The hydrolytic disproportionation of gaseous NO 2 on water's surface (2 NO 2 + H 2 O -HONO + NO 3 À + H + ) (R1) has long been deemed to play a key, albeit unquantifiable role in tropospheric chemistry. We recently found that (R1) is dramatically accelerated by anions in experiments performed on aqueous microjets monitored by online electrospray mass spectrometry. This finding let us rationalize unresolved discrepancies among previous laboratory results and suggested that under realistic environmental conditions (R1) should be affected by everpresent surfactants. Herein, we report that NO 2 (g) uptake is significantly enhanced by cationic surfactants, weakly inhibited by fulvic acid (FA, a natural polycarboxylic acid) and anionic surfactants, and unaffected by 1-octanol. Surfactants appear to modulate interfacial anion coverage via electrostatic interactions with charged headgroups. We show that (R1) should be the dominant mechanism for the heterogeneous conversion of NO 2 (g) to HONO under typical atmospheric conditions throughout the day. The photoinduced reduction of NO 2 into HONO on airborne soot might play a limited role during daytime.

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Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O((3)P(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO(2). Two different adsorption states of CO(2), i.e., physisorbed CO(2) on the surface of amorphous solid water and trapped CO(2) in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H(2)O. Since the absorption cross section of CO(2) is smaller than that of H(2)O at 157 nm, the CO(2) surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.

Nanosecond laser ablation and subsequent ion formation processes of submicron aerosol particles containing multicomponent ammonium and sodium salts relevant to atmospheric aerosols were studied using a laser-ionization single-particle aerosol mass spectrometer. In the case of ammonium salts, chemical species with a low ionization energy are predominately observed in the positive-ion mass spectra, while those having a high electron affinity are predominately observed in the negative-ion mass spectra. Dissociative electron attachment processes take place. However, as far as sodium salts are concerned, the energetically preferred ions are not necessarily dominant, and the ion formation processes are suggested to be more complex than the ammonium salts.