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. Oxidation flow reactors (OFRs) complement environmental smog chambersas a portable, low-cost technique for exposing atmospheric compoundsto oxidants such as ozone (O3), nitrate (NO3)radicals, and hydroxyl (OH) radicals. OH is most commonly generatedin OFRs via photolysis of externally added O3 atλ=254 nm (OFR254) or combined photolysis ofO2 and H2O at λ=185 nm plusphotolysis of O3 at λ=254 nm (OFR185) usinglow-pressure mercury (Hg) lamps. Whereas OFR254 radical generation isinfluenced by [O3], [H2O], and photon flux atλ=254 nm (I254), OFR185 radical generationis influenced by [O2], [H2O], I185, andI254. Because the ratio of photon fluxes,I185:I254, is OFR-specific, OFR185 performance variesbetween different systems even when constant [H2O] andI254 are maintained. Thus, calibrations and models developedfor one OFR185 system may not be applicable to another. To investigatethese issues, we conducted a series of experiments in whichI185:I254 emitted by Hg lamps installed in an OFR wassystematically varied by fusing multiple segments of lamp quartztogether that either transmitted or blocked λ=185 nmradiation. Integrated OH exposure (OHexp) valuesachieved for each lamp type were obtained using the tracer decaymethod as a function of UV intensity, humidity, residence time, andexternal OH reactivity (OHRext). Following previousrelated studies, a photochemical box model was used to develop ageneralized OHexp estimation equation as a function of[H2O], [O3], and OHRext that isapplicable for I185:I254≈0.001 to 0.1.

atmospheric chemistry and physics

Technical Note: Effect of varying the &amp;lt;i&amp;gt;λ&amp;lt;/i&amp;gt; = 185 and 254 nm photon flux ratio on radical generation in oxidation flow reactors