Michael A. Robinson, Matthew M. Coggon, Kelvin H. Bates, Jeff Peischl, Christopher M. Jernigan, Gordon Novak, Subi Thakali, James M. Roberts, J. Andrew Neuman, Patrick R. Veres, Kristen Zuraski, Eleanor M. Waxman, Wyndom Chace, Andrew W. Rollins, Victoria Treadaway, Morgan Selby, Colby Francoeur, Jessica B. Gilman, Shang Liu, Erin R. Delaria, Abby E. Sebol, N S Desai, J Kaiser, Kathryn E. Kautzman, Jason M. St. Clair, Glenn M. Wolfe, Lu Xu, Chelsea E. Stockwell, Carsten Warneke, Han N. Huynh, Ming Lyu, Adam Ahern, C. A. Brock, Alison Piasecki, Sarah Albertin, A. M. Middlebrook, Amy P. Sullivan, M. Mohan, Rodney Weber, Emily Lill, Ilana Pollack, Katherine Ball, John D. Crounse, Paul O. Wennberg, Anna Novelli, Aaron Stainsby, Hendrik Fuchs, Birger Bohn, Georgios I. Gkatzelis, Joshua P. DiGangi, Glenn S. Diskin, J. Jerrold M. Acdan, R. Bradley Pierce, Chia‐Hua Hsu, Siyuan Wang, Rebecca Schwantes, Gonzalo González Abad, Caroline R. Nowlan, Xiong Liu, Nathan Howard, Steven S. Brown
Declining nitrogen oxide (NO x = NO + NO 2 ) emissions have transformed oxidation pathways in urban atmospheres, with implications for air quality. Organic peroxy radicals (RO 2 ), key intermediates in volatile organic compound oxidation, typically react with NO to form ozone (O 3 ). Under lower-NO conditions, alternative RO 2 fates, including isomerization forming highly oxidized organic molecules (HOMs), can enhance secondary organic aerosol (SOA) production. We combine aircraft observations over four major North American cities with geostationary satellite data to characterize isoprene-derived RO 2 fate across urban environments. We infer RO 2 bimolecular lifetimes (τ bi ) as a proxy for isomerization potential, finding longer τ bi (17 ± 11 seconds) in New York, Chicago, and Toronto compared to Los Angeles (7 ± 6 seconds). Satellite measurements reveal that long τ bi is widespread across urban North America, suggesting that declining NO x is likely to lead to greater HOM formation in urban regions. These findings indicate that atmospheric models omitting RO 2 isomerization chemistry may incorrectly simulate organic oxidation and the subsequent oxidation state of volatile organic compounds and SOA.