Contribution of Primary and Secondary Organic Aerosol to Global Cloud Condensation Nuclei Concentrations

Cloud condensation nuclei (CCN) prediction is the key to estimating aerosol indirect effects, which remain very uncertain. A significant fraction of CCN is organic aerosol (OA) originating from both primary and secondary sources, and the relative contribution of primary organic aerosol (POA) and secondary organic aerosol (SOA) to the overall organic aerosol budget is still controversial. Even if total OA levels can be well described in models, different microphysical processes form SOA than POA, thereby affecting aerosol size distributions and CCN concentrations. A global aerosol model is used to study the sensitivity of POA versus SOA to CCN concentrations. This work develops further the TwO-Moment Aerosol Sectional (TOMAS) global aerosol microphysical model (Adams and Seinfeld, 2002), which was integrated into the GEOS-CHEM model, a global three-dimensional tropospheric chemistry-transport model (Bey et al., 2001). The TOMAS model conserves both number and mass concentrations of aerosol segregated into 30 size bins from 0.01 μm to 10 μm. The microphysical processes include coagulation, condensation, evaporation, nucleation, size-resolved wet deposition and size- resolved dry deposition. In this work, a simulation of elemental carbon (EC), primary organic aerosol (POA), and secondary organic aerosol (SOA) is added to the model, which previously included sulfate and sea-salt aerosols. TOMAS predicted aerosol number, carbonaceous mass concentrations, and size distributions are compared with observational data from a variety of field campaigns. Year-round observations from various sites are used in the comparison to test the model ability to reproduce the observed seasonality. Additionally, we explore the contribution of secondary organic aerosol to global CCN. By varying the fraction of SOA within the total OA source, we can examine how it affects CCN production. The findings will shed some light on the range of uncertainty resulting from the currently uncertain tropospheric SOA levels.