A publication of the Deutsche Meteorologisc:he CJesellschaft

The net effect of atmospheric aerosols in the radiation balwlce is detj~rmined by both their scattering and absorption of solar radiation. The combined optical effect is expressed in the single scatter albedo, 0), of the particles. Currently available data on 0) are insufficient for definitive 'use in climate models because most of them are not corrected for the method-dependent 'I:ffect of Ithe scattering portion of the aerosol on the measured absorption, most refer to the dry state of the aerosol, and the coverage of the globe is far from being complete. Standardisation and calibration of tile measurements is needed. Modelling exercises using currently available data on 0) should clearly state that corrections are required. The purpose of this review is not to suggest a particular range of values for single: scatter albedo. Rather, it is to illustrate that the uncertainties are currently imbedded in various data sets because of the lack of calibration, the possibility that many of the extant methods systematically overe~;timal:e light absorption coefficients, and the necessity of including the influence of humidity in models. Rationale: The Importance of Single Scattering Albedo of Aerosols to the Problem of Radiative Forcing O"sp (m-l), to the extinction coefficient, O"ep (m-I). In l:um, O"e,D = asp + O"ap where O"ap is the absorption coe~fficient:z. The co-albedo of single scatter (1 -00) is often usc~d in radiative transfer calculations and, at typical values of 00 close to unity, it is a sensitive indicator of the importance of light absorption. All of the above optical aerosol parameters are functions of wavelengtJil, A. Indirect radiative forcing by the same anthropogc~nic aerosols may be caused by their inftuenc:e on cloud albedo, cloud area or cloud lifetime; ho1,vever these will not be considered further here. 1 Aerosol particles in the atmosphere both scatter and absorb sunlight, and may absorb infrared radiation as well. Aerosols deriving from human activities cause radiative jorcinj;' that is they impose a change in heat balance (Wm) above and beyond effects of natural aerosols by directly reflecting sunlight backward into space, or by absorbing sunlight or infrared radiation and heating the atmosphere. The relative magnitudes of these two processes -scattering and absorption are embodied in the concept of the single scattering albedo, (I), which is defined as the dimensionless ratio of the scattering coefficient of the aerosol particles, 1 Irhis reviewed l~tOrial article summarizes the outcome of an intematiollal w'Drkshop held in March 1997 at the Institute for ~rroposphl~ric B~esearch (Iff) Leipzig 2 As we only deal with the optical effects of aerosol particles index p is left out in the rest of this text. 250 J. Heintzenberg et al. Beitr. Phys. Atmosph. -.. Prior to ca. 1990, most estimates of the effects of atmospheric aerosols on heat balance considered the aerosol as a whole (natural plus anthropogenic) (Coakley Jr. et al., 1983; Pollack et al., 1976). The single scattering albedo of the aerosols figured prominently in these papers, and was usually estimated by assuming an imaginary part of the refractive index, nim, of the aerosol material. Typical values if nim of 0.004 to 0.2 yielded single scattering albedos in the range 0.97 -0.5. The issue of heating versus cooling by aerosols fonned a substantial part of the efforts in the 1970's and 1980's, with a consensus existing that cooling dominated, except perhaps close to sources of strong light absorbers like black carbon (BC). The critical influence of the ratio of aerosol scattering to absorption was already pointed out by Yamamoto and Tanaka (1972). Nonlight-absorbing aerosols (primarily sulfates and organic matter) reflect sunlight and have a cooling effect that is greatest over low albedo surfaces (e.g. oceans) and smallest over high albedos (e.g. clouds or snow). Light absorbing aerosols (probably mainly BC but possibly also soil dust) also scatter light and their net effect depends on the relative magnitudes of scattering and absorption (e.g. single scattering albedo) as well as the albedo of the underlying surface. Because emissions of these aerosols depend on latitude and season the optical effects also have this