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Biomass burning (BB) is one of the most important contributors to atmosphericaerosols on a global scale, and wildfires are a large source of emissionsthat impact regional air quality and global climate. As part of the BiomassBurning Observation Project (BBOP) field campaign in summer 2013, we deployeda high-resolution time-of-flight aerosol mass spectrometer(HR-AMS) coupled with a thermodenuder at theMt. Bachelor Observatory (MBO, ∼  2.8 km above sea level) tocharacterize the impact of wildfire emissions on aerosol loading andproperties in the Pacific Northwest region of the United States. MBOrepresents a remote background site in the western US, and it is frequentlyinfluenced by transported wildfire plumes during summer. Very cleanconditions were observed at this site during periods without BB influencewhere the 5 min average (±1σ) concentration of non-refractorysubmicron aerosols (NR-PM1) was 3.7 ± 4.2 µg m−3.Aerosol concentration increased substantially (reaching up to210 µg m−3 of NR-PM1) for periods impacted bytransported BB plumes, and aerosol composition was overwhelmingly organic.Based on positive matrix factorization (PMF) of the HR-AMSdata, three types of BB organic aerosol (BBOA) were identified, including afresh, semivolatile BBOA-1 (O ∕ C  =  0.35; 20 % of OA mass) thatcorrelated 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 volatilitywith only  ∼  40 % mass loss at 200 °C. The remaining32 % 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 anda low-volatility oxygenated OA (LV-OOA; O ∕ C  =  1.09) representingregional aerosols in the free troposphere.The mass spectrum of BBOA-3 resembled that of LV-OOA and had negligiblecontributions 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 highlightsthe possibility that the influence of BB emission could be underestimated inregional air masses where highly oxidized BBOA (e.g., BBOA-3) might be asignificant aerosol component but where primary BBOA tracers, such as levoglucosan, are depleted. Wealso examined OA chemical evolution for persistent BB plume eventsoriginating from a single fire source and found that longer solar radiationled to higher mass fraction of the chemically aged BBOA-2 and BBOA-3 and moreoxidized aerosol. However, an analysis of the enhancement ratios of OArelative to CO (ΔOA ∕ΔCO) showed little difference betweenBB plumes transported primarily at night versus during the day, despiteevidence of substantial chemical transformation in OA induced byphotooxidation. These results indicate negligible net OA production inphotochemically aged wildfire plumes observed in this study, for which apossible reason is that SOA formation was almost entirely balanced by BBOAvolatilization. Nevertheless, the formation and chemical transformation of BBOAduring atmospheric transport can significantly influence downwind sites with important implications for health and climate

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