New accurate values of the imaginary part, k, of the refractive index of water at T = 22 °C, supercooled water at T = -8 °C and polycrystalline ice at T = -25 °C are reported. The k spectrum for water in the spectral region 0.65-2.5 µm is found to be in excellent agreement with those of previous studies. The k values for polycrystalline ice in the 1.44-2.50-µm region eliminate the large uncertainties existing among previously published conflicting sets of data. The imaginary part of refractive index of supercooled water shows a systematic shift of absorption peaks toward the longer wavelengths compared with that of water at warmer temperatures.

[1] A size-segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below-cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol-cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size-segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process-splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea-salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96-bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea-salt mass accompanies this paper [Gong et al., 2002].

[1] The optical properties and hence the radiative forcing of atmospheric aerosols are determined, in part, by the way in which the various constituents are externally or internally mixed. The mixing state must be known to compute the effective refractive index, water activity, and size distribution of the aerosols. In this study we found that the percentage difference in the optical properties, including extinction, single scattering albedo, and asymmetry parameter, between an internal mixture and external mixture of black carbon and ammonium sulfate can be over 25% for the dry case and over 50% for the wet case for typical mass mixing ratios. The differences are a result of a complicated combination of nonlinear Mie theory on the refractive index, assumptions about the coagulated particle sizes for internal mixtures, and the role of water uptake and deliquescence as a function of relative humidity. The computed optical properties are used to estimate the globally average clear-sky direct radiative forcing for different mixing assumptions. The results are displayed as a function of relative humidity to conveniently see the mixing effects for dry aerosols at less than the crystallization point, for dry internal and wet external mixtures between the crystallization and deliquescence points, and for fully wet mixtures above the deliquescence point. For a 9:1 ammonium sulfate to black carbon mass ratio, nearly all the cooling effect predicted for an external mixture is lost for the internally mixed assumption, especially for relative humidities less than the deliquescence point.

An expression for a globally averaged value of direct radiative forcing by absorbing aerosols is derived and applied to the case of smokes produced by biomass burning. It is shown that the direct radiative forcing due to the biomass burning aerosols is a sensitive function of the size distribution of aerosol particles. For the range of measured size distributions of smoke aerosols the direct radiative forcing varies between −0.2 and −1.1 W/m².