A13G-06
Comprehensive Modeling Study of Chemistry in Oxidation Flow Reactors
Monday, 14 December 2015: 14:55
3004 (Moscone West)
Zhe Peng1, Douglas A Day2, Julia M Lee-Taylor2,3, Harald Stark1, Brett B Palm1, Rui Li4, Amber M Ortega5,6, Weiwei Hu7, Kostas Tsigaridis8, William H Brune9 and Jose L Jimenez10, (1)University of Colorado at Boulder, Boulder, CO, United States, (2)CIRES, Boulder, CO, United States, (3)NCAR-ACD, Boulder, CO, United States, (4)NOAA Boulder, Boulder, CO, United States, (5)University of Colorado Boulder, Boulder, CO, United States, (6)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (7)University of Colorado, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (8)Columbia University of New York, Palisades, NY, United States, (9)Pennsylvania State University Main Campus, University Park, PA, United States, (10)University of Colorado at Boulder, Dept. of Chemistry and Biochemistry, Boulder, CO, United States
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 atmospheric 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. Parametric uncertainty of modeled OH is ~20%. Using different residence time distributions only changes OH by ~10% on average, compared to plug flow. Calibration experiments with SO2 may introduce an error of a factor of ~2 on OH due to non-plug flow. We also investigate the contribution of non-OH reactants (UV, O(1D), O(3P), and O3) to the VOC fate in OFR. For field studies in forested regions or the Los Angeles urban area, VOCs are predominantly consumed by OH. The impact of O atoms can be neglected under most conditions. At low H2O and/or high OHRext, the importance of non-OH reactants is enhanced. Under these conditions, some biogenics can have substantial contributions from O3. Also, non-tropospheric photolysis (at 185 and 254 nm) of some aromatics may be significant. Non-tropospheric VOC photolysis may have been a problem in some lab and source studies, but can be avoided by humidifying and lowering precursor concentrations. SOA photolysis is insignificant for most functional groups. In OFRs NOx is rapidly oxidized to HNO3, and RO2 fate is similar to the low-NOx atmosphere. A comparison 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.
