Towards a more detailed description of tropospheric aqueous phase organic chemistry: CAPRAM 3.0

Abstract

CAPRAM 3.0 is the latest development of the chemical aqueous phase radical mechanism (CAPRAM) series which is incorporating CAPRAM 2.4 (Ervens et al., 2003a, Journal of Geophysical Research—Atmospheres 108) and a new extended reaction mechanism for atmospherically relevant hydrocarbons containing more than two and up to six carbon atoms. The chemistry of organics containing three and four carbon atoms is now described in detail. Almost 400 new reactions are now implemented considering the chemistry of organic compounds containing different functional groups, i.e. alcohols, carbonyl compounds, mono- and dicarboxylic acids, polyfunctional compounds as well as some esters and one heterocyclic compound. The aqueous chemistry has been coupled to the gas phase mechanism RACM (regional atmospheric chemistry modeling) (Stockwell et al., 1997, Journal of Geophysical Research—Atmpspheres 102, 25847–25879), and phase exchange is treated using the resistance model of Schwartz (1986. In: Jaeschke, W. (Ed.), Chemistry of Multiphase Atmospheric Systems, NATO ASI Series, Springer, Berlin, pp. 415–471). The CAPRAM remote scenario which was chosen as the standard scenario showed that the introduction of the higher organic chemistry has a relevant influence on the standard subsystems. The diurnal peak concentration of OH radical in the droplets decreases with about 40% and the reactions of OH with hydrocarbons containing 3 or 4 carbon atoms account for about 10% out of the total sinks of OH in the droplets. A slightly stronger acidification of the aqueous phase in comparison to CAPRAM 2.4 is observed. The simulations for the standard scenario showed that there is an increase of organic mass within the droplets where the organic compounds containing 4 carbon atoms represent the 67.5% of the total mass, whereas in the urban and in the marine scenario the contribution of two carbon atom compounds is dominating. The formation and accumulation of substituted mono- and dicarboxylic acids such as tartaric, mesoxalic and acetic acid in the aqueous phase are also observed.