Satellite-Derived Photolysis Rates As Constraints on Atmospheric Photochemical Budgets

Photolysis drives reactive atmospheric chemistry, including the production and loss of the key atmospheric oxidants O₃ and OH. Despite the importance of photolysis rates (J-values), global models frequently differ by a factor 3 in these rates and there are currently no long-term datasets to evaluate these model. To address this shortcoming, we have developed SatJ, a global dataset of photolysis rates derived from satellite measurements of clouds and aerosols. The satellite instruments used here—CALIPSO, CloudSat, CERES, and MODIS (CCCM)—resolve individual clouds. As a result, SatJ circumvents the major uncertainties associated with cloud distributions and properties in atmospheric models and reanalysis datasets. SatJ is tightly correlated with J values measured in situ from 18 aircraft campaigns. For J(NO₂) and J(O₃O(¹D)), the correlations are R² = 0.89 and 0.97, respectively, and the mean biases are under 10%. We present a monthly, global climatology of photolysis rates from 8 years of SatJ data (38 million satellite cloud scenes) and use this to evaluate oxidant budgets in the GEOS-Chem global chemical transport model. We identify where the model likely has biases in photolysis rates and the implications of those biases for the model’s representation of atmospheric oxidants.