| Recent model predictions indicate that sulfate aerosol particles cause radiative forcing of the same magnitude but of opposite sign than the forcing caused by anthropogenic greenhouse gases. The spatial and temporal variability of parameters used in the models are highly uncertain resulting in uncertain model predictions of the aerosol radiative forcing. Such parameters related to aerosol particle properties and direct radiative forcing are: the hygroscopic growth factor (f(RH)), the upscatter fraction (;To take and validate such measurements: (i) an ambient aerosol monitoring station was designed and built for continuous ambient aerosol measurements, south of Bondville, in Central Illinois, U.S.A., (ii) instrumentation that measures aerosol particle light scattering coefficients under controlled relative humidity conditions was designed, built, calibrated and tested, (iii) a model was developed that estimates hygroscopic particle growth under the assumption of thermodynamic equilibrium and for metastable particles, (iv) experiments with laboratory generated aerosol particles were completed to calibrate the instrumentation and test the models, and (v) tests for closure were completed using the experimental and modeled results.;Examination of the measurements to date, showed that about 95% of the aerosol gravimetric mass (dp ;A model, was developed to test closure. The model integrates results from thermodynamic equilibrium calculations and experimental data on metastable particles, with light scattering modeling. Tests for closure indicated that the water insoluble component of the aerosol, which is not accounted for by ion chromatography, can be very important in modifying the optical properties of the aerosol. |
