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

摘要

pstrongAbstract./strong Oxidation flow reactors (OFRs) using low-pressure Hg lamp emission at 185 and 254span class="thinspace"/spannm 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(sup1/supD), O(sup3/supP), and Osub3/sub. 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 (Hsub2/subO) and external OH reactivity (OHRsubext/sub), 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(sup1/supD), O(sup3/supP), and Osub3/sub 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 a??riskier OFR conditionsa?? as those with either low Hsub2/subO (&span class="thinspace"/span0.1span class="thinspace"/span%) or high OHRsubext/sub (a??a?￥a??span class="thinspace"/span100span class="thinspace"/spanssupa??1/sup in OFR185 and &span class="thinspace"/span200span class="thinspace"/spanssupa??1/sup 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 254span class="thinspace"/spannm) may play a significant (&span class="thinspace"/span20span class="thinspace"/span%) 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 Osub3/sub, similarly to the troposphere. Working under low Osub2/sub (volume mixing ratio of 0.002) with the OFR185 mode allows OH to completely dominate over Osub3/sub reactions even for the biogenic species most reactive with Osub3/sub. 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 (&span class="thinspace"/span20span class="thinspace"/span%) under the upper limit assumption of unity quantum yield at medium (1a????a??10sup13/sup and 1.5a????a??10sup15/sup??photonsspan class="thinspace"/spancmsupa??2/supspan class="thinspace"/spanssupa??1/sup at 185 and 254span class="thinspace"