AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Observational Constraints on High- and Low-NOx Aerosol Formation from Isoprene
DAVID WORTON, Allen H. Goldstein, Jason Surratt, Brian LaFranchi, Arthur Chan, Yunliang Zhao, Robin Weber, Jeong-Hoo Park, Jessica Gilman, Joost de Gouw, Changhyoun Park, Gunnar Schade, Melinda Beaver, Jason St. Clair, John Crounse, Paul Wennberg, Glenn Wolfe, Sara Harrold, Joel A. Thornton, Delphine Farmer, Kenneth Docherty, Michael Cubison, Jose-Luis Jimenez, Amanda Frossard, et al., University of California, Berkeley
Abstract Number: 368 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Isoprene photooxidation in the atmosphere is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic emissions can enhance this SOA formation. In this work we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE), an oxidation product of methacryloyl peroxy nitrate (MPAN), and a broad suite of chemical measurements to investigate the relative importance of high- and low- NO$_x SOA formation pathways at a forested site in California. Gas phase IEPOX and MPAN concentrations were present in substantial excess (> 100 times) over their particle phase organosulfate products indicating that precursor abundances were not the limiting factor for SOA formation. Concentrations of the IEPOX- and MAE-derived organosulfates were similar (~1 ng m$^(-3)) under cooler conditions, while the IEPOX-derived organosulfates were much more abundant (~10 – 20 ng m$^(-3)) under warmer conditions, consistent with larger isoprene emissions. In contrast to the IEPOX-derived organosulfates, the MAE-derived organosulfate concentrations were negligible at elevated temperatures as a result of the short thermolysis lifetime of MPAN, which competes with the aerosol-forming reaction of MPAN with the hydroxyl radical. As the majority of the high NO$_x SOA from isoprene goes through MPAN, with the exception of aqueous processing of glyoxal, the strong anti-correlation between temperature-driven isoprene emissions and the thermal lifetime of MPAN suggests that the low NO$_x SOA formation pathway via IEPOX is more important at the surface. However, in regions of high NO$_x, high isoprene emissions and strong vertical mixing the longer MPAN thermal lifetime aloft could lead to a vertically varying isoprene SOA source.