GACP Projects
Defining a Climatology and the Effects of Absorbing Aerosols: Models and Measurements
Joyce E. Penner, PI
Jay Herman, Co-PI
Abstract: The presence of absorbing material in aerosols decreases their single scattering albedo, thereby reducing the amount of sunlight scattered back to space and leading to absorption of solar radiation in the atmosphere within the atmospheric column. In this proposal we aim to develop a better understanding of the sources of absorbing aerosols by comparison of model-predicted concentrations with the measure of absorbing aerosols provided by the TOMS instrument.
The major sources of absorbing aerosols are dust aerosols and aerosols from biomass burning. The major source of uncertainty in the estimates of total abundance of these aerosols derives from an uncertainty in the magnitude of the total source strength of the absorbing aerosols. Thus, our present ability to test the model's representation of biomass aerosols and their source strength is unsatisfactory. In Liousse et al. (1996) we found reasonable agreement between our predicted aerosol concentrations and measurement of absorbing aerosols at Amsterdam Island, but our concentrations at the South Pole were underestimated for the time period October - February. Tegen et al. (1997) showed that the optical depths from our smoke aerosols were considerably smaller than those measured locally at several sites in South America (possibly due to an overestimate of the grid-averaged optical depth by a localized measurement); but the model-predicted optical depths were higher than measured off the Western South Atlantic. Another example of the uncertainty in biomass-burning source strengths derives from the comparison of model-predicted CO concentrations and measurements. Saylor and Easter (1996) compared model-predicted CO (which is primarily from biomass burning in Africa and South America) with data from the MAPS instrument and found that their inventory for burning may be substantially under predicted.
Dust source strength representations are also uncertain. Representations of dust flux are highly dependent on the threshold velocity of wind, as well as the local vegetation cover. Different regions are expected to have different threshold velocities and fluxes as a result of different soil and ground-cover characteristics. There is a need to better quantify dust flux from different regions.
Our project will use wind and precipitation fields for a time period overlapping with the available data from TOMs to compare predicted absorbing aerosol with the presence of measured absorbing aerosol by the TOMs instrument for specific days in regions where cloud does not occur. This will provide a measure of the accuracy of the sources of absorbing aerosol at the surface, assuming the transport and removal of aerosols between the location of burning and detection by TOMS is sufficiently accurate. We will also examine the effects of absorbing aerosol on the thermal structure of the atmosphere.
References
- Liousse, C., J.E. Penner, C. Chuang, J.J. Walton, H. Eddleman, and H. Cachier, 1996: A Three-dimensional model study of carbonaceous aerosols, J. Geophys. Res., 101, 19,411-19,432.
- Saylor, R.D. and R.C. Easter, Air quality models for atmospheric chemistry, Research Summaries: 1994-1996, NASA Upper Atmospheric Research Program and Atmospheric Chemistry Modeling and Analysis Program, 483-484, 1996.
- Tegen, I., P. Hollrigl, M. Chin, I. Fung, D. Jacob, and J. Penner, Contribution of different aerosol species to the global aerosol extinction optical thickness: Estimates from model results, 1997: J. Geophys. Res., 102, 23,895-23,915.
