An evaluation of the FAST-J photolysis algorithm for predicting nitrogen dioxide photolysis rates under clear and cloudy sky conditions

The FAST-J algorithm was developed to quickly and accurately calculate photolysis rates under both clear and cloudy sky conditions. In this paper, photolysis rates of nitrogen dioxide were calculated using FAST-J and compared with measurements taken at two sites in the United States: Phoenix, Arizona, and Houston, Texas. The measurements were derived from either an actinic flux filter radiometer (Phoenix) or a spectroradiometer (Houston). A sun photometer sited nearby these radiometers provided irradiance measurements from which aerosol and cloud optical thicknesses were obtained. Aerosol single scattering albedo was not known, but was taken to be either 0.79 or 0.94, representative of either soot- or sulfate-like aerosols, respectively. These optical properties served as input to the FAST-J algorithm, which in turn was used to calculate photolysis rates. For both clear and cloudy sky cases, the modeled and measured photolysis rates agree within the uncertainties of the measurements for a single scattering albedo of 0.94. For a single scattering albedo of 0.79, the agreement is again within the uncertainty limits except for the cloudy sky case in Houston. The results suggest that the FAST-J code may be a practical algorithm for use in atmospheric chemical transport models that make repeated calls to photolysis rate subroutines.