Emissions of Volatile Organic Compounds from Western U.S. Wildfires Measured During WE-CAN

Abstract:

Wildfires are a significant source of atmospheric volatile organic compounds (VOCs). However, due to analytical limitations, there are few field observations of many newly identified VOCs in wildfire smoke. Meanwhile, more commonly reported species lack statistics describing their natural emissions variability. We present an overview of western U.S. wildland fire VOC emissions measurements 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, the Trace Organic Gas Analyzer (TOGA) and the Advanced Whole Air Sampler (AWAS). We determine emission ratios (ERs), emission factors (EFs), and speciation profiles for 161 individual VOCs and ions measured in 31 emission transects through plumes associated with 24 specific individual fires sampled in the afternoon when burning conditions are typically most active. We find that approximately 66 % of the total VOC EF is accounted for by oxygenated VOCs, while the top 10 most abundantly measured species and ions contributed to more than half of the total VOC EF. Modified combustion efficiency (MCE) alone explained nearly 70 % of the observed variance for total VOC emissions (R² = 0.67) and > 50 % for 55 individual VOC EFs, representing more than half the organic carbon mass. Finally, we find little fire-to-fire variability for the mass fraction of individual species to the total VOC emissions, suggesting that a single speciation profile can describe VOC emissions for the wildfires in coniferous ecosystems sampled during WE-CAN.