Separate quantification of isoprene oxidation products via the hydroperoxyl and nitric-oxide pathways over Amazonia and the implication for aerosol formation

Photooxidation of isoprene is a large carbon source of secondary organic material (SOM) of atmospheric particles over the globe. Depending on the fate of the intermediate isoprene peroxy radicals (ISOPOO), isoprene photooxidation leads to chemically different gaseous products and hence particulate composition. Recent laboratory studies have shown an unexpected instrument bias in the measurement of some of these gaseous products. Isoprene-derived hydroxylhydroperoxides (ISOPOOH; C5H10O3), the major first-generation products via the HO2 pathway of ISOPOO reaction, are detected as their equivalent NO-pathway products, methyl vinyl ketone (MVK, C4H6O) and methacrolein (MACR, C4H6O), by commonly-used instruments. Here we present separate measurements of ISOPOOH and MVK/MACR in Amazonia by deploying a cold trap upstream of a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS), as a part of the GoAmazon2014/5 Experiment. To our knowledge, this study provides the first field evaluation of ISOPOOH contributions to apparent MVK/MACR signals. The ratio of these species with respect to reactive nitrogen compounds (NOy) provides a means to examine the influence of anthropogenic pollutants on oxidation chemistry of isoprene. Based on these and other complementary measurements, the implication of the results on SOM formation from isoprene was discussed.