Improving Langley calibrations by reducing diurnal variations of aerosol Angstrom parameters

摘要

Errors in the sun photometer calibration constant lead to artificial diurnal variations, symmetric around solar noon, of the retrieved aerosol optical depth (AOD) and the associated Angstrom exponent α and its curvature γ. We show in simulations that within the uncertainty of state-of-the-art Langley calibrations, these diurnal variations of α and γ can be significant in low AOD conditions, while those of AOD are negligible. We implement a weighted Monte Carlo method of finding an improved calibration constant by minimizing the diurnal variations in α and γ and apply the method to sun photometer data of a clear day in Innsbruck, Austria. The results show that our method can be used to improve the calibrations in two of the four wavelength channels by up to a factor of 3.6. needed, the Langley methods require clear and highly stable atmospheric aerosol conditions, typically only found above the planetary boundary layer on high mountain sites. Here, we introduce a new method that may alleviate this restriction and improve the SPM calibration beyond the Langley uncertainty.