Atmospheric Environment 38 (2004) 3393–3403 An evaluation of the FAST-J photolysis algorithm for predicting nitrogen dioxide photolysis rates under clear and cloudy sky conditions James C. Barnard a, *, Elaine G. Chapman a , Jerome D. Fast a , John R. Schmelzer a , James R. Slusser b , Richard E. Shetter c a Pacific Northwest National Laboratory, MSIN: K:9-30, 902 Battelle Boulevard, P. O. Box 999, Richland, WA 99352, USA b Natural Resource Ecology Laboratory, USDA UVB Monitoring and Research Network, Colorado State University, Ft. Collins, CO 80523, USA c National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80305, USA Received 3 December 2003; accepted 12 March 2004 Abstract 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. r 2004 Elsevier Ltd. All rights reserved. Keywords: NO 2 photolysis; Radiative transfer model; Cloudy sky; Cloud optical thickness; Single scattering albedo 1. Introduction Photolytic reactions play a critical role in controlling the abundance of many atmospheric pollutants. In particular nitrogen dioxide (NO 2 ) photodissociation into nitric oxide and atomic oxygen is known to substantially affect ozone levels, because the only significant tropospheric ozone source is the reaction of atomic and molecular oxygen (Finlayson-Pitts and Pitts, 2000). Chemical transport models have been designed to simulate the formation and destruction of pollutants such as ozone, and for these models to be accurate, they need temporally and spatially specific photolysis rates that explicitly reflect current conditions of aerosol loading and cloud cover. This need has spurred the development of a number of numerical routines designed to calculate photolysis rates. The performance of these routines has been assessed either by comparing one algorithm against another (e.g., Olson et al., 1997), ARTICLE IN PRESS *Corresponding author. Fax: +(509)-372-6168. E-mail address: james.barnard@pnl.gov (J.C. Barnard). 1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.03.034