Abstract. Nighttime reaction of nitrate radicals (NO3) with biogenic volatile organic compounds (BVOC) has been proposed as a potentially important but also highly uncertain source of secondary organic aerosol (SOA). The southeast United States has both high BVOC and nitrogen oxide (NOx) emissions, resulting in a large model-predicted NO3-BVOC source of SOA. Coal-fired power plants in this region constitute substantial NOx 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 NO3 radicals rapidly remove isoprene, to obtain field-based estimates of the secondary organic aerosol yield from NO3 + isoprene. Observed in-plume increases in nitrate aerosol are consistent with organic nitrate aerosol production from NO3 + isoprene, and these are used to determine molar SOA yields, for which the average over 9 plumes is 9 %. Corresponding mass yields depend on the assumed molecular formula for isoprene-NO3-SOA, but the average over 9 plumes is 27 %, larger than those previously measured in chamber studies (12–14 % 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 ambient peroxy radical lifetimes and heterogeneous reactions of NO3-isoprene gas phase products. Future studies of aerosol composition from NO3 radical + isoprene are needed to better understand the oxidation chemistry producing this potentially important coupled anthropogenic – biogenic source of SOA.;
