GOCART Model Simulation of Atmospheric Aerosols
The Goddard Chemistry Aerosol Radiation and Transport (GOCART) model simulates major tropospheric aerosol components, including sulfate, dust, black carbon (BC), organic carbon (OC), and sea-salt aerosols. The following is a brief description of the model (details can be found in the references listed at the end).
The GOCART model uses the assimilated meteorological fields of the Goddard Earth Observing System Data Assimilation System (GEOS DAS), generated by the Goddard Global Modeling and Assimilation Office. The model has a horizontal resolution of 2 deg latitude by 2.5 deg longitude or 1 deg by 1 deg, and 20-55 vertical sigma layers (depending on the version of GEOS DAS). The following processes are included in the model:
(Figure 1: Global annual emission rates for sulfur, dust, OC+BC, and sea salt)
Anthropogenic emission of SO2 from fossil fuel and biofuel combustions and transportations is taken from the Emission Data Base for Global Atmospheric Research (EDGAR) (Olivier et al., 1996). Biomass burning emission of SO2 is scaled to the seasonal variations of burned biomass data (Duncan et al., 2003; Yevich and Logan, 2003). Volcanic emission of SO2 is from continuously erupting volcanoes (Andres and Kasgnoc, 1998) and sporatically erupting volcanoes (when data available). Oceanic emission of dimethyl sulfide (DMS) is calculated based on the surface seawater concentrations of DMS and 10-m winds over the ocean using an empirical formula (Liss and Merlivat, 1986).
Dust particles ranging from 0.1 to 10 micrometer in radius are considered in the model with 8 size groups (0.1-0.18, 0.18-0.3, 0.3-0.6, 0.6-1, 1-1.8, 1.8-3, 3-6, and 6-10 micrometer). The emission flux Fp for a size group p is expressed as
Fp = S sp u2 ( u - ut ) if u > ut
Where S is the probability source function, which is the probability of sediments accumulated at the topographic depression regions with bare surface; sp is the fraction of size group p within the soil; u is the surface wind speed, and ut is the threshold velocity of wind erosion, determined by particle size and surface wetness.
3. OC and BC
The biomass burning emissions of OC and BC are estimated from the database of seasonal and interannual variations in the burned biomass (Duncan et al., 2002), developed from long-term satellite observations of global fire-counts and aerosol index and an annual mean burned biomass inventory. Anthropogenic emissions of OC and BC are taken from a global data set (Cooke et al., 1999). In addition to direct emissions, the production of OC from terrestrial source is estimated from the emission of volatile organic compounds (Guenther et al.,1995).
Sea-salt emission from the ocean is highly dependent on the surface wind speed, which is calculated as follows:
dF/dr = 1.37u3.41r-3 (1+0.057 r1.05) 101.19 exp(-B2)
where F is the emission flux, r is the particle radius, u is the 10-m winds, and B = (0.380 - log r) / 0.65. Four size bins are considered in the model with radius of 0.1-0.5, 0.5-1.5, 1.5-5, and 5-10 micrometer.
Transport, Chemistry, Dry and Wet Removal
Advection is computed by a flux-form semi-Lagrangian method (Lin and Rood, 1996). Boundary layer turbulent mixing is treated by a second-order closure scheme (Helfand and Labraga, 1998). Moist convection is calculated using the cloud mass flux archived in the GEOS DAS data. Dry deposition includes gravitational settling as a function of particle size and air viscosity, and surface deposition as a function of surface type and meteorological conditions (Wesely, 1989). Wet deposition accounts for the scavenging of aerosols in convective updrafts and rainout/washout in large-scale precipitation (Giorgi and Chameides, 1986; Balkanski et al., 1993). Chemical reactions, including reactions of DMS and SO2 with OH in the air and SO2 with H2O2 in cloud, are calculated using prescribed oxidant fields from the IMAGES model (Müller and Brasseur, 1995).
For a given aerosol type and mass, the wavelength dependent aerosol optical thickness tau is calculated with the following equation:
tau = B Md
where Md is the aerosol dry mass, B is the specific or mass extinction coefficient (m2/g), which is a function of refractive indices, size distributions, particle density, and relative humidity. The value of B is calculated with a Mie code based on the optical property database in the Global Aerosol Data Set (Kopke et al., 1997). We also assume a maximum B as the value at RH = 99 %.
(Figure 2: mass extinction efficiencies for sulfate, OC, BC, and sea salt as a function of RH and wavelength.)
(Figure 3: mass extinction efficiencies for dust at different sizes).
Please refer to the following papers for detailed model description, model results, and all the references listed above.
Chin, M., P. Ginoux, S. Kinne, B. N. Holben, B. N. Duncan, R. V. Martin, J. A. Logan, A. Higurashi, and T. Nakajima, Tropospheric aerosol optical thickness fromt he GOCART model and comparisons with satellite and sunphotometer measurements, J. Atmos. Sci. 59, 461-483, 2002. [download pdf or send email to Mian Chin for reprint.]
Chin, M., R. B. Rood, S.-J. Lin, J. F. Muller, and A. M. Thomspon, Atmosphericsulfur cycle in the global model GOCART: Model description and global properties, J. Geophys. Res., 105, 24,671-24,687, 2000. [download preprint (pdf) (ps) or send email to Mian Chin for reprint.]
Chin, M., D. Savoie, D. Thornton, A. Bandy, B. Huebert, Atmospheric sulfur cycle inthe global model GOCART: Comparison with observations, J. Geophys. Res., 105, 24,698-24,712, 2000. [download preprint (pdf) (ps) or send email to Mian Chin for reprint.]
Ginoux, P., M. Chin, I. Tegen, J. Prospero, B. Holben, O. Dubovik, and S.-J. Lin, Sources and global distributions of dust aerosols simulated with the GOCART model, J. Geophys. Res., 106, 20,255-20,273, 2001. [download preprint (pdf) (ps) or send email to Paul Ginoux for reprint.]