We measured photochemical mineralization of dissolved organic carbon in a humic lake in situ. At a depth of 1 cm, solar radiation mineralized 19 mmol C m Ϫ3 d Ϫ1 . The rate of mineralization decreased with increasing depth with an attenuation coefficient of 23 m Ϫ1 . Consequently, most photochemical mineralization in the water column (0.99 mmol C m Ϫ2 d Ϫ1 ) took place in the top 10 cm. The rate of photochemical mineralization was also modeled as a product of three spectra: (1) scalar photon flux density, (2) the apparent quantum yield ( ), and (3) the absorption of chromophoric dissolved organic matter. We described the spectrum for apparent quantum yield as ϭ c ϫ 10 Ϫ d , where c (dimensionless) and d (nm Ϫ1 ) are positive constants. Mathematical optimization for the best fit between the measured and the modeled photochemical mineralization resulted in of 7.52 ϫ 10 Ϫ0.0122 . The based on the measurements in situ agreed with determined in a laboratory at 320, 355, and 390 nm. Using the determined , we calculated that UV-B contributed 9%, UV-A 68%, and visible light 23% to the photochemical mineralization. Half of total photochemical mineralization was due to wavelengths Ͻ360 nm. Our method for the determination of is applicable in situ, improves the prediction of photochemical reaction rates in surface waters, and offers an alternative to the determination of quantum yields at discrete wavelengths. helsinki.fi). AcknowledgmentsWe thank Anne Ojala for the global radiation measurements, Jorma Keskitalo for many kinds of assistance, Heikki Haario for advice with the Matlab software, and Martti Heikinheimo for advice in the problems of radiative transfer in the atmosphere. The quantum yields at single wavelengths were determined in the laboratory of Helge Lemmetyinen under his supervision.

Abstract. During recent years, methods have been developed for estimating UV irradiance reaching the Earth's surface using satellite-measured backscattered UV radiances. The NASA-developed method is based on radiative transfer calculations and satellite measurements of parameters affecting UV radiation: extraterrestrial solar irradiance, atmospheric ozone, cloud reflectivity, aerosol amounts, and ground albedo. In this work a comparison is made between daily UV erythemal doses estimated from Nimbus-7/TOMS measurements (from 1991 to May 1993) and those calculated from ground-based spectroradiometer data. Three stations operated by the National Science Foundation were chosen for this comparison: Ushuaia, Argentina (for 573 days), Palmer, Antarctica (for 450 days), and San Diego, California, (for 149 days). These stations were selected to illustrate the differences between ground-based measurements using the same type of instrument, SUV-100 double monochromator spectroradiometers, and satellite When the reflectivity at all three sites is low (no snow), the TOMS irradiance estimate is larger than the SUV-100 measurements consistent with previously analyzed Brewer data at Toronto. The effects of local fog or clouds smaller than the satellite field of view and undetected UV-absorbing aerosols near the ground are discussed. In addition to uncertainties in radiometric calibrations of the spectrometers, none of the SUV-100 data are corrected for deviations of diffuser-transmittance from true cosine response. IntroductionChanges in the Earth's atmosphere caused by anthropogenic and natural pollutants has led to the well-documented decline in ozone and the corresponding small increase in UV irradiance at the Earth's surface at higher latitudes greater than about 40 ø [Herman et al., 1996]. These increases can affect the human health, crop yields, ocean productivity, and materials aging.In addition to the satellite estimation of UV irradiance, there has been a long-term well-established network of ground-based instrumentation. The ground-based network has the advantage that it more accurately represents the local conditions of the region than the currently large-pixel (100 km) estimates from TOMS (total ozone mapping spectrometer). UV radiation at ground level is traditionally measured using a variety of ground-based instruments. However, the network of high-quality ground-based UV instruments is not dense enough for a monitoring of the global distribution of solar UV radiation and is almost completely lacking over the oceans. Because of this limitation, methods using satellite measurements of extraterrestrial solar radiation, atmospheric ozone, cloud reflectivity, aerosol amounts, and ground albedo combined with radiative transfer modeling are needed to estimate the daily global distribution of UV irradiance. It is important that the satellite estimates of surface UV irradiance are validated under the widest set of conditions.The longest continuous time series of satellite-based surface UV data has been calculated using th...

No abstract

[1] A method for estimating daily erythemal UV doses using total ozone, sunshine duration, and snow depth has been developed. The method consists of three steps: (1) daily clear-sky UV doses were simulated using the UVSPEC radiative transfer program, with daily values of total ozone as input data, (2) an empirical relationship was sought between the simulated clear-sky UV doses, the measured UV doses, and the duration of bright sunshine, and (3) daily erythemal UV doses were estimated using this relationship. The method accounts for the varying surface albedo by dividing the period of interest into winter and summer days, depending on the snow depth. Using this method, the daily erythemal UV doses at Sodankylä were estimated for the period 1950-1999. This was done using Tromsø's total ozone together with Sodankylä's own sunshine duration and snow depth as input data. Although the method is fairly simple, the results are in good agreement, even on the daily scale, with the UV radiation measured with the Brewer spectrophotometer at Sodankylä. Over the period 1950-1999 a statistically significant increasing trend of 3.9% per decade in erythemal UV doses was found for March. The fact that this trend is much more pronounced during the latter part of the period, which is also the case for April, suggests a connection to the stratospheric ozone depletion. For July, on the other hand, a significant decreasing trend of 3.3% per decade, supported by the changes in both total ozone and sunshine duration, was found.

Abstract. Recent analysis of the total ozone observations indicate a negative trend of about 4%/decade in the Northern Hemisphere midlatitudes during the last two decades [WMO, 1999]. The effect of this decline on surface UV levels is of interest to a variety of applications. In this work the long-term variation of UV radiation at three stations located in northern Europe (Belsk, Norrk6ping, and Jokioinen) has been studied using data from (1) ground-based observations, (2) surface UV doses determined using TOMS satellite measurements, and (3) reconstructed UV doses using observations of global radiation, total ozone, and radiative transfer modeling. For each station the estimates of daily UV doses from various sources have been intercompared, and a trend analysis has been performed to reveal long-term changes in the UV radiation. Data sets, which start in the late 1970s or early 1980s, show a general positive trend in annual doses of UV radiation. Some of these upward trends are statistically significant. For Belsk the increases are in the range of 5-15% per decade during spring and summer. The largest increases, about 20%/ decade, has been observed in Norrk6ping during spring. At Jokioinen there has been a slight upward trend in UV throughout the year. The analysis of reconstructed Belsk data from 1966 onward shows that the positive trend since late 1970s was preceeded by a negative trend. The reason for such changes is probably not only related to the changes in the total ozone but also to changes in aerosol content and cloudiness. The agreement of the UV series based on different data sources is good. This was studied using a subset of data in which it was required that data from all possible sources were available. The different trend estimates were in very close agreement with each other. However, there were often differences in absolute values, which is probably related to problems in calibration and limitations of the models. IntroductionThe springtime depletion of ozone in Antarctica is a regular phenomenon in the present atmosphere. As a consequence, the levels of ultraviolet (UV) radiation have been increasing focuses on the past changes in UV radiation in Northern Europe using data from various sources. The determination of UV radiation at ground level is probably done with most accuracy using in situ measurements with well-calibrated spectroradiometers. However, these instruments are expensive and laborious to maintain and the spatial and temporal coverage of the measurements is limited. The procurement of broadband UV meters is much cheaper, but maintenance with high-quality control is also expensive [WMO, 1996]. Because of the high costs of maintaining in situ measurement systems it is questionable if a global network with high regional coverage will ever be established. Consequently, there is lack of data over large parts of the Earth, such as oceans and many continental areas, and the knowledge of the long-term changes in UV radiation reaching the ground is limited. However, the UV radiation aff...