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We compare three different methods for determining an average ‘effective‧ UV albedo. These methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. The first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow-covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present the differences and the correlations between the various methods as well as the results for the different locations.

Seven spectroradiorneters measured simultaneous surface UV irradiances at six different sites in the vicinity of Garmisch-Partenkirchen, Germany, during spring 1999. The measured clear-sky irradiance variability between the sites was analyzed with respect to altitude, aerosol optical depth, solar zenith angle, effective albedo, and tropospheric ozone. For conditions of low aerosol loading the increase of irradiance per 1000 m altitude difference was 9% at 400 nm. 20% at 320 nm, and 30% at 300 nm in this season. Effective albedo differences of 0.15, 0.29, and 0.65 were found between the snow-covered stations and the snow-free ground station with the higher effective albedo values determined at the two mountain stations. Clean continental aerosols with a single-scatter albedo of 0.95 were observed during this campaign. The measurements and the observed variations between the sites should enable more accurate modeling studies to be performed for an Alpine environment.

[1] The International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) took place at Boulder, Colorado, from 15 to 19 June 1998 and offered the opportunity to test how well experimental techniques and theoretical models can determine the photolysis frequency of O-3 --> O(D-1) in the troposphere. Different techniques measured the downwelling 2pi sr component of j((OD)-D-1) at the ground and were blindly compared to each other. Moreover, theoretical j((OD)-D-1) model results were blindly compared to data measured under clear sky at relatively low aerosol optical density. Six experimental groups operated one chemical actinometer (CA), six spectroradiometers (SR), and four filter radiometers (FR). General good agreement with deviations less than 10% among the radiometers (SR and FR) was found for solar zenith angles (SZA) less than 60degrees, provided that the instruments used similar absorption cross sections, quantum yields, and temperatures for deriving j((OD)-D-1). The deviations were generally larger at high solar zenith angles and reached a factor of 2 in some cases. Two spectroradiometers and one filter radiometer showed excellent agreement with each other and with the chemical actinometer at all solar zenith angles up to at least 80degrees within typically 5%. These radiometers used recently published O(D-1) quantum yield data and explicitly considered the temperature dependence of j((OD)-D-1). This good agreement shows that each of the different categories of instruments (CA, SR, and FR) is in principle capable of accurate determinations of j((OD)-D-1). A large sensitivity was found to the choice of data for the O(D-1) quantum yield. The best agreement between spectroradiometry and chemical actinometry was obtained when recently published quantum yield data were used. The IPMMI study thus supports the quantum yield recommendation by National Aeronautics and Space Administration-Jet Propulsion Laboratory [Sander et al., 2003] and International Union of Pure and Applied Chemistry (IUPAC) (http://www.iupac-kinetic.ch.cam.ac.uk; data sheet POx2 from 2001). Fifteen models that were operated by 12 model groups participated in the comparison of modeled j((OD)-D-1) with measured data. Most models agreed within 15% with the spectroradiometer-derived j((OD)-D-1) values under clear sky at SZA < 75 degrees, provided that they used similar absorption cross sections, quantum yields, and temperatures. While most models simulated the measured actinic flux quite well, significant deviations in j((OD)-D-1) were observed in cases when outdated O(D-1) quantum yield data or inappropriate temperature data were used.

[1] Four different satellite-UV mapping methods are assessed by comparing them against ground-based measurements. The study includes most of the variability found in geographical, meteorological and atmospheric conditions. Three of the methods did not show any significant systematic bias, except during snow cover. The mean difference (bias) in daily doses for the Rijksinstituut voor Volksgezondheid en Milieu (RIVM) and Joint Research Centre (JRC) methods was found to be less than 10% with a RMS difference of the order of 30%. The Deutsches Zentrum fur Luft- und Raumfahrt (DLR) method was assessed for a few selected months, and the accuracy was similar to the RIVM and JRC methods. It was additionally used to demonstrate how spatial averaging of high-resolution cloud data improves the estimation of UV daily doses. For the Institut d'Aeronomie Spatiale de Belgique (IASB) method the differences were somewhat higher, because of their original cloud algorithm. The mean difference in daily doses for IASB was about 30% or more, depending on the station, while the RMS difference was about 60%. The cloud algorithm of IASB has been replaced recently, and as a result the accuracy of the IASB method has improved. Evidence is found that further research and development should focus on the improvement of the cloud parameterization. Estimation of daily exposures is likely to be improved if additional time-resolved cloudiness information is available for the satellite-based methods. It is also demonstrated that further development work should be carried out on the treatment of albedo of snow-covered surfaces.

[1] Experimental observations have shown photochemical production in the upper layers of firn. We use a modeling approach for Antarctica in summer, calculating the actinic flux in snowpack, and estimating NOX production from nitrate photolysis. Assuming nitrate photolysis is about as efficient for ice as for aqueous solution, and that all nitrate in the ice is available for photolysis, we find good agreement with measured fluxes of NOX from the snow surface at Neumayer Station, Antarctica. We estimate fluxes for other sites, and confirm that they could significantly affect boundary layer chemistry, especially where an atmospheric surface temperature inversion is present. We find that there is considerably more NOX production in the upper snowpack than in the entire tropospheric column above it. Photolysis explains a proportion of the nitrate loss observed from snow at some sites, but other processes (sorption/desorption) are necessary to explain the magnitude and depth profiles observed.