Aerosol-Radiation-Chemistry Interactions: Impact of Size-Resolved Particle Microphysics

Aerosol-radiation-chemistry is an interactive system and there exists large uncertainty in our understanding of these interaction processes. The representation of size-resolved particle microphysics in global models has been identified as an important step toward reducing this uncertainty. Under the support of the NASA-ACMAP program, we have integrated a size-resolved (sectional) advanced particle microphysics (APM) model into GEOS-Chem and employed the resulting model to investigate the impact of key particle microphysical processes on aerosol properties, radiation and chemistry. The model has been updated to include recently available nucleation mechanisms derived from laboratory/theoretical studies and to take into account explicitly the contribution of highly oxidized low volatile organics to the particle growth.  We show that secondary particles formed via nucleation dominate cloud condensation nuclei (CCN) number concentrations in most parts of the troposphere but different nucleation parameterization predict quite different spatial patterns and temporal variations of CCN as well as aerosol first indirect radiative forcing. We find that the H₂SO₄-Organics nucleation parameterization derived from recent laboratory measurements significantly over-predicts new particle formation and particle number concentrations in North America in summer. Comparisons of GEOS-Chem/APM simulated particle size distributions based on different nucleation schemes with those measured at a number of locations in different seasons, along with implications to aerosol first indirect radiative forcing, will be presented. Finally, GEOS-Chem/APM simulated size-resolved particle properties have been used to derive cloud droplet number concentration and, together with inputted GEOS-5 cloud water content, to recalculate cloud optical depth (COD) used in GEOS-Chem for photolysis rate calculation. We show that the recalculated COD values are in much better agreement with those from satellite (MODIS) measurements. With the recalculated COD that includes the aerosol effect on cloud droplet effective radius, GEOS-Chem simulated global mean boundary layer OH concentration decreases by ~10% and surface OH concentrations over major anthropogenic regions such as Eastern US, Europe, and East Asia decrease by up to 19%, 14%, and 20%, respectively. Seasonal variations and regional characteristics of the impact of size-resolved aerosol microphysics on cloud properties and implications to atmospheric oxidation capacity and tropospheric compositions will be discussed.