Aerosol Optical Thickness Results from Chemistry/Transport Models
Current retrievals of aerosol optical properties from satellites lack global coverage and are especially difficult over land. Therefore, results from 3-dimensional aerosol chemistry/transport models can be useful to fill in coverage gaps. These models compute aerosol distributions from source emissions using prescribed meteorological fields to calculate aerosol transport, mixing, transformation, and deposition. Such calculations can be carried out for each of the important aerosol types individually. Ground-based measurements and satellite-derived aerosol distributions are used to validate and constrain these models. On the other hand, the results from transport models can be used as "first guess" scenarios in satellite retrievals. In this way both satellite retrievals and model transport calculations of distributions of aerosol properties can be improved iteratively. When better satellite instruments, retrieval algorithms, and models become available, the combined aerosol product will become increasingly accurate.
As a first step, we provide below model-derived distributions of aerosol optical thickness as described by Tegen et al., 1997. There, resulting distributions for the main aerosol species (sulfate, soil dust, carbonaceous aerosols and sea salt) from different transport models by several authors were combined to estimate the individual contributions to the total aerosol optical thickness. The aerosol transport models, from which the results were derived, were developed by Chin et al. (1996) for sulfates; Tegen and Fung (1995) for soil dust; Tegen et al (1997) for sea salt; and Liousse et al. (1996) for carbonaceous aerosols and black carbon.
Model-derived aerosol mass loads for the individual species were converted at each model gridbox into optical thickness τ using:
τ = B × m,
where m is the aerosol column mass load in g/m2 and B is the specific extinction cross section in m2/g. The B values used along with source strengths, global and annual mean mass loads, and global and annual mean aerosol optical thicknesses, are given in the following table. To see maps of annual averages of the optical thickness for the individual aerosol types and the total aerosol optical thickness, click here.
Table 1. Results from Global Transport Models for the Different Aerosol Types, Assumptions for Specific Extinction Efficiencies B Used for Estimating Extinction Optical Thicknesses, and Global Mean and Maximum of Calculated Extinction Optical Depths Using these Values.
| Mean Load
| Optical Thickness
| Optical Thickness
|Soil dust (1-10 µm)||1000||21.6||0.2-0.4(0.3)||0.007||0.59|
|Soil dust (<1µm)||250||14.7||1-2(1.5)||0.022||0.85|
a Numbers in parentheses are used to calculate optical thicknesses given in this table; values are for the 0.55-µm wavelength.
b Sulfate aerosols are assumed to consist of H2SO4.
c Values are 5-8.5 (6.0) m2g-1 over land and 7-14 (10) m2g-1 over oceans.
d Carbonaceous aerosol includes black carbon.
Model-Derived Aerosol Optical Thickness
The following files contain monthly mean values of the aerosol optical thickness derived from the transport models cited above. For information on model validation and model uncertainties, please refer to the original publications.
The files are located in the transport directory at gacp.giss.nasa.gov/data/transport. All are gzipped plain text. Click on the following links to download them individually:
- Total sulfate aerosol
- Anthropogenic fraction sulfate aerosol
- Total soil dust aerosol
- Fraction of soil dust from disturbed soils
- Submicron soil dust
- Fraction of submicron soil dust from disturbed soils
- Organic aerosol
- Black carbon aerosol
- Sea salt aerosol
The results are gridded at 4°×5° resolution. The following sample FORTRAN code explains how to read the data in:
PARAMETER (lon=72, lat=46) REAL*4 AOT(lon,lat) CHARACTER*80 title OPEN (10, FILE='file_name', * FORM='FORMATTED') DO MONTH = 1,12 READ (10, 100) title READ (10, 101) AOT WRITE(6,*) title ENDDO 100 FORMAT(a80) 101 FORMAT(12e10.3) STOP END
The horizontal grid arrangement is as follows:
- lon increases eastward, lat increases northward.
- lon = 1 is centered at 180°W.
- lat = 1 is centered at 89°S.
- Note: lat = 1 and lat = 46 are only 2° tall. All others are 4&$176; tall.
- Chin, M., D. J. Jacob, G. M. Gardner, P. A. Spiro, M. Foreman-Fowler and D. L. Savoie 1996. A global three-dimensional model of tropospheric sulfate. J. Geophys. Res., 101, 18667-18690.
- J. Geophys. Res., 101, 19411-19432, 1996.
- Tegen, I. and I. Fung 1995. Contribution to the mineral aerosol load from land surface modification. J. Geophys. Res., 100, 18707-18726.
- Tegen, I., P. Hollrigl, M. Chin, I. Fung, D. Jacob, and J. Penner 1997. Contribution of different aerosol species to the global aerosol extinction optical thickness: Estimates from model results. J. Geophys. Res., 102, 23895-23915.
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