Estimating the influence of the secondary organic aerosols on present climate using ECHAM5-HAM
1Max Planck Institute for Meteorology, Bundesstrasse 55, 20146 Hamburg, Germany
2Laboratoire des Sciences du Climat et de l'Environnement (LSCE), 91191 Gif-sur-Yvette, France
*now at: Institute for Atmospheric Science and Climate, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland
**now at: Center for Climate System Research, Columbia University and NASA Goddard Institute for Space Studies, 2880 Broadway, New York NY10025, USA
Abstract. In recent years, several field measurement campaigns have highlighted the importance of the organic fraction of aerosol mass, and with such spatial diversity that one may assert that these aerosols are ubiquitous in the troposphere, with particular importance in continental areas. Investigation of the chemical composition of organic aerosol remains a work in progress, but it is now clear that a significant portion of the total organic mass is composed of secondary organic material, that is, aerosol chemically formed from gaseous volatile organic carbon (VOC) precursors. A number of such precursors, of both biogenic and anthropogenic origin, have been identified. Experimental, inventory building and modelling studies have followed. Laboratory studies have yielded information on the chemical pathways that lead to secondary organic aerosol (SOA) formation, and provided the means to estimate the aerosol yields from a given precursor-oxidant reaction. Global inventories of anthropogenic VOC emissions, and of biogenic VOC emitter species distribution and their emission potential have been constructed. Models have been developed that provide global estimates of precursor VOC emissions, SOA formation and atmospheric burdens of these species. This paper estimates the direct and indirect effects of these aerosols using the global climate-aerosol model ECHAM5-HAM. For year 2000 conditions, we estimate a global annual mean shortwave (SW) aerosol direct effect due to SOA of −0.3 W m−2. The model predicts a positive SW indirect effect due to SOA amounting to +0.23 W m−2, arising from enlargement of particles due to condensation of SOA, together with an enhanced coagulation sink for small particles. Longwave effects are small. Finally, we indicate of areas of research into SOA that are required in order to better constrain our estimates of the influence of aerosols on the climate system.