AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Comprehensive Modeling Study of Chemistry of Oxidation Flow Reactors
ZHE PENG, Julia Lee-Taylor, Douglas Day, Amber Ortega, Brett Palm, Weiwei Hu, Harald Stark, Rui Li, Kostas Tsigaridis, Richard Valorso, Bernard Aumont, Sasha Madronich, William Brune, Jose-Luis Jimenez, University of Colorado-Boulder
Abstract Number: 191 Working Group: Aerosol Chemistry
Abstract Oxidation flow reactors (OFRs) using OH produced from low-pressure Hg lamps at 254 nm (OFR254) or both 185 and 254 nm (OFR185) are commonly used in aerosol chemistry and other fields. OFR254 requires the addition of externally formed O3 since OH is formed from O3 photolysis, while OFR185 does not. We perform a comprehensive modeling study of OFR chemistry and provide guidelines for OFR operation. OFR radical chemistry is systematically characterized as a function of H2O concentration (H2O), UV intensity (UV), external OH reactivity (OHRext), and initial O3 concentration (only for OFR254). OH suppression due to OHRext can be orders of magnitude, which can be understood in terms of increase in OH loss rate. The similarities and differences between the evolution of OH reactivity and HOx recycling across VOCs is explored with the fully explicit GECKO-A model. We also investigate the contribution of non-OH reactants (UV, O3 etc.) to the VOC fate in OFR. For field studies in forested regions or the LA urban area, VOCs are predominantly consumed by OH. At low H2O and/or high OHRext, the importance of non-OH reactants is enhanced. Non-tropospheric VOC photolysis (at 185 and 254 nm) may have been a problem in some laboratory and source studies, but can be avoided by experimental planning, which may be aided by the OFR exposure estimator (https://sites.google.com/site/pamwiki/hardware/estimation-equations). RO2 fate under most conditions is similar to the low-NOx atmosphere. Self-reactions of primary RO2 may be significant at high OHRext. NO is rapidly oxidized in OFR under most conditions, while it may have a lifetime >10 s and react with RO2 significantly under very specific conditions in OFR185. A comparison of multiple aspects of OFRs with typical chamber studies and with the atmosphere is presented. This study further establishes OFRs’ usefulness and enables better experiment design and interpretation.