Interannual variation of ambient aerosol characteristics on the east coast of the United States

We investigate the interannual variability of the seasonally recurring aerosol haze of the United States East Coast. The data consist of observations collected by five Sun/sky radiometer stations. We compare the data collected in 1996 in conjunction with the TARFOX experiment to similar data collected at the same locations in 1993. The regional mean optical thickness remains essentially constant in the two years. The Mount Pinatubo stratospheric mode decreases spectral dependence in 1993, especially for optical thickness below 0.20. The volume size distributions inverted from the sky measurements show an increase in accumulation mode particle size with increasing aerosol optical thickness in both years. At moderate to high optical thickness (tau(670)>0.20) the particles in the fine particle mode, 0.1-0.6 mu m radius range, from both years have the same lognormal parameters (r(m)=0.21, sigma=0.45-0.50). The volume of these particles increases linearly with tau. These size particles dominate the optical properties of the aerosol in the visible spectrum and cause remarkable interannual consistency in the aerosol optical characteristics. The 1993 and 1996 phase functions agree to within 7-10% for optical thicknesses greater than 0.2.