Investigation of the Photoenhanced Reduction of Nitrogen Dioxide (NO2) on Organic Films and Above Soils as the Missing Source of Daytime Tropospheric Nitrous Acid (HONO)
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Recent observed unpredictably high HONO daytime concentrations, demanding its ordinarily proposed heterogeneous source to proceed > 60 times faster at noon than during the night, prompted this study, concerning the effect of UV-A radiation on the uptake kinetics of gas-phase NO2 on various phenolic-containing organic films using a wetted-wall flow tube (WWFT) photoreactor. Experimental methods are discussed in detail. A time-dependent model for non-equilibrium conditions incorporating both the chemistry and diffusion for predicting a best-fit reaction probability, ɣbest-fit, as a function of the experimental parameters is detailed. Emphasis is placed on the kinetics of the photoenhanced NO2 uptake reaction under acidic conditions for humic acids (HA), a ubiquitous group of environmental compounds. The linear correlation of HONO production rate with [NO2]0 (k1 = 6.7×10-3 s-1) for commercial HA suggest NO2- particles only diffuse throughout the surface layer depth, as experimentally verified. In general, the pH-dependent results were qualitatively coherent with those of Stemmler et al. (2006), monitoring increases in the photoenhanced HONO formation with pH, owing to increases in reactivity in the reaction with NO2 with several carboxyl groups within the commercial HA with deprotonation, however centred on more authentic pH conditions of anaerobic humic soils (between pH 1.5 and 4.3). A value of krxn = 2.70×10-3 s-1 at pH 3 was obtained, indicative that there was no competition with the hydrolysis reaction, for the tested conditions. Assuming that ɣrxn was rate-limiting, ɣdiffusion was estimated to account for ~ 19% of the total uptake, consistent with the model results. The humification-generated bacteria likely functioned as multicellular aggregates on the acidic HA substrate, producing a biofilm containing numerous chromophoric sensitizer units capable of photochemically reducing NO2 to HONO, defending the observed exponential dependence of HONO yield on irradiance under the tested conditions. The datasets presented for the photoenhanced reaction of NO2 on acidic HA films provide a rather complete kinetic ‘picture’ of an important surface reaction (ɣbest-fit, max of 10-8 at pH 4 under the tested conditions). The scaling up assessments of the kinetic results for the small-scale photoreactor to that of both urban and rural scales are discussed.