UVA/Vis-induced nitrous acid formation on polyphenolic films exposed to gaseous NO2

Abstract

Photochemical processes on ground and airborne surfaces have been suspected to lead to production of HONO in the sunlit lower troposphere, e.g. upon light activation of humic acids followed by reaction with adsorbed NO(2). Here, we used tannic and gentisic acids as proxies for atmospheric polyphenolic compounds to obtain further insights into the photoenhanced NO(2) conversion to HONO, which is a significant tropospheric hydroxyl radical (OH) source. The coated wall flow tube technique was used in combination with online detection of gas-phase HONO and NO(x) under different irradiation conditions. Photoenhanced HONO formation rates of 0.1 to 2 ppbv s(-1) were measured upon NO(2) (0-400 ppbv) uptake on tannic and gentisic acid coatings under irradiation with UV light. The data allow identification of three pathways of light-induced HONO formation: (I) photolysis of a nitroaromatic intermediate formed by a non-photochemical process in the dark, with a photolysis frequency of 10(4) s(-1) at 2 × 10(20) photons m(-2) photon flux; (II) direct photo-oxidation, presumably through electron or hydrogen transfer of the excited substrate; and (III) sensitized electron or hydrogen transfer as suggested before but also demonstrated for visible irradiation here. Aging of tannic acid under oxygen in the dark led to products which promoted light-induced HONO formation in the visible.

General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales

“…Photo-enhanced aging was observed in terms of different observables: Soot, humic acids and proxy components of biomass burning aerosols showed enhanced uptake of nitrogen dioxide and ozone under UV-A or visible light (Stemmler et al, 2007;Monge et al, 2010;Zelenay et al, 2011;Sosedova et al, 2011). The initial step is energy, electron or hydrogen atom transfer from a partially oxidized organic precursor (which thereby gets oxidized) to an acceptor, catalyzed by an activated chromophore as a photosensitizer.…”

Photosensitised heterogeneous oxidation kinetics of biomass burning aerosol surrogates by ozone using an irradiated rectangular channel flow reactor

“…Recent studies have shown that reactive uptake of gas phase oxidants by adsorbed phenolic and polyphenolic compounds such as catechols, gentisic acid, tannic acid, 4-phenoxyphenol, and phenol in the presence of a photosensitiser can be significantly enhanced when exposed to ultraviolet (UV) or visible (VIS) radiation compared to dark conditions (George et al, 2005;Jammoul et al, 2008;Net et al, 2010;Sosedova et al, 2011).…”

“…A photosensitiser is a light-absorbing compound that can further enhance heterogeneous kinetics through photosensitive interactions with other species (George et al, 2005;Jammoul et al, 2008;Sosedova et al, 2011). A photosensitising compound can absorb radiation and transfer its excitation energy to a neighbouring molecule that might not have otherwise absorbed radiation (Vione et al, 2006;Hoffmann, 2008;Gómez-Alvarez et al, 2012).…”

Tropospheric aerosol as a reactive intermediate

“…Since R6 is fast (HONO half-life $10 min) this proposal in effect calls for an unspecied process that causes HONO daytime production to be competitive with NO 2 photolysis, R2. [31][32][33][34] The missing process is likely heterogeneous, 19,[35][36][37][38][39] must obviously track sunlight and be able to reduce N(IV)O 2 into HON(III)O under the prevailing oxidizing conditions. 40 Reaction R1:…”