Does the POA–SOA split matter for global CCN formation? W. Trivitayanurak1,* and P. J. Adams1,2 1Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA 2Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA *now at: Department of Highways, Ministry of Transport, Thailand
Abstract. A model of carbonaceous aerosols has been implemented in the TwO-Moment
Aerosol Sectional (TOMAS) microphysics module in the GEOS-Chem chemical transport model (CTM), a model
driven by assimilated meteorology. Inclusion of carbonaceous emissions
alongside pre-existing treatments of sulfate and sea-salt aerosols increases
the number of emitted primary aerosol particles by a factor of 2.5 and raises
annual-average global cloud condensation nuclei at 0.2% supersaturation (CCN(0.2%)) concentrations by a factor of two.
Compared to the prior model without carbonaceous aerosols, this development
improves the model prediction of condensation nuclei with dry diameter larger than 10 nm (CN10) number concentrations significantly
from −45% to −7% bias when compared to long-term observations.
Inclusion of carbonaceous particles also largely eliminates a tendency for
the model to underpredict higher cloud condensation nuclei (CCN) concentrations. Similar to other
carbonaceous models, the model underpredicts organic carbon (OC) and elemental carbon (EC) mass concentrations by
a factor of 2 when compared to EMEP and IMPROVE observations. Because primary organic aerosol (POA) and secondary organic aerosol (SOA) affect aerosol number size distributions via different
microphysical processes, we assess the sensitivity of CCN production, for a
fixed source of organic aerosol (OA) mass, to the assumed POA–SOA split in the model. For a
fixed OA budget, we found that CCN(0.2%) decreases nearly everywhere as
the model changes from a world dominated by POA emissions to one dominated by
SOA condensation. POA is about twice as effective per unit mass at CCN
production compared to SOA. Changing from a 100% POA scenario to a
100% SOA scenario, CCN(0.2%) concentrations in the lowest model
layer decrease by about 20%. In any scenario, carbonaceous aerosols
contribute significantly to global CCN. The SOA–POA split has a significant
effect on global CCN, and the microphysical implications of POA emissions
versus SOA condensation appear to be at least as important as differences in
chemical composition as expressed by the hygroscopicity of OA. These findings
stress the need to better understand carbonaceous aerosols loadings, the
global SOA budget, microphysical pathways of OA formation (emissions versus
condensation) as well as chemical composition to improve climate modeling.
Citation: Trivitayanurak, W. and Adams, P. J.: Does the POA–SOA split matter for global CCN formation?, Atmos. Chem. Phys., 14, 995-1010, doi:10.5194/acp-14-995-2014, 2014.