Analysis of Aerosol Optical Depth and Forward Scattering in an Ultraviolet Band Based on Sky Radiometer Measurements

The sky-radiometer/sun-photometer is the most widely used instrument for obtaining aerosol optical depth (AOD) or aerosol optical properties worldwide. Due to the existence of field of view (FOV, 1 degrees), the radiation received by the sky-radiometer includes the forward scattering in addition to direct solar irradiance. This leads to more diffuse light errors of retrieved AODs, especially for shorter wavelength and heavily polluted weather conditions. Using simulation data of three typical aerosol particles (dust, soot, water-soluble), we first verified the accuracy of the Monte Carlo method for calculating the forward scattering effect. Based on the sky-radiometer data collected in Xi'an (2015-2020) where heavy pollution weather is common, the relative errors and correction factors of the AOD were obtained under different conditions, including various short wavelengths (<= 400 nm), solar zenith angles (SZAs) and AODs. Our analysis indicates the close dependence of AOD correction factors on wavelength, SZA, AOD and the optical properties of aerosol particles. The mean relative error in Xi'an increases with the decrease of wavelength (similar to 16.1% at 315 nm) and decreases first and then increases with the increase of the SZA. The relative errors caused by forward scattering can exceed 10% when the AOD is greater than 1 and 25% when the AOD is larger than 2 in the ultraviolet (UV) band. The errors with a wavelength greater than 400 nm and an AOD below 1.0 can be within 5%, which can be ignored. The correlation coefficients of AODs before and after a correction from 315 nm to 400 nm are greater than 0.96, which basically increase with the increase of the wavelength. This indicates that the significance of the forward scattering effect in the Xi'an area with heavy pollution cannot be ignored for short wavelengths. However, such effect is negligible at the longer wavelengths and lower AODs (<1.0) of a sky-radiometer.