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

We investigate the interannual variability of the seasonally recurring aero sol haze of the United States East Coast. The data consist of observations collected by five Sun/sky radiometer stations. We compare the data collecte d in 1996 in conjunction with the TARFOX experiment to similar data collect ed at the same locations in 1993. The regional mean optical thickness remai ns essentially constant in the two years. The Mount Pinatubo stratospheric mode decreases spectral dependence in 1993, especially for optical thicknes s below 0.20. The volume size distributions inverted from the sky measureme nts show an increase in accumulation mode particle size with increasing aer osol 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 radi us range, from both years have the same lognormal parameters (r(m)=0.21, si gma=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 aeroso l optical characteristics. The 1993 and 1996 phase functions agree to withi n 7-10% for optical thicknesses greater than 0.2.