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

Abstract

Abstract. 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, ∼ 2.8 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 min average (±1σ) concentration of non-refractory submicron aerosols (NR-PM1) was 3.7 ± 4.2 µg m−3. Aerosol concentration increased substantially (reaching up to 210 µg m−3 of NR-PM1) 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 ∕ C = 0.35; 20 % of OA mass) that correlated well with ammonium nitrate; an intermediately oxidized BBOA-2 (O ∕ C = 0.60; 17 % of OA mass); and a highly oxidized BBOA-3 (O ∕ C = 1.06; 31 % of OA mass) that showed very low volatility with only ∼ 40 % mass loss at 200 °C. The remaining 32 % of the OA mass was attributed to a boundary layer (BL) oxygenated OA (BL-OOA; O ∕ C = 0.69) representing OA influenced by BL dynamics and a low-volatility oxygenated OA (LV-OOA; O ∕ C = 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 – C2H4O2+ (m∕z = 60.021) and C3H5O2+ (m∕z = 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 ∕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.

Insight into the in-cloud formation of oxalate based on in situ measurement by single particle mass spectrometry

“…The observed oxalatecontaining particles likely represented aged biomass-burning particles, associated with enhanced aliphatic acids (Paglione et al, 2014). Continuous evolution of primary organics to highly oxidized organics is widely observed for biomassburning particles (Cubison et al, 2011;Zhou et al, 2017). Significant correlations between these OAs were observed in aged biomass-burning particles (Zauscher et al, 2013) and also cloud water samples (Sorooshian et al, 2013).…”

Chemical characterization of long-range transport biomass burning emissions to the Himalayas: insights from high-resolution aerosol mass spectrometry

“…As mentioned above, atmospheric aerosols in the TPH region were significantly influenced by BB emissions from South Asia during the sampling periods. BBs would emit large amounts of nitrogen-containing organic compounds (Fleming et al, 2018;Zhou et al, 2017) and as discussed in Sect. 3.2.…”

Ubiquitous influence of wildfire emissions and secondary organic aerosol on summertime atmospheric aerosol in the forested Great Lakes region

“…6). Freshly emitted biomass burning aerosol has an O/C ratio of ∼ 0.2, which can increase to ∼ 0.6 in only a few hours as the emissions undergo photochemical aging and oxidized material condenses onto the particles (Grieshop et al, 2009;Pratt et al, 2011;Liu et al, 2016;Zhou et al, 2017). While AMS levoglucosan ion markers (m/z 60 (C 2 H 4 O + 2 ) and 73 (C 3 H 5 O + 2 )) (Alfarra et al, 2007) were observed, consistent with the ATOFMS observation of biomass burning particles, the levels were low (Fig.…”