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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 Ångström exponent &alpha; and its curvature &gamma;. We show in simulations that within the uncertainty of state-of-the-art Langley calibrations, these diurnal variations of &alpha; and &gamma; 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 &alpha; and &gamma; 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

Improving Langley calibrations by reducing diurnal variations of aerosol Ångström parameters