1School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
2Department of Atmospheric Science, Colorado State University, Ft. Collins, CO, USA
3Department of Applied Environmental Science & Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden
4Institute of Physics, University of São Paulo, São Paulo, Brazil
5Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
6Department of Earth and Exact Sciences, Institute of Environmental, Chemical and Pharmaceutics Sciences, Federal University of São Paulo, UNIFESP, Diadema, Brazil
7Division of Nuclear Physics, Lund University, P.O. Box 118, 221 00 Lund, Sweden
8Centre for Environmental and Climate Research, Lund University, P.O. Box 118, 221 00 Lund, Sweden
9National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
Received: 25 Jan 2015 – Discussion started: 17 Feb 2015
Abstract. The oxidation of biogenic volatile organic compounds (BVOCs) gives a range of products, from semi-volatile to extremely low-volatility compounds. To treat the interaction of these secondary organic vapours with the particle phase, global aerosol microphysics models generally use either a thermodynamic partitioning approach (assuming instant equilibrium between semi-volatile oxidation products and the particle phase) or a kinetic approach (accounting for the size dependence of condensation). We show that model treatment of the partitioning of biogenic organic vapours into the particle phase, and consequent distribution of material across the size distribution, controls the magnitude of the first aerosol indirect effect (AIE) due to biogenic secondary organic aerosol (SOA). With a kinetic partitioning approach, SOA is distributed according to the existing condensation sink, enhancing the growth of the smallest particles, i.e. those in the nucleation mode. This process tends to increase cloud droplet number concentrations in the presence of biogenic SOA. By contrast, an approach that distributes SOA according to pre-existing organic mass restricts the growth of the smallest particles, limiting the number that are able to form cloud droplets. With an organically mediated new particle formation mechanism, applying a mass-based rather than a kinetic approach to partitioning reduces our calculated global mean AIE due to biogenic SOA by 24 %. Our results suggest that the mechanisms driving organic partitioning need to be fully understood in order to accurately describe the climatic effects of SOA.
Revised: 14 Oct 2015 – Accepted: 19 Oct 2015 – Published: 24 Nov 2015
Scott, C. E., Spracklen, D. V., Pierce, J. R., Riipinen, I., D'Andrea, S. D., Rap, A., Carslaw, K. S., Forster, P. M., Artaxo, P., Kulmala, M., Rizzo, L. V., Swietlicki, E., Mann, G. W., and Pringle, K. J.: Impact of gas-to-particle partitioning approaches on the simulated radiative effects of biogenic secondary organic aerosol, Atmos. Chem. Phys., 15, 12989-13001, doi:10.5194/acp-15-12989-2015, 2015.