Observations and analysis of organic aerosol evolution in some prescribed fire smoke plumes

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Abstract

Open biomass burning is a significant source of primary air pollutants such as particulate matter and non-methane organic gases. However, the physical and chemical atmospheric processing of these emissions during transport is poorly understood. Atmospheric transformations of biomass burning emissions have been investigated in environmental chambers, but there have been limited opportunities to investigate these transformations in the atmosphere. In this study, we deployed a suite of real-time instrumentation on a Twin Otter aircraft to sample smoke from prescribed fires in South Carolina, conducting measurements at both the source and downwind to characterize smoke evolution with atmospheric aging. Organic aerosol (OA) within the smoke plumes was quantified using an Aerosol Mass Spectrometer (AMS), along with refractory black carbon (rBC) using a Single Particle Soot Photometer and carbon monoxide (CO) and carbon dioxide (CO<sub>2</sub>) using a Cavity Ring-Down Spectrometer. During the two fires for which we were able to obtain aerosol aging data, normalized excess mixing ratios and "export factors" of conserved species (rBC, CO, CO<sub>2</sub>) were unchanged with increasing sample age. Investigation of AMS mass fragments indicated that the in-plume fractional contribution (<i>f</i><sub><i>m/z</i></sub>) to OA of the primary fragment (<i>m/z</i> 60) decreased downwind, while the fractional contribution of the secondary fragment (<i>m/z</i> 44) increased. Increases in <i>f</i><sub>44</sub> are typically interpreted as indicating chemical production of secondary OA (SOA). Likewise, we observed an increase in the O : C elemental ratio downwind, which is usually associated with aerosol aging. However, the rapid mixing of these plumes into the background air suggests that these chemical transformations may be attributable to the different volatilities of the compounds that fragment to these <i>m/z</i> in the AMS. The gas-particle partitioning behavior of the bulk OA observed during the study was consistent with the predictions from a parameterization developed for open biomass burning emissions in the laboratory. Furthermore, we observed no statistically-significant increase in total organic mass with atmospheric transport. Hence, our results suggest that dilution-driven evaporation likely dominated over chemical production of SOA within our smoke plumes, likely due to the fast dilution and limited aging times (<~5 h) that we could sample.

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May, A. A., Lee, T., McMeeking, G. R., Akagi, S., Sullivan, A. P., Urbanski, S., … Kreidenweis, S. M. (2015). Observations and analysis of organic aerosol evolution in some prescribed fire smoke plumes. Atmospheric Chemistry and Physics, 15(11), 6323–6335. https://doi.org/10.5194/acp-15-6323-2015

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