The Fate of RO2 Radicals from α-Pinene and Naphthalene Precursors under Constant Branching Ratio with Continues NOx Injection

SAHAR GHADIMI, David R. Cocker III, University of California, Riverside

     Abstract Number: 438
     Working Group: Aerosol Chemistry

Abstract
The fate of gas-phase organic oxidation reactions is dependent on a large number of branching ratios within each molecule’s reaction mechanism. Branching ratio, ß, is defined as the fraction of organic peroxy (RO₂) radical consumption by NO, which depends on NO_x conditions. Indeed, RO₂ radicals can react bimolecularly with NO_x, HO₂, other RO₂ or undergo unimolecular rearrangement, which results in different secondary organic products with different volatility. For instance, during the photo-oxidation of biogenic compounds, low vapor pressure species formed in peroxide rich (low-NO, low ß) conditions favor RO₂+HO₂ reactions, and organic nitrates form in higher NO_x (higher ß) environments by favoring RO₂+NO reactions.

SOA yield from α-pinene and naphthalene is evaluated in this work in a new 118 m³ fixed-volume environmental chamber at UCR/CE-CERT. The SOA yield experiments were conducted at fixed ß, which were controlled via continuous NO_x injection designed using SAPRC-11 simulation.

The SOA formation results are compared to classical experiments in which the NO_x is added instantaneously at the beginning of the experiment. It is observed that the SOA yield was substantially suppressed for high ß conditions for both the monoterpene and the polycyclic aromatic hydrocarbon. Moreover, the continuous NO_x injection method (constant ß) revealed lower SOA yield than variable ß (traditional VOC-NOx) experiments at lower initial hydrocarbon (HC) concentration and higher SOA yield at higher HC conditions.