1Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
2Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
3Department of Physics, University of Helsinki, Helsinki, Finland
4Department of Chemical Engineering, Columbia University, New York, NY, USA
5Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA
6Atmospheric Sciences Division, Brookhaven National Laboratory, Building 815E, Upton, NY, USA
7Department of Civil Engineering, Indian Institute of Technology, Kanpur, India
8Environment Canada, Toronto, Ontario, Canada
9School of Earth and Environment, University of Leeds, Leeds, UK
10Department of Applied Environmental Science and Bert Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
Received: 20 Jun 2013 – Published in Atmos. Chem. Phys. Discuss.: 16 Jul 2013
Abstract. Recent research has shown that secondary organic aerosols (SOA) are major contributors to ultrafine particle growth to climatically relevant sizes, increasing global cloud condensation nuclei (CCN) concentrations within the continental boundary layer (BL). However, there are three recent developments regarding the condensation of SOA that lead to uncertainties in the contribution of SOA to particle growth and CCN concentrations: (1) while many global models contain only biogenic sources of SOA (with annual production rates generally 10–30 Tg yr−1), recent studies have shown that an additional source of SOA around 100 Tg yr−1 correlated with anthropogenic carbon monoxide (CO) emissions may be required to match measurements. (2) Many models treat SOA solely as semi-volatile, which leads to condensation of SOA proportional to the aerosol mass distribution; however, recent closure studies with field measurements show nucleation mode growth can be captured only if it is assumed that a significant fraction of SOA condenses proportional to the Fuchs-corrected aerosol surface area. This suggests a very low volatility of the condensing vapors. (3) Other recent studies of particle growth show that SOA condensation deviates from Fuchs-corrected surface-area condensation at sizes smaller than 10 nm and that size-dependent growth rate parameterizations (GRP) are needed to match measurements. We explore the significance of these three findings using GEOS-Chem-TOMAS global aerosol microphysics model and observations of aerosol size distributions around the globe. The change in the concentration of particles of size Dp > 40 nm (N40) within the BL assuming surface-area condensation compared to mass-distribution net condensation yielded a global increase of 11% but exceeded 100% in biogenically active regions. The percent change in N40 within the BL with the inclusion of the additional 100 Tg SOA yr−1 compared to the base simulation solely with biogenic SOA emissions (19 Tg yr−1) both using surface area condensation yielded a global increase of 13.7%, but exceeded 50% in regions with large CO emissions. The inclusion of two different GRPs in the additional-SOA case both yielded a global increase in N40 of < 1%, however exceeded 5% in some locations in the most extreme case. All of the model simulations were compared to measured data obtained from diverse locations around the globe and the results confirmed a decrease in the model-measurement bias and improved slope for comparing modeled to measured CCN number concentration when non-volatile SOA was assumed and the extra SOA was included.
Revised: 21 Oct 2013 – Accepted: 29 Oct 2013 – Published: 27 Nov 2013
D'Andrea, S. D., Häkkinen, S. A. K., Westervelt, D. M., Kuang, C., Levin, E. J. T., Kanawade, V. P., Leaitch, W. R., Spracklen, D. V., Riipinen, I., and Pierce, J. R.: Understanding global secondary organic aerosol amount and size-resolved condensational behavior, Atmos. Chem. Phys., 13, 11519-11534, doi:10.5194/acp-13-11519-2013, 2013.