Anthropogenic aerosols and the scattering and absorption of solar radiation: Estimates of the climatic impacts through a synthesis of models and satellite observations

Accurate estimates of the impact of aerosols on the radiation balance of the Earth, or aerosol radiative forcing, are essential for future predictions of climate. Anthropogenic aerosols, produced largely through the burning of fossil fuels and biomass, decrease the amount of radiation absorbed by the climate system, partially offsetting the warming from greenhouse gases. The anthropogenic aerosols are thus an important factor, along with volcanic aerosols and variations in solar insolation, that combined with the greenhouse effect determine the long term trends in the Earth's temperature. Ascertaining the magnitude of the aerosol forcing is complicated by the highly variable distribution of the aerosols about the globe, along with imprecise knowledge of the scattering and absorption properties of the aerosol particles. Clouds and the brightness of the underlying land or ocean surface, which dominate the overall radiation balance, must also be well represented in the calculation of the aerosol perturbation.; This thesis estimates the aerosol direct shortwave radiative forcing of anthropogenic aerosols, composed of sulfates, organic carbons and soot, along with estimates for the naturally occurring mineral dust and sea-salt particles. The computations are based on a synthesis of model simulations with modern satellite observations of aerosols, the land surface and the radiation reflected by clouds.; The results of six present-day aerosol simulations from the chemical transport model MATCH (Model for Atmospheric Transport and Chemistry), each for a time span of three years, from March 2000 through February 2003, are presented for comparison. They are based on two different sets of fossil-fuel and industrial emission inventories, two sets of biomass burning emissions, and two separate dust mobilization schemes. For each set of emissions an additional simulation is performed in which the aerosol mass is constrained through the blending of MODIS (Moderate Resolution Imaging Spectro-radiometer) observations of aerosol optical depth with the optical depth determined by the model, through a process of assimilation. The use of several emission sets demonstrates the ability of the assimilation to constrain the model generated aerosol distributions, as well as establishing an expected range of forcing estimates due to uncertainties in the emission inventories.; The fidelity of the present-day simulation is assessed through comparisons to the AERONET (Aerosol Robotic Network) ground network of sun-photometers. The aerosol assimilation improves the simulation relative to both the non-assimilated model and the pure gridded MODIS observations.; A pre-industrial simulation based on emission estimates circa 1850 supplements the present-day simulations and isolates the anthropogenic contribution to the aerosol mass.; The radiative effects of the aerosols are further constrained through the use of MODIS estimates of the land-surface albedo as well as a method of cloud adjustment in which model computed fluxes in the presence of clouds are made to match the all-sky fluxes seen by the CERES (Clouds and the Earth's Radiant Energy Budget) radiometer. The MATCH/MODIS aerosol analysis is now apart of the NASA CERES/SARB (Surface and Atmospheric Radiation Budget) project that generates in-atmosphere and surface fluxes constrained directly by both the CERES all-sky and clear-sky observations.