Aerosols affect the Earth's temperature and climate by altering the radiative properties of the atmosphere. A large positive component of this radiative forcing from aerosols is due to black carbon--soot--that is released from the burning of fossil fuel and biomass, and, to a lesser extent, natural fires, but the exact forcing is affected by how black carbon is mixed with other aerosol constituents. From studies of aerosol radiative forcing, it is known that black carbon can exist in one of several possible mixing states; distinct from other aerosol particles (externally mixed) or incorporated within them (internally mixed), or a black-carbon core could be surrounded by a well mixed shell. But so far it has been assumed that aerosols exist predominantly as an external mixture. Here I simulate the evolution of the chemical composition of aerosols, finding that the mixing state and direct forcing of the black-carbon component approach those of an internal mixture, largely due to coagulation and growth of aerosol particles. This finding implies a higher positive forcing from black carbon than previously thought, suggesting that the warming effect from black carbon may nearly balance the net cooling effect of other anthropogenic aerosol constituents. The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH4, suggesting that black carbon may be the second most important component of global warming after CO2 in terms of direct forcing.

This well-received and comprehensive textbook on atmospheric processes and numerical methods has been thoroughly revised. This edition includes a wide range of new numerical techniques for solving problems in areas such as cloud microphysics, ocean-atmosphere exchange processes and atmospheric radiative properties. It also contains improved descriptions of atmospheric physics, dynamics, radiation, and aerosol and cloud processes. It is essential reading for researchers, scientists and advanced students to successfully study air pollution and meteorology.

To date, global models of direct radiative forcing have treated elemental carbon (EC) as completely externally mixed or well‐mixed internally. No global study has treated EC as a core in an internal mixture. It is hypothesized that the well‐mixed treatment is unphysical and reality lies between the externally‐mixed and core treatments. It is also suggested, but not proven, that most EC particles are coated to some degree; hence, the core treatment may be more representative than the external‐mixture treatment. Global simulations with the core treatment resulted in EC forcing 50% higher and 40% lower than forcings obtained with the externally‐mixed and well‐internally‐mixed treatments, respectively. In the core case, EC's positive forcing more than offset negative forcing due to all other anthropogenic aerosol components combined. Further studies are needed to understand the mixing state of EC and determine the accuracy of the core treatment.

Abstract. Global simulations of the composition of and direct forcing due to aerosols containing natural and/or anthropogenic sulfate, nitrate, chloride, carbonate, ammonium, sodium, calcium, magnesium, potassium, black carbon, organic matter, silica, ferrous oxide, and aluminum oxide were carried out. Chloride and natural sulfate were found to be the most important natural aerosol constituents in the atmosphere in terms of solar plus thermal-infrared forcing. Sea spray was the most important natural aerosol type, indicating that it should be accounted for in weather and climate calculations. Ammonium was found to have a positive direct forcing, since it reduces water uptake in sulfate-containing solutions; thus, anthropogenic ammonium contributes to global warming. The magnitudes of ammonium and nitrate forcing were smaller than those of chloride or sulfate forcing. When organics were divided into three groups with different assumed UV absorption characteristics, total aerosol direct forcing at the tropopause increased by about +0.

Abstract. Measurements in 1973 and 1987 showed that downward ultraviolet (UV) irradianceswithin the boundary layer in Los Angeles were up to 50% less than those above the boundary layer. Downward total solar irradiances were reduced by less than 14% in both studies. It is estimated that standard gas and particulate absorbers and scatterers accounted for only about 52-62% of the observed UV reductions at Claremont and Riverside. It is hypothesized that absorption by nitrated and aromatic aerosol components and nitrated aromatic gases caused at least 25-30% of the reductions (with aerosols accounting for about 4/5 of this percent). The remaining reductions are still unaccounted for. Absorbing aerosol components include nitrated aromatics, benzaldehydes, benzoic acids, aromatic polycarboxylic acids, phenols, polycyclic aromatic hydrocarbons, and nitrated inorganics. Many of these species have been observed to date in atmospheric particles, and absorption coefficient data indicate many are strong absorbers at long UV wavelengths. Since aerosols containing nitrated or aromatic aerosols have been observed widely in many areas aside from Los Angeles the finding may account for a portion of UV extinction in those regions as well. In Los Angeles, the finding may be important for predicting smog evolution, since UV reductions associated with high aerosol loadings were estimated to cause a 5-8% decrease in ozone mixing ratios in August 1987. Further laboratory and field studies are needed to quantify better the extent of UV absorption due to nitrated and aromatic aerosols and nitrated aromatic gases.

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ABSTRACT. A comparative review of algorithms currently used in air qu ality models to simulate aerosol dynamics is presented. This review addresses coagulation, condensational growth, nucleation, an d gas r r r r rparticle mass transfer. Two major approaches are used in air qu ality models to represent the particle size ( ) distribution: 1 the sectional approach in wh ich the size distribution is discretized into sections and particle properties are assumed to be constant over particle size ( ) sections and 2 the modal approach in wh ich the size distribution is approximated by several modes and particle properties are assumed to be uniform in each mode. The section al approach is accur ate for coagulation an d can reproduce the major ch aracteristics of the evolution of the particle size distribution for condensational growth with the moving-center an d hybrid algorithms. For coagulation and condensation al growth, the modal approach provides more accurate results when the standard deviations of the modes are allowed to vary than it does when they are ® xed. Predictions of H SO nucleation rates are highly sensitive to environ -2 4 mental variables and simulation of relative rates of condensation on existing particles and nucleation is a preferable approach. Explicit treatment of mass transfer is recommended for cases where volatile species undergo different equilib-( rium reactions in different particle size ranges e.g., in the presence of coarse salt ) particles . The results of this study provide useful information for use in selecting algorithms to simulate aerosol dynamics in air qu ality models and for improving the accuracy of existing algorithms.