Particulate contributions to light extinction and local forcing at a ruralIllinois site

The light extinction and direct forcing properties of the atmospheric aeros ol were investigated for a midwestern rural site (Bondville, IL) using fiel d measurements, a semi-empirical light extinction model, and a radiative tr ansfer code. Model inputs were based on the site measurements of the physic al and chemical characteristics of atmospheric aerosol during the spring, s ummer, fall and winter of 1994. The light scattering and extinction coeffic ients were calculated and apportioned using the elastic light scattering in teractive efficiency (ELSIE) model (Sloane and Wolff, 1985, Atmospheric Env ironment 19(4), 669-680). The average efficiencies calculated for organic c arbon (OC, carbon measured as organic multiplied by 1.2) ranged from 3.81 m (2)/g OC at lower relative humidities (<63%) to 6.90 m2/g QC at higher rela tive humidities (>75%) while sulfate (assumed as ammonium sulfate) efficien cies ranged from 1.23 m(2)/g (NH4)(2)SO4 to 5.78 m(2)/g (NH4)(2)SO4 for the same range of relative humidities. Radiative transfer calculations showed that the rural aerosol at Bondville is most likely to have an overall negat ive (cooling) forcing effect on climate. Elemental carbon (EC), however, ac ts to counter sulfate forcing to a degree that has a significant seasonal v ariation, primarily due to the seasonal variation in the sulfate concentrat ions. Taking the loading to be the mean summer EC + ammonium sulfate loadin g and assuming [EC]/[(NH4)(2)SO4] to be zero in one case (i.e. no soot pres ent) and 0.025 (summer mean at Bondville) in another leads to a 37% differe nce in calculated forcing. (C) 1999 Elsevier Science Ltd. All rights reserv ed.