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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 16, issue 7 | Copyright
Atmos. Chem. Phys., 16, 4283-4305, 2016
https://doi.org/10.5194/acp-16-4283-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 06 Apr 2016

Research article | 06 Apr 2016

Non-OH chemistry in oxidation flow reactors for the study of atmospheric chemistry systematically examined by modeling

Zhe Peng1,2, Douglas A. Day1,2, Amber M. Ortega1,3,a, Brett B. Palm1,2, Weiwei Hu1,2, Harald Stark1,2,5, Rui Li1,3,4,b, Kostas Tsigaridis6, William H. Brune7, and Jose L. Jimenez1,2 Zhe Peng et al.
  • 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
  • 2Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA
  • 3Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO 80309, USA
  • 4Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 5Aerodyne Research, Inc., Billerica, MA 01821, USA
  • 6Center for Climate Systems Research, Columbia University, and NASA Goddard Institute for Space Studies, New York, NY 10025, USA
  • 7Department of Meteorology, Pennsylvania State University, University Park, PA 16802, USA
  • anow at: Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USA
  • bnow at: Markes International, Inc., Cincinnati, OH 45242, USA

Abstract. Oxidation flow reactors (OFRs) using low-pressure Hg lamp emission at 185 and 254nm produce OH radicals efficiently and are widely used in atmospheric chemistry and other fields. However, knowledge of detailed OFR chemistry is limited, allowing speculation in the literature about whether some non-OH reactants, including several not relevant for tropospheric chemistry, may play an important role in these OFRs. These non-OH reactants are UV radiation, O(1D), O(3P), and O3. In this study, we investigate the relative importance of other reactants to OH for the fate of reactant species in OFR under a wide range of conditions via box modeling. The relative importance of non-OH species is less sensitive to UV light intensity than to water vapor mixing ratio (H2O) and external OH reactivity (OHRext), as both non-OH reactants and OH scale roughly proportionally to UV intensity. We show that for field studies in forested regions and also the urban area of Los Angeles, reactants of atmospheric interest are predominantly consumed by OH. We find that O(1D), O(3P), and O3 have relative contributions to volatile organic compound (VOC) consumption that are similar or lower than in the troposphere. The impact of O atoms can be neglected under most conditions in both OFR and troposphere. We define “riskier OFR conditions” as those with either low H2O (<0.1%) or high OHRext ( ≥ 100s−1 in OFR185 and >200s−1 in OFR254). We strongly suggest avoiding such conditions as the importance of non-OH reactants can be substantial for the most sensitive species, although OH may still dominate under some riskier conditions, depending on the species present. Photolysis at non-tropospheric wavelengths (185 and 254nm) may play a significant (>20%) role in the degradation of some aromatics, as well as some oxidation intermediates, under riskier reactor conditions, if the quantum yields are high. Under riskier conditions, some biogenics can have substantial destructions by O3, similarly to the troposphere. Working under low O2 (volume mixing ratio of 0.002) with the OFR185 mode allows OH to completely dominate over O3 reactions even for the biogenic species most reactive with O3. Non-tropospheric VOC photolysis may have been a problem in some laboratory and source studies, but can be avoided or lessened in future studies by diluting source emissions and working at lower precursor concentrations in laboratory studies and by humidification. Photolysis of secondary organic aerosol (SOA) samples is estimated to be significant (>20%) under the upper limit assumption of unity quantum yield at medium (1 × 1013 and 1.5 × 1015 photonscm−2s−1 at 185 and 254nm, respectively) or higher UV flux settings. The need for quantum yield measurements of both VOC and SOA photolysis is highlighted in this study. The results of this study allow improved OFR operation and experimental design and also inform the design of future reactors.

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Oxidation flow reactors (OFRs) are promising tools of studying atmospheric oxidation processes. Elevated concentrations of both OH and non-OH oxidants in OFRs leave room for speculation that non-OH chemistry can play a major role. Through systematic modeling, we find conditions where non-OH VOC fate is significant and show that, in most field studies of SOA using OFRs, non-OH VOC fate in OFRs was insignificant. We also provide guidelines helping OFR users avoid significant non-OH VOC oxidation.
Oxidation flow reactors (OFRs) are promising tools of studying atmospheric oxidation processes....
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