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

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... mental 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.