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Atmospheric Peroxyacetyl Nitrate (PAN): a Global Budget and Source Attribution

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Atmospheric Peroxyacetyl Nitrate (PAN): a Global Budget and Source Attribution Atmospheric Peroxyacetyl Nitrate (PAN): A Global Budget and Source Attribution The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Fischer, E. V., Daniel James Jacob, Robert M. Yantosca, Melissa Payer Sulprizio, D. B. Millet, J. Mao, F. Paulot, et al. 2014. “Atmospheric Peroxyacetyl Nitrate (PAN): A Global Budget and Source Attribution.” Atmospheric Chemistry and Physics 14 (5): 2679–2698. Published Version doi:10.5194/acp-14-2679-2014 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:13792758 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA Open Access Atmos. Chem. Phys., 14, 2679–2698, 2014 Atmospheric www.atmos-chem-phys.net/14/2679/2014/ doi:10.5194/acp-14-2679-2014 Chemistry © Author(s) 2014. CC Attribution 3.0 License. and Physics Atmospheric peroxyacetyl nitrate (PAN): a global budget and source attribution E. V. Fischer1, D. J. Jacob2, R. M. Yantosca2, M. P. Sulprizio2, D. B. Millet3, J. Mao4, F. Paulot1, H. B. Singh5, A. Roiger6, L. Ries7, R.W. Talbot8, K. Dzepina9, and S. Pandey Deolal10 1Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA 3Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN, USA 4Princeton University, GFDL, Princeton, NJ, USA 5NASA Ames Research Center, Moffett Field, CA, USA 6Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany 7Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA 8Federal Environment Agency, GAW Global Station Zugspitze/Hohenpeissenberg, Zugspitze, Germany 9Department of Chemistry, Michigan Technological University, Houghton, MI, USA 10Bluesign Technologies AG, St. Gallen, Switzerland Correspondence to: E. V. Fischer ([email protected]) Received: 25 August 2013 – Published in Atmos. Chem. Phys. Discuss.: 15 October 2013 Revised: 18 January 2014 – Accepted: 30 January 2014 – Published: 14 March 2014 Abstract. Peroxyacetyl nitrate (PAN) formed in the at- Lightning NOx is the dominant contributor to the observed mospheric oxidation of non-methane volatile organic com- PAN maximum in the free troposphere over the South At- pounds (NMVOCs) is the principal tropospheric reservoir lantic. for nitrogen oxide radicals (NOx = NO + NO2). PAN enables the transport and release of NOx to the remote troposphere with major implications for the global distributions of ozone 1 Introduction and OH, the main tropospheric oxidants. Simulation of PAN is a challenge for global models because of the dependence Peroxyacetic nitric anhydride (CH3COO2NO2), commonly of PAN on vertical transport as well as complex and uncer- known by its misnomer peroxyacetyl nitrate (PAN), is the tain NMVOC sources and chemistry. Here we use an im- principal tropospheric reservoir species for nitrogen oxide proved representation of NMVOCs in a global 3-D chemical radicals (NOx = NO + NO2) with important implications for transport model (GEOS-Chem) and show that it can simu- the production of tropospheric ozone (O3) and the hydroxyl late PAN observations from aircraft campaigns worldwide. radical OH (the main atmospheric oxidant) (Singh and Hanst, The immediate carbonyl precursors for PAN formation in- 1981). PAN is formed by oxidation of non-methane volatile clude acetaldehyde (44 % of the global source), methylgly- organic compounds (NMVOCs) in the presence of NOx. oxal (30 %), acetone (7 %), and a suite of other isoprene and NMVOCs and NOx have both natural and anthropogenic terpene oxidation products (19 %). A diversity of NMVOC sources. Fossil fuel combustion is the principal NOx source, emissions is responsible for PAN formation globally in- with additional contributions from biomass burning, light- cluding isoprene (37 %) and alkanes (14 %). Anthropogenic ning and soils (van der A et al., 2008). The organic side of sources are dominant in the extratropical Northern Hemi- PAN formation involves many stages of NMVOC oxidation. sphere outside the growing season. Open fires appear to play Most NMVOCs can serve as PAN precursors, but the yields little role except at high northern latitudes in spring, although vary widely (Roberts, 2007). results are very sensitive to plume chemistry and plume rise. Published by Copernicus Publications on behalf of the European Geosciences Union. 2680 E. V. Fischer et al.: PAN: a global budget and source attribution PAN enables the long-range transport of NOx at cold tem- the atmospheric oxidants and nitrogen deposition will be the peratures, and PAN decomposition releases NOx in the re- focus of a subsequent paper. mote troposphere where it is most efficient at producing O3 and OH (Singh and Hanst, 1981; Hudman et al., 2004; Fis- cher et al., 2010; Singh, 1987). NOx abundance controls the 2 Model description balance of O3 production and destruction. Without PAN for- We use the GEOS-Chem global 3-D CTM including de- mation the distributions of tropospheric NOx,O3 and OH tailed ozone–NOx–VOC–aerosol chemistry (version 9.01.01, would be very different, with higher values in NOx source re- gions and lower values in the remote troposphere (Kasibhatla www.geos-chem.org) with significant modifications as de- et al., 1993; Moxim et al., 1996; Wang et al., 1998a). PAN scribed below. chemistry can also be important for oxidant formation on a 2.1 Chemistry regional scale. In polluted environments, PAN formation is a sink for both NOx and hydrogen oxide radicals (HOx). Ob- GEOS-Chem uses a chemical scheme originally described servations show that O3 concentrations increase when tem- by Horowitz et al. (1998) and Bey et al. (2001), with re- perature increases, and this has been in part related to PAN cent updates outlined in Mao et al. (2010). Following Marais thermal instability (Sillman and Samson, 1995). Observa- et al. (2012) we have updated the rate coefficients for the tions also show that the production of PAN becomes more reactions of HO2 with the > C2 peroxy radicals to Eq. (iv) efficient relative to O3 in highly polluted air masses (Roberts in Saunders et al. (2003). We also include nighttime reac- et al., 1995). Thus a comprehensive understanding of PAN is tions of organic peroxy radicals with NO3 following Stone needed to understand oxidant distributions on a spectrum of et al. (2013). To implement the Stone et al. (2013) night- scales. time chemistry, we went through each of the RO2 + NO reac- A large body of PAN observations worldwide has accu- tions in the GEOS-Chem chemical mechanism, copied each mulated over the years, including in particular from aircraft of these reactions, and changed the RO2 reactants to react platforms and mountaintop sites. There have also been recent with NO3 rather than NO. The Master Chemical Mechanism retrievals of PAN concentrations in the upper troposphere (MCM) considers three different reactions rates for this class, (UT) from satellites (Glatthor et al., 2007; Tereszchuk et al., one for CH3O2, one for RC(O)O2 and one for all other RO2. 2013). Concentrations vary from pptv levels in warm remote There is no temperature dependence included, and all prod- locations such as tropical oceans to ppbv levels in polluted ucts are assumed to be the same as the corresponding reaction source regions. Despite the relatively large database of mea- of the RO2 radical with NO (Bloss et al., 2005). We replaced surements compared to other photochemical indicators, sim- the isoprene chemical mechanism with one based on Paulot ulation of PAN in global chemical transport models (CTMs) et al. (2009a, b), as described by Mao et al. (2013b). has been a difficult challenge because of the complexity of PAN is produced reversibly by reaction of the peroxy- PAN chemistry. Recent model intercomparisons show very acetyl (PA) radical CH3C(O)OO with NO2: large difference among themselves and with observations in many regions of the atmosphere (Thakur et al., 1999; Singh CH3C(O)OO + NO2 + M PAN + M, (R1) et al., 2007; von Kuhlmann et al., 2003; Sudo et al., 2002), but confirm the very important role for PAN in sustaining O3 where M is a third body (typically N2 or O2). The dominant production in remote air (Zhang et al., 2008; Hudman et al., sources of CH3C(O)OO are the oxidation of acetaldehyde 2004). (CH3CHO) and the photolysis of acetone (CH3C(O)CH3) Here we exploit a worldwide collection of PAN observa- and methylglyoxal (CH3COCHO): tions to improve the PAN simulation in the GEOS-Chem O2 (Goddard Earth Observing System) CTM, which has been CH3CHO + OH −→ CH3C(O)OO + H2O (R2) used extensively in global studies of tropospheric oxidants O2 (Bey et al., 2001; Sauvage et al., 2007; Murray et al., 2012). CH3C(O)CH3 + hυ −→ CH3C(O)OO + CH3 (R3) The earliest global models that included PAN chemistry (Ka- O2 CH3COCHO + hυ −→ CH3C(O)OO + HCO. (R4) sibhatla et al., 1993; Moxim et al., 1996) relied on highly simplified NMVOC budgets. Our improvements involve new PAN can also be produced at night via reaction of acetalde- treatments of NMVOC sources and chemistry, a well-known hyde with the nitrate radical. Acetaldehyde, acetone and weakness even in current CTMs (Williams et al., 2013; Ito et methylglyoxal are all directly emitted (“primary” sources) al., 2007). Our new simulation, which captures the major fea- and produced in the atmosphere from oxidation of primary tures of the existing observations, affords a new opportunity emitted NMVOCs (“secondary” sources). These different to understand the factors driving the global PAN distribution sources will be discussed below. There are also other minor and the essential chemistry that needs to be described. A de- sources of the PA radical, again to be discussed below.
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