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%.

[1] Approximately 10,000 UV irradiance spectra resulting from 2 years of continuous measurements in Germany were used as a database to analyze the effect of cloudiness on spectral UV irradiance. Values of spectral cloud modification factors (CMF) were derived by modeling a corresponding clear-sky irradiance spectrum for every UV measurement under cloudy conditions. The total set of CMF values was used to train neural networks using different sets of input data ( parameter records) to describe the clouds, resulting in different, optimized, algorithms (CMF parameterizations). These different CMF parameterizations were evaluated by asking how the quality of the derived CMFs depended on the information content of different parameter records. It was shown that a visual description of cloudiness is not adequate to determine CMFs for an actual case (deviations of 50% and more), even if it was known whether or not the solar disk was obscured by clouds. Improvements for the determination of actual CMFs are possible, with deviations mostly below 15% if the parameter record comprises an actual broadband irradiance measurement. It was shown that such a CMF parameterization is able to provide a good estimation of actual CMFs, also for places with a different cloud climatology. The sensitivity of CMFs to wavelength and solar zenith angle was investigated on the basis of the derived CMF parameterizations. The relations found depend on the kind of CMF parameterization, i.e., the parameter record. In particular the separation of those cases when the solar disk is visible from those cases when the solar disk is obscured may lead to different dependencies of CMFs on solar zenith angle and wavelength.

Abstract. The effect of variable tropospheric aerosol conditions on UV radiation is analyzed using both model simulations and UV radiation measurements. A sensitivity study of the aerosol effect on UV radiation is performed by numerical modeling with a focus on the dependence on wavelength, solar zenith angle, and detector geometry. Potential day to day variability in atmospheric aerosols produces changes of spectral integrated radiation quantities of the order of 20% to 45%. Equivalent effects are induced by total ozone variations between 40 Dobson units (DU) and 90 DU for those spectral integrals having a high sensitivity in the UV-B wavelength range. At least 80% of the aerosol effect is caused by typical variations in aerosol optical depth and single scattering albedo. The variation of other aerosol optical properties such as phase function, spectral extinction, and extinction profile is of minor importance. Our results indicate that information on both aerosol optical depth and single scattering albedo is necessary in most cases for an adequate description of aerosol impact on UV radiation. The sensitivity study supplies a tool for the comparison of aerosol effects derived fi'om various UV radiation measurements with respect to spectral weighting and/or detector geometry. The results of the sensitivity study contribute to the discussion of a local aerosol study performed using measurements we made at Hoher Peissenberg, Germany. Two years of ozone and nitrogen dioxide photolysis fi'equency measurements are analyzed with respect to aerosol-induced radiation changes in the UV-B and UV-A wavelength region. A procedure is introduced to extract the aerosol influence from the impact of other atmospheric parameters, particularly fi'om the ozone amount for investigations in the UV-B. A comparison of the derived aerosol effect with model simulations shows good agreement.

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