Emissions of Trace Organic Gases From Western U.S. Wildfires Based on WE-CAN Aircraft Measurements

We present emission measurements of volatile organic compounds (VOCs) for western U.S. wildland fires made on the NSF/NCAR C-130 research aircraft during the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) field campaign in summer 2018. VOCs were measured with complementary instruments onboard the C-130, including a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) and two gas chromatography (GC)-based methods. Agreement within combined instrument uncertainties (<60%) was observed for most co-measured VOCs. GC-based measurements speciated the isomeric contributions to selected PTR-ToF-MS ion masses and generally showed little fire-to-fire variation. We report emission ratios (ERs) and emission factors (EFs) for 161 VOCs measured in 31 near-fire smoke plume transects of 24 specific individual fires sampled in the afternoon when burning conditions are typically most active. Modified combustion efficiency (MCE) ranged from 0.85 to 0.94. The measured campaign-average total VOC EF was 26.1 ± 6.9 g kg⁻¹, approximately 67% of which is accounted for by oxygenated VOCs. The 10 most abundantly emitted species contributed more than half of the total measured VOC mass. We found that MCE alone explained nearly 70% of the observed variance for total measured VOC emissions (r² = 0.67) and >50% for 57 individual VOC EFs representing more than half the organic carbon mass. Finally, we found little fire-to-fire variability for the mass fraction contributions of individual species to the total measured VOC emissions, suggesting that a single speciation profile can describe VOC emissions for the wildfires in coniferous ecosystems sampled during WE-CAN.