*Corresponding author. Current address: Center for Atmospheric Sciences (CAS), Hampton University, 23 Tyler St, Hampton, VA 23668, USA. Tel.: # 1-757-7275127; fax: # 1-757- 7275090; e-mail: aomar@hamptonu.ed. Atmospheric Environment 33 (1999) 2637 — 2646 Particulate contributions to light extinction and local forcing at a rural Illinois site Ali H. Omar*, Steve Biegalski, Susan M. Larson, Sheldon Landsberger Department of Civil Engineering, University of Illinois at Urbana, Urbana, I1 61801, USA Department of Nuclear Engineering, University of Illinois at Urbana, USA Abstract The light extinction and direct forcing properties of the atmospheric aerosol were investigated for a midwestern rural site (Bondville, IL) using field measurements, a semi-empirical light extinction model, and a radiative transfer code. Model inputs were based on the site measurements of the physical and chemical characteristics of atmospheric aerosol during the spring, summer, fall and winter of 1994. The light scattering and extinction coefficients were calculated and apportioned using the elastic light scattering interactive efficiency (ELSIE) model (Sloane and Wolff, 1985, Atmospheric Environment 19(4), 669—680). The average efficiencies calculated for organic carbon (OC, carbon measured as organic multiplied by 1.2) ranged from 3.81 m/g OC at lower relative humidities ( ( 63%) to 6.90 m/g OC at higher relative humidities ( ' 75%) while sulfate (assumed as ammonium sulfate) efficiencies ranged from 1.23 m/g (NH  )  SO  to 5.78 m/g (NH  )  SO  for the same range of relative humidities. Radiative transfer calculations showed that the rural aerosol at Bondville is most likely to have an overall negative (cooling) forcing effect on climate. Elemental carbon (EC), however, acts to counter sulfate forcing to a degree that has a significant seasonal variation, primarily due to the seasonal variation in the sulfate concentrations. Taking the loading to be the mean summer EC#ammonium sulfate loading and assuming [EC]/[(NH  )  SO  ] to be zero in one case (i.e. no soot present) and 0.025 (summer mean at Bondville) in another leads to a 37% difference in calculated forcing.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Direct aerosol forcing; Extinction efficiency; Soot; Sulfate; Organic carbon 1. Introduction In this paper we present (1) data on midwestern rural aerosol concentrations and size distributions, (2) calcu- lations of optical properties for this aerosol, including the light scattering and absorption coefficients and their apportionments, single scattering albedos, and scattering efficiencies, and (3) the seasonal direct forcing for elemen- tal carbon (EC) and ammonium sulfate for this site as a function of diurnal sun angle. Such aerosol data and calculations for midwestern regions are sparse but neces- sary for regional predictions of climate change. Aerosol particles affect climate in two ways. Aerosols directly scatter incoming solar radiation away from the earth, thereby contributing to a ‘direct’ negative radiative forc- ing of climate. Aerosol particles also enhance the number density of cloud condensation nuclei (CCN), thereby enhancing cloud albedo (Twomey, 1977; Charlson et al., 1987). This ‘indirect’ effect of aerosols also leads to 1352-2310/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 3 2 7 - 6