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.

The DC-8 mission of September 27, 1991, was designed to sample air flowing into Typhoon Mireille in the boundary layer, air in the upper tropospheric eye region, and air emerging from the typhoon and ahead of the system, also in the upper troposphere. The objective was to find how a typhoon redistributes trace constituents in the West Pacific region and whether any such redistribution is important on the global scale. The boundary layer air (300 m), in a region to the SE of the eye, contained low mixing ratios of the tracer species 03, CO, C2H6, C2H2, C3H8, C6H6 and CS2 but high values of dimethylsulfide (DMS). The eye region relative to the boundary layer, showed somewhat elevated levels of CO, substantially increased levels of 03, CS2 and all nonmethane hydrocarbons (NMHCs), and somewhat reduced levels of DMS. Ahead of the eye, CO and the NMHCs remained unchanged, 03 and CS2 showed a modest decrease, and DMS showed a substantial decrease. There was no evidence from lidar cross sections of ozone for the downward entrainment of stratospheric air into the eye region; these sections show that low ozone values were measured in the troposphere. The DMS data suggest substantial entrainment of boundary layer air into the system, particularly into the eye wall region. Estimates of the DMS sulphur flux between the boundary layer and the free troposphere, based on computations of velocity potential and divergent winds, gave values of about 69 gg S m -2 d-•averaged over a 17.5 ø grid square encompassing the typhoon. A few hours a•er sampling with the DC-8, Mireille passed over Oki Island, just to the north of Japan, producing surface values of ozone of 5.5 ppbv. These 03 levels are consistent with the low tropospheric values found by lidar and are more typical of equatorial regions. We suggest that the central eye region may act like a Taylor column which has moved poleward from low latitudes. The high-altitude photochemical environment within Typhoon Mireille was found to be quite active as evidenced by significant levels of measured gas phase H202 and CH300H and model-computed levels of OH.

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It would appear that the statistical treatment employed herein is capable of defining the primary sources that contribute to roadway dust and of achieving semiquantitative estimates of those anthropogenic sources of primary environmental significance. The resolving power of the analysis is, however, insufficient to distinguish between sources that produce particulate matter having closely similar physical characteristics. Better definition of test vectors and/or more extensive sample fractionation is suggested as a means of improving resolving power. Quantitative assessment of the major source components is generally poor, and it is clear that analytical data for bulk matrix elements are necessary for improved quantitation.Finally, it must be stressed that, while statistical analysis provides a more detailed and more quantitative description of the data, graphical presentation of the mass and concentration distributions of individual elements in a fractionated particulate sample is frequently more useful in assigning physical significance. It is strongly recommended, therefore, that both interpretive approaches be employed in an interactive configuration. AcknowledgmentWe wish to thank the operating staff of the Illinois Advanced TRIGA Reactor facility for their help in making the irradiations and the Illinois State Geological Survey for access to their counting equipment on which most of the reported analyses were performed.