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 (O-3), nitrate (NO3) radicals, and hydroxyl (OH) radicals. OH is most commonly generated in OFRs via photolysis of externally added O-3 at lambda = 254 nm (OFR254) or combined photolysis of O-2 and H2O at lambda = 185 nm plus photolysis of O-3 at lambda = 254 nm (OFR185) using low-pressure mercury (Hg) lamps. Whereas OFR254 radical generation is influenced by [O-3], [H2O], and photon flux at lambda = 254 nm (1254), OFR185 radical generation is influenced by [O-2], [H2O], I-185, and I-254. Because the ratio of photon fluxes, I-185 : I-254, is OFR-specific, OFR185 performance varies between different systems even when constant [H2O] and I-254 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 I-185 : I-254 emitted by Hg lamps installed in an OFR was systematically varied by fusing multiple segments of lamp quartz together that either transmitted or blocked lambda = 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], [O-3], and OHR ext that is applicable for I-185 : I-254 approximate to 0.001 to 0.1.