Aerosol properties at a midlatitude Northern Hemisphere continental site

Measurements of chemical, physical, and optical properties of ambient aerosol particles were obtained at Bondville, Illinois. This research was completed to increase the spatial and temporal resolution of measured aerosol. Results from measurements describe (1) the physical and chemical characteristics of the aerosol and (2) the dependence of light scattering and backscattering on wavelength of light lambda, controlled relative humidity RH, and aerosol particle chemical composition. Formulations for the hygroscopic growth factor f(RH), and backscatter ratio b, as functions of lambda and RH and estimates of the upscatter fraction <(<beta>)over bar>, and Angstrom exponent (a) over circle are also provided. For aerosol sampled at the site from January to December 1995, the mean gravimetric mass concentration for particles with aerodynamic diameter (d(pa)) less than or equal to 1 mum had an arithmetic mean and standard deviation of 10.6 +/- 6.5 mug/m(3) respectively. Ion chromatography (IC) speciated 53% of the total gravimetric mass for particles with d(pa) less than or equal to 1 mum. Most of the IC-identified material (88.0 +/- 13.5%) consisted of NH4+ and SO42-. Material not identified with IC was primarily elemental and organic carbon. The total aerosol light-scattering coefficient at lambda = 550 nm and RH less than or equal to 40% was 51.6 +/- 43.2 Mm(-1) for d(pa) less than or equal to 10 mum and 42.0 +/- 34.9 Mm(-1) for d(pa) less than or equal to 1 mum. Mean values of f(RH = 82.5%, lambda) for total scattering ranged between 1.4 and 1.5 and for back scattering between 1.1 and 1.2. Mean values of b(lambda) ranged from 0.11 to 0.18 for RH < 40% and from 0.09 to 0.14 for RH = 82.5%. Mean values of <(beta )over bar> ranged from 0.21 to 0.30 depending on lambda, RM, and the particle size distribution. Mean values of (a) over circle ranged between 1.8 and 2.4 for RH < 40% and 1.8 and 2.1 for RH = 82.5%. These measured properties are now available for use in models to reduce uncertainties: when quantifying direct aerosol radiative forcing at a continental site influenced by aerosol with anthropogenic origin.