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

摘要

Oxidation flow reactors (OFRs) using low-pressure Hg lamp emissionat 185 and 254 nm produce OH radicals efficiently and are widely used inatmospheric chemistry and other fields. However, knowledge of detailed OFRchemistry is limited, allowing speculation in the literature about whethersome non-OH reactants, including several not relevant for troposphericchemistry, may play an important role in these OFRs. These non-OH reactantsare UV radiation, O(D), O(P), and O. In this study, weinvestigate the relative importance of other reactants to OH for the fate ofreactant species in OFR under a wide range of conditions via box modeling.The relative importance of non-OH species is less sensitive to UV lightintensity than to water vapor mixing ratio (HO) and external OHreactivity (OHR), as both non-OH reactants and OH scale roughlyproportionally to UV intensity. We show that for field studies in forestedregions and also the urban area of Los Angeles, reactants of atmosphericinterest are predominantly consumed by OH. We find that O(D),O(P), and O have relative contributions to volatile organic compound (VOC) consumption thatare similar or lower than in the troposphere. The impact of O atoms can beneglected under most conditions in both OFR and troposphere. We define“riskier OFR conditions” as those with either low HO ( 0.1 %) or high OHR( ≥  100 s in OFR185 and  200 s in OFR254). We strongly suggest avoiding such conditions as theimportance of non-OH reactants can be substantial for the most sensitivespecies, although OH may still dominateunder some riskier conditions, depending on the species present. Photolysis at non-tropospheric wavelengths(185 and 254 nm) may play a significant ( 20 %) role in thedegradation of some aromatics, as well as some oxidation intermediates,under riskier reactor conditions, if the quantum yields are high. Underriskier conditions, some biogenics can have substantial destructions byO, similarly to the troposphere. Working under low O (volume mixing ratio of 0.002) with the OFR185 mode allows OH to completely dominate over O reactions even for the biogenic species most reactive with O. 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 precursorconcentrations in laboratory studies and by humidification. Photolysis ofsecondary organic aerosol (SOA) samples is estimated to be significant ( 20 %) under the upper limit assumption of unity quantum yield at medium (1 × 10 and 1.5 × 10 photons cm s at 185 and 254 nm, 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.