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Decreasing Particle Number Concentrations in a Warming Atmosphere and Implications F Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Atmos. Chem. Phys. Discuss., 11, 27913–27936, 2011 Atmospheric www.atmos-chem-phys-discuss.net/11/27913/2011/ Chemistry doi:10.5194/acpd-11-27913-2011 and Physics © Author(s) 2011. CC Attribution 3.0 License. Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Decreasing particle number concentrations in a warming atmosphere and implications F. Yu1, G. Luo1, R. P. Turco2, J. A. Ogren3, and R. M. Yantosca4 1Atmospheric Sciences Research Center, State University of New York at Albany, USA 2Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, USA 3Global Monitoring Division (GMD), Earth System Research Laboratory (ESRL), NOAA, USA 4School of Engineering and Applied Sciences, Harvard University, USA Received: 22 August 2011 – Accepted: 29 September 2011 – Published: 14 October 2011 Correspondence to: F. Yu ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 27913 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract New particle formation contributes significantly to the number concentration of conden- sation nuclei (CN) as well as cloud CN (CCN), a key factor determining aerosol indirect radiative forcing of the climate system. Using a physics-based nucleation mechanism 5 that is consistent with a range of field observations of aerosol formation, it is shown that projected increases in global temperatures could significantly inhibit new particle, and CCN, formation rates worldwide. An analysis of CN concentrations observed at four NOAA ESRL/GMD baseline stations since the 1970s and two other sites since 1990s reveals long-term decreasing trends consistent with these predictions. The analysis 10 also suggests, owing to larger observed CN reductions at remote sites than can be explained by the basic nucleation mechanism, that dimethylsulphide (DMS) emissions may be decreasing worldwide with increasing global temperatures, implying a posi- tive DMS-based cloud feedback forcing of the climate (“CLAW”). The combined effects of rising temperatures on aerosol nucleation rates, and possibly on DMS emissions, 15 may imply substantial decreases in future tropospheric particle abundances associated with global warming, delineating a potentially significant feedback mechanism that in- creases Earth’s climate sensitivity to greenhouse gas emissions. Further research is needed to quantify the magnitude of such a feedback process. 1 Introduction 20 Atmospheric particles affect the Earth’s energy budget directly by scattering solar radi- ation, and indirectly by acting as cloud condensation nuclei (CCN), which affect cloud optical properties and influence precipitation (e.g., Twomey, 1977). The magnitude of aerosol indirect radiative forcing is poorly constrained in climate models, and repre- sents one of the greatest remaining uncertainties in assessing climate change (IPCC, 25 2007). A major source of atmospheric CCN is the secondary particle formation fre- quently observed in the atmosphere (Pierce and Adams, 2007; Makkonen et al., 2009; 27914 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Wang and Penner, 2009; Yu and Luo, 2009; Spracklen et al., 2010; Kazil et al., 2010). The non-linear dependence of aerosol nucleation rates on key atmospheric parame- ters, and the influence of aerosol formation on CCN populations, points to a potentially important climate feedback mechanism involving interactions between particle nucle- 5 ation, CCN production, cloud microphysics, and climatic responses. Ambient atmospheric temperatures (T ) and sulfuric acid vapor concentrations ([H2SO4]) are the dominant factors controlling particle nucleation events in many re- gional settings (although other parameters, such as relative humidity and ionization rates, can also contribute in determining nucleation rates; e.g., Yu, 2010). In the 10 past, it was suggested that future climatic warming might change oceanic emissions of dimethylsulphide (DMS), thus influencing [H2SO4], sulfate aerosols, and the indi- rect radiative forcing of marine stratus clouds, defining a climate feedback process (i.e., the CLAW hypothesis; Charlson et al., 1987). After more than two decades of study, however, the strength and even the sign (i.e., negative or positive) of the CLAW 15 feedback process remain ambiguous (e.g., Ayers and Cainey, 2007; Kloster et al., 2007; Woodhouse et al., 2010). In contrast to the large number of studies consid- ering CLAW (∼ 1500 publications since 1987, according to Ayers and Cainey, 2007), very few investigations have been carried out to quantify the direct effects of climate change (specifically, temperature variations) on new particle and CCN formation rates, 20 while the associated climate feedback processes, which offer a more direct physical linkage, have never been investigated. Hence, in this study, we seek to quantify the magnitude of a positive climate system feedback involving the influence of temperature on particle nucleation rates. Long term trends in CN concentrations, derived for the first time from six observational stations with ∼ 15–35 yr records of CN data, appear to 25 support such a feedback mechanism. Moreover, a consistent interpretation of the CN data, as presented below, may also indicate that the sign of CLAW feedback process is likely to be positive – that is, global warming might be enhanced through temperature sensitive changes in DMS emissions (although no direct evidence of the nature of the temperature-DMS relationship is presented here). 27915 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 Effect of temperature increase on global particle formation rates and concentrations Physically, temperature (T ) is a key parameter determining particle nucleation rates (J). Figures 1 and 2 show the dependence of J on T based on several different nucleation 5 theories. In Fig. 1, the sensitivity of J to T has been determined using a physics- based ion-mediated nucleation (IMN) model that incorporates recently available ther- modynamic data (Yu, 2010) and has been tested against detailed multiple-instrument measurements of freshly nucleated particles, including their electrical charge state (Yu and Turco, 2011). The IMN reduces to binary homogeneous nucleation (BHN) (Yu, 10 2007) when the ionization rate is zero. The percentage change in the nucleation rate (J) (i.e., PCJ ) per degree increase in T is a non-linear function of temperature. IMN rates are very sensitive to temperature as J values increase from insignificant (<∼ 0.01 cm−3 s−1) to significant (>∼ 1 cm−3 s−1). At J = 0.5 cm−3 s−1, under the condi- tions considered in Fig. 1, the values of PCJ based on the IMN model are in the range ◦ −3 −1 15 of −80–−20 % per C. After JIMN reaches a significant value (∼ 1–20 cm s ), further decreases in T (other parameters fixed) have small effects, because nucleation under such conditions is limited by the ionization rate. When T is very low, binary homoge- neous nucleation (BHN) becomes dominant and PCJ for IMN approaches that for BHN. Compared to BHN alone, IMN yields larger PCJ at higher ambient temperatures, but 20 smaller values at lower temperatures. If averaged over the entire ambient temperature range of interest, PCJ values for BHN are greater than those for the integrated IMN mechanism. Figure 2 shows a similar or larger effect of temperature on nucleation rates based on a different BHN (Vehkamaki et al., 2002) and several ternary homogeneous nucleation 25 (THN) models (Napari et al., 2002; Yu, 2006; Merikanto et al., 2007). The THN model of Napari et al. (2002), now known to overestimate the THN rates significantly (Yu, 2006; Merikanto et al., 2007), gives the largest values of PCJ . It should be noted that some unknown or yet to be established nucleation mechanisms may also contribute 27916 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | to the production of new particles in the atmosphere. We expect that such nucleation mechanisms would also be quite sensitive to temperature, as this is a key parame- ter controlling the saturation vapor pressures of precursors, stability of pre-nucleation clusters, and thus nucleation rates. 5 In order to confidently assess the significance of the temperature feedback mecha- nism associated with new particle formation and CCN evolution, a global aerosol model that can treat particle microphysical processes – especially nucleation and growth – is essential. Baseline aerosol simulations should also be carefully validated against ob- servational data. Yu and Luo (2009) incorporated an advanced particle microphysics 10 (APM) model into GEOS-Chem, a global 3-D model of atmospheric composition driven by assimilated meteorological data from the NASA Goddard Earth Observing System 5 (GEOS-5). The GEOS-Chem model has been developed and used by many research groups and contains a number of state-of-the-art modules treating various chemical and aerosol processes (e.g., Bey et al., 2001; Liu et al., 2001; Martin et al., 2003; 15 Park et al., 2004; Evans and Jacob, 2005; Liao et al., 2007; Fountoukis and Nenes, 2007; Trivitayanurak et al., 2008; Henze et al., 2008; Yu and Luo, 2009) with up-to-date key emission inventories (e.g., Guenther et al., 2006; Bond et al., 2007; Zhang et al., 2009). The APM model in GEOS-Chem
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