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

Oxidation flow reactors (OFRs) using low-pressure Hg lamp emission at 185 and 254 nm 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 O 3. 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 (H 2O) and external OH reactivity (OHR ext), 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 O 3 have relative contributions to volatile organic compound (VOC) consumption that are similar or lower than in the troposphere. The impact of Omore » atoms can be neglected under most conditions in both OFR and troposphere. We define “riskier OFR conditions” as those with either low H 2O (< 0.1%) or high OHR ext (≥ 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 254 nm) 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 O 3, similarly to the troposphere. Working under low O 2 (volume mixing ratio of 0.002) with the OFR185 mode allows OH to completely dominate over O 3 reactions even for the biogenic species most reactive with O 3. 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 × 10 13 and 1.5 × 10 15 photons cm –2 s –1 at 185 and 254 nm, respectively) or higher UV flux settings. Furthermore, 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.« less
 [1] ;  [1] ;  [2] ;  [1] ;  [1] ;  [3] ;  [4] ;  [5] ;  [6] ;  [1]
  1. Univ. of Colorado, Boulder, CO (United States)
  2. Univ. of Colorado, Boulder, CO (United States); Univ. of Arizona, Tucson, AZ (United States)
  3. Univ. of Colorado, Boulder, CO (United States); Aerodyne Research, Inc., Billerica, MA (United States)
  4. Univ. of Colorado, Boulder, CO (United States); National Oceanic and Atmospheric Administration, Boulder, CO (United States); Markes International, Inc., Cincinnati, OH (United States)
  5. Columbia Univ., New York, NY (United States); NASA Goddard Institute for Space Studies, New York, NY (United States)
  6. Pennsylvania State Univ., University Park, PA (United States)
Publication Date:
Grant/Contract Number:
Published Article
Journal Name:
Atmospheric Chemistry and Physics (Online)
Additional Journal Information:
Journal Name: Atmospheric Chemistry and Physics (Online); Journal Volume: 16; Journal Issue: 7; Journal ID: ISSN 1680-7324
European Geosciences Union
Research Org:
Univ. of Colorado, Denver, CO (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
Country of Publication:
United States
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1258747