Sources and Fate of Reactive Carbon in an Isoprene-Dominated Forest

On a global scale, biogenically emitted isoprene is the most abundant nonmethane volatile organic compound (VOC), and therefore a key source of reactive carbon in the earth's atmosphere. Because there are significant uncertainties in the oxidation mechanism of isoprene, and little work examining the deposition rates of the key oxidation products, predicting the total amount, the speciation, and the fate of isoprene oxidation products is difficult. Therefore, the fate of reactive carbon is also poorly constrained. During the 2013 Southern Oxidant and Aerosol Study (SOAS) field campaign, OH reactivity, isoprene, and key first-generation and later-generation isoprene oxidation products were measured over the South-East United States. The observed OH reactivity can largely be accounted for, with an average "missing" reactivity of ~25%. OH reactivity was largely dominated by isoprene, with only small contributions from isoprene-derived oxidation products. In contrast, not all oxidation products show good agreement, despite well parameterized dry deposition and dilution rates. Using the viewpoint of OH reactivity, we discuss the sources of reactive carbon at this site. Then, from the viewpoint of oxidized VOCs, we discuss its fate. Measurement and model discrepancies are used to explore the plausibility of unknown physical and chemical carbon sinks.