AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
The Dependence of SOA Formation on NOx Conditions: Effects of Branching Ratio of RO2 +NO Pathway
WEIHAN PENG, William Porter, David R. Cocker III, University of California, Riverside
Abstract Number: 746 Working Group: Aerosol Chemistry
Abstract Secondary organic aerosol formation is dependent on NOx conditions because oxidation products of SOA precursors with different volatility form when NOx condition varies. For example, during the photo-oxidation of aromatic compounds, low vapor pressure species form in peroxide rich (low-NO) conditions by favoring RO2+HO2 reactions and organic nitrates form in higher NO environments by favoring RO2+NO reactions. Different NOx indicators have been used to identify high and low NOx conditions and quantify the effects of NOx on SOA formation, including absolute NOx concentrations, NOx/VOC ratio, and HO2/NO ratio. However, it is the branching ratio (β) of RO2+NO reaction that determines composition and volatility of oxidation products from SOA precursors, because, instead of sole NOx, NOx/VOC or HO2/NO, β decides the fraction of RO2+NO reactions in RO2 chemistry.
In this work, the global branching ratio distribution was estimated using the chemical transport model from GEOS-Chem. Four major categories of branching ratio ranges, as well as other environmental conditions including temperature and relative humidity, were simulated in the dual 90m3 environmental chamber at UCR/CE-CERT with the addition of typical anthropogenic or biogenic SOA precursors (xylene, α-pinene and isoprene). Volatility basis set mass yields for SOA products at different branching ratios were investigated and are in the process of being applied to a regional air quality model to improve the sensitivity and accuracy of SOA prediction at different NOx conditions. Additionally, the bulk chemical composition of the aerosol formed was studied to improve the understanding of oxidation mechanism of SOA precursors at different branching ratios.