Persistent Influence of Wildfire Emissions in the Western United States and Characteristics of Aged Biomass Burning Organic Aerosols under Clean Air Conditions

Abstract

Wildfire-influenced air masses under regional background conditions were characterized at the Mt. Bachelor Observatory (∼2800 m a.s.l.) in summer 2019 to provide a better understanding of the aging of biomass burning organic aerosols (BBOAs) and their impacts on the remote troposphere in the western United States. Submicron aerosol (PM 1 ) concentrations were low (average ± 1σ = 2.2 ± 1.9 μg sm −3 ), but oxidized BBOAs (average O/C = 0.84) were constantly detected throughout the study. The BBOA correlated well with black carbon, furfural, and acetonitrile and comprised above 50% of PM 1 during plume events when the peak PM 1 concentration reached 18.0 μg sm −3 . Wildfire plumes with estimated transport times varying from ∼10 h to >10 days were identified. The plumes showed ΔOA/ΔCO values ranging from 0.038 to 0.122 ppb ppb −1 with a significant negative relation to plume age, indicating BBOA loss relative to CO during longrange transport. Additionally, increases of average O/C and aerosol sizes were seen in more aged plumes. The mass-based size mode was approximately 700 nm (D va ) in the most oxidized plume that likely originated in Siberia, suggesting aqueous-phase processing during transport. This work highlights the widespread impacts that wildfire emissions have on aerosol concentration and properties, and thus climate, in the western United States.

Comment on acp-2022-167

“…Nucleation events at Storm Peak, a mountaintop observatory in the Western USA, in spring are most likely associated with sulfuric acid, whereby NPF events in summer are driven by lowvolatility organics (Yu and Hallar, 2014). Farley et al, 2022 found Siberian BB to be associated with low volatility oxidized organic aerosol, greater O:C elemental ratio, and greater secondary 500 sulfate mass, the later consistent with observations by Sihto et al, 2006 andSchill et al, 2020. BB smoke transported long-range distances thus likely mixes with other aerosol sources, such as sulfate from marine or other sources, leading to an enhancement in small particles.…”

“…• The higher lower SAE, and higher MSE of Siberian BB events reflected contributions from large particles, for which we propose two hypotheses: o Enhanced secondary processing during long-range transport leading to particle growth, which is supported by SP-AMS measurements of enhanced low-volatility 530 organics and sulfate in Siberian BB reported by Farley et al, 2022. o Fine dust transported alongside smoke, which is supported by CALIPSO and Dust Score satellite products of dust mixed with smoke originating from Siberian Boreal Forest fires.…”

“…This result suggests that net PM1 loss through deposition, evaporation and/or cloud processing was greater than secondary aerosol production during >14-day transport in the FT from Siberia. Concurrent SP-AMS measurements in summer 2019 demonstrated a clear decreasing 285 trend of ΔOA/ΔCO with increased transport time ranging from a few hours to over 10 days, for which PM losses during transport was provided as a possible reason (Farley et al, 2022). Wigder et al, 2013 andWeiss-Penzias et al, 2006 previously reported a similar association of decreased ΔPM1/ΔCO with increased transport >750 km for wildfire smoke arriving at MBO in 2004-2011.…”

“…Bachelor (43.981 • N 121.691 • W, 2764 m a.s.l) from 1 July to 10 September 2019. These observations were 100 part of the larger Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) experiment (Liao et al, 2021;Xu et al, 2021;Decker et al, 2021;Wiggins et al, 2021;Makar et al, 2021), which included extensive observations at MBO (Farley et al, 2022). Due to its remote location and limited anthropogenic influence, MBO is an ideal site for measurements of wildfire plumes ranging from locally emitted to long-range transport events (Laing et al, 2016;Baylon et 105 al., 2017;Wigder et al, 2013).…”

“…August and September OPC data were determined to be artificially biased high, which coincided with the installation of a thermodenuder in the aerosol sampling line on July 31, 2019. Correction of Aug. -Sep. OPC PM1 measurements using coinciding high-resolution time-of-flight soot particle aerosol mass spectrometer (SP-AMS) PM1 170 measurements (Farley et al, 2022) is presented in the Supporting Information.…”