Understanding Secondary Organic Aerosol Formation of Isoprene and Nitrate Radical Reactions: Role of Oxidants

TIANCHANG XU, Masayuki Takeuchi, Nga Lee Ng, Georgia Institute of Technology

     Abstract Number: 177
     Working Group: Aerosol Chemistry

Abstract
Owing to its abundance in the atmosphere, isoprene and its oxidation products are important constituents in our understanding of atmospheric chemistry. Previous studies have investigated its reactions with nitrate radicals (NO₃) to form secondary organic aerosol (SOA) but focused mainly on how different levels of precursors would affect SOA formation. However, the role of varying oxidant concentrations has not been thoroughly investigated. In this study, we discover that with constant isoprene concentration, the SOA yield and reaction products tend to change with the level of oxidant (NO₃) when VOC:N₂O₅ (oxidant source) ratio varies between 1:1 and 1:14. As oxidant level increases, SOA yield increases from 5.71% (1:1) to 15.2% (1:3) and then remains constant as we further raise oxidant concentration. We attribute such variations to: (1) suppression of SOA formation through first-generational RO₂ unimolecular 1,6 H-shift pathways; (2) enhancement of SOA formation due to multi-generation oxidation. RO₂ unimolecular pathways tend to form more-volatile carbonyl products, hydroperoxyl aldehydes (cis-HPALD-I), and can reduce SOA formation in higher VOC:N₂O₅ conditions as confirmed by both FIGAERO-CIMS measurements and simulated RO₂ fates in 0-D model. On the other hand, when the products undergo multi-generation oxidation in excess NO₃, more oxidized products with lower volatilities will partition more into the particle phase and enhance SOA formation: we identify from FIGAERO-CIMS particle phase results that more oxidized compounds (3N-monomers and 4N-dimers) increase from 4% to 33% of all organic signals, as VOC: N₂O₅ ratio changes from 1:1 to 1:14. However, it is possible that the two effects counteract each other after VOC:N₂O₅ decreases beyond 1:3, which results in a constant SOA yield. Results from this study highlight the importance of considering different degrees of oxidation to assess SOA formation from nitrate radical oxidation of isoprene.