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The photolysis frequency of NO2, j(NO2), was determined by various instrumental techniques and calculated using a number of radiative transfer models for 4 days in June 1998 at the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) in Boulder, Colorado. Experimental techniques included filter radiometry, spectroradiometry, and chemical actinometry. Eight research groups participated using 14 different instruments to determine j(NO2). The blind intercomparison experimental results were submitted to the independent experimental referee and have been compared. Also submitted to the modeling referee were the results of NO2 photolysis frequency calculations for the same time period made by 13 groups who used 15 different radiative transfer models. These model results have been compared with each other and also with the experimental results. The model calculation of clear‐sky j(NO2) values can yield accurate results, but the accuracy depends heavily on the accuracy of the molecular parameters used in these calculations. The instrumental measurements of j(NO2) agree to within the uncertainty of the individual instruments and indicate the stated uncertainties in the instruments or the uncertainties of the molecular parameters may be overestimated. This agreement improves somewhat with the use of more recent NO2 cross‐section data reported in the literature.

Abstract. Uncertainties in modeled spectral UV irradiances under cloud-free conditions are analyzed with respect to limited measurement accuracy of actual atmospheric input parameters or their nonavailability under the assumption that no uncertainty results from the used model or from the spectral values of the extraterrestrial solar irradiance and the gaseous absorption coefficients. The resulting mean uncertainty of spectral UV irradiance is calculated using a root-mean-square (rms) procedure for various scenarios, defined by differing qualities of the used sets of input values. The results are discussed with respect to the possibility of reducing the uncertainty in modeled UV irradiances by additional measurements of input parameters and, on the other hand, assessing which of such measurements may be redundant since greater measurement expense leads to no significant improvement in accuracy of modeled irradiances. The uncertainties in modeled UV irradiances are mainly produced by the uncertainties of the measured ozone amount, by the aerosol optical depth if it is not directly measured, and by the soot concentration of the aerosol in the haze layer. Additional uncertainties can arise where snow cover is present. If 03 and SO2 contents, spectral aerosol optical depth, and aerosol soot concentration near the ground are measured under actual conditions, the uncertainties in input parameters result in a mean uncertainty of about 5% for spectral integrals of UV irradiance. These results cannot be improved significantly, even when measured values of vertical profiles of all atmospheric constituents are used. Using only the observed visibility without the measurement of aerosol optical properties, the mean uncertainty for modeled UV integrals is about 10-15%.

Abstract. The validity of a radiative transfer model can be checked either by comparing its results with measurements or with solutions for artificial cases. Unfortunately, neither type of comparison can guarantee that the spectral UV surface irradiance is accurately calculated for real atmospheric cases. There is a need therefore for benchmarks, i.e., standard results that can be used as a validation tool for UV radiation models. In this paper we give such benchmarks for six cloud-free situations. The chosen cases are characterized by different values of solar zenith angle, ozone column, aerosol loading, and surface albedo. Observations are also available for these cases to allow a further comparison between model results and measurements. An intercomparison of 12 numerical models is used to construct the benchmarks. Each model is supplied with identical input data, and a distinction is made between models that assume a planeparallel geometry and those that use a pseudospherical approximation. Differences remain between the model results, because of different treatments of the input data set. Calculations of direct and global transmission and direct and global irradiance are within 3% for wavelengths longer than 320 nm. For the low-Sun cases the calculations are within 10% for wavelengths longer than 300 nm. On the basis of these calculations, six benchmark UV spectra (295-400 nm) are established with a standard deviation of 2%. Relative standard deviations are higher for the lowest absolute intensities at low Sun (5% at 300 nm). The variation between models is typically less than the variation seen between model and measurement. Differences between the benchmarks and the observed spectra are mainly due to the uncertainty in the input parameters. In four of the six cases the benchmarks agree with the observed spectra within 13% over the whole UV spectral region. IntroductionIn this paper we compare 12 radiative transfer models that are used in various institutes throughout Europe to calculate the surface solar spectral UV irradiance. For this study all Several groups involved in SUVDAMA have a radiation code, which they use for analyzing their spectral irradiance measurements. The objective of this paper is to intercompare these codes, as they are usually run, using the best (and always limited) ancillary data as input parameters, and to compare the model results with actual irradiance measurements. The paper shows that the interpretation of ancillary measurements such as aerosol optical depth and total ozone, which are often performed simultaneously with irradiance measurements, can lead to differences in the calculated spectra. 4915

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