Improvements in star photometry for aerosol characterizations

This work addresses the applicability of the Astronomical Langley method to retrieve accurate calibration constants for a star photometer at a high mountain site. For this application, this method provides more accurate and reliable calibration constants than the classical Langley method. The Astronomical Langley method has been also tested in an urban environment using one year of star photometer measurements in the city of Granada (37.16°N, 3.60°W, 680 m a.s.l.). Under low and stable aerosol load conditions the calibration constants obtained are close to those retrieved at altitude. Finally, an analysis of the applicability of the one-star and classical two-star methods to retrieve aerosol optical depth (δA(λ)) at night is performed. For the one-star method the estimated errors in δA(λ) are close to 0.02 for λ<800 nm and 0.01 for λ>800 nm. For the two-star method the uncertainties are larger than those obtained by the one-star method, and thus can lead to unreliable values of the δA(λ). As a concluding remark, we consider that the one-star method is more appropriate, especially in an urban environment.