The dark reaction of NO, and HzO vapor in 1 atm of air was studied for the purpose of elucidating the recently discussed unknown radical source in smog chambers. Nitrous acid and nitric oxide were found to be formed by the reaction of NO2 and H2O in an evacuable and bakable smog chamber. No nitric acid was observed in the gas phase. The reaction is not stoichiometric and is thought to be a heterogeneous wall reaction. The reaction rate is first order with respect to NO2 and HzO, and the concentrations of HONO and NO initially increase linearly with time. The same reaction proceeds with a different rate constant in a quartz cell, and the reaction of NO2 and H2180 gave H180N0 exclusively. Taking into consideration the heterogeneous reaction of NO2 and HzO, the upper limit of the rate constant of the third-order reaction NO + NO2 + HzO -2HONO was deduced to be (3.0 f 1.4) X ppm-2.min-1, which is one order of magnitude smaller than the previously reported value.Nitrous acid formed by the heterogeneous dark reaction of NO2 and H20 should contribute significantly to both an initially present HONO and a continuous supply of OH radicals by photolysis in smog chamber experiments.

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The surface reaction of NO2 and H 2 0 vapor to emit HONO into the gas phase was studied in the evacuable and bakeable photochemical chamber under the irradiation of UV-visible light ( 2 290 nm). Kinetic analysis of the NO, NOz, and HONO with the aid of computer modeling strongly suggested that the formation of HONO by the surface reaction is photoenhanced. When a linear regression was assumed, the photoenhancement factor defined by {(kh1/k2,) -1) was expressed as (6.8 ? 2.5)k1 under our experimental conditions, where k , is the primary photolysis rate of NO,, and k2,, k h are the second-order-equivalent rate constants of the HONO formation reaction in dark and under irradiation, respectively. The discussion was made that this photocatalitic enhancement of HONO formation would explain the nature of the extra OH radical flux in the smog chamber experiments, which has been discussed as "unknown radical source" and has still been unexplained by the surface dark reaction of NOz and H 2 0 to emit HONO.

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[1] Aerial measurement of atmospheric pollutants was carried out over the East China Sea on 20, 21, and 22 March 2001. High concentrations of not only gaseous species (maximum concentrations of SO 2 > 10 ppb and O 3 > 100 ppb in the marine boundary layer) but also particulate matter (PM) (average concentration of PM 2.5 $ 30 mg m À3 and PM 10 $ 1000 mg m À3 in the marine boundary layer) were observed on 21March. Such high concentrations were observed only at low altitude. A cold front passing over the Korean Peninsula and extension of a high-pressure system from central China to the East China Sea were responsible for the transport of pollutants. High concentrations of pollutants were confined to lower altitude by the high-pressure system. This result is in marked contrast to our previous observation: A highly polluted air mass was transported from central China when a low-pressure system originating near Taiwan moved northeast to Japan along the south coast of the main islands. Anthropogenic sulfate aerosols were transported ahead of Kosa particles (fine yellow sand dust particles). A humid air mass containing a high concentration of sulfate (>20 mg m ) was followed by a dry air mass containing a high concentration of calcium (>1000 mg m À3). Interactions between aerosol components and anthropogenic gaseous species gave insight into the formation process of aerosols over the East China Sea. Nitrate showed a good correlation with calcium except in the highly polluted air mass, indicating that gaseous nitric acid was adsorbed on the surface of coarse particles. No correlation between nitrate and ammonium supported this contention. In contrast, sulfate showed an excellent linear correlation with ammonium, indicating quick neutralization of sulfuric acid by ammonia in the polluted air mass. Exceptions were observed when the calcium concentration was >4 mg m À3 , suggesting that SO 2 gas was adsorbed on the surface of yellow sand dust particles and that oxidation took place on the surface.