Measurement of the seasonal and annual variability of total column aerosol in a northeastern U.S. network

A network of multi-filter rotating shadowband radiometers has operated since late 1991 in the northeastern US. The data acquired are simultaneous measurements of total and diffuse horizontal irradiances in six narrowband filtered detectors and one broadband shortwave detector. The direct normal irradiances are calculated from these measurements. These direct data are corrected for cosine response and used to calculate extraterrestrial irradiance (I{sub o}) using the Langley method of regressing the natural logarithm of direct irradiance versus air mass. With frequent determinations of I{sub o}, changes in I{sub o} caused by soiling and filter degradation, for example, can be tracked. Using these I{sub o}`s, total optical depth is calculated for every clear 30-minute period in the record. Consequently, total optical depth may be obtained on a fair number of days throughout the year. Using daily average total optical depth the authors have calculated aerosol optical depths for five wavelengths by subtracting Rayleigh scattering optical depths and Chappuis ozone absorption optical depths at each wavelength. The aerosol pattern at nearly every site is an annual cycle superimposed on a decaying stratospheric loading associated with the Mount Pinatubo volcanic eruption. An attempt is made to remove the volcanic signal using data from another site