10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Hydroxyl and Nitrate Radical Aging of Organic Emissions from Wildfires
SHANTANU JATHAR, Ali Akherati, Shiva Tarun, Liam Lewane, Abril Galang, Timothy Onasch, Scott Herndon, Joseph Roscioli, Tara Yacovitch, Edward Fortner, Philip Croteau, Wen Xu, Conner Daube, Berk Knighton, Benjamin Werden, Ezra Wood, Christopher Lim, David Hagan, Christopher Cappa, Jesse Kroll, Daniel S. Tkacik, Christopher Hennigan, Allen Robinson, Colorado State University
Abstract Number: 1316 Working Group: Carbonaceous Aerosol
Abstract Wildfires are the largest combustion-related source of organic emissions to the atmosphere; these include direct emissions of primary organic aerosol (POA) and semi-volatile, intermediate-volatility, and volatile organic compounds (SVOCs, IVOCs, and VOCs). However, there are large uncertainties surrounding the evolution of these organic emissions as they are physically and chemically transformed in the atmosphere. To understand these transformations, we performed sixteen experiments using an environmental chamber to simulate day- and night-time chemistry of wildfire emissions from 6 different fuels at the Fire Laboratory in Missoula, MT.
Across the test matrix, the experiments simulated 0.5 to 9 hours of equivalent day-time aging (with the hydroxyl radical and ozone) or several hours of night-time aging (with the nitrate radical). Day-time aging resulted in an average organic aerosol (OA) mass enhancement of 72% although the full range of OA mass enhancements varied between ~0% and 400%. The distribution of OA mass enhancements was consistent with chamber and flow reactor experiments performed at the Fire Laboratory in 2010 and 2012 but similar to previous studies offered no evidence to link the OA mass enhancement to fuel type, burn conditions, or oxidant exposure. The chamber OA mass enhancements were nearly half of those observed with flow reactor experiments performed in conjunction with this study and the differences could be explained by significant differences in oxidant exposure (median age of 6.1 hours for the chamber versus 3.8 days for the flow reactor). Night-time aging resulted in an average OA mass enhancement of 20%, which was lower than that observed with day-time aging. We found strong evidence for production of organic nitrates during the night-time experiments where they accounted for ~10% of the total OA emitted and formed by the end of the experiment. Both day- and night-time aging resulted in only modest changes in the OA composition measured by the aerosol mass spectrometer. The median fraction of the OA mass measured at a mass-to-charge ratio of 44 and 60 changed by +65% and -25% respectively and the median oxygen-to-carbon ratio increased by 25%. The flow reactor experiments witnessed much larger changes in the OA composition (e.g., median oxygen-to-carbon ratio more than doubled), which again could be attributed to substantial differences in oxidant exposure.
Ongoing work is focussed on integrating chamber and flow reactor wildfire experiments from 2010, 2012, and 2016 (~130 individual experiments) to examine the relationship between OA mass enhancement ratios, modified combustion efficiency, initial aerosol concentrations and composition, aerosol size, oxidant exposure, VOC:NOx ratios, and emissions and speciation of SOA precursors.