Conversion of gaseous nitrogen dioxide to nitrate and nitrite on aqueous surfactants

Abstract

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.

Tropospheric aerosol as a reactive intermediate

“…93 However, recent work has provided strong constraints on the unknown HONO daytime source and its relation to NO 2 chemistry. 26 Key ndings were that the rates of HONO production during daytime, R HONO , at a given site (1) are an order of magnitude faster than extrapolated nighttime rates of the hydrolytic disproportionation of NO 2 on wet surfaces, 27 (2) systematically increase with solar ux, peaking at noon but, however, (3) can be accounted neither by the reaction of excited NO 2 * with H 2 O(g), 28 reaction R9 nor by the reduction of NO 2 photosensitized by irradiated soot, [C-H] red *, reaction R10: 46,94 …”

“…The objective of this paper is to present an explicit chemical mechanism for R1, and show that its operation on wet SOA accounts for a suite of apparently unrelated, hitherto unexplained observations. The paper is organized as follows: we (1) report new laboratory results obtained by a novel technique developed in our laboratory [41][42][43][44] on the catalysis by dicarboxylic acid anions of the hydrolytic disproportionation of NO 2 (g) on aqueous surfaces (reaction R1) [45][46][47] as well as supporting quantum mechanical calculations and (2) show that such anions are normally produced in urban SOA from the photochemical oxidation of volatile organic compounds (VOCs). 48 We then review and summarize pertinent information on aerosol optical depth time series data over megacities, 49,50 and solar photolysis frequencies J(NO 2 ), J(O 1 D) and J(HONO) as functions of season and latitude.…”

“…46,59 Typical (S/V) values vary from <2 Â 10 À5 cm À1 in clear weather to >5 Â 10 À5 cm À1 during foggy events. It should be realized, however, that (S/V) and total electrolyte concentrations of typical cloud and fog droplets and, hence, g will display wide variability over daily and weekly periods.…”

“…From the ratio of I 62 signal intensities in the presence and absence of carboxylate anions: I 62 (anions)/I 62 (water) $ g/g w , we estimate that reactive NO 2 uptake coefficients increase with anion concentrations from g w $ 10 À7 up to g $ 10 À4 -10 À3 values in the mM to mM range. 46,47 The range of our reported g values is a conservative estimate based on the reproducibility of a series of experiments in which we alternated the use of water and electrolytes as liquids.…”

“…72 Perhaps not unexpectedly, Moscow is less typical: atmospheric aerosol number densities show more pronounced seasonal variability, peaking in winter at values 3-4 times higher than at midsummer. 74 We have shown in the laboratory that all anions, organic and inorganic, [45][46][47] catalyze reaction R1. Organic acids are produced as ubiquitous components of SOA via in situ oxidation of the water-soluble compounds generated in the photo-oxidation of volatile organic compounds VOCs.…”