Incorporation of a Cloud Optical Depth Parameterization in GEOS-Chem and Its Impacts on Photochemistry and Aerosol Microphysics

GEOS-Chem is a state-of-the-art global 3-D model of atmospheric composition driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling and Assimilation Office (GMAO). The model has been developed and used by many research groups and contains a number of state-of-the-art modules treating various chemical and aerosol processes. GEOS-5 has become the most widely used meteorological data in GEOS-Chem studies since 2009. However, cloud optical depth (COD) in GEOS-5, used in photolysis rate calculation in GEOS-Chem and known to be important for modeling of atmospheric oxidation ability, has not been fully validated with observations yet. Our comparisons of GEOS-5 cloud properties with MODIS and ISCCP measurements show that GEOS-5 significantly underestimated COD (by a factor of >2) over most continental and marine regions but its liquid and ice water contents are close to observations. To improve the agreement of COD, with observations, an alternative COD parameterization (called NewC thereafter), which re-calculate COD from GEOS-5 cloud water content, GEOS-Chem simulated cloud droplet number concentration, and satellite-derived ice particle effective radius, has been developed and incorporated into GEOS-Chem. Our study indicates that CODs averaged between 60šS and 60šN for year 2006 from MODIS, ISCCP, GEOS-5, and NewC are 4.3, 4.1, 1.8, and 4.2, respectively (see Figure 1 below). MODIS and ISCCP CODs are 2.3 times higher than GEOS-5 COD and close to the value of NewC COD. Due to the change of NewC COD, tropospheric photolysis frequencies and oxidants in GESO-Chem are changed, which then impacts precursors of secondary aerosol particles and associated aerosol microphysics. Compared to the simulation using original GEOS-5 COD, global mean boundary layer J(O1D) and OH are reduced ~8% when COD from NewC is used. At tropic regions (Amazon, the Congolian forests, and Western Pacific), J(O1D) and OH are reduced 20-30%. At Alaska, Eastern USA, North Europe, Eastern China, and the South Ocean, J(O1D) and OH are reduced 10-20%. It is in agreement with the enhanced COD over above regions. The less solar radiation penetrated through the clouds significantly decrease boundary layer photolysis rate and then decrease J(O1D) and OH. We also noticed that the changes over East Pacific, South Atlantic, South India, North Africa, Middle East, and Antarctica are less than 1%. It is because above regions are dominated by clear-sky condition, cloud radiation shows negligible impact on photolysis rate over there. The corresponding reductions of boundary layer CCN0.4 mainly appear over the Tropic and North Hemisphere middle-high latitude continent regions with the values reaching 5-15%. Zonal averaged percentage changes in photolysis frequencies, oxidants, precursors of secondary aerosol particles, and particle number concentrations have also been studied. The backscattering effect above clouds and attenuation effect below clouds and associated influence on photochemistry and aerosol microphysics in different seasons will be presented. Our study indicated that the radiative effects of clouds on ultrafine particles which is dominated by gaseous precursors and new particle formation process is very close to that on photochemistry, while the radiative effects of clouds on CCN0.4 is quite different from others. A clear humped shape of CCN0.4 found at the Tropics is caused by the downward shift of CCN0.4 from tropopause layer and the helping of enhanced gaseous species such as low volatility secondary organic gases, which contributes significantly to particle growth.