeley, CA, USA Back Close 4University of Illinois, Urbana-Champaign, IL, USA Full Screen / Esc Received: 20 January 2010 – Accepted: 8 February 2010 – Published: 15 February 2010 Correspondence to: S. E. Bauer ([email protected]) Printer-friendly Version

Published by Copernicus Publications on behalf of the European Geosciences Union. Interactive Discussion

4543 Abstract ACPD Recently, attention has been drawn towards black carbon aerosols as a short-term climate warming mitigation candidate. However the global and regional impacts of the 10, 4543–4592, 2010 direct, cloud-indirect and semi-direct forcing eﬀects are highly uncertain, due to the 5 complex nature of aerosol evolution and the way that mixed, aged aerosols interact Carbonaceous with clouds and radiation. A detailed aerosol microphysical scheme, MATRIX, em- aerosol microphysics bedded within the GISS climate model is used in this study to present a quantitative assessment of the impact of microphysical processes involving black carbon, such as S. E. Bauer et al. emission size distributions and optical properties on aerosol cloud activation and ra- 10 diative forcing. Our best estimate for net direct and indirect aerosol radiative forcing between 1750 Title Page 2 and 2000 is −0.56 W/m . However, the direct and indirect aerosol eﬀects are quite sen- Abstract Introduction sitive to the black and organic carbon size distribution and consequential mixing state. The net radiative forcing can vary between −0.32 to −0.75 W/m2 depending on these Conclusions References 15 carbonaceous particle properties at emission. Assuming that sulfates, nitrates and Tables Figures secondary organics form a coating around a black carbon core, rather than forming a uniformly mixed particle, changes the overall net aerosol radiative forcing from negative to positive. Taking into account internally mixed black carbon particles let us simulate J I

correct aerosol absorption. Black carbon absorption is ampliﬁed by sulfate and nitrate J I 20 coatings, but even more strongly by organic coatings. Black carbon mitigation scenarios generally showed reduced radiative forcing when sources with a large proportion Back Close

of black carbon, such as diesel, are reduced; however reducing sources with a larger Full Screen / Esc organic carbon component as well, such as bio-fuels, does not necessarily lead to climate beneﬁts. Printer-friendly Version

Interactive Discussion

4544 1 Introduction ACPD Anthropogenic and natural aerosols impact the Earth’s radiation balance and thus exert a forcing on global climate. Black carbon (BC) has a positive radiative forcing, while 10, 4543–4592, 2010 other aerosol species mainly lead to atmospheric cooling. Therefore cleaning up most 5 aerosol emissions, which is much needed due to their severe health impacts, could Carbonaceous strengthen the global warming trend. However, mitigation of BC has the potential to be aerosol microphysics beneﬁcial in mitigating both climate warming and air pollution. Emission reductions that target light-absorbing aerosol might also reduce warming quickly (Hansen et al., 2000; S. E. Bauer et al. Jacobson, 2002; Bond and Sun, 2005). However, BC’s indirect (cloud microphysics) 10 and semi-direct contributions to net climate forcing remain an outstanding uncertainty. For example, the model study of Penner et al. (2003) suggested that “smoke” did not Title Page

produce net warming, considering both direct and cloud effects. In addition, model Abstract Introduction experimental results are very sensitive to treatment of aerosol microphysics. The aerosol direct effect (ADE) is caused by absorption and scattering of solar ra- Conclusions References 15 diation by liquid and solid aerosol particles in the atmosphere. Most absorbing, and Tables Figures therefore enhancing climate warming, are black carbon (BC) particles. The mass absorption strength and scattering ability of a BC particle strongly depends on the particle effective size and mixing state. Both of those quantities depend on the size and chem- J I

ical composition of black carbon as it is released into the atmosphere as well as its J I 20 microphysical and chemical evolution during transport through the atmosphere. Most other aerosol species scatter radiation back to space, such as sulfate, nitrate, organic Back Close

carbon (although some organics can be slightly absorbing), sea salt, aerosol water and Full Screen / Esc to a certain extent mineral dust, and therefore these counterbalance climate warming, by cooling the atmosphere. However, if BC is mixed with these scattering components, Printer-friendly Version 25 the result is enhanced absorption. Laboratory studies by Khalizov et al. (2009) showed that for soot particles with an initial mobility diameter of 320 nm and a 40% H2SO4 mass Interactive Discussion coating fraction, absorption and scattering are increased by 1.4 – and 13 fold at 80% RH, respectively. Also, the single scattering albedo of soot aerosol increases from 0.1

4545 to 0.5 after coating and humidiﬁcation. Jacobson (2000) performed global simulations with core-shell treatment and found BC forcings 50% higher and 40% lower than forc- ACPD ings obtained with externally mixed and well-internally mixed treatments, respectively. 10, 4543–4592, 2010 Therefore, correct treatment of the aerosol size and mixing state is crucial in order to 5 calculate their net radiative forcing. Attempts to quantify the aerosol direct radiative forcing (ADF), the diﬀerence in top of Carbonaceous the atmosphere forcing between pre-industrial and present day conditions, by the Aero- aerosol microphysics Com initiative gave estimates between −0.41 and +0.04 W/m2 (Schulz et al. 2006) and by the IPCC AR4 report a range from −0.9 to −0.1 W/m2 (IPCC, 1007). The wide range S. E. Bauer et al. 10 of uncertainty is caused on the one hand by large discrepancies among the model sys- tems, ranging from emission strength, transport, aerosol transformation, removal and Title Page optical properties. However, the models are also poorly constrained by limited avail- ability of aerosol measurements. Furthermore, Myhre (2009) explains large deviations Abstract Introduction between models and satellite retrieved ADF estimates by the lack of considering cor- Conclusions References 15 rect pre-industrial aerosol distributions in the satellite data derived forcing calculations

and in failure to sample the model like the retrieval. Tables Figures The aerosol indirect effect (AIE) is caused by an increase in the number of hygroscopic aerosols, with a corresponding increase in cloud droplet number concentrations, J I reduction in cloud droplet size, increase in cloud albedo (first indirect effect) and sup- 20 pression of precipitation and increased cloud life time (second indirect effect), thereby J I cooling the planet (IPCC, 2007). The role of BC in determining cloud droplet number Back Close concentrations (CDNC) is unknown (Adams and Seinfeld, 2003), as it depends upon the size distribution and mixing state of BC with soluble species such as sulfate, nitrate Full Screen / Esc and organic carbon. Furthermore, insoluble black carbon or dust aerosols can act as 25 ice nuclei and therefore alter cirrus and mixed-phase clouds; however these effects are Printer-friendly Version poorly known (e.g. Lohmann et al., 2008). Here again the microphysical evolution of the aerosol population in terms of size and mixing state is crucial in order to identify Interactive Discussion the number of particles are suitable for cloud activation. The IPCC AR4 report estimates the aerosol indirect radiative forcing (AIF) to lie between −1.8 and −0.3 W/m2.

4546 The AeroCom initiative, linking models to satellite-based estimates gives a range of −0.7±0.5 W/m2 for the AIF (Quaas et al., 2009). The models on which these estimates ACPD were based generally did not include sophisticated treatment of aerosol mixing state, 10, 4543–4592, 2010 and the contribution of BC mixing to the AIF uncertainty was not evaluated. 5 In this study we are particularly interested in the microphysical evolution of black carbon. The size and mixing state of carbonaceous particles at emission must be Carbonaceous assumed for this investigation. Once emitted, BC can grow by coagulation and con- aerosol microphysics densation. Although freshly emitted pure BC is hydrophobic, inorganic and organic coatings will attract water and convert particles to be hygroscopic. Materials likely to S. E. Bauer et al. 10 condense on a BC particle, as on any other particle, are sulfate and nitrate precursors as well as secondary organics (Riemer et al., 2009). Those particles are likely to form Title Page core – shell particles, where the insoluble BC fraction would form the core and the other species would enclose that core with a shell. Abstract Introduction In addition to surface condensation processes, coagulation is a very eﬀective mixing Conclusions References 15 pathway. In the atmosphere every aerosol shape is unique, complex and not necessar-

ily spherical. Transmission electron microscopy (TEM) images support the theory that Tables Figures BC particles become coated once emitted. Although BC may be internally mixed with other components, BC cannot be “well-mixed” (diluted) in the particle, since soot, which J I contains BC, is irregularly shaped and mostly solid. Thus most likely BC is distinct and 20 not well mixed within particles. J I As we explore the role of BC in aerosol forcing we will study the following questions: Back Close How important is the size distribution of carbonaceous particles at emission time (Sect. 3)? How sensitive are the forcings to mixing assumptions of BC particles Full Screen / Esc (Sect. 4)? How well can we constrain our simulations with observational data (Sect. 5)? 25 And how well can we assess BC reduction strategies given the previously discussed Printer-friendly Version microphysical modeling uncertainties (Sect. 6)? This paper is linked to a study by Menon et al. (2010) (hereafter referred to as SM10) Interactive Discussion where the same model version and conﬁguration as in this study is used, but the analysis is more detailed with respect to the AIF. SM10 includes evaluation and comparison

4547 of the performance of a new AIE scheme coupled to aerosol microphysics or to the previous mass based GISS aerosol model (Koch et al., 2006; Menon and Rotstayn, ACPD 2006). 10, 4543–4592, 2010

2 Model description Carbonaceous aerosol microphysics 5 The Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) climate modelE (Hansen et al., 2005; Schmidt et al., 2006) coupled to the aerosol micro- S. E. Bauer et al. physics and chemistry model MATRIX (Multiconﬁguration Aerosol TRacker of mIXing state) (Bauer et al., 2008), hereafter BA08, is used in this study. MATRIX is designed to support model calculations of the direct and indirect eﬀect and permit detailed treat- Title Page 10 ment of aerosol mixing state, size and aerosol-cloud activation. MATRIX is based on the quadrature methods of moments. For each aerosol pop- Abstract Introduction

ulation, deﬁned by mixing state and size distribution, the tracked species are number Conclusions References concentration, and mass concentration of sulfate, nitrate, ammonium, aerosol water, black carbon, organic carbon, mineral dust, and sea salt. Here we use the aerosol Tables Figures 15 population setup called “mechanism 1” (BA08), given in Table 1. MATRIX dynamics includes nucleation, new particle formation, particle emissions, gas-particle mass J I transfer, aerosol phase chemistry, condensational growth, coagulation, and cloud activation. New additions to BA08 are linking the aerosol scheme to a recent aerosol J I indirect eﬀect scheme that uses prognostic equations (Morrison and Gettelman, 2008) Back Close 20 to calculate the cloud droplet number concentrations, as described in SM10. A further addition to BA08 is the coupling of mixed aerosol populations to the radiation scheme Full Screen / Esc as described below. Printer-friendly Version 2.1 Aerosol radiation coupling Interactive Discussion Previously the GISS radiation scheme treated only externally mixed aerosol popula- 25 tions, suitable for our mass based aerosol scheme (Koch et al., 2006). Here we de-

4548 scribe the new coupling scheme for internally mixed aerosol populations. The GISS model radiation scheme (Hansen et al., 1983) includes explicit multiple scattering cal- ACPD culations for solar radiation (shortwave – SW) and explicit integrations over both the 10, 4543–4592, 2010 SW and thermal (longwave – LW) spectral regions. Gaseous absorbers of LW ra- 5 diation are H2O, CO2,O3,O2, and NO2. Size dependent scattering properties of clouds and aerosols are computed from Mie scattering, ray tracing, and T-matrix the- Carbonaceous ory (Mishchenko et al., 1996) to include non-spherical cirrus and dust particles. The aerosol microphysics k-distribution approach (Lacis and Oinas, 1991) utilizes 15 non-contiguous spectral intervals to model overlapping cloud aerosol and gaseous absorption. S. E. Bauer et al. 10 The MATRIX module calculates the optical properties single scattering albedo, asym- metry factor and extinction for the 16 aerosol populations (see Table 1) for six wave- Title Page length bands in the SW and 33 bands in LW and passes those to the GISS radiation code. Each population can include multiple chemical species. Each aerosol popula- Abstract Introduction tion, as indicated in Table 1, is treated as an external mixture, a homogeneous internal Conclusions References 15 mixture or a core-shell particle, depending on its chemical composition. If a particle

contains only one chemical species, for example a freshly emitted pure organic car- Tables Figures bon particle, then the optical properties are those of an externally mixed particle. In those cases MATRIX calculates the refractive index for each population and then uses J I Mie-code pre-calculated lookup tables in order to assign the corresponding optical pa- 20 rameters. A homogeneous internal mixture is assumed to be a well-mixed particle, J I most likely containing soluble species including aerosol water. MATRIX calculates the Back Close optical properties for those particles by using the volume mixing approach. Black carbon containing particles can exist in various shapes. A pure BC particle is externally Full Screen / Esc mixed (part of population BC1). BC coated by condensed sulfate, nitrate, organics and 25 aerosol water can form a core-shell particle, where the insoluble BC forms the core and Printer-friendly Version the soluble material the shell. Pre-calculated lookup tables using the core-shell model by Toon and Ackerman (1981) provide optical speciﬁcations for all possible combina- Interactive Discussion tions of BC core sizes and shell thicknesses of various compositions formed from sulfate, nitrate, organics and aerosol water. Alternatively, we may assume that BC is ho-

4549 mogeneously mixed with the other species. The following refractive indices at 550 nm are used in this study: Sulfate (1.528-1.e-7i), nitrate (1.528-1.e-7i), OC (1.527-0.014i), ACPD BC (1.85-0.71i), sea salt (1.45-0.i), dust (1.564-0.002i) and water (1.334-3.91e-8i). 10, 4543–4592, 2010 Here, we did not perform special optical calculations for core-shell particles involv- 5 ing mineral dust. This was not necessary as the potential shell thicknesses of such particles would be too thin for them to optically make any signiﬁcant impact (Bauer et Carbonaceous al., 2007). Therefore the optical properties for the populations DD1, DS1, DD2 and aerosol microphysics DS2 are close to those of externally mixed mineral dust. However, this might not be true for the dust particles included in the mixed mode (MXX), as here mineral dust is S. E. Bauer et al. 10 mainly mixed with sea salt, and therefore mineral dust is not necessarily the dominating species. Population MXX is treated as homogeneously internally mixed, due to Title Page the complicated mixing involving all eight components, dust, sea salt, sulfate, nitrate, ammonium, BC, OC, and water. In this study the base case simulation uses the vol- Abstract Introduction ume mixing approach for all internally mixed particles. The impact of BC-containing Conclusions References 15 core-shell particles is discussed separately in Sect. 4.

Tables Figures 2.2 Model conﬁguration

Anthropogenic and natural emissions for present day and pre-industrial conditions are J I taken from the AeroCom project (Dentener et al., 2006 and http://dataipsl.ipsl.jussieu. J I fr/AEROCOM/). We use fluxes for “natural” emissions of dust, sea salt and dimethyl 20 sulfide (DMS), and organic carbon (OC) assuming secondary organic aerosol as a 15% Back Close yield from terpene emissions, and in addition, anthropogenic emissions from biomass Full Screen / Esc burning and fossil and bio fuel burning of SO2, OC and BC. The inventory provides data for the year 2000 (present-day conditions) and for the year 1750 (pre-industrial conditions). Printer-friendly Version 25 Model simulations for present day and pre-industrial conditions only differ by emis- Interactive Discussion sion levels. The model runs are performed fully interactive, therefore aerosol and cloud changes will impact the general circulation. Present day atmospheric conditions, including present day sea surface temperature, are also applied for the pre-industrial 4550 emission runs. The model is employed on a horizontal resolution of 4◦×5◦ latitude by longitude and 23 vertical layers. The model uses a 30 min time step for all physics ACPD calculations. Every model simulation is integrated for 5.5 years, and if not otherwise 10, 4543–4592, 2010 noted, five-year mean conditions are discussed in this paper. 5 We will discuss direct and indirect aerosol effects by looking at aerosol direct radiative impacts (ADE) and cloud radiative impacts (AIE) at present-day conditions. ADE Carbonaceous is calculated by taking the difference between radiative transfer calculations at the top aerosol microphysics of the atmosphere with and without aerosols. AIE is calculated from changes to the S. E. Bauer et al. net cloud forcing obtained from differences between total and clear skies for each call 10 to the radiation excluding aerosols. Aerosol direct (ADF) and indirect radiative forcing (AIF) is calculated as the difference in forcing between pre-industrial and present day Title Page conditions. AIF and AIE include indirect as well as semi-direct effects. Semi-direct effects are cloud changes caused by changed vertical heating profiles in the atmosphere Abstract Introduction caused by aerosol forcing changes. Conclusions References

Tables Figures 15 3 Eﬀects of chosen particle size for emission

In order to calculate emission fluxes in microphysical aerosol models, the mass, num- J I ber concentration and the mixing state of those emissions need to be known. Infor- mation about size distributions is very important for particulate emission fluxes such J I as dust, sea salt and carbonaceous emissions. Usually some size information is pro- Back Close 20 vided for dust and sea salt emissions, or interactively calculated in models, but not for carbonaceous aerosols. Current emission inventories, such as Bond et al. (2004) or Full Screen / Esc Cooke et al. (1999) provide only mass emission information, so that each modeler has to choose what sizes to assign for carbonaceous emission fluxes. Textor et al. (2006) Printer-friendly Version

summarized the sizes of the emitted particles as used by 16 different models that par- Interactive Discussion 25 ticipated in the AeroCom study. Mass median diameter ranging from 0.02 to 0.85 µm were used for BC and OC emissions. Some AeroCom models used different sizes for fossil and biomass burning sources, and some models emitted BC and OC emissions 4551 into several size bins. These choices differed greatly among the models; furthermore there is very little information available about actual emission sizes. Bond et al. (2006, ACPD Table 3.) collected particle size distribution observations at combustion sources from 10, 4543–4592, 2010 the literature and reported mass median diameters of 0.038–0.32 µm for diesel vehi- 5 cles, 0.02–1.5 µm for gasoline vehicles, 0.1–1.3 µm for small solid fuel combustors (excluding coal briquettes), such as wood fireplaces or cooking stoves, and 0.05–0.78 µm Carbonaceous for large stationary sources such as industrial boilers. However these sizes obtained aerosol microphysics close to emitters may not be appropriate for global model gridbox-scale initial particle sizes. S. E. Bauer et al. 10 In order to understand the importance of emission size distribution of carbonaceous particles, we performed a series of sensitivity studies where only the sizes of carbona- Title Page ceous emissions are varied. Note that those sizes are just the emission sizes, and that aged particle sizes are determined by the subsequent microphysical processes. In our Abstract Introduction simulations, carbonaceous aerosols grow by roughly a factor of three, through coagu- Conclusions References 15 lation, condensation and water uptake. We assume that fossil and bio fuel BC and OC

emissions enter the atmosphere as external mixtures, entering population BC1 and Tables Figures OCC, and biomass burning emissions are internally mixed, entering population BOC in our model. However the model is rather insensitive towards this last assumption, as J I BC and OC from biomass burning sources coagulate very quickly even if emitted as 20 external mixtures (not shown here). J I The geometric mean emission particle diameters chosen for the single experiments Back Close are reported in Table 2. Three sensitivity experiments are performed around the base case simulation (BASE), one experiment with smaller emission sizes (S1) and two Full Screen / Esc experiments with larger emission sizes (S3, S4). The set of experiments covers ap- 25 proximately the range of observed emission sizes. Printer-friendly Version The design of the sensitivity experiments is rather simple, but analyzing the results is difficult as nearly everything in the aerosol simulation is affected. First we will analyze Interactive Discussion the differences between the base case BASE and S1, the experiment with the smallest BC/OC emission sizes. The largest differences occur in the coagulation calculation.

4552 Smaller carbonaceous particles in experiment S1 impact the mixing state and size distribution in all of MATRIX’s populations. Size is the most important particle property ACPD affecting coagulation. The differences in sulfate, OC and BC mixing state between 10, 4543–4592, 2010 BASE and S1 (S1-BASE) are presented in Fig. 1 for present day conditions. Smaller 5 initial OC/BC sizes lead to more coagulation between sulfate and BC. The sulfate coagulation rate to form BCS (a BC-sulfate mixture) increases from 1087 (BASE) to 8655 Carbonaceous (S1) Gg/a, leaving less externally mixed sulfate in the system (sulfate ACC and AKK aerosol microphysics loads decrease from 188 (BASE) to 41 Gg (S1)). Table 3 in BA08 may be consulted to put those budget numbers into perspective. Net sulfate condensation rates are rather S. E. Bauer et al. 10 similar between the experiments, but the particles on which surfaces condensation happens changes. In BASE most H2SO4 condenses on ACC, OCC and BOC parti- Title Page cles and aqueous phase H2SO4 condensations within clouds favors ACC and BOC. In S1 most dry and wet condensation forms sulfates on BOC surfaces. Again lead- Abstract Introduction ing to more internally mixed particles, due to less condensation of H2SO4 on pure Conclusions References 15 sulfate (ACC) particles. Changes in H2SO4 condensation rates also affect sulfate primary particle production, and decreases sulfate nucleation rates (1800 Gg/a (BASE), Tables Figures 1168 Gg/a (S1)), hence changing the size distribution. BC (Fig. 1 row 2) itself favors coagulation with OC for S1, this however increases the sulfate ratio in the BCS popu- J I lation. So overall sulfate predominantly mixes with BC and BC mixes more with OC, 20 leaving fewer externally mixed OC (OCC) in the atmosphere. Furthermore, the smaller J I overall OC/BC size leads to more coagulation of OC and BC with coarse particles. Fig- Back Close ure 3 shows the zonal mean BC mass mixing concentrations, the sum over BC from all BC containing populations, of the BASE case and the differences between BASE Full Screen / Esc and the size experiments S1, S3 and S4. The total BC mass increases from the BASE 25 case 1195 Gg to 1287 Gg in S1, with all of the increase happening in the Northern Printer-friendly Version Hemisphere. This is caused by the extended lifetime of smaller pollution BC particles and the fact that smaller particles can be transported over larger distances. Interactive Discussion Figure 3 presents the differences of experiments S1 and S3 with BASE. Comparing BASE and S1, AOT increases over the oceans and decreases in biomass burning

4553 areas. The aerosol direct effect shows a similar pattern but of opposite sign. ADE increases over land, due to internal mixing of BC and therefore enhancing overall BC ACPD absorption, which leads to more warming due to BC particles. However the dominating 10, 4543–4592, 2010 feature here is the enhanced cooling effect over the oceans, which is caused by a 5 strong contribution from the mixed population, MXX. The smaller particles in S1 lead to overall more mixing. If all 7 species are mixed together, or none of the other 15 Carbonaceous populations can be used, MXX is populated. However MXX is always representing a aerosol microphysics coarse mode as usually some sea salt or dust is present in MXX. Changes in AIE depend on particle chemistry and number concentrations, and there- S. E. Bauer et al. 10 fore strongly on the size distribution, leading to different changes than seen in the mass concentrations as shown in Fig. 1. Regionally the negative or cooling AIE (Fig. 3) is Title Page caused by the increased cloud droplet number concentration (CDNC) in those regions. Most important is the shift in size and mixing state. Particles are smaller, but still large Abstract Introduction enough to activate clouds, and more internally mixed in S1, leading to a larger number Conclusions References 15 of cloud activating particles in polluted sulfate rich regions, but to regional decreases in

CDNCs in biomass burning, OC/BC rich regions. In the BASE case CDNCs in biomass Tables Figures burning regions are dominated by externally mixed OC, e.g. OCC, whereas in S1 more of OC is mixed with BOC leading to overall smaller number concentrations in biomass J I burning areas. Globally cloud-forcing increases over the oceans leading to a diﬀerence 2 20 of −0.83 W/m in AIE. J I The second set of experiments, S3 and S4, tests the sensitivity of our model at Back Close larger BC/OC emission sizes. These experiments show many similarities towards each other, so that it is suﬃcient here to discuss the results of case S3. Larger initial OC/BC Full Screen / Esc particles lead to less internal mixing of the aerosol populations, leaving more externally 25 mixed sulfate and OC particles. Externally mixed sulfate increases from 188 Gg/a Printer-friendly Version (BASE) to 575 Gg/a (S4) and more sulfate condenses on coarse particles. Also OC and BC are less mixed with each other, but BC in the biomass burning regions is more Interactive Discussion mixed with sulfate, leading again to an increase of sulfate in the BCS population. These changes in mixing state are generally the reverse of case S1; however this does not

4554 translate into a reverse response for the aerosol optics (Fig. 3). AOT globally decreases due to less extinction by the slightly larger and more externally mixed particles. The ACPD direct aerosol (ADE) effect relative to BASE is slightly positive, with more warming 10, 4543–4592, 2010 in dust regions. This is caused by less coagulation with the overall larger OC/BC/SU 5 particles with dust, hence increasing dust lifetime in S4. The BC load (Fig. 2) is strongly reduced in S3 to 1041 Gg and 822 Gg in S4. Carbonaceous CDNC particle concentrations are reduced by −15% (S3) and −20% (S4) leading aerosol microphysics to a weakened indirect effect, the aerosol indirect effect is approximated from changes to net radiation at the TOA and therefore also includes semi-direct effects, by 3% (S3) S. E. Bauer et al. 10 and 4% (S4); however there was also an increase in liquid water path (LWP) (0.5% (S3) and 0.8% (S4)) and increased cloud cover (1% (S3) and 0.3% (S4)). Title Page After discussing the complex interactions a simple summary (Fig. 4) can be given for the impact of carbonaceous aerosol emission sizes on radiative forcings: Smaller Abstract Introduction BC/OC emission sizes lead to larger, more negative direct and indirect aerosol forcings Conclusions References 15 and larger BC/OC emission sizes to smaller forcings and therefore less aerosol cooling.

Tables Figures 4 Particle mixing state and radiative transfer J I Black carbons absorption depends strongly on its mixing state. It has been demonstrated in numerous publications (Fuller et al., 1999; Bond et al., 2006; Lesins et al., J I 2002; Jacobson, 2000) that when BC is treated solely as an external mixture, aerosol Back Close 20 absorption can be underestimated by a factor of 1.5 to 4. We will not repeat these comparison of external versus internal mixtures, because it is well established that most BC Full Screen / Esc is internally mixed; instead we will test the diﬀerences between homogeneous internal mixtures and core-shell structured particle. Printer-friendly Version A large fraction of BC mass is mixed with other aerosol species, either at emission Interactive Discussion 25 or during transport in the atmosphere. The solubility of the involved species and the processes that lead to the formation of an internally mixed particle determines the structure of the mixed aerosol. Here we distinguish between three diﬀerent basic types 4555 of particles. First, an externally mixed particle, e.g. a dry sulfate, BC, etc is a particle without any coating or added components. Second, a homogeneously internally mixed ACPD particle consists of two or up to eight constituents, such as sulfate, nitrate, ammonium, 10, 4543–4592, 2010 OC, BC, sea salt, dust and water. In order to form a homogeneously internally mixed 5 particle, all involved species need to be soluble. For example, we would assume that an ammonium-sulfate aerosol that is coated by nitrate and OC and has taken up aerosol Carbonaceous water could form a homogeneously mixed droplet. The third type of particle is made aerosol microphysics of an insoluble particle, such as BC, which is more likely to form a core-shell particle. The insoluble core is always assumed to be at the center of the particle, with either one S. E. Bauer et al. 10 species, or well-mixed multiple species in its shell. Most aerosol models that treat internally mixed aerosol populations (Stier et al, 2005) Title Page use a volume mixing rule approach or use for example other mixing-based approaches like the Maxwell-Garnett (Garnett 1904, 1906) or Bruggeman (1935) mixing rule to Abstract Introduction calculate the optical properties of aerosols. In this study, we included core-shell model Conclusions References 15 calculations, which should more accurately describe BC including particles (Kim et al.,

2008). Tables Figures In this section we will test the assumptions of homogenously internally mixed (HI) and core-shell (CS) structures of particles containing BC. The following experiments J I are tested: J I 20 – BASE (HI): The volume mixing approach is used for all internally mixed aerosols. Back Close – CS100: BC coated by sulfate, nitrate, organics and water has a core – shell structure, all other particles are either externally or homogeneously internally mixed Full Screen / Esc (see Table 1). Printer-friendly Version – CS10: as CS100 but only 10% of the organics are considered for coating a BC 25 particle. The remaining 90% will be considered homogeneously internally mixed. Interactive Discussion

Figure 5 and Table 3 shows the present day short wave TOA radiative impacts for certain species and mixing states for our BASE experiment. The net ADE is −1.78 W/m2, 4556 with contributions from externally mixed sulfates and nitrates −0.05 W/m2 (ACC), organics −0.16 W/m2, black carbon sulfate mixtures 0.05, BC-OC mixtures 0.1 W/m2, ACPD 2 2 mineral dust −1.35 W/m and sea salt −0.4 W/m . Note that sea salt and dust forc- 10, 4543–4592, 2010 ing numbers are given for aerosols that mostly include sea-salt or dust, but still can 5 be mixed with other species. Sulfate, nitrate, organic carbon, dust and sea salt lead to cooling of the atmosphere; only over the poles a positive warming appears due to Carbonaceous the strong surface reﬂection. BC – sulfate (BC-SU) mixtures have positive forcing, but aerosol microphysics BC-OC mixtures have positive or negative forcing depending on the dominant mixing S. E. Bauer et al. component, OC or BC, in the aerosol particle. 10 In order to understand the radiative impacts of black carbon, we have to know its

mixing state. In BASE, the total global load of BC is 0.12 Tg, with roughly 75% of Title Page the BC mass in population BOC. Therefore, BC is mixed mostly with OC but also with sulfate and nitrate, and the remaining 25% is present in mixtures including sulfates and Abstract Introduction nitrates, such as population BCS, BC1, BC2 and BC3. Conclusions References 15 Figure 6 gives the fractions of BC, sulfate/nitrate, and OC (which is only included in BOC) in populations BOC and BCS. The remaining fraction is taken up by aerosol Tables Figures water. Neglecting aerosol water, BC is the dominant component in population BCS,

and that component has a positive radiative impact globally. The BOC population is J I dominated by OC and SU. BC mass contributes only 1% on a global average, and up 20 to 10% in biomass burning areas. The mixing state greatly inﬂuences the radiative J I

impact. Small BC fractions in the aerosol mixture are sufficient to result in a positive Back Close forcing, so the BOC mixture can also have a positive impact for very small fractions of BC. Full Screen / Esc In simulation CS100 and CS10 only the optical properties of BC mixtures are changed. 25 Therefore the radiative forcings of non-BC species remain mainly unchanged, however Printer-friendly Version small changes occur due to feedbacks from the radiative impacts to the climate sys- Interactive Discussion tem. Table 3 gives global forcing numbers for BASE, CS100 and CS10, showing that only population BOC gives significantly different forcing numbers. Furthermore the differences between CS100 and CS10 are similar, so it will be sufficient here to discuss 4557 only experiment CS10. Even coatings that result from only 10% of the available OC mass are sufficient to change the optical properties between HI and CS significantly. ACPD First we will take a look at sulfate coatings. In the left column of Fig. 7 the absorption 10, 4543–4592, 2010 optical thickness for BCS particles are displayed. BCS absorption is higher in the 5 core-shell case, CS10, in sulfate rich areas, and slightly lower in the biomass burning areas. Globally BCS absorption is increased by a factor of 1.1. For BC particles that Carbonaceous are mainly coated by non-absorbing materials like sulfate and nitrate, the HI or CS Mie aerosol microphysics calculations give fairly similar results. The difference between BOC and BCS particles is that BOC particles can include S. E. Bauer et al. 10 OC in addition to sulfates, nitrates and water. OC is 10 times more abundant in the atmosphere than BC, so it is not surprising that these particles have a much smaller Title Page BC fraction than BCS (Fig. 6). The combination of a very small BC fraction and a large fraction of slightly absorbing organics in the shell leads to a large difference in Abstract Introduction absorption between the HI and CS assumption. Figure 7 shows that AAOT is enhanced Conclusions References 15 by a factor of 3.3 between BASE and CS10. However we were surprised to see such a strong impact on aerosol absorption, especially as several studies (Lesins et al., Tables Figures 2002; Fuller et al., 1999) have demonstrated that absorption calculated for coated BC particles are higher when they were treated as HI mixtures. To understand this J I behavior we plotted the percentage difference between CS and HI Mie calculations 20 (Fig. 8) for BC/OC mixtures (left) and BC/sulfate mixtures (right). Previous studies J I (Fuller et al., 1999; Bond et al., 2006; Lesins et al., 2002; Jacobson, 2000), mainly Back Close looked at sulfate coated BC particles. For BC-sulfate mixtures and for most sizes and BC mixing fractions below 40% we also see stronger absorption for HI, and therefore Full Screen / Esc agree with those studies. However this is different when BC is coated by OC, which is 25 less scattering than sulfate and slightly absorbing itself. Printer-friendly Version In our model most BOC particle radii are around 0.2 µm, and the BC fraction is below 1%. In this case we get a much stronger absorption when treating the particles as CS. Interactive Discussion However in our case this leads to such a strong positive BOC forcing (see Table 3) that the total net aerosol radiative forcing change between present day and pre-industrial

4558 would be positive. The strong forcing of coated BOC particles raises the question on how realistic this ACPD scenario could be. We suspect that in the real atmosphere such small BC fractions 10, 4543–4592, 2010 within a particle might be unrealistic. If the core is much smaller than the shell, the lo- 5 cation of the core within the particle matters. The Toon and Ackerman model assumes the core to be in the center of the particle, and therefore strongly enhances absorption, Carbonaceous this enhancement would be much reduced, when the core is not in the center. The aerosol microphysics smaller the core is relative to the particle, the less likely it is to be exactly in the center. The high enhancements occur only when the particle is perfectly concentric (Fuller, S. E. Bauer et al. 10 1995, 1999). Before we can draw any conclusions about the core – shell simulations we need to Title Page evaluate the simulated mixing state. We assume that we model unrealistically low BC fractions in the BOC populations and therefore will explore this problem in our ongoing Abstract Introduction work. Conclusions References

Tables Figures 15 5 Observational constraints

The experiments performed in this study show large sensitivities of the overall aerosol J I simulations to the chosen carbonaceous emission sizes and towards optical particle mixing configuration assumptions. In this section the results of sensitivity experi- J I ments are compared to observational datasets, including BC mass measurements and Back Close 20 AERONET (Holben et al., 1998) products. Table 4 compares the model with the averaged surface BC mass concentrations of Full Screen / Esc the European EMEP (2002–2003) and the North American IMPROVE (1995–2001) network. The base model underestimates European BC concentrations by a factor Printer-friendly Version of 1.6. Decreasing BC emission sizes (S1) improves the simulation, but surface BC Interactive Discussion 25 concentrations in Europe are still too low by a factor of 1.5. The model also underestimates surface BC concentrations in North America by a factor of 1.8, but with improved correlation coefficients compared to the European network. The IMPROVE 4559 network includes mainly rural stations and therefore can better be simulated by coarse models, whereas the EMEP network includes more urban stations, that observe more ACPD detailed sub-grid scale features. Comparing surface observations in polluted regions 10, 4543–4592, 2010 to a coarse 4◦×5◦ model is questionable, but we can conclude that the model under- 5 estimates North American and European surface concentrations. The S1 experiment leads to increased and therefore slightly improved surface concentrations in those re- Carbonaceous gions. aerosol microphysics Comparisons between surface concentrations, particularly in regions distant from sources, raise the question of how the BC is distributed vertically. Figure 2 already S. E. Bauer et al. 10 demonstrated the large variable of BC loads among the single experiments. The causes for the large diversity in aerosol load are the differences in aerosol mixing state, Title Page solubility and size distributions, which impacts BC lifetime through wet and dry removal changes and transport. Recent aircraft measurements provide BC profile measure- Abstract Introduction ments with Single Particle Soot absorption Photometers (SP2) onboard NASA and Conclusions References 15 NOAA research aircrafts (Schwartz et al., 2006; Slowik et al., 2007). In this paper

we used the same campaign data and averaging technique as in Koch et al. (2009). Tables Figures Figure 9 shows profiles measured in North American mid-latitude regions (a, d), in the tropics (b, c) and at high latitudes (e–i). The base model simulation generally agrees J I better with some observed profiles in the mid- and high-latitude regions, overestimates 20 BC concentrations in the tropics and underestimates BC concentrations in some cases J I at high latitudes. The sensitivity experiments show a rather uniform response globally. Back Close Smaller particle size BC/OC emissions (exp. S1) increases the overall BC mass in the troposphere while larger particles (S3, S4) leads to a decrease. However, no simula- Full Screen / Esc tion significantly improves the vertical distribution of BC mass concentrations, as we 25 already get over and underestimations of BC mass concentrations in different latitude Printer-friendly Version regions. Some measurements (g and i) are strongly impacted by biomass burning events, and these instantaneous events are not reflected in the climate model simula- Interactive Discussion tion. In summary we do see some systematic errors in the vertical distribution of BC (However our regional observational coverage is quite limited.):

4560 – The model overestimates upper-level BC in regions near sources, while surface concentrations in those regions look reasonable. ACPD – The model usually, but not always, underestimates surface BC in the Arctic. 10, 4543–4592, 2010 – The model always fails to reproduce the measured discontinuity (or rapid change) 5 in the vertical distribution in the Arctic. Carbonaceous aerosol microphysics – Finally, the changes in microphysics do not aﬀect the smoothness of the vertical proﬁle, so microphysics does not explain the model’s failure to reproduce the ob- S. E. Bauer et al. served sharp discontinuities, however we have to keep in mind that we compare observed snap shots to modeled average mean conditions. Title Page 10 AERONET (Dubovik et al., 2002) observations provide column amounts and absorption of aerosols. These are seasonally averaged for the year 2000. We use the aerosol Abstract Introduction optical thickness (AOT), which is the most reliable measurement from this network, and Conclusions References the absorption AOT (AAOT), the non-scattering part of the aerosol optical thickness, which involves a higher degree of uncertainty at low AOT conditions. Tables Figures 15 Figures 10 and 11 show the seasonal comparison between the AERONET data and the base model simulations. The simulation of AOT shows a good seasonal compari- J I son with AERONET. Many seasonal and regional features are well captured. Modeled AOT is underestimated in some megacities, including some European sites and in J I South America. Table 4. Presents the global as well as regional comparisons of AOT Back Close 20 and AAOT. AOT is best simulated in S1. With larger BC/OC emission sizes, BASE, S3 Full Screen / Esc and S4 AOT gets further underestimated. The core-shell experiments CS100 and CS10 show similar AOT values compared to BASE. S1 shows as well the best simulation of AAOT. Modeled AAOT values are systemati- Printer-friendly Version

cally too low in South America and Asia in all experiments. However, AOT was already Interactive Discussion 25 too low in South America and Asia and therefore the AAOT values cannot be evaluated in those two regions.

4561 The model simulates AOT and AAOT in the strongly dust inﬂuenced regions of Africa and the Arabian Peninsula best in experiments S3, and absorption is too strong in ACPD experiments CS and CS . 100 10 10, 4543–4592, 2010 In summary, BC surface concentrations are systematically underestimated in Europe 5 and the US, but the vertical distribution over the North American continent do not show a systematic bias. Aerosol optical measures, such as AOT and AAOT, are fairly well Carbonaceous simulated, and improve with the assumption of smaller BC particles, which results in aerosol microphysics larger BC loads and more internal mixing. Larger BC particles (experiments S3 and S. E. Bauer et al. S4) or extremely strong absorption due to BC (CS100, CS10) worsen the simulation. 10 Mineral dust is an important absorber and also impacts the comparisons in this study, but we have not carefully examined these eﬀects here. Title Page

Abstract Introduction 6 Black carbon reduction experiments Conclusions References Recently, attention has been drawn towards black carbon aerosols as a short-term climate warming mitigation candidate. Although the BC direct effect is definitely warm- Tables Figures 15 ing, there remains high uncertainty in estimates of indirect (cloud) changes associated with reductions, due to the complex nature of aerosol evolution and its climate interac- J I tions. Furthermore, BC emission reductions must be achieved by controlling individual sources. Therefore co-emitted, cooling species would be reduced as well. Here we J I conduct two idealized experiments where we simply reduce BC emissions from fossil Back Close 20 and biofuel burning by 50% (BC 50), and a scenario where BC fossil fuel emissions are reduced by 100% (BC BCFF). Because OC is not reduced, these experiments Full Screen / Esc evaluate the net effect of BC but not of individual sources. We also test two more realistic cases where both OC and BC from biofuel sources Printer-friendly Version are reduced by 50% (BCOCBF), and a scenario where BC and OC emissions from Interactive Discussion 25 on-road and off-road diesel were removed (BC diesel). Particles emitted from diesel engines have the highest BC fraction of any major source, and the OC:BC ratio of 2:1 is common to all engines. Thus, BC diesel reflects the likely impact of any control 4562 strategy that reduces diesel emissions, although it does not account for reductions in sulfur emissions which would be required to implement advanced controls. On the ACPD other hand, biofuel sources have different OC:BC ratios, ranging from 3:1 or 4:1 for 10, 4543–4592, 2010 wood cooking stoves to 6:1 for fireplaces or cooking with animal waste. The average 5 OC:BC reduction of 5.6:1 in the BCOCBF case provides an estimate of the response to reductions in high-OC sources, but may not exactly reflect the impact of individual Carbonaceous sources. See Table 5 for emission budgets. aerosol microphysics The BC reduction experiments are performed under present day conditions and are compared to PD to PI changes of the BASE experiment. In addition to the fuel emis- S. E. Bauer et al. 10 sions, biomass burning emissions for PD runs are 3 Tg/a BC and 24.8 Tg/a OC. Note that also the non-biomass burning OC emissions levels are reduced in experiment Title Page BCOCBF and BC diesel. The results of all BC reduction experiments are summarized in Table 5 and Fig. 12. In all experiments reducing BC emissions leads to less pos- Abstract Introduction itive direct aerosol impacts (ADE), with changes ranging from −0.05 to −0.15 W/m2. Conclusions References 15 BC 50 and BCFF show very similar results for ADE, whereas BCOCBF and BC diesel

show much weaker effects. This is caused in the diesel case by smaller BC emis- Tables Figures sion changes as well by a slight decrease in OC emissions. In the BCOCBF case some of the negative forcing change that is achieved by the BC emission reduction is J I counterbalanced by the positive forcing change caused by the reduced co-emitted OC 20 emissions. In both cases the reduced OC emissions over Europe and the US lead to J I reduced (BC diesel) or reversed (BCOCBF) BC mitigation effects, less positive forcing, Back Close in those regions, whereas in Asia and the high latitudes both measures would weaken the positive direct aerosol forcing. Full Screen / Esc Table 5 and Fig. 12 also give changes in cloud diagnostics. Reducing BC emissions 25 leads to a slight increase in CDNCs in all experiments, except BCOCBF. Changes to Printer-friendly Version CDNC come from a change in the aerosol mixing state. Reducing the number concentration of BC particles leads to less internally mixed BC-sulfate particles, and therefore Interactive Discussion to an increase in the number concentrations of externally mixed sulfate particles, which serve very effectively as CDNC.

4563 First we will discuss the experiments which resulted in reduced CDNCs: BC 50, BCFF, and BC diesel. Regionally, for example over Europe where the strongest change ACPD in CDNCs occurs, more CDNCs lead to an increase in cloud cover and therefore to 10, 4543–4592, 2010 a negative cloud forcing. However, globally, even though there are more CDNCs, 5 there is reduction in cloud cover and positive and negative AIEs ranging from 0.11 to −0.05 W/m2 per experiment. These cloud cover changes are caused by a compe- Carbonaceous tition between indirect and semi-direct eﬀects. Figure 12 shows the surface forcing aerosol microphysics changes in the single experiments. Reduced BC loads lead to surface heating, due to the missing absorbers in the atmosphere. More radiation can reach and warm the S. E. Bauer et al. 10 Earth surface, heat the lower atmosphere and lead to a decrease of low-cloud cover and LWP, leading to a positive cloud radiative forcing. These semi-direct eﬀect cloud Title Page changes dominate over the smaller cloud changes that result from CDNC enhancement. Abstract Introduction Case BCOCBF, where BC as well as OC emissions are reduced, shows a decrease Conclusions References 15 of −5% in CDNCs. Less CDNCs, leads to reduced LWP and cloud cover and therefore

logically to a smaller (less negative) overall cloud forcing, resulting in a cloud radiative Tables Figures impact (AIE) change of 0.2 W/m2 compared to the BASE case. In Summary: J I – Reducing BC emissions by 50% (BC 50) compared to the BASE case leads glob- J I 20 ally to increased CDNC, decreased cloud cover and LWP and a positive AIE change of +0.12 W/m2. Together with the direct aerosol impact of −0.15 W/m2 this Back Close leads to net forcing difference of −0.03 W/m2. In principle we would expect a neg- Full Screen / Esc ative AIE with increased CDNCs. However we see a negative AIE (Fig. 12) in the regions with increased CDNCs, e.g. Europe, South East Asia, Easter US, how- Printer-friendly Version 25 ever the global mean AIE is positive due to positive AIE contributions from remote ocean regions that are not directly impacted by CDNC changes. We speculate, Interactive Discussion that those changes, e.g. ADE leads to cooling at the surface and TOA over the Southern Hemispheric ocean and positive AIF, are caused by semi-direct effects that are related to aerosol direct impacts. 4564 – Removing fossil fuel emissions (BC FF) leads as well to an increase in CDNC, decreased cloud cover but an allover negative AIE of −0.05 W/m2. Together with ACPD the direct aerosol impact of −0.15 W/m2 this leads to net forcing difference of 10, 4543–4592, 2010 −0.2 W/m2.

5 – Removing diesel emissions (BC diesel) still leads to globally increased CDNCs Carbonaceous but to a lesser extend than the above discussed cases. India and South East Asia aerosol microphysics show a decrease in CDNC. Allover this leads to a strong negative AIE over Europe 2 and a global mean cloud forcing of −0.05 W/m . Combined with a negative ADE S. E. Bauer et al. this results in a net forcing diﬀerence of −0.1 W/m2.

10 – Reducing biofuel sources (BCOCBF) is the only scenario where we see de- Title Page creased CDNC concentrations. The strong reduction in BC and especially OC particles ﬁnally resulted in a reduced number concentration of activated particles, Abstract Introduction which was not the case in the prior discussed scenarios. Eventually this leads to Conclusions References an positive AIE +0.2 W/m2 and a net radiative change of +0.13 W/m2. Tables Figures 15 These model experiments show that the success of BC mitigation in reducing warming depends on the combination of direct and indirect eﬀects, and the resulting change in the net radiative forcing. As a reference, our BASE model estimates a net forcing of J I 2 −0.56 W/m for PD–PI. Our simple BC mitigation scenarios which ignore co-emitted J I species, BC 50 and BCFF, show an increase in aerosol cooling and therefore reduce 2 Back Close 20 global warming by −0.03 and −0.2 W/m . The two more “realistic” scenarios, BCOCBF and BC diesel, show that eliminating diesel emissions will also slow global warming by Full Screen / Esc −0.1 W/m2, whereas reducing biofuel sources would even accelerate global warming,

due to the combined reduction of BC and OC emissions and the resulting reductions Printer-friendly Version in cloud cover. Interactive Discussion

4565 7 Discussion and conclusions ACPD In this paper we studied the sensitivity of BC eﬀects on climate forcings with a climate model that includes detailed microphysical processes. Since BA08, MATRIX internally 10, 4543–4592, 2010 and externally mixed aerosol populations are now coupled to the radiation scheme and 5 to cloud indirect eﬀects (SM10). We examined single processes and particle properties Carbonaceous and their impacts on cloud and aerosol forcings. This study allows us to evaluate which aerosol microphysics processes need to be further studied, and what observational data are needed. First we discussed the importance of initial size distributions for particulate emissions S. E. Bauer et al. such as BC and OC. Evaluating the model with ambient BC mass and AERONET AOT 10 and AAOT measurements leads to the conclusion that best results are seen when BC/OC emission sizes are assumed to be between 0.01 and 0.05 µm. Aerosol mi- Title Page

crophysical models such as MATRIX now require particulate emission inventories that Abstract Introduction include information about aerosol mass, number, surface area, composition, and mixing state, and possibly including subgrid scale eﬀects, such as source-plume mixing, Conclusions References 15 that can not be treated by current climate models. Tables Figures Koch et al. (2009) showed that the previous generation of aerosol models without aerosol microphysics and mixing state inﬂuencing optical properties generally underestimate AAOT. In contrast, this study demonstrated a greatly improved agreement with J I

retrieved AAOT, as the internal mixing of BC enhances absorption. On the other hand, J I 20 Koch et al. (2009) found that these older models generally did not underestimate BC surface concentration; the rapid BC aging of the microphysical scheme in our model Back Close

caused faster washout near source regions and consequent underestimation of BC Full Screen / Esc surface concentrations. Thus it seems that the microphysics required to improve BC optical properties has excessively reduced model BC surface concentrations. Careful Printer-friendly Version 25 examination of regional tendencies may help resolve the new discrepancy. We found extremely large differences in aerosol radiative forcing amounts, −0.56 Interactive Discussion up to 0.35 W/m2, when testing different BC mixing configuration assumptions when internally mixed with other species. In the extreme case, assuming that all internally

4566 mixed OC forms a shell structure around BC, the net anthropogenic aerosol forcing would be positive. Simply assuming volume mixing rules for internally mixed particles ACPD is physically not realistic, but we found that for mainly sulfate coated BC particles the 10, 4543–4592, 2010 calculations using a simple volume mixing treatment are approximately equal to the 5 more complex core-shell calculations. However, we found large differences between assuming homogeneously mixed or core-shell configuration, when OC formed a large Carbonaceous coating around a very small BC particle. Absorption can be amplified by a factor of 3 aerosol microphysics when BC cores take up only 1% or less of the particle volume. In our model this is typically the case in the BC-OC population we simulate. However this is physically un- S. E. Bauer et al. 10 realistic and comparison to AeroNet absorption optical depth showed that this extreme simulation overestimates absorption. However in order to better understand this behav- Title Page ior we will need to better evaluate the aerosol mixing state simulated by our model. The largest volume fraction is taken up by aerosol water, a quantity that is hardly validated Abstract Introduction in aerosol models. Chemical and morphological analysis of different individual parti- Conclusions References 15 cles is needed, along with size selective bulk analysis, to understand such processes,

taking into account aerosol microphysical processes. Tables Figures One possible approach to observe mixing state including BC coating estimates was discussed by Shiraiwa et al. (2008), where the mixing state of BC in Asian air masses J I was examined by a combination of aerosol mass spectroscopy (AMS) and single- 20 particle soot photometer SP2 data. The AMS data provide a detailed chemical analysis J I of the measured species, and the SP2 was able to measure the size distribution, and Back Close the mixing state of BC. Shiraiwa et al. (2008) found a median value of the shell/core diameter ratio increased to 1.6 in Asian continental and maritime air masses with a Full Screen / Esc core diameter of 200 nm, while in free tropospheric and in Japanese air masses it was 25 1.3–1.4. It was estimated that internal mixing enhanced the BC absorption by a factor Printer-friendly Version of 1.5–1.6 compared to external mixing. The calculated absorption coeﬃcient was 2–3 times higher in Asian continental air masses than in clean air. A further approach would Interactive Discussion be the combination of AMS with single particle mass spectroscopy ATOF-MS (Prather et al., 1994), or chemical ionization mass spectroscopy, CIMS (Hearn and Smith, 2004)

4567 data. Our ongoing research will focus on the validation of mixing state with these newly emerging datasets. ACPD In light of the analysed model sensitivities towards microphysical BC characteriza- 10, 4543–4592, 2010 tion, we performed a set of BC mitigation studies. The model suggests that BC mitiga- 5 tion is always beneficial if only BC sources are reduced, as the direct radiative forcing is lowered, and the effects on clouds are weak. BC mitigation led to enhanced CDNC Carbonaceous production and a slightly stronger indirect effect; this finding is in contrast to assump- aerosol microphysics tions about reducing primary particle concentrations. However, semi-direct effects, induced by changed vertical heating profiles in the atmosphere, as well as the first and S. E. Bauer et al. 10 second indirect effect, are very important components of the response to altered BC emissions. The two more ‘realistic’ scenarios which include reduction of OC show that Title Page mitigating diesel emissions will slow global warming by −0.1 W/m2, whereas reducing biofuel sources could even accelerate global warming, mainly due to the impacts on Abstract Introduction clouds. Conclusions References 15 This study did not include ice cloud feedbacks or BC-snow ice albedo feedbacks

(Koch et al., 2009) that may enhance warming by BC. Despite the various limitations Tables Figures explored in this study regarding emission sizes and mixing state of BC particles, our model results suggest that a reduction in BC diesel emissions would help reduce pos- J I itive forcing even when including cloud changes. However, we stress that strategies 20 that include a reduction in BC emissions should not delay any GHG reduction plans, J I as the likely reductions in positive forcing from BC emissions are small und uncertain Back Close (from +0.12 up to −0.19 W/m2) compared to GHG forcings of 2.7 W/m2 and growing. Furthermore, BC reductions also reduce co-emitted species, many of which have neg- Full Screen / Esc ative forcings, such as OC and sulfate. For sources sufficiently rich in black carbon, 25 mitigation is beneficial, but aerosol microphysical processes and their interactions with Printer-friendly Version atmospheric dynamics need to be better constrained in order to estimate warming ben- efits from those measures. Interactive Discussion

4568 Acknowledgements. This work has been supported by the NASA MAP program Model- ing, Analysis and Prediction Climate Variability and Change (NN-H-04-Z-YS-008-N) and ACPD (NNH08ZDA001N-MAP). KT was supported by an appointment to the NASA Postdoctoral Program at the Goddard Institute for Space Studies, administered by Oak Ridge Associated 10, 4543–4592, 2010 5 Universities through a contract with NASA. We thank Andy Lacis for never getting tired of ex- plaining the GISS radiation code to us, and Jessica Sagona for her work with the AERONET data sets. We acknowledge AERONET data, available at http://aeronet/gsfc.nasa.gov; IM- Carbonaceous PROVE data available from http://vista.cira.colostate.edu/IMPROVE; and EMEP data from aerosol microphysics http://tarantula.nilu.no/projects/ccc. We are acknowledging the aircraft measurements made S. E. Bauer et al. 10 available to us by groups at NOAA: David Fahey, Ru-shan Gao, Joshua Schwarz, Ryan Spack- man, Laurel Watts; University of Tokyo: Yutaka Kondo, Nobuhiro Moteki; and University of Hawaii: Antony Clarke, Cameron McNaughton, Steﬀen Freitag. Title Page

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L., Cakmur, R., Perlwitz, J., Geogdzhayev, I., Ginoux, P.,Kohfeld, K., Koch, D., Prigent, 20 C., Ruedy, R., Schmidt, G., and Tegen, I.: Mineral dust aerosols in the NASA Goddard Institute for Space Sciences ModelE atmospheric general circulation model, J. Geophys. J I Res., 111, D06208, doi:10.1029/2005JD005796, 2006. Mishchenko, M. I., Travis, L. D., and Mackowski, D. W.: Tmatrix computations of light scattering J I by nonspherical particles: A review, J. Quant. Spectrosc. Radiat. Transfer, 55, 535–575, Back Close 25 1996. Penner, J. E., Zhang, S. Y., and Chuang, C. C.: Soot and smoke aerosol may not warm climate, Full Screen / Esc J. Geophys. Res., 108(D21), 4657, doi:10.1029/2003JD003409, 2003. Prather, K. A., Nordmeyer, T., and Salt, K.: Real-Time Characterization of Individual Aerosol- Printer-friendly Version Particles Using Time-Of-Flight Mass-Spectrometry, Anal. Chem., 66(9), 1403–1407, 1994. 30 Quaas, J., Ming, Y., Menon, S., Takemura, T., Wang, M., Penner, J. E., Gettelman, A., Lohmann, Interactive Discussion U., Bellouin, N., Boucher, O., Sayer, A. M., Thomas, G. E., McComiskey, A., Feingold, G., Hoose, C., Kristjansson,´ J. E., Liu, X., Balkanski, Y., Donner, L. J., Ginoux, P. A., Stier, P., Grandey, B., Feichter, J., Sednev, I., Bauer, S. E., Koch, D., Grainger, R. G., Kirkevag,˚ 4573 A., Iversen, T., Seland, Ø., Easter, R., Ghan, S. J., Rasch, P. J., Morrison, H., Lamarque, J.-F., Iacono, M. J., Kinne, S., and Schulz, M.: Aerosol indirect effects general circulation ACPD model intercomparison and evaluation with satellite data, Atmos. Chem. Phys., 9, 8697– 8717, 2009, http://www.atmos-chem-phys.net/9/8697/2009/. 10, 4543–4592, 2010 5 Riemer, N., West, M.,Zaveri, R. A., and Easter, R. C.: Simulating the evolution of soot mixing state with a particle-resolved aerosol model, J. Geophysical Res., 114, D09202, doi:10.1029/2008/JD011073, 2009. Carbonaceous Schulz, M., Textor, C., Kinne, S., Balkanski, Y., Bauer, S., Berntsen, T., Berglen, T., Boucher, aerosol microphysics O., Dentener, F., Guibert, S., Isaksen, I. S. A., Iversen, T., Koch, D., Kirkevag,˚ A., Liu, X., S. E. Bauer et al. 10 Montanaro, V., Myhre, G., Penner, J. E., Pitari, G., Reddy, S., Seland, Ø., Stier, P., and Takemura, T.: Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations, Atmos. Chem. Phys., 6, 5225–5246, 2006, http://www.atmos-chem-phys.net/6/5225/2006/. Title Page Schmidt, G. A., Ruedy, R., Hansen, J., Aleinov, I., Bell, N., Bauer, M., Bauer, S., Cairns, B., 15 Cheng, Y., DelGenio, A., Faluvegi, G., Friend, A., Hall, T. M., Hu, Y., Kelley, M., Kiang, N., Abstract Introduction Koch, D., Lacis, A. A., Lerner, J., Lo, K. K., Miller, R. L., Nazarenko, L., Oinas, V., Perlwitz, J., Perlwitz, J., Rind, D., Romanou, A., Russell, G. L., Shindell, D. T., Stone, P. 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R.: Radiative impact of mixing state of black carbon aerosol in Asian outflow, J. Geophys. Res., 113, D24210, doi:10.1029/2008JD010546, 2008. Slowik, J. G., Cross, E. S., Han, J. H., Davidovits, P., Onasch, T. B., Jayne, J. T., Williams, L. Carbonaceous R., Canagaratna, M. R., Worsnop, D. R., Chakrabarty, R. K., Moosmuller, H., Arnott, W. P., aerosol microphysics Schwarz, J. P., Gao, R. S., Fahey, D. W., Kok, G. L., and Petzold, A.: An intercomparison S. E. Bauer et al. 10 of instruments measuring black carbon content of soot particles, Aerosol Sci. Technol., 41, 295, doi:10.1080/02786820701197078, 2007. Stier, P., Feichter, J., Kinne, S., Kloster, S., Vignati, E., Wilson, J., Ganzeveld, L., Tegen, I., Werner, M., Balkanski, Y., Schulz, M., Boucher, O., Minikin, A., and Petzold, A.: The aerosol- Title Page climate model ECHAM5-HAM, Atmos. Chem. Phys., 5, 1125–1156, 2005, 15 http://www.atmos-chem-phys.net/5/1125/2005/. 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B. and Ackerman, T. P.: Algorithms for the calculation of scattering by stratified Full Screen / Esc spheres, Appl. Opt. 20, 3657–3660, 1981. van der Werf, G. R., Randerson, J. T, Collatz, G. C., and Giglio, L.: Carbon emissions from fires Printer-friendly Version in tropical and subtropical ecosystems, Global Change Biol., 9(4), 547–562, 2003. Interactive Discussion

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Table 1. Populations, constituents, and possible particle mixing state considerations in the Carbonaceous radiation calculations: homogeneously internally mixed (HI), externally mixed (EM), core shell aerosol microphysics particles (CS). S. E. Bauer et al. population description symbol constituents mixing state −2 − sulfate Aitken mode AKK SO4 , NO3 ,H2O EM/ HI −2 − sulfate accum. mode ACC SO4 , NO3 ,H2O EM/HI Title Page −2 − dust accum. mode (≤%5 inorg.) DD1 dust, SO4 , NO3 ,H2O EM −2 − Abstract Introduction dust accum. mode (>%5 inorg.) DS1 dust,SO4 , NO3 ,H2O EM/HI −2 − dust coarse mode (≤%5 inorg.) DD2 dust,SO4 , NO3 ,H2O EM −2 − Conclusions References dust coarse mode (>%5 inorg.) DS2 dust,SO4 , NO3 ,H2O EM/HI −2 − sea salt accum. mode SSA sea salt, SO4 , NO3 ,H2O EM/HI −2 − Tables Figures sea salt coarse mode SSC sea salt, SO4 , NO3 ,H2O EM/HI −2 − organic carbon (OC) OCC OC, SO4 , NO3 ,H2O EM/HI −2 − Black carbon (BC) (≤5% inorg.) BC1 BC,SO4 , NO3 ,H2O EM/HI or CS J I −2 − BC (5–20% inorg.) BC2 BC, SO4 , NO3 ,H2O EM/HI or CS −2 − J I BC (>20% inorg.) BC3 BC,SO4 , NO3 ,H2O HI or CS −2 − BC-OC BOC BC, OC, SO4 , NO3 ,H2O HI or CS −2 − Back Close BC-sulfate BCS BC,SO4 , NO3 ,H2O HI or CS (formed by coagulation) −2 − Full Screen / Esc BC-mineral dust DBC dust, BC,SO4 , NO3 ,H2O HI −2 mixed MXX dust, sea salt, BC, OC, SO4 , NO3,H2O HI Printer-friendly Version

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Table 2. Emission sizes, geometric mean diameters in [µm] of carbonaceous aerosols. Title Page OC [OCC] BC BC-OC Abstract Introduction fossil & [BC1] [BOC] biofuel fossil & biomass Conclusions References biofuel burning Tables Figures S1 0.01 0.01 0.025 BASE 0.05 0.05 0.10 S3 0.1 0.1 0.25 J I S4 0.5 0.5 1 J I

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S. E. Bauer et al. Table 3. Global annual average values for the net aerosol direct eﬀect (ADE), for sulfate and nitrate (ACC), OC (OCC), BC, sulfate and nitrate (BC-SU, population BC1, BC2, BC3, BCS), BC, sulfate, nitrate and OC (BC-OC, population BOC); AIF is indirect and semi-direct eﬀect; NR Title Page is net radiative forcing (AIF + ADF)TOA.

Abstract Introduction ADEtoa ADEtoa ADEtoa ADEtoa ADEtoa AIF ADFtoa NR ACC OCC BC-SU BC-OC total PD- PI PD-PI PD-PI PD PD PD PD PD Conclusions References

BASE −0.05 −0.16 0.05 0.1 −1.78 −0.45 −0.11 −0.56 Tables Figures Emission size experiments S1 −0.01 −0.01 0.01 0.36 −2.68 −0.65 −0.09 −0.74 S3 −0.15 −0.34 0.09 0.09 −1.71 −0.35 −0.25 −0.60 J I S4 −0.37 −0.05 0.03 0.08 −1.18 −0.13 −0.19 −0.32 Radiation experiments J I CS −0.05 −0.16 0.05 1.61 −0.27 −0.60 1.09 0.49 CS10 −0.05 −0.16 0.05 1.33 −0.55 −0.53 0.88 0.35 Back Close

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S. E. Bauer et al. Table 4. Comparison between model simulations and EMEP (EU) and IMPROVE (US) surface BC concentrations, and AERONET absorption optical thickness (AAOT) and optical thickness (AOT). Year 2000 AERONET data are used. Model data are averaged at station locations. Gray shadings mark model values that are lower than the observation, red shadings show Title Page higher modeled values, and yellow shading indicates best simulation. Global and regional comparisons are presented for, North America, South America, Europe, Africa (including Arabian Abstract Introduction peninsula) and Asia. Conclusions References

EU US AERONET Tables Figures BC mass [µg/m3] AAOT AOT EU US Global N. Am. S. Am. Europe Africa Asia Global N. Am. S. Am Europe Africa Asia # stat. 14 55 77 33 9 13 10 6 77 33 9 13 10 6 Obs. 0.65 0.29 0.018 0.0061 0.0014 0.0035 0.0032 0.0031 0.19 0.063 0.011 0.041 0.035 0.031 J I BASE 0.41 0.16 0.014 0.0054 0.0004 0.0026 0.0037 0.0016 0.16 0.057 0.003 0.032 0.041 0.017 S1 0.44 0.17 0.018 0.0066 0.0005 0.0036 0.0032 0.0022 0.19 0.066 0.004 0.040 0.041 0.020 S3 0.38 0.14 0.010 0.0032 0.0003 0.0018 0.0032 0.0013 0.14 0.047 0.004 0.028 0.035 0.015 S4 0.31 0.12 0.008 0.0019 0.0002 0.0013 0.0033 0.0009 0.11 0.033 0.003 0.022 0.033 0.011 J I CS100 0.42 0.16 0.023 0.0090 0.0007 0.0043 0.0045 0.0028 0.16 0.057 0.003 0.032 0.042 0.017 CS10 0.43 0.16 0.021 0.0082 0.0006 0.0043 0.0044 0.0024 0.16 0.057 0.003 0.033 0.043 0.017 Back Close

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Table 5. Global mean budgets of the BASE experiment, for present day conditions (PD), differ- S. E. Bauer et al. ences between PD and PI, and differences between BASE run and BC reduction experiments under PD conditions. The following variables are listed: Non-biomass burning BC and OC emissions, (⊗ Biomass burning emissions are 3 Tg/a BC and 24.8 Tg/a OC for present-day and Title Page 1. T/g BC and 9.1 Tg/a OC for pre-industrial times), CDNC (cm3), Liquid water path [mg/m2], total cloud cover [%], the indirect and semi-direct effect (AIE), aerosol direct effect (ADE) at the Abstract Introduction surface and TOA, and total net radiative change (NR) [W/m2]. Conclusions References 3 2 BC1 OCC CDNC [cm ] LWP [mg/m ] Cloud Cover [%] AIETOA ADETOA NR TOA ADESURF Emission Emission [W/m2] [W/m2] [W/m2] [W/m2] Tables Figures [Tg/a] [Tg/a] BASE (PD) 4.6 22.1 162 702 60.1 −1.78 −1.75 −4.62 BASE 0.4 14.6 41 7.0 0.2 −0.45 −0.11 −0.56 −1.32 (PD-PI)⊗ J I Mitigation experiments: Delta between experiment and BASE, (BASE-EXP) BC 50 (PD) −2.3 0 4 −0.2 −0.05 0.12 −0.15 −0.03 0.13 J I BCFF (PD) −3.0 0 3 −0.43 −0.07 −0.05 −-0.15 −0.20 0.20 BCOCBF (PD) −1.6 −6.4 −5 −2.52 −0.14 0.20 −0.07 0.13 0.21 Back Close BC diesel(PD) −1.3 −0.4 2 0.98 0 −0.05 −0.05 −0.10 0.11

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J I 2 Fig. 1.FDiigurﬀerencese 1. Diff ineren presentces in daypresen totalt day column total co aerosollumn aero masssol concentrationsmass concentrati [µg/mons [µg]/ perm2] mixingper J I state betweenmixing stat experimente between ex S1peri andment BASES1 and (S1-BASE). BASE (S1–B TheASE) ﬁrst. The row first shows row sh externallyows external mixedly sulfatemi (populationxed sulfate (p ACCopulat andion A AKK),CC and sulfate AKK), predominantlysulfate predominan mixedtly mi withxed w OCith O (OCC,C (OCC, BOC), BOC),BC Back Close (BC1,BC BC2, (BC1 BC3,, BC2 BCS),, BC3 and, BCS sulfate), and mixedsulfate withmixed coarse with co aerosolsarse aero (DS1,sols (D DS2,S1, D SSC,S2, SS MXX).C, MXX Rows). Full Screen / Esc two andRo threews two show and thethree corresponding show the corres changesponding ch inan mixingges in statemixing for st BCate fo andr BC OC an mixtures.d OC mixt Graphsures. show onlyGraph thes sh sulfate,ow only orthe BC, sulfat OCe, respectively,or BC, OC res masspectivel presenty, mass inpres thoseent in mixtures. those mix Sometures. So aerosolme concentrationsaerosol con arecen dividedtrations are by adi factorvided b ofy tena fact toor match of ten theto mat colorch barthe co aslo indicatedr bar as in indicat theed individual in the Printer-friendly Version title ofi thendiv mapidual title of the map. Interactive Discussion

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J I Figure 2 Zonal mean BC mass mixing ratios for the BASE case and the differences between Fig. 2. Zonal mean BC mass mixing ratios for the BASE case and the diﬀerences between Back Close BASE (BA) and the size experiments, S1, S3, S4 and the radiation experiments CS100 and CS10. BASEGl (BA)obal andmean the BC size mas experiments,s concentratio S1,ns S3,per S4exper andimen thet are radiation given experimentsin the figure CS t100itlesand. Full Screen / Esc CS10. Global mean BC mass concentrations per experiment are given in the ﬁgure titles.

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Figure 3 Changes between BASE and S1 (S1-BASE) (upper panels) and (S3 - BASE) (lower Printer-friendly Version panels) for AOT, CDNC [#/cm3], ADE [W/m2] and AIE [W/m2]. All maps show differences Fig. 3. Changes betweenbetw BASEeen the exp anderiment S1s for p (S1-BASE)resent day condition (uppers. panels) and (S3-BASE) (lower 3 2 2 Interactive Discussion panels) for AOT, CDNC [#/cm ], ADE [W/m ] and AIE [W/m ]. All maps show diﬀerences between the experiments for present day conditions.

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Carbonaceous aerosol microphysics

S. E. Bauer et al.

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Back Close Fig. 4. Global mean AIE and ADE [W/m2] values for all size experiments, (S1 – 4) and the base experiment, BA, forFig presenture 4 day Gl conditions.obal mean AIE and ADE [W/m2] values for all size experimenFull Screents, (S1 / Esc – 4) and the base experiment, BA, for present day conditions. Printer-friendly Version

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Carbonaceous aerosol microphysics

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J I Fig. 5. FPresentigure 5 Pres dayen radiativet day radi forcing,ative forci ADE,ng, A atDE the, at t tophe to ofp theof th atmospheree atmosphere byby s speciespecies fo forr the the BASE experiment.BASE experim Externallyent. Extern mixedally mix sulfatesed sulfat andes an nitratesd nitrates (ACC), (ACC), organics organics ( (OCC),OCC), b blacklack carb carbon/on/ Back Close sulfate/nitratesulfate/n mixtureitrate mix (BCture –(B SU),C – S blackU), bl carbon/organicack carbon/organi carbonc carbon mixture mixture (BC(BC –– OC),OC), d dustust an andd sea sea 2 2 salt. Unitssalt. areUnit [W/ms are [W]./m ]. Full Screen / Esc

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Carbonaceous aerosol microphysics

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Fig. 6. Fractional contributions of BC, sulfate/nitrate and OC, in population BOC (upper row), and BCS (lower row). Aerosol water is not displayed. Back Close Figure 6 Fractional contributions of BC, sulfate/nitrate and OC, in population BOC (upper row), and BCS (lower row). Aerosol water is not displayed. Full Screen / Esc

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Full Screen / Esc Fig. 7. Absorption optical thickness of BC – sulfate/nitrate mixtures (left column), and BOC (right column), for experiments BASE and CS10 Printer-friendly Version Figure 7 Absorption optical thickness of BC – sulfate/nitrate mixtures (left column), and BOC (right column), for experiments BASE and CS . 10 Interactive Discussion

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Carbonaceous aerosol microphysics

S. E. Bauer et al.

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Figure 8 Percentage difference [((CS-HI)–CS)/CS] in absorption between a core-shell and a J I Fig. 8. Percentagehomogeneo diusﬀ mierencexing Mi [((CS-HI)–CS)/CS]e calculation, for BC co inresabsorption with OC coat betweening (left) an ad core-shellsulfate coatin andg a homogeneous(righ mixingt). Mie calculation, for BC cores with OC coating (left) and sulfate coating J I (right). Back Close

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ACPD 10, 4543–4592, 2010

Carbonaceous aerosol microphysics

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Figure 9 Modeled (colored) and observed (black lines) BC mass profiles in [ng/kg] in the tropics Full Screen / Esc Fig. 9. Modeledan (colored)d mid-latitudes and (a–d) an observedd ARCPAC and (black ARCTASlines) campaign BC(e–i). massObservatio profilesns are averag ined [ng/kg] in the tropics for the respective campaigns, with standard deviations where available. The two black lines in Fig and mid-latitudesa) s(a–d)how measandurement ARCPACs on two different and days. A ARCTASll other profiles campaigngive multiple day av(e–i)erages.. M Observationsean are aver- (solid) and median (dashed) observed profiles are provided except that (g) the ARCPAC Printer-friendly Version aged for the respectivecampaign has campaigns, distinct profiles for with the mean standard of the 4 flig deviationshts that probed lo whereng-range bi available.omass The two black lines in (a) showbu measurementsrning plumes (dashed) and on mean two for the1 difliffgherentt that samp days.led aged A Allrctic otherair (solid). profiles give multiple day av- erages. Mean (solid) and median (dashed) observed profiles are provided except that (g) the Interactive Discussion ARCPAC campaign has distinct profiles for9 the mean of the 4 flights that probed long-range biomass burning plumes (dashed) and mean for the1flight that sampled aged Arctic air (solid).

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Fig. 10. Aerosol optical thicknessFigure 10 Aero atso 550l opti nmcal th foricknes clears at 55 sky0 nm conditionsfor clear sky co shownnditions sh forown thefor th foure four seasons. Interactive Discussion seasons. Left column shows model data and the filled circles in the right column give the Left column shows modelA dataERONE andT data. the ﬁlled circles in the right column give the AERONET data.

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Interactive Discussion Fig. 11. Aerosol absorptionFigure 11 opticalAerosol ab thicknesssorption opticalat thick 550ness nmat 550 for nm clearfor clear skysky co conditionsnditions shown shownfor the for the four seasons. Left columnfour seas showsons. Left modelcolumn sho dataws mod andel data in an thed in th righte right co columnlumn AER AERONETONET data data.

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ACPD BASE PD – PI -3 2 2 2 CDNC [cm ] Cloud Cover *10 [%] AIF [W/m ] ADFTOA[W/m ] ADFsurf[W/m ] 10, 4543–4592, 2010

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BC_50 -3 2 2 2 aerosol microphysics CDNC [cm ] Cloud Cover *10 [%] AIE [W/m ] ADETOA[W/m ] ADEsurf[W/m ] S. E. Bauer et al.

BCFF -3 2 2 2 CDNC [cm ] Cloud Cover *10 [%] AIE [W/m ] ADETOA[W/m ] ADEsurf[W/m ] Title Page

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BCOCBF -3 2 2 2 CDNC [cm ] Cloud Cover *10 [%] AIE [W/m ] ADETOA[W/m ] ADEsurf[W/m ] Tables Figures

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BC_diesel -3 2 2 2 CDNC [cm ] Cloud Cover *10 [%] AIE [W/m ] ADETOA[W/m ] ADEsurf[W/m ] J I

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Figure 12 Differences in C−D3NC [cm-3], total cloud cover [%], top of the atmosphere cloud (AIE), Fig. 12. Differencesaero insol CDNC(ADE) and [cmsurface aero],so totall forcin cloudg [W/m2], coverbetween th [%],e PD an topd PI ofin th thee BAS atmosphereE case cloud (AIE), Printer-friendly Version aerosol (ADE) and(fi surfacerst panel) andaerosol the differenceforcing between BA [W/mSE and th2e], fou betweenr BC reduction cas thees. Cl PDoud co andver and PI in the BASE case ADE are multiplied by a factor of 10 to fit the color scheme. Interactive Discussion (first panel) and the difference between BASE and the four BC reduction cases. Cloud cover and ADE are multiplied by a factor of 10 to fit the color scheme.

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