Regional influence of wildfires on aerosol chemistry in the western US and insights into atmospheric aging of biomass burning organic aerosol

Shan Zhou, Sonya Collier, Daniel A. Jaffe, Nicole L. Briggs, Jonathan Hee, Arthur J. Sedlacek III, Lawrence Kleinman, Timothy B. Onasch, Qi Zhang
2017 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Biomass burning (BB) is one of the most important contributors to atmospheric aerosols on a global scale, and wildfires are a large source of emissions that impact regional air quality and global climate. As part of the Biomass Burning Observation Project (BBOP) field campaign in summer 2013, we deployed a high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) coupled with a thermodenuder at the Mt. Bachelor Observatory (MBO, ∼ <span
more » ... lass="thinspace"></span>2.8<span class="thinspace"></span>km above sea level) to characterize the impact of wildfire emissions on aerosol loading and properties in the Pacific Northwest region of the United States. MBO represents a remote background site in the western US, and it is frequently influenced by transported wildfire plumes during summer. Very clean conditions were observed at this site during periods without BB influence where the 5<span class="thinspace"></span>min average (±1<i>σ</i>) concentration of non-refractory submicron aerosols (NR-PM<sub>1</sub>) was 3.7<span class="thinspace"></span>±<span class="thinspace"></span>4.2<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup>. Aerosol concentration increased substantially (reaching up to 210<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup> of NR-PM<sub>1</sub>) for periods impacted by transported BB plumes, and aerosol composition was overwhelmingly organic. Based on positive matrix factorization (PMF) of the HR-AMS data, three types of BB organic aerosol (BBOA) were identified, including a fresh, semivolatile BBOA-1 (O<span class="thinspace"></span>∕<span class="thinspace"></span>C<span class="thinspace"></span> = <span class="thinspace"></span>0.35; 20<span class="thinspace"></span>% of OA mass) that correlated well with ammonium nitrate; an intermediately oxidized BBOA-2 (O<span class="thinspace"></span>∕<span class="thinspace"></span>C<span class="thinspace"></span> = <span class="thinspace"></span>0.60; 17<span class="thinspace"></span>% of OA mass); and a highly oxidized BBOA-3 (O<span class="thinspace"></span>∕<span class="thinspace"></span>C<span class="thinspace"></span> = <span class="thinspace"></span>1.06; 31<span class="thinspace"></span>% of OA mass) that showed very low volatility with only ∼ <span class="thinspace"></span>40<span class="thinspace"></span>% mass loss at 200<span class="thinspace"></span>°C. The remaining 32<span class="thinspace"></span>% of the OA mass was attributed to a boundary layer (BL) oxygenated OA (BL-OOA; O<span class="thinspace"></span>∕<span class="thinspace"></span>C<span class="thinspace"></span> = <span class="thinspace"></span>0.69) representing OA influenced by BL dynamics and a low-volatility oxygenated OA (LV-OOA; O<span class="thinspace"></span>∕<span class="thinspace"></span>C<span class="thinspace"></span> = <span class="thinspace"></span>1.09) representing regional aerosols in the free troposphere. The mass spectrum of BBOA-3 resembled that of LV-OOA and had negligible contributions from the HR-AMS BB tracer ions – C<sub>2</sub>H<sub>4</sub>O<sub>2</sub><sup>+</sup> (<i>m</i>∕<i>z</i> = 60.021) and C<sub>3</sub>H<sub>5</sub>O<sub>2</sub><sup>+</sup> (<i>m</i>∕<i>z</i> = 73.029); nevertheless, it was unambiguously related to wildfire emissions. This finding highlights the possibility that the influence of BB emission could be underestimated in regional air masses where highly oxidized BBOA (e.g., BBOA-3) might be a significant aerosol component but where primary BBOA tracers, such as levoglucosan, are depleted. We also examined OA chemical evolution for persistent BB plume events originating from a single fire source and found that longer solar radiation led to higher mass fraction of the chemically aged BBOA-2 and BBOA-3 and more oxidized aerosol. However, an analysis of the enhancement ratios of OA relative to CO (ΔOA<span class="thinspace"></span>∕ΔCO) showed little difference between BB plumes transported primarily at night versus during the day, despite evidence of substantial chemical transformation in OA induced by photooxidation. These results indicate negligible net OA production in photochemically aged wildfire plumes observed in this study, for which a possible reason is that SOA formation was almost entirely balanced by BBOA volatilization. Nevertheless, the formation and chemical transformation of BBOA during atmospheric transport can significantly influence downwind sites with important implications for health and climate.</p>
doi:10.5194/acp-17-2477-2017 fatcat:do2u6yuir5gwvefzx4bqhu565q