Overview and conclusions of the international photolysis frequency measurement and modeling intercomparison (IPMMI) study

An overview is given of the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) study which was held at the Marshall field site of the National Center for Atmospheric Research near Boulder, Colorado, for 4 days during the period 15-19 June 1998. The major objectives of the study were (1) to compare in blind experiments the results of different techniques of measuring j(NO2) and j((OD)-D-1) under identical and well-defined experimental conditions, (2) to compare results of blind theoretical modeling of j(NO2) and j((OD)-D-1) expected at the experimental site using several current models, (3) to compare the experimental and model results for the j values, (4) to compare the measured and modeled downwelling actinic flux, and (5) on the basis of the results, make recommendations for the choices between alternative quantum yield and cross-section data. The findings of the IPMMI experiment are described in a series of papers in this issue [ e. g., Shetter et al., 2003; Bais et al., 2003], and the overall conclusions are summarized here. Actinic fluxes can be measured to accuracies of similar to5% and can be estimated with radiative transfer models to similar to10%. Photolysis rate coefficients can be measured and modeled with accuracies of similar to10% for high-sun, clear-sky, and low-aerosol loadings. Comparisons between chemical actinometers and spectroradiometers favor more recent, larger cross-section and quantum yield values for NO2 and O-3.