Impact of Biomass Burning Plumes on Photolysis Rates and Ozone

PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE Impact of Biomass Burning Plumes on Photolysis 10.1002/2017JD027341 Rates and Ozone Formation at the Mount Key Points: Bachelor Observatory • Biomass burning (BB) aerosols 1,2 1,2 3 3 4 5 increase local noontime j(NO2) P. Baylon , D. A. Jaffe , S. R. Hall , K. Ullmann , M. J. Alvarado , and B. L. Lefer • At high solar zenith angle, BB plumes – decrease j(NO2)by14 21% 1Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA, 2School of Science, Technology, • – We calculate 49 185 pptv of HO2 and 3 RO in BB plumes and an Engineering and Mathematics, University of Washington Bothell, Bothell, WA, USA, Atmospheric Chemistry Observations 2 4 instantaneous O3 production rate of and Measurements Laboratory, National Center for Atmospheric Research, Boulder, CO, USA, Atmospheric and 2.0–3.6 ppbv/h Environmental Research, Lexington, MA, USA, 5NASA Earth Science Division, Washington, DC, USA Supporting Information: Abstract In this paper, we examine biomass burning (BB) events at the Mt. Bachelor Observatory (MBO) • Supporting Information S1 • Table S1 during the summer of 2015. We explored the photochemical environment in these BB plumes, which remains poorly understood. Because we are interested in understanding the effect of aerosols only (as Correspondence to: opposed to the combined effect of aerosols and clouds), we carefully selected three cloud-free days in P. Baylon, August and investigate the photochemistry in these plumes. At local midday (solar zenith angle (SZA) = 35°), [email protected] j(NO2) values were slightly higher (0.2–1.8%) in the smoky days compared to the smoke-free day, presumably due to enhanced scattering by the smoke aerosols. At higher SZA (70°), BB aerosols decrease j(NO2)by Citation: 14–21%. We also observe a greater decrease in the actinic flux at 310–350 nm, compared to 360–420 nm, Baylon, P., Jaffe, D. A., Hall, S. R., Ullmann, K., Alvarado, M. J., & Lefer, B. L. presumably due to absorption in the UV by brown carbon. We compare our measurements with results from (2018). Impact of biomass burning the Tropospheric Ultraviolet-Visible v.5.2 model. As expected, we find a good agreement (to within 6%) plumes on photolysis rates and ozone during cloud-free conditions. Finally, we use the extended Leighton relationship and a photochemical model formation at the Mount Bachelor Observatory. Journal of Geophysical (Aerosol Simulation Program v.2.1) to estimate midday HO2 and RO2 concentrations and ozone production – Research: Atmospheres, 123, 2272 2284. rates (P(O3)) in the fire plumes. We observe that Leighton-derived HO2 and RO2 values (49–185 pptv) and https://doi.org/10.1002/2017JD027341 instantaneous P(O3) (2.0–3.6 ppbv/h) are higher than the results from the photochemical model. Received 23 JUN 2017 Plain Language Summary Biomass burning can emit huge amounts of aerosol particles and trace Accepted 30 JAN 2018 gases to the atmosphere. These plumes are rich in nitrogen oxides and volatile organic compounds that can Accepted article online 7 FEB 2018 Published online 18 FEB 2018 react with sunlight to produce ozone, a greenhouse gas and a health hazard to sensitive individuals. However, photochemistry in biomass burning plumes is poorly understood. While most studies rely on model simulations, there are very few in situ measurements aimed at investigating how these aerosols affect photolysis rates. Our study aims to fill this knowledge gap. We use measurements from a mountaintop station in central Oregon (Mount Bachelor Observatory, 2.8 km above sea level) and run a box model to verify our estimates of radicals and ozone production rates. Our results show that biomass burning aerosols increase photolysis rates during midday and decrease the rates during early morning/late afternoon. The ozone production rates derived from the photochemical model are in the same order of magnitude but lower than our calculations. In our paper, we present explanations for the discrepancies between measured and modeled values. 1. Introduction Biomass burning (BB) can emit huge amounts of aerosol particles and trace gases to the atmosphere (Andreae, 1991; Andreae & Merlet, 2001; Crutzen & Andreae, 1990; Holzinger et al., 1999). BB aerosols are composed predominantly of organic carbon and black carbon, with lesser amounts of inorganic components (Reid et al., 2005; Vakkari et al., 2014). Primary organic aerosols are emitted directly from the fires while secondary organic aerosols are formed from the oxidation of organic gases (Akagi et al., 2011, 2013). BB plumes are rich in primary emissions such as carbon monoxide (CO), volatile organic compounds (VOCs), and nitrogen oxides (NOx; Hodnebrog et al., 2012; Martins et al., 2012; Real et al., 2007). Photochemical reactions of these pollutants can produce secondary pollutants, such as O3, which can be transported over large distances (McKeen et al., 2002; Mebust & Cohen, 2014; Pfister et al., 2008; Val Martín et al., 2006). In the troposphere, O3 is in a delicate balance between photolytic destruction and photochemical formation. Whether the tropo- ©2018. American Geophysical Union. All Rights Reserved. sphere is a net source or sink of O3 depends on the balance between the two processes (Thompson, 1984). BAYLON ET AL. 2272 Journal of Geophysical Research: Atmospheres 10.1002/2017JD027341 1 The rate-limiting step in the photolytic destruction of O3 is the reaction between O( D) and H2O to form OH radicals, which are important oxidizing agents in the troposphere: ÀÁ 1 O3 þ hν→O D þ O2 (1) ÀÁ 1 O D þ H2O→2OH (2) Reaction of OH with O3 produces HO2, which in turn reacts with O3 resulting in a catalytic destruction cycle. The loss of O3 from its reaction with OH and HO2 is generally of minor importance in the lower troposphere. OH þ O3→HO2 þ O2 (3a) HO2 þ O3→OH þ 2O2 (3b) Net : 2O3→3O2 (3c) On the other hand, the rate-limiting step in the photochemical formation of O3 is the reaction of NO with HO2 or RO2. HO2 OH þ NO→NO2 þ (4) RO2 RO NO2 þ hν→NO þ O (5) O þ O2 þ M→O3 þ M (6) 1 The photolysis rate for O3 (equation (1)) is termed j(O D), while for NO2 (equation (5)), it is called j(NO2). Tropospheric ozone photolysis is in the 290–340 nm spectral range, while NO2 photolysis is in the 290–420 nm range. Because BB plumes contain large amounts of NOx and VOCs, they have the potential to produce O3 down- wind (Jaffe & Wigder, 2012). For example, the Siberian wildfires in 2003 caused enhancements of summer- time ozone in the western U.S. from 9 to 17 ppbv (Jaffe et al., 2004). On January 2014, intense wildfires in forests located 70 km southwest of Santiago, Chile were transported by low-level winds and led to a striking decrease in visibility and a marked increase in ozone well above 80 ppbv in the urban atmosphere (Rubio et al., 2015). The September 2012 Pole Creek Fires, which originated 20 km north of Mount Bachelor Observatory (MBO; 2.8 km above sea level), resulted in ozone enhancements of 17–32 ppbv at MBO (Baylon et al., 2015). Studies have found that ozone formation ranges can vary sharply from very low in fresh plumes to low in moderately aged plumes to high in well-aged plumes (Val Martín et al., 2006; Wigder et al., 2013). However, quantifying ozone production from wildfires is complicated by variations and uncertainties such as fire emissions (type of species emitted), combustion efficiency, meteorological patterns, chemical and photochemical reactions, aerosol effects on chemistry, and solar radiation (Jaffe & Wigder, 2012). Aside from NOx and VOCs, sunlight is an important factor for ozone production. Baylon et al. (2015) examined wildfire plumes observed at MBO during the summers of 2012 and 2013 and found that plumes that are not asso- ciated with ozone formation were transported during the nighttime, when there is minimal photochemistry. Numerous studies have evaluated the effects of aerosols on photolysis rates and O3 formation. These effects are dependent on different factors. First is the type of aerosol. Model simulations by Dickerson et al. (1997) show that scattering particles in the boundary layer (BL) can increase photolysis rates and can therefore accel- erate ozone production. However, UV-absorbing aerosols such as mineral dust and soot have also been found to reduce ozone mixing ratios by up to 24 ppbv (Dickerson et al., 1997). Similarly, aerosol particles containing black carbon (BC) have been suggested to reduce surface ozone in Los Angeles by 5–8% (Jacobson, 1998). Simulations in Mexico City by Castro et al. (2001) suggest an even larger reduction in NO2 photolysis rates (10–30%) and surface ozone mixing ratios (30–40 ppbv) due to urban aerosols. A more recent field campaign study by Ying et al. (2011) analyzed the impact of dust aerosols in Mexico City on photolysis rates and confirmed that carbonaceous aerosols lead to significant reductions in photolysis rates and surface concentrations of OH and O3 in the city. Using a three-dimensional regional chemical transport model, Tang et al. (2003) found that the accumulated impact of BB aerosols from Southeast Asia, which contain large amounts of carbonaceous material, is the reduction of surface ozone mixing ratios by about 6 ppbv. Smaller reductions in j(NO2) have been simulated by Li et al. (2011) for urban aerosols in Central Eastern China. They found reductions less than 10% at noon; however, they also note that aerosols can BAYLON ET AL. 2273 Journal of Geophysical Research: Atmospheres 10.1002/2017JD027341 slightly enhance photolysis rates near midday when scattering dominates absorption by aerosols at ultraviolet wavelengths (Li et al., 2011).

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