Characteristics, impacts and direct radiative forcing of aerosols at the ARM Southern Great Plains Central Facility

Abstract. Based on the analysis of five years of field measurements of aerosol and meteorological variables, we present the characteristics, impacts and direct radiative forcing of aerosols at the Southern Great Plains (SGP) Central Facility (CF) of the Atmospheric Radiation Measurement (ARM) Program. Annual means of total submicron aerosol concentration for particles with aerodynamic diameter (Dp) < 1 mm, particle concentration (Np) for aerosols with 0.1 <−Dp<−10 mm, submicron light absorption coefficient (σ_a) and single scattering albedo at the SGP CF amount to 5306±392 cm−2, 654&olusmn;290 cm−3, 2.0±0.7 Mm−1 and 0.94±0.02, respectively, while those of submicron total scattering coefficient, hemispheric backscatter fraction, submicrometer scattering fraction (Rsp), Angström exponent, hygroscopic growth factor and visibility at 550 nm are 36±2 Mm−1, 0.12± 0.01, 0.84±0.03, 2.25±0.09 and 1.84±0.10, 38±2 km, respectively. Although they exhibit a considerable year-to-year variability, ,i>Rsp, σp, Np and ozone show some increase over the period examined here. This increase is accompanied by a decline in annual precipitation, column integrated water vapor, relative humidity, and cloud cover (particularly from 1998 to 2001) at the site. In particular, the marked precipitation deficit at the SGP CF in 2000 and 2001 is evident of drought associated with La Nina conditions. We compare the diabatic impacts and optical characteristics for smoke and dust aerosols. In general, aerosols at the ARM site produce a net diabatic cooling, with an estimated direct radiative forcing ranging from about −0.7 W m−2 in winter to −2.4 W m−2 in summer.