Review of aerosol mass scattering efficiencies from ground-based measurements since 1990

We performed a survey of ground-based estimates of aerosol mass scattering efficiencies (alpha(sp), m(2) g(-1)) for various aerosol species and size modes from peer-reviewed literature published since 1990. Accurate estimates of alpha(sp) are important in aerosol modules of global circulation and chemical transport models to compute radiative forcing effects of aerosols and in chemical extinction budgets used for visibility regulatory purposes. The variety of techniques used to compute alpha(sp) can be categorized into four basic methods. We separate reported alpha(sp) on the basis of the methods used to derive them and normalize estimates of alpha(sp) to a common dry relative humidity ( RH, RH approximate to 0%) and mass composition basis, removing some of the variability inherent in alpha(sp) reported under different monitoring conditions. The values of alpha(sp) reviewed here represent common aerosol species and correspond to data from a variety of time periods and global locations. For the 60 studies reviewed, the average (and one standard deviation) alpha(sp) (at visible wavelengths) for fine mode dry ammonium sulfate and dry ammonium nitrate are 2.5 +/- 0.6 m(2) g(-1) and 2.7 +/- 0.5 m(2) g(-1), respectively. The fine mode particulate organic matter alpha(sp) is 3.9 +/- 1.5 m(2) g(-1). The fine mode dust alpha(sp) is 3.3 +/- 0.6 m(2) g(-1) and the fine mode dry sea salt alpha(sp) is 4.5 +/- 0.9 m(2) g(-1). Coarse mode alpha(sp) estimates are also reported. The range of asp may reflect differences in aerosol morphology, age, physicochemical properties and mixing state; however, this survey suggests that the type of method used to derive alpha(sp) can contribute considerably to the variability observed.