Sources and sinks of acetone in the troposphere: Behavior of reactive hydrocarbons and a stable product

Abstract

Acetone, a relatively unreactive carbonyl compound in the atmosphere, has been detected in concentrations of ≃500 parts per trillion in surface air over the central Atlantic and several hundred ppt (volume) near the tropopause. This represents a substantially higher mixing ratio than anticipated from previous descriptions of its sources which are primarily much more reactive hydrocarbon emissions and skinks. We assimilate new information about the tropospheric sources and sinks of this substance into a numerical model of its chemistry, cloud transport, and removal. Simulations relevant to the tropical and the mid‐latitude atmosphere are presented. Photolysis provides a rather slow sink, giving a first‐order decay time scale of 40 days (near the surface) to 10 days (at 200 mbar). Attack by HO radicals is of comparable importance, with a time scale of about 20 days (near the surface) to a hundred days (at 200 mbar). A significant amount of carbon, approximately 70 Tg (C) yr−1, appears to be cycled as acetone. Many of the sources of acetone involve the oxidation of higher molecular weight hydrocarbons, but the relative contribution of primary, nonanthropogenic, sources, e.g., biogenic and oceanic, may be significant. Acetone, like CO, is a useful tracer recording previous photochemical activity in an air parcel, a tracer that might be used to describe the photochemical production of compounds like ozone. The mid‐latitude simulations required some adaptation of the Chatfield (1982) transport model. The activity of vertical transport was constrained to reproduce available 222Rn data. Simulations of these data suggest that mid‐latitude vertical transport is less active than tropical, but rather more active than Kz parameterizations suggest.