Technical Note: Effect of varying the λ = 185 and 254 nm photon flux ratio on radical generation in oxidation flow reactors

Oxidation flow reactors (OFRs) complement environmental smog chambers as a portable, low-cost technique for exposing atmospheric compounds to oxidants such as ozone (inline-formulaO₃), nitrate (inline-formulaNO₃) radicals, and hydroxyl (OH) radicals. OH is most commonly generated in OFRs via photolysis of externally added inline-formulaO₃ at inline-formulaλ=254 inline-formulanm (OFR254) or combined photolysis of inline-formulaO₂ and inline-formulaH₂O at inline-formulaλ=185 inline-formulanm plus photolysis of inline-formulaO₃ at inline-formulaλ=254 inline-formulanm (OFR185) using low-pressure mercury (Hg) lamps. Whereas OFR254 radical generation is influenced by [inline-formulaO₃], [inline-formulaH₂O], and photon flux at inline-formulaλ=254 inline-formulanm (inline-formulaI₂₅₄), OFR185 radical generation is influenced by [inline-formulaO₂], [inline-formulaH₂O], inline-formulaI₁₈₅, and inline-formulaI₂₅₄. Because the ratio of photon fluxes, inline-formulaI₁₈₅:I₂₅₄, is OFR-specific, OFR185 performance varies between different systems even when constant [inline-formulaH₂O] and inline-formulaI₂₅₄ are maintained. Thus, calibrations and models developed for one OFR185 system may not be applicable to another. To investigate these issues, we conducted a series of experiments in which inline-formulaI₁₈₅:I₂₅₄ emitted by Hg lamps installed in an OFR was systematically varied by fusing multiple segments of lamp quartz together that either transmitted or blocked inline-formulaλ=185 inline-formulanm radiation. Integrated OH exposure (OHinline-formula_exp) values achieved for each lamp type were obtained using the tracer decay method as a function of UV intensity, humidity, residence time, and external OH reactivity (OHRinline-formula_ext). Following previous related studies, a photochemical box model was used to develop a generalized OHinline-formula_exp estimation equation as a function of [inline-formulaH₂O], [inline-formulaO₃], and OHRinline-formula_ext that is applicable for inline-formula $M37inlinescrollmathml{I}_{\mathrm{normal\; 185}}:I_{\mathrm{normal\; 254}}\approx \mathrm{normal\; 0.001}$ 84pt12ptsvg-formulamathimg527352cf59ee64effbab94ffad7ea3f7 acp-20-13417-2020-ie00001.svg84pt12ptacp-20-13417-2020-ie00001.png to 0.1.