Gas-phase low volatility organic compounds (LVOC), produced from oxidation of isoprene 4-hydroxy-3-hydroperoxide (4,3-ISOPOOH) under low-NO conditions, were observed during the FIXCIT chamber study. Decreases in LVOC directly correspond to appearance and growth in secondary organic aerosol (SOA) of consistent elemental composition, indicating that LVOC condense (at OA below 1 μg m–3). This represents the first simultaneous measurement of condensing low volatility species from isoprene oxidation in both the gas and particle phases. The SOA formation in this study is separate from previously described isoprene epoxydiol (IEPOX) uptake. Assigning all condensing LVOC signals to 4,3-ISOPOOH oxidation in the chamber study implies a wall-loss corrected non-IEPOX SOA mass yield of ∼4%. By contrast to monoterpene oxidation, in which extremely low volatility VOC (ELVOC) constitute the organic aerosol, in the isoprene system LVOC with saturation concentrations from 10–2 to 10 μg m–3 are the main constituents. These LVOC may be important for the growth of nanoparticles in environments with low OA concentrations. LVOC observed in the chamber were also observed in the atmosphere during SOAS-2013 in the Southeastern United States, with the expected diurnal cycle. This previously uncharacterized aerosol formation pathway could account for ∼5.0 Tg yr–1 of SOA production, or 3.3% of global SOA.

We use a large laboratory, modeling, and field dataset to investigate the isoprene + O3 reaction, with the goal of better understanding the fates of the C1 and C4 Criegee intermediates in the atmosphere.

Abstract. The lifetime of nitrogen oxides (NO x ) affects the concentration and distribution of NO x and the spatial patterns of nitrogen deposition. Despite its importance, the lifetime of NO x is poorly constrained in rural and remote continental regions. We use measurements from a site in central Alabama during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013 to provide new insights into the chemistry of NO x and NO x reservoirs. We find that the lifetime of NO x during the daytime is controlled primarily by the production and loss of alkyl and multifunctional nitrates ( ANs). During SOAS, AN production was rapid, averaging 90 ppt h −1 during the day, and occurred predominantly during isoprene oxidation. Analysis of the AN and HNO 3 budgets indicate that ANs have an average lifetime of under 2 h, and that approximately 45 % of the ANs produced at this site are rapidly hydrolyzed to produce nitric acid. We find that AN hydrolysis is the largest source of HNO 3 and the primary pathway to permanent removal of NO x from the boundary layer in this location. Using these new constraints on the fate of ANs, we find that the NO x lifetime is 11±5 h under typical midday conditions. The lifetime is extended by storage of NO x in temporary reservoirs, including acyl peroxy nitrates and ANs.