Secondary organic aerosol (SOA) yields from NO ₃ radical + isoprene based on nighttime aircraft power plant plume transects

Nighttime reaction of nitrate radicals (inline-formulaNO₃) with biogenic volatile organic compounds (BVOC) has been proposed as a potentially important but also highly uncertain source of secondary organic aerosol (SOA). The southeastern United States has both high BVOC and nitrogen oxide (inline-formulaNO_x) emissions, resulting in a large model-predicted inline-formulaNO₃-BVOC source of SOA. Coal-fired power plants in this region constitute substantial inline-formulaNO_x emissions point sources into a nighttime atmosphere characterized by high regionally widespread concentrations of isoprene. In this paper, we exploit nighttime aircraft observations of these power plant plumes, in which inline-formulaNO₃ radicals rapidly remove isoprene, to obtain field-based estimates of the secondary organic aerosol yield from inline-formulaNO₃ inline-formula+ isoprene. Observed in-plume increases in nitrate aerosol are consistent with organic nitrate aerosol production from inline-formulaNO₃ inline-formula+ isoprene, and these are used to determine molar SOA yields, for which the average over nine plumes is 9 % (inline-formula±5 %). Corresponding mass yields depend on the assumed molecular formula for isoprene-inline-formulaNO₃-SOA, but the average over nine plumes is 27 % (inline-formula±14 %), on average larger than those previously measured in chamber studies (12 %–14 % mass yield as inline-formulaΔOA inline-formula∕ inline-formulaΔVOC after oxidation of both double bonds). Yields are larger for longer plume ages. This suggests that ambient aging processes lead more effectively to condensable material than typical chamber conditions allow. We discuss potential mechanistic explanations for this difference, including longer ambient peroxy radical lifetimes and heterogeneous reactions of inline-formulaNO₃-isoprene gas phase products. More in-depth studies are needed to better understand the aerosol yield and oxidation mechanism of inline-formulaNO₃ radical inline-formula+ isoprene, a coupled anthropogenic–biogenic source of SOA that may be regionally significant.