Quantification of the unknown HONO daytime source and its relation to NO<sub>2</sub>

Abstract

Abstract. During the DOMINO (Diel Oxidant MechanismIn relation to Nitrogen Oxides) campaign in southwest Spain we measured simultaneously all quantities necessary to calculate a photostationary state for HONO in the gas phase. These quantities comprise the concentrations of OH, NO, and HONO and the photolysis frequency of NO 2 , j (NO 2 ) as a proxy for j (HONO). This allowed us to calculate values of the unknown HONO daytime source. This unknown HONO source, normalized by NO 2 mixing ratios and expressed as a conversion frequency (% h −1 ), showed a clear dependence on j (NO 2 ) with values up to 43 % h −1 at noon. We compared our unknown HONO source with values calculated from the measured field data for two recently proposed processes, the light-induced NO 2 conversion on soot surfaces and the reaction of electronically excited NO 2 * with water vapour, with the result that these two reactions normally contributed less than 10 % (<1 % NO 2 + soot + hν; and <10 % NO 2 * + H 2 O) to our unknown HONO daytime source. OH production from HONO photolysis was found to be larger (by 20 %) than the "classical" OH formation from ozone photolysis (O( 1 D)) integrated over the day.Correspondence to: M. Sörgel

Tropospheric aerosol as a reactive intermediate

“…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. 20 On this factual basis, we provide a novel interpretation of recently reported data on the quantication of the unknown HONO daytime source, 26 and seasonally-averaged daytime NO 2 decay lifetimes over several megacities. 51 …”

“…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 …”

“…26 Since a 50% NO 2 / HONO conversion is the stoichiometric limit imposed by R1, the implication is that the 'unknown HONO daytime source' must also be a signicant daytime sink (as NO 3 À ) for NO 2 , and should therefore impact NO 2 decay lifetimes. At present, it is not obvious how the heterogeneous 'dark' reaction R1 could be signicantly enhanced by sunlight, 26 or whether such enhancements are compatible with the kinetics of NO 2 removal, 95 which is generally deemed to occur exclusively in the gas-phase via reaction R11:…”

“…25 Given the key role of _OH, this represents a major deciency in our understanding of tropospheric chemistry. It has been suggested that the diminished solar photolysis of O 3 in winter could be supplemented by that of HONO, 16,[26][27][28][29][30] reaction R6,…”

“…97 Thus, by adopting a representative average value for the uptake coefficient of NO 2 (g) on tropospheric aerosols: g $ 3 Â 10 À4 (see above), we estimate k d $ 6 Â 10 À5 s À1 , which corresponds to $20% NO 2 (g) hourly conversions in clear-day conditions, i.e., in the range of those observed in the eld. 26 By assuming that all HONO produced in reaction R1 is rapidly photolyzed via R6, the rate of _OH production at the terminus of this route, r +OH (R1), will therefore be given by eqn (E3):…”