Dust aerosols constitute a major fraction of particulate matter in the troposphere and play a major role in tropospheric chemistry by providing an active surface for heterogeneous reactions. Heterogeneous reactions on dust aerosol particles change the concentration of trace gases in the gas phase and alter the physical/chemical properties of aerosols and so affect atmospheric oxidation capacity, air quality and climate. To better understand the impact of heterogeneous reactions, the current coupled gas and aqueous phase chemistry mechanism (MCM-CAPRAM) was expanded by the addition of a heterogeneous module (CAPRAM-HET), which, at its initial stage, now contains 19 reactions, and includes the uptake of inorganic trace gases such as NO2 and HOx species, and also the uptake of organic species such as formaldehyde, acetaldehyde and acetone. The uptake coefficients used were obtained from the IUPAC task group on atmospheric kinetic data evaluation. Simulations with MCMv3.3.1-CAPRAM-HET were performed for low and high dust conditions in an urban regime. The results obtained show that the concentrations of trace gases barely change under low dust conditions, but generally decrease in the high dust case. For the high dust case, daytime concentrations of NO and NO2 on average decrease by 28% and 30%, respectively. The average daytime decrease is lower for OH, with about 3% loss. O3 also decreased on average by about 10%. On the other hand, HO2 increased on average during daytime by about 5%, although the peak production at midday was lower during the second and third day. The greatest decrease in concentration was observed in NO3 and N2O5, with average losses of 60% and 79%, respectively. Further investigations showed that the concentration decrease observed in most of the species is mainly caused by changes in the gas-phase process rates rather than direct heterogeneous uptake. Changes in the chemical production and loss rate fluxes were analyzed, showing e.g., a decrease of about 33% for NOx when considering heterogeneous reactions. The chemical rates of production and destruction of OH and HO2 decreased on average by 10% and 16%, implying a reduction in the oxidizing capacity of the atmosphere. An outlook for the further development of CAPRAM-HET will be given.

Heterogeneous reactions on dust aerosols under polluted conditions

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