Characterization of temporal and spatial variability of aerosols from ground-based climatology: towards evaluation of satellite mission requirements

The high variability of atmospheric aerosol in space and time poses significant challenges for aerosol observation and simulation, as well as for the design of aerosol monitoring systems. Multi-Angular Polarimeters (MAP) have been identified to provide highly accurate data for characterizing in detail columnar properties of atmospheric aerosol. Obtaining such multi-angular observations at high spatial resolution is very challenging, and even more so from satellite observations. At present, the most advanced MAP instruments are intended to provide observations at the spatial resolution of about 2 km to 4 km. The practical understanding of aerosol loading and type variability at fine to moderate spatial scales is still limited. In this paper, we provide insight on the spatial variability of ambient aerosol by combining the full archive of AERONET observations with ancillary wind speeds from the Modern-Era Retrospective Analysis for Research Application, version 2 (MERRA-2) reanalysis dataset. First, the temporal variability of aerosol observations at the smallest AERONET time scale of 15-30 minutes was used to estimate maximum temporal variability of the aerosol loading (aerosol optical depth - AOD), size (Angstrom exponent - AE) and absorption (single scattering albedo - SSA) over a selection of 30 typical AERONET sites. In the subsequent step, the derived aerosol temporal variability for AOD, AE and SSA are converted to maximum spatial variability using the mean wind speed from MERRA-2. In the final step, the mean aerosol variability difference was analyzed at spatial scales of 2 km and 4 km, which are the spatial scales considered for the MAP instrument to be deployed as part of the Copernicus Anthropogenic CO2 Monitoring (CO2M) mission. The mean aerosol parameters obtained at these spatial scales showed very small differences: only 0.004 for AOD (440 nm), 0.004 for AE (440/870), and 0.00 05 for SSA (440 nm). The analysis of maximum spatial variation of aerosol concentrations showed some non-negligible spikes, up to similar to 0.2 for AOD (440 nm) at spatial scales of 4 km. However, those high fluctuations correspond to highly polluted urban sites (i.e. Beijing and Mexico City), and the maximum AOD changes per km remain at similar to 6% with respect to the total AOD. The maximum spatial variability for AE and SSA also showed no significant deviations at 4 km (<0.2 for AE; <0.03 for SSA). Therefore, we conclude that using a 4 km spatial resolution for MAP sensors is sufficient for capturing the main features of aerosol variability that is required for the CO2M mission. (C) 2021 The Author(s). Published by Elsevier Ltd.