Broadband Extinction Method to Determine Aerosol Optical Depth from Accumulated Direct Solar Radiation

Abstract

There are two important problems in the aerosol optical depth (AOD) retrievals from hourly/daily/monthly accumulated pyrheliometer data, that is, how to select a suitable cosine of the solar zenith angle (?0) and how to eliminate or minimize cloud effects. In this paper, two models of &ldquo=uivalent? ?0 are developed for two cases of hourly and daily pyrheliometer data. As shown in retrieval simulations, relative standard errors of 0.75-?m-wavelength AOD solutions caused by the model uncertainties are, respectively, 0.77% and 1.93% in the two cases. Then, an approach is proposed to select AOD data from the total optical depths retrieved from hourly/daily pyrheliometer data probably containing cloud contribution. The approach uses additional cloud fraction, surface visibility, and relative humidity data to constrain the AOD upper limit and then to minimize the cloud effect on the monthly/yearly mean AOD estimations. The limit decreases with an increase of cloud fraction. Furthermore, in the case of the AOD retrieval using joint data of total monthly direct solar radiation and sunshine duration, two error factors, that is, the optically thin (solar transparent) cirrus cloud effect and the difference between radiation limits of pyrheliometer and sunshine recorder, are analyzed and an improved inversion method is proposed. The AOD selection approach is tested in two kinds of practical AOD retrieval comparisons. Eleven representative meteorological observatories are selected in the first kind of comparisons of AODs from hourly/daily/monthly pyrheliometer data during 1993?2000. It is found that the AOD selection approach is available to minimize the cloud effect for stable and reasonable monthly/yearly mean AOD estimations. In the case of hourly pyrheliometer data for 1995, the maximum relative deviation between yearly mean AODs, retrieved by using different cloud fraction limits from 0.0 to 60%, is less than 7.9% for all 11 sites. Another kind of comparison between AODs from sun photometer and pyrheliometer data also shows the effectiveness of the AOD selection approach to minimize the cloud effect.