Measurements of in-situ SOA Formation and Chemistry Using an Oxidation Flow Reactor at GoAmazon2014 and Other Campaigns
Tuesday, 16 December 2014
During several recent field campaigns including GoAmazon2014, ambient gases and particles were exposed to controlled concentrations of OH, O3 or NO3 in-situ using a Potential Aerosol Mass oxidation flow reactor. Oxidant exposure in the reactor ranged from an hour to several weeks of equivalent atmospheric residence time, allowing the study of SOA formation and chemistry over long time scales. Oxidized air from the reactor was sampled directly (e.g., HR-AMS, ACSM, PTR-TOFMS, SMPS, CCN), and these results were compared with collocated biogenic and anthropogenic tracers (e.g., SV-TAG sesquiterpenes and PTR-TOFMS aromatics, isoprene, and monoterpenes). In all studies, OH oxidation of ambient air in the reactor led to substantial SOA mass production (often several µg/m3 of SOA) during times of high precursor gas concentrations. While SOA production correlated with measured gas-phase precursors, the total mass formed in the reactor was generally several times larger than could be explained by the aerosol yields of measured VOC’s. This suggests that a majority of gases that formed SOA in the reactor were not the primary VOCs considered as traditional SOA precursors. Additionally, most of the SOA mass increase occurred in the first day of equiv. atmospheric aging, suggesting that ambient SOA is predominantly formed close to emission sources of precursors with gas-phase reaction lifetimes of <1 day. At a remote Colorado pine forest site (during BEACHON-RoMBAS), the mainly biogenic aerosol formed in the reactor from <1 equivalent day of oxidation had an atomic O:C of 0.54, similar to the existing ambient aerosol O:C of 0.61. As OH exposures increased (up to 10-20 equivalent days), the OA became highly oxidized (O:C>1) and partially revolatilized, demonstrating the competing effects of functionalization/condensation at low exposures vs. fragmentation/evaporation reactions for high exposures. SOA formation from O3 and NO3 oxidation correlated with biogenic gas-phase precursors, but led to smaller (<0.5 µg/m3) SOA production, consistent with the ability for OH to achieve more generations of oxidation than O3 or NO3. Measurements taken in a variety of biogenic ecosystems with a wide range of anthropogenic influence were compared, allowing investigation of the effects of anthropogenic pollution on SOA formation.