Abstract
Abstract. Oxidation flow reactors (OFRs) complement environmental smog chambers as a portable, low-cost technique for exposing atmospheric compounds to oxidants such as ozone (O3), nitrate (NO3) radicals, and hydroxyl (OH) radicals. OH is most commonly generated in OFRs via photolysis of externally added O3 at λ=254 nm (OFR254) or combined photolysis of O2 and H2O at λ=185 nm plus photolysis of O3 at λ=254 nm (OFR185) using low-pressure mercury (Hg) lamps. Whereas OFR254 radical generation is influenced by [O3], [H2O], and photon flux at λ=254 nm (I254), OFR185 radical generation is influenced by [O2], [H2O], I185, and I254. Because the ratio of photon fluxes, I185:I254, is OFR-specific, OFR185 performance varies between different systems even when constant [H2O] and I254 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 I185: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 λ=185 nm radiation. Integrated OH exposure (OHexp) 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 (OHRext). Following previous related studies, a photochemical box model was used to develop a generalized OHexp estimation equation as a function of [H2O], [O3], and OHRext that is applicable for I185:I254≈0.001 to 0.1.
Highlights
Hydroxyl (OH) radicals govern the concentrations of most atmospheric organic compounds, including those that lead to secondary organic aerosol (SOA) formation
We designed a series of experiments in which I185 : I254 was systematically varied over a wide range using multiple novel Hg lamp configurations
Integrated OH exposure (OHexp) values were obtained as a function of OFR185 conditions, and a photochemical box model was used to develop a system of OHexp estimation equations that are applicable to OFR185 systems with I185 : I254 ≈ 0.001 to 0.1
Summary
Hydroxyl (OH) radicals govern the concentrations of most atmospheric organic compounds, including those that lead to secondary organic aerosol (SOA) formation. OFRs have employed the secondary λ = 185 nm emission line present in low-pressure mercury (Hg) lamps to generate radicals from the following reactions in addition to those listed above that are employed in OFR254: H2O + hν185 → H + OH (R4). Calibrations and models developed for one OFR185 system may not be applicable to another, making it more difficult to evaluate results or plan experiments. To investigate these issues, we designed a series of experiments in which I185 : I254 was systematically varied over a wide range using multiple novel Hg lamp configurations. Integrated OHexp values were obtained as a function of OFR185 conditions, and a photochemical box model was used to develop a system of OHexp estimation equations that are applicable to OFR185 systems with I185 : I254 ≈ 0.001 to 0.1
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