Using Satellite-Derived Photolysis Rates (SatJ) to Evaluate Constraints on Atmospheric Oxidants Based on Photolysis Mechanisms in Chemistry Models

Abstract:

SatJ is a global dataset of photolysis rates derived from satellite measurements of clouds and aerosols (from CALIPSO, CloudSat, CERES, and MODIS; C3M) with the FastJ algorithm. Since the satellites resolve individual clouds, SatJ circumvents the major uncertainty associated with cloud properties in atmospheric models and reanalysis. For each day, SatJ provides vertically resolved photolysis rates of 62 chemical species in 13,000 daytime satellite footprints, each of which contains up to 4 cloud scenes with 16 aerosol layers. SatJ is tightly correlated with collocated J(NO₂) and J(O₃O(¹D)) measurements(R²= 0.97) with minimal biases (4% and -10% respectively) sensitive mainly to small-scale differences in non-uniform cloud field andO₃column estimates.

The global, multi-year coverage of SatJ enables evaluation of atmospheric chemistry models on a scale not previously possible. We report the sensitivity of global model cloud scheme approximations for photochemistry and show the limitations of coarsening model resolution. We examine interannual variability of photolysis rates in SatJ data and the ability of a global atmospheric model (GEOS-Chem) to reproduce them. Finally, we quantify and correct statistical biases in photolysis rates across the globe for GEOS-Chem and highlight its implications for the production of atmospheric oxidants and the lifetime of trace gases. This analysis demonstrates the potential of SatJ for benchmarking atmospheric photochemistry models and understanding the budgets of photochemical oxidants.