A Robust Solar Zenith Angle Dependency for Passive versus LiDAR Sensor AOD Retrieval Biases, and it’s Implications for Arctic Aerosol Seasonality

Proper constraints on the seasonality of aerosol burdens are essential for quantifying the radiative effects of aerosols in the high latitudes, where insolation varies dramatically by season. Satellite measures of aerosol optical depth (AOD) are an integral component of global aerosol monitoring systems, and are particularly important in remote regions where other observational constraints are limited. In situ observations of surface-level aerosol optical properties at Arctic stations have long shown a pronounced seasonal cycle. This seasonality is also evident in high latitude (>60N) AODs from the LiDAR-based instrument CALIOP, but is not reflected in reanalysis AOD products (MERRA-2 and CAMSRA), or AODs from the passive sensors they assimilate (e.g. AERONET, AVHRR, MODIS, MISR, POLDER, SeaWiFS, VIIRS), even when controlling for seasonal differences in data availability. We find that seasonal differences in the bias between passive and active sensors may be explained by a significant and substantial dependency on the solar zenith angle, which is also apparent in the midlatitudes. We further consider the limits of high latitude observations and provide important context for interpreting the seasonality of passive sensor observations in the mid- to high latitudes.