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Regional Influence of Wildfires on Aerosol Chemistry in the Western US

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Regional Influence of Wildfires on Aerosol Chemistry in the Western US Atmos. Chem. Phys., 17, 2477–2493, 2017 www.atmos-chem-phys.net/17/2477/2017/ doi:10.5194/acp-17-2477-2017 © Author(s) 2017. CC Attribution 3.0 License. Regional influence of wildfires on aerosol chemistry in the western US and insights into atmospheric aging of biomass burning organic aerosol Shan Zhou1, Sonya Collier1, Daniel A. Jaffe2,3, Nicole L. Briggs2,3,4, Jonathan Hee2,3, Arthur J. Sedlacek III5, Lawrence Kleinman5, Timothy B. Onasch6, and Qi Zhang1 1Department of Environmental Toxicology, University of California, Davis, CA 95616, USA 2School of Science, Technology, Engineering, and Mathematics, University of Washington Bothell, Bothell, WA 98011, USA 3Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA 4Gradient, Seattle, WA 98101, USA 5Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, NY 11973, USA 6Aerodyne Research Inc., Billerica, MA 01821, USA Correspondence to: Qi Zhang ([email protected]) Received: 19 September 2016 – Discussion started: 23 September 2016 Revised: 2 January 2017 – Accepted: 19 January 2017 – Published: 16 February 2017 Abstract. Biomass burning (BB) is one of the most im- OA mass); and a highly oxidized BBOA-3 (O = C D 1.06; portant contributors to atmospheric aerosols on a global 31 % of OA mass) that showed very low volatility with only scale, and wildfires are a large source of emissions that im- ∼ 40 % mass loss at 200 ◦C. The remaining 32 % of the OA pact regional air quality and global climate. As part of the mass was attributed to a boundary layer (BL) oxygenated Biomass Burning Observation Project (BBOP) field cam- OA (BL-OOA; O = C D 0.69) representing OA influenced by paign in summer 2013, we deployed a high-resolution time- BL dynamics and a low-volatility oxygenated OA (LV-OOA; of-flight aerosol mass spectrometer (HR-AMS) coupled with O = C D 1.09) representing regional aerosols in the free tro- a thermodenuder at the Mt. Bachelor Observatory (MBO, posphere. The mass spectrum of BBOA-3 resembled that of ∼ 2.8 km above sea level) to characterize the impact of wild- LV-OOA and had negligible contributions from the HR-AMS C D C fire emissions on aerosol loading and properties in the Pa- BB tracer ions – C2H4O2 (m=z 60:021) and C3H5O2 cific Northwest region of the United States. MBO repre- (m=z D 73:029); nevertheless, it was unambiguously related sents a remote background site in the western US, and it to wildfire emissions. This finding highlights the possibil- is frequently influenced by transported wildfire plumes dur- ity that the influence of BB emission could be underesti- ing summer. Very clean conditions were observed at this mated in regional air masses where highly oxidized BBOA site during periods without BB influence where the 5 min (e.g., BBOA-3) might be a significant aerosol component but average (±1σ / concentration of non-refractory submicron where primary BBOA tracers, such as levoglucosan, are de- −3 aerosols (NR-PM1/ was 3.7 ± 4.2 µg m . Aerosol concen- pleted. We also examined OA chemical evolution for persis- tration increased substantially (reaching up to 210 µg m−3 of tent BB plume events originating from a single fire source NR-PM1/ for periods impacted by transported BB plumes, and found that longer solar radiation led to higher mass frac- and aerosol composition was overwhelmingly organic. Based tion of the chemically aged BBOA-2 and BBOA-3 and more on positive matrix factorization (PMF) of the HR-AMS data, oxidized aerosol. However, an analysis of the enhancement three types of BB organic aerosol (BBOA) were identified, ratios of OA relative to CO (1OA =1CO) showed little dif- including a fresh, semivolatile BBOA-1 (O = C D 0.35; 20 % ference between BB plumes transported primarily at night of OA mass) that correlated well with ammonium nitrate; versus during the day, despite evidence of substantial chem- an intermediately oxidized BBOA-2 (O = C D 0.60; 17 % of ical transformation in OA induced by photooxidation. These Published by Copernicus Publications on behalf of the European Geosciences Union. 2478 S. Zhou et al.: Regional influence of wildfires on aerosol chemistry in the western US results indicate negligible net OA production in photochem- al., 2009; DeCarlo et al., 2010), depletion (e.g., Akagi et al., ically aged wildfire plumes observed in this study, for which 2012; Jolleys et al., 2015), or no change (Brito et al., 2014; a possible reason is that SOA formation was almost entirely May et al., 2015) of dilution-adjusted OA mass in BB plumes balanced by BBOA volatilization. Nevertheless, the forma- after emissions. tion and chemical transformation of BBOA during atmo- In order to decipher what factors affect BBOA evolution spheric transport can significantly influence downwind sites and reconcile discrepancies in previous laboratory and at- with important implications for health and climate. mospheric observational results, the US Department of En- ergy (DOE) sponsored the Biomass Burning Observation Project (BBOP) campaign, which combined aircraft-based measurements with mountaintop observations to character- 1 Introduction ize the downstream evolution of the chemical, microphysical, and optical properties of carbonaceous aerosol generated by Biomass burning (BB) is estimated to be the largest source of BB. Wildfires across the western US have been linked to in- primary carbonaceous aerosols and a major source of reac- creased PM2:5 concentrations at various receptor sites (Jaffe tive trace gases in the Earth’s atmosphere (Bond et al., 2004; et al., 2008) and high-pollution episodes that exceeded the Akagi et al., 2011). Emissions from wildfires and other BB National Ambient Air Quality Standards (Jaffe and Wigder, sources, such as residential wood combustion and agricul- 2012). Furthermore, due to changes in precipitation, temper- tural burning, have been shown to affect the global radiation ature, and other meteorological conditions as a result of cli- budget (IPCC, 2013) and degrade air quality in both rural mate change, wildfire activities in this region have been in- areas and populated locations (e.g., Jaffe et al., 2008; Jaffe creasing (Westerling et al., 2006; Dennison et al., 2014) and and Wigder, 2012). The environmental impacts of BB emis- are predicted to increase summertime OA concentration by sions are strongly correlated with the chemical, optical, and 40 % from 2000 to 2050 (Spracklen et al., 2009). microphysical properties of BB aerosols, which are in turn A large number of wildfire events originating in the west- dependent in a complex manner on fuel type, combustion ern US were observed during BBOP from the Mount Bach- phase, and atmospheric aging of emitted particles and gas elor Observatory (MBO) – a remote mountaintop site that species (e.g., Petters et al., 2009; Liu et al., 2014; Collier et serves to characterize western US background conditions and al., 2016). is frequently impacted by transported BB plumes during the Organic compounds are a dominant component of BB summer fire season (Wigder et al., 2013). Continuous mea- aerosols (Bond et al., 2004; De Gouw and Jimenez, 2009), surements of BB plumes at MBO allow for the study of but the chemical and physical properties of primary organic BBOA with different source, age, and formation pathways aerosol (POA) released directly from burning and secondary under realistic atmospheric conditions and can provide rich organic aerosol (SOA) formed from gaseous precursors emit- data for evaluating the impact of BB emissions on regional ted by BB are dramatically different. For example, BB POA aerosol properties and elucidating their atmospheric aging tends to be semivolatile, smaller in size, and composed of processes. A number of recent studies were conducted at less oxidized compounds, whereas SOA from BB is gener- fixed locations in the western US and investigated impacts ally more oxidized, larger in size, and less volatile (Abel of BB on ozone, gaseous nitrogen species, and organic and et al., 2003; Heringa et al., 2011; May et al., 2013). Fur- elemental carbon (e.g., Wigder et al., 2013; Timonen et al., thermore, aerosol composition, optical properties, and hy- 2014; Hallar et al., 2015). Yet only a few ground-based mea- groscopicity have been found to change substantially in BB surements have examined the chemical composition and evo- plumes undergoing photooxidation and cloud processing, lution of BBOA, including a filter-based study of wildfire and the changes are mostly driven by the organic fraction aerosols in Yosemite National Park (Engling et al., 2006) and (Abel et al., 2003; De Gouw et al., 2006; Engelhart et al., a single-particle mass spectrometry study on the mixing state 2012; Gilardoni et al., 2016). Understanding the chemical and aging of particles during the 2007 San Diego wildfires properties and atmospheric processing of organic aerosols (Zauscher et al., 2013). (OAs) from BB sources (i.e., BBOA) is thus crucial for im- In this study, we provide an overview of the chemical proving our ability to quantitatively assess and predict the and physical characteristics of non-refractory submicrome- impacts of BB emissions on climate and air quality. How- ter particles (NR-PM1/ at MBO and examine the changes in ever, the chemical processing of BBOA is highly complex, ambient aerosol concentration and composition influenced and the net effect of aging on BBOA mass is highly vari- by BB emissions. The sources of OA are investigated via able. For example, while several laboratory studies reported factor analysis of the high-resolution time-of-flight aerosol substantial formation of SOA during chamber aging, others mass spectrometer (HR-AMS), and the aging of BBOA is observed a very small increase or even a decrease of BBOA discussed via combining real-time measurements with trajec- mass (Grieshop et al., 2009; Cubison et al., 2011; Henni- tory analysis.
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