A secondary organic aerosol formation model considering successive oxidation aging and kinetic condensation of organic compounds: Global scale implications
The widely used two-product secondary organic aerosol (SOA) formation model has been extended in this study to consider the volatility changes of secondary organic gases (SOG) arising from the aging process as well as the kinetic condensation of low volatile SOG (LV-SOG). In addition to semi-volatile SOG (SV-SOG) with saturation vapor pressure at 290 K (C*(290)) in the range of similar to 3 ppt-3 ppb and medium-volatile SOG (MV-SOG) with C*(290) in the range of similar to 0.3-300 ppb, we add a third component representing LV-SOG with C*(290) below similar to 3 ppt and design a scheme to transfer MV-SOG to SV-SOG and SV-SOG to LV-SOG associated with oxidation aging. This extended SOA formation model has been implemented in a global aerosol model (GEOS-Chem) and the co-condensation of H2SO4 and LV-SOG on pre-existing particles is explicitly simulated. We show that, over many parts of the continents, LV-SOG concentrations are generally a factor of similar to 2-20 higher than those of H2SO4 and the kinetic condensation of LV-SOG significantly enhances particle growth rates. Comparisons of the simulated and observed evolution of particle size distributions at a boreal forest site (Hyytiala, Finland) clearly show that LV-SOG condensation is critical in order to bring the simulations closer to the observations. With the new SOA formation scheme, annual mean SOA mass increases by a factor of 2-10 in many parts of the boundary layer and reaches above 0.5 mu g m(-3) in most parts of the main continents, improving the agreement with aerosol mass spectrometer (AMS) SOA measurements. While the new scheme generally decreases the concentration of condensation nuclei larger than 10 nm by 3-30% in the lower boundary layer as a result of enhanced surface area and reduced nucleation rates, it substantially increases the concentration of cloud condensation nuclei at a water supersaturation ratio of 0.2%, ranging from similar to 5-20% over a large fraction of oceans and high latitude continents to more than 50% over some parts of South America, Australia, and Indonesia. Our study highlights the importance for global aerosol models to explicitly account for the oxidation aging of SOGs and their contribution to particle growth.