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

Rowe, Jake P.; Lambe, Andrew T.; Brune, William H.

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=254inline-formulanm (OFR254) or combined photolysis of inline-formulaO2 and inline-formulaH2O at inline-formulaλ=185inline-formulanm plus photolysis of inline-formulaO3 at inline-formulaλ=254inline-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=254inline-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=185inline-formulanm radiation. Integrated OH exposure (OHinline-formulaexp) 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-formulaext). Following previous related studies, a photochemical box model was used to develop a generalized OHinline-formulaexp estimation equation as a function of [inline-formulaH2O], [inline-formulaO3], and OHRinline-formulaext that is applicable for inline-formula M37inlinescrollmathml I normal 185 : I normal 254 normal 0.001 84pt12ptsvg-formulamathimg527352cf59ee64effbab94ffad7ea3f7 acp-20-13417-2020-ie00001.svg84pt12ptacp-20-13417-2020-ie00001.png to 0.1.

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Rowe, Jake P. / Lambe, Andrew T. / Brune, William H.: Technical Note: Effect of varying the λ = 185 and 254 nm photon flux ratio on radical generation in oxidation flow reactors. 2020. Copernicus Publications.

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