Characterizing 185 nm Irradiance and Alternative Attenuation Methods in Oxidation Flow Reactors

Oxidation Flow Reactors (OFRs) are emerging as a complementary technique to conventional environmental chambers for the simulation of atmospheric oxidative aging and secondary organic aerosol (SOA) formation processes that are initiated by the hydroxyl radical (OH). In OFRs, OH radicals are produced via the reactions H₂O+hν→OH+H (λ=185 nm) and O(¹D)+H₂O→2OH, with O(¹D) radicals produced from the reaction O₃+hν→O₂+O(¹D) (λ=254 nm). Low-pressure mercury lamps are used to initiate photolysis of O₂, O₃ and H₂O at both λ= 185 and 254 nm. In these reactors OH concentrations are typically ∼100-1000 times higher than ambient OH concentrations, with reactor residence times ranging from τ = 1-4 min for most applications. With this range of OH concentrations and exposure times, flow reactors can simulate multiple days of equivalent atmospheric OH oxidation. One disadvantage of conventional OFRs is the difficulty in achieving controlled, shorter (<1 day) oxidative aging processes that are also relevant to atmospheric SOA formation processes. To investigate this issue, this study characterizes two alternative mercury lamp configurations designed to attenuate 185 and/or 254 nm irradiance relative to standard low-pressure UVC mercury lamps used in the Aerodyne Potential Aerosol Mass (PAM) OFR. In the first configuration, the irradiance at 185 and 254 nm was attenuated by applying segments of Viton heat shrink tubing along standard UVC lamps. In another configuration, the 185 nm lamp output was attenuated relative to the 254 nm output by splicing segments of lamp glass that transmit either 185 and 254 nm or only 254 nm radiation. OH exposures attained by photolysis of O₂/O₃/H₂O with these lamps were calculated from the reactive loss of CO and SO₂ input to the reactor and measured under steady-state conditions as a function of photon flux and [H₂O]. At a midrange RH ~ 50% and τ ~ 2 min, the attainable lower-range photochemical age decreased from ~4-5 to ~0.3-0.4 days of equivalent atmospheric exposure. Thus, the combined usage of attenuated and standard UVC mercury lamps -- as demonstrated here with the PAM OFR -- significantly extends the range of attainable photochemical aging timescales in OFRs with no additional modification of OFR conditions.