Light-induced ozone depletion by humic acid films and submicron aerosol particles

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Abstract

[1] The interactions of ozone with submicron particles and films consisting of humic acids of various origins were investigated under near-ultraviolet and visible irradiation using aerosol and coated wall flow-tube systems. Ozone loss to these surfaces was strongly activated in the presence of irradiation. Under simulated atmospheric conditions with respect to irradiance, relative humidity, and O3 mixing ratios, the reactive uptake coefficients of the coatings ranged from γ ∼ 10−6 (in the dark) to γ ∼ 10−4 (under near-UV and visible irradiation) and were inversely dependent on the ozone mixing ratio in the 20- to 270-ppbv range. For the aerosol experiment the uptakes were an order of magnitude smaller. Light and ozone exposure promoted emissions of volatile organic compounds including small aldehydes (formaldehyde, acetaldehyde, hexanal, octanal, nonanal), methanol, and acetone. These results together suggest the existence of photoinduced ozone removal at the surface of various humic substances, which may be a potentially important ozone sink in the continental boundary layer and can represent a possible pathway for processing the organic aerosol.

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