Total Column Ozone Measurements Research Articles

Evidence of gravity waves into the atmosphere during the March 2006 total solar eclipse

Abstract. This study aims at providing experimental evidence, to support the hypothesis according to which the movement of the moon's shadow sweeping the ozone layer at supersonic speed, during a solar eclipse, creates gravity waves in the atmosphere. An experiment was conducted to study eclipse induced thermal fluctuations in the ozone layer (via measurements of total ozone column, ozone photolysis rates and UV irradiance), the ionosphere (Ionosonde Total Electron Content – ITEC, peak electron density height – hmF2), and the troposphere (temperature, relative humidity), before, during and after the total solar eclipse of 29 March 2006. We found the existence of eclipse induced dominant oscillations in the parameters related to the ozone layer and the ionosphere, with periods ranging between 30–40 min. Cross-spectrum analyses resulted to statistically significant square coherences between the observed oscillations, strengthening thermal stratospheric ozone forcing as the main mechanism for GWs. Additional support for a source below the ionosphere was provided by the amplitude of the oscillations in the ionospheric electron density, which increased upwards from 160 to 220 km height. Even though similar oscillations were shown in surface temperature and relative humidity data, no clear evidence for tropospheric influence could be derived from this study, due to the modest amplitude of these waves and the manifold rationale inside the boundary layer.

Spatial variation of total column ozone on a global scale

The spatial dependence of total column ozone varies strongly with latitude, so that homogeneous models (invariant to all rotations) are clearly unsuitable. However, an assumption of axial symmetry, which means that the process model is invariant to rotations about the Earth's axis, is much more plausible and considerably simplifies the modeling. Using TOMS (Total Ozone Mapping Spectrometer) measurements of total column ozone over a six-day period, this work investigates the modeling of axially symmetric processes on the sphere using expansions in spherical harmonics. It turns out that one can capture many of the large scale features of the spatial covariance structure using a relatively small number of terms in such an expansion, but the resulting fitted model provides a horrible fit to the data when evaluated via its likelihood because of its inability to describe accurately the process's local behavior. Thus, there remains the challenge of developing computationally tractable models that capture both the large and small scale structure of these data.

A quarter century of TOMS total column ozone measurements over Brazil

The Earth's atmosphere has been probed since 1978 by the total ozone mapping spectrometer (TOMS) experiment. In polar orbits around the Earth, Nimbus 7, Meteor 3, and Earth Probe satellites have carried TOMS to measure the backscattered solar ultraviolet radiation, which is processed by Version 8 TOMS algorithm for the retrieval of column ozone values. These TOMS data and their behavior are reported and investigated in this work for some sites in the equatorial and tropical zones of Brazil. Comparison with measurements from Brewer and Dobson spectrophotometers in some cases is provided. An average difference as low as 1.5% between TOMS and Brewer measurements was obtained. In addition, more than two decades of TOMS monthly average column ozone measurements were applied to the calculation of ozone trends. Although small and in general negative, these trends were statistically significant for all sites in the winter season.

Satellite Measurements of Tropospheric Column O3 and NO2 in Eastern and Southeastern Asia: Comparison with a Global Model (MOZART-2)

Satellite measurements of tropospheric column O3 and NO2 in eastern and southeastern Asia are analyzed to study the spatial and seasonal characteristics of pollution in these regions. Tropospheric column O3 is derived from differential measurements of total column ozone from Total Ozone Mapping Spectrometer (TOMS), and stratospheric column ozone from the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS). The tropospheric column NO2 is measured by Global Ozone Monitoring Experiment (GOME). A global chemical and transport model (Model of Ozone and Related Chemical Tracers, version 2; MOZART-2) is applied to analyze and interpret the satellite measurements. The study, which is based on spring, summer, and fall months of 1997 shows generally good agreement between the model and satellite data with respect to seasonal and spatial characteristics of O3 and NO2 fields. The analysis of the model results show that the industrial emission of NOx (NO + NO2) contributes about 50%–80% to tropospheric column NO2 in eastern Asia and about 20%–50% in southeastern Asia. The contribution of industrial emission of NOx to tropospheric column O3 ranges from 10% to 30% in eastern Asia. Biomass burning and lightning NOx emissions have a small effect on tropospheric O3 in central and eastern Asia, but they have a significant impact in southeastern Asia. The varying effects of NOx on tropospheric column ozone are attributed to differences in relative abundance of volatile organic compounds (VOCs) with respect to total nitrogen in the two regions.

Ozone column retrieval from solar UV measurements at ground level: Effects of clouds and results from six European sites

A differential absorption method, developed by the Laboratoire d'Optique Atmosphérique (LOA), is applied to retrieve the total ozone column from UV global irradiance spectra under clear and cloudy sky conditions. Contrarily to clear sky, cloudy sky generates a high uncertainty in the retrieved ozone column. This study shows that under cloudy conditions the daily ozone mean is a rather good estimation of the true value. The standard deviation allows us to estimate the relative uncertainty of this mean value, i.e., about 7%. Results for all conditions from 3 years at Villeneuve d'Ascq are presented, as well as a comparison with TOMS (Total Ozone Mapping Spectrometer) data. The results are very similar to those obtained for clear days: In 75% of cases both values agree to within 5%; however, some cases present larger relative differences. Over the 3 years considered, there is a bias less than 3% (LOA > TOMS). This method is applied to five other European sites involved in the EDUCE (European Database for UV Climatology and Evaluation) project. Three of them are Brewer stations. The results of the comparison between the LOA‐retrieved ozone and TOMS data show that a relative difference smaller than 5% is obtained in 63–80% of the cases, depending on site and year. Biases smaller than 3% are observed (LOA > TOMS). The comparison between the LOA‐retrieved ozone and total ozone data from direct sun and zenith sky observations of the Brewer instruments shows better agreement. For more than about 80% of cases, except at one site, the relative difference is smaller than 5%, and the biases are smaller than 1%. The LOA method allows us to obtain a complementary data set and hence to provide time series of reliable measurements of total ozone column under all sky conditions.

Kelvin wave signatures in ECMWF meteo fields and Global Ozone Monitoring Experiment (GOME) ozone columns

This study investigates the vertical structure of the Kelvin wave signals previously found in total ozone column measurements from the Global Ozone Monitoring Experiment (GOME) instrument. For this, zonal wind and temperature measurements from the European Centre for Medium‐Range Weather Forecasts (ECMWF) Re‐Analysis data set are analyzed by using the same bidimensional spectral method as was used to analyze the GOME total ozone columns. These fields are available on 60 levels from the surface to 0.1 hPa. For the three high Kelvin wave activity periods identified in the GOME data we found spectral features in the ECMWF fields associated with Kelvin waves with zonal wave numbers 1 or 2 and periods around 15–20 days. These characteristics correspond to the characteristics of the Kelvin waves detected in GOME. The signals are significant throughout the lower stratosphere between ∼100 and 10 hPa and, depending on the period, are largest around 15, 45, or 65 hPa. There is a good correlation between the Kelvin wave signals in the ECMWF zonal wind and temperature and the GOME total ozone column. The induced fluctuations in zonal wind and temperature are, respectively, up to 8 m/s and 2 K. From these induced zonal wind fluctuations, expected total ozone column fluctuations of around 1 DU are calculated, corresponding to the ozone fluctuations found in the GOME data. The results indicate that the analyzed total ozone column fluctuations are mainly caused by transport effects in the lower stratosphere. This study shows that combined use of ECMWF Re‐Analysis data and GOME ozone columns provides a possibility to study the three‐dimensional structure of Kelvin wave activity.

Bias in Dobson total ozone measurements at high latitudes due to approximations in calculations of ozone absorption coefficients and air mass

The Dobson spectrophotometer is the primary standard instrument for ground‐based measurements of total column ozone. The accuracy of its data depends on the knowledge of ozone absorption coefficients used for data reduction. We document an error in the calculations that led to the set of absorption coefficients currently recommended by the World Meteorological Organisation (WMO). This error has little effect because an empirical adjustment was applied to the original calculations before the coefficients were adopted by WMO. We provide evidence that this adjustment was physically sound. The coefficients recommended by WMO are applied in the Dobson network without correction for the temperature dependence of the ozone absorption cross sections. On the basis of data measured by Dobson numbers 80 and 82, which were operated by the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory at the South Pole, we find that omission of temperature corrections may lead to systematic errors in Dobson ozone data of up to 4%. The standard Dobson ozone retrieval method further assumes that the ozone layer is located at a fixed height. This approximation leads to errors in air mass calculations, which are particularly relevant at high latitudes where ozone measurements are performed at large solar zenith angles (SZA). At the South Pole, systematic errors caused by this approximation may exceed 2% for SZAs larger than 80°. The bias is largest when the vertical ozone distribution is distorted by the “ozone hole” and may lead to underestimation of total ozone by 4% at SZA = 85° (air mass 9). Dobson measurements at the South Pole were compared with ozone data from a collocated SUV‐100 UV spectroradiometer and Version 8 overpass data from NASA's Total Ozone Mapping Spectrometer (TOMS). Uncorrected Dobson ozone values tend to be lower than data from the two other instruments when total ozone is below 170 Dobson units or SZAs are larger than 80°. When Dobson measurements are corrected for the temperature dependence of the ozone absorption cross section and accurate air mass calculations are implemented, data from the three instruments agree with each other to within ±2% on average and show no significant dependence on SZA or total ozone.

Elevated ozone in the troposphere over the Atlantic and Pacific oceans in the Northern Hemisphere

Tropospheric column ozone (TCO) is derived from differential measurements of total column ozone from Total Ozone Mapping Spectrometer (TOMS), and stratospheric column ozone (SCO) from the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS). It is shown that TCO during late spring and summer months over the Atlantic and Pacific oceans at northern mid‐latitudes is about 50–60 Dobson Units (DU) which is about the same as over the continents of North America, Europe and Asia (except high altitude mountain regions), where surface emissions of NOx from industrial sources, biomass and biofuel burning, and biogenic emissions are significantly larger. The zonal characteristics of TCO derived from satellite measurements are generally simulated by a global chemical transport model called MOZART‐2, but some discrepancies are also shown. The model results are analyzed to delineate the relative importance of surface NOx emission, lightning NOx and stratospheric flux.

Dilution of the Antarctic ozone hole into southern midlatitudes, 1998–2000

The reduction in southern midlatitude ozone is quantified by evaluating the trajectories of ozone‐depleted air masses, assuming that photochemical recovery of ozone in advected air parcels can be ignored. This procedure is carried out for the 3 months from 15 October to 15 January for each of the years 1998, 1999, and 2000. Two distinct source regions, the vortex core and the vortex edge, are considered, and for each day, diabatic reverse domain filling calculations are performed for an ensemble of parcels between 30°S and 60°S and 400–700 K in altitude. In 1998, 1999, and 2000 the mean calculated ozone reduction is 16, 18, and 19 DU, respectively. Air parcels from the vortex edge region are significant contributors to the reduction, especially during spring. Results for four longitudinal and three latitudinal midlatitude subregions are also presented. A comparison with the Total Ozone Mapping Spectrometer measurements of total column ozone shows that without the dilution, ozone over Southern Hemisphere midlatitudes would be 5–6% higher during spring and summer. This result is probably an overestimate due to the neglect of photochemical recovery.

Ozone column retrieval from solar UV irradiance measurements at ground level: Sensitivity tests and uncertainty estimation

Since the discovery of the stratospheric ozone depletion in the 1980s [World Meteorological Organization/United Nations Environment Programme (WMO/UNEP), 1994; Solomon, 1999; Staehelin et al., 2001], the interest of its measurement has substantially increased. Indeed, this gas highly absorbs the dangerous UV radiation and preserves the biosphere. Long series of measurements of total ozone column and of solar UV radiation are needed to define climatologies of both UV and ozone and to study the impact of the ozone depletion at the Earth surface. Total ozone column is determined at Villeneuve d'Ascq, France, by a differential absorption method, using global spectral UV measurements performed with a Jobin Yvon spectroradiometer by the Laboratoire d'Optique Atmospherique. This method is based on the comparison between two irradiance ratios, one simulated beforehand and stored in a lookup table (LUT) and one calculated from the measured spectrum [Stamnes et al., 1991]. This paper presents the method used to improve the choice of the different irradiances used to calculate the ratios and presents the five irradiance ratios that are finally kept. Some sensitivity tests are performed in order to find the validity range of a LUT and the number of LUTs needed. These tests reveal that the aerosol Ångström coefficient, the aerosol optical depth, and the atmospheric vertical profiles may induce errors in the ozone retrieval. That makes it necessary to calculate 21 LUTs and to select one of them depending on the season and on the aerosol properties. Then an estimation of the uncertainty of the method is performed, enabling us to conclude that the method leads to approximately 3% relative uncertainty. The results for the clear days of the last 3 years at Villeneuve d'Ascq are presented, as well as a comparison with Total Ozone Mapping Spectrometer data. In most of the cases, both values agree to within 5%, but some cases present larger relative differences.

A Madden‐Julian Oscillation in tropospheric ozone

This study shows evidence of a Madden‐Julian Oscillation (MJO) in tropospheric ozone. Tropospheric ozone is derived using differential measurements of total column ozone and stratospheric column ozone from total ozone mapping spectrometer (TOMS) and microwave limb sounder (MLS) instruments. Two broad regions of significant MJO signal are identified in the tropics, one in the western Pacific and the other in the eastern Pacific. Over both regions, MJO variations in tropospheric ozone represent 5–10 Dobson Unit (DU) peak‐to‐peak anomalies. These variations are significant compared to mean background amounts of around 20 DU or less over most of the tropical Pacific. MJO signals of this magnitude would need to be considered when investigating and interpreting particular pollution events since ozone is a precursor of the Hydroxyl (OH) radical, the main oxidizing agent of pollutants in the lower atmosphere.

Near-real-time approach to assimilation of satellite-retrieved 3D ozone fields in a global model using a simplified Kalman filter

In this paper, a near-real-time approach to data assimilation for the estimation of the three-dimensional global ozone distribution based on the transport model TM3 is presented. TM3 uses the meteorological data from European Centre for Medium-Range Weather Forecasts to transport the ozone field. The ozone profiles used in the assimilation are retrieved from measurements of the Global Ozone Monitoring Experiment instrument (GOME) on the European Space Agency's European Remote Sensing-2 satellite. A simplified Kalman filter technique is chosen as a data assimilation technique since it not only estimates the ozone distribution globally but also quantifies the uncertainty in its estimate. The NRT data assimilation technique is demonstrated for the period of March–June 2000. The results presented in this paper show that the estimated profiles deviate from the measurements by, on a global average, 15% of the ozone values in pressure layer 163–119 hPa and ranging from 6% to 2% of the ozone values between 119 hPa and 40 hPa. The deviation of the estimate from the measurements decreases with altitude and reaches 0.05% of the ozone value in the layer 2–0 hPa. Comparison with ozone-sonde observations shows that the model driven by observed meteorological fields is capable of describing the main spatial and temporal variations. Comparison with data from the Halogen Occultation Experiment instrument on the Upper Atmosphere Research Satellite shows that assimilating the retrieved profiles from GOME improves the vertical description of ozone profiles mainly in the stratosphere and the upper troposphere. Moreover, the integrated ozone-profile estimate is compared with the total-column ozone measurements made by the Total Ozone Mapping Spectrometer. This shows that assimilating the ozone profiles results in a better description of the total column than the ten-day forecast. Copyright © 2003 Royal Meteorological Society

Tropospheric ozone at tropical and middle latitudes derived from TOMS/MLS residual: Comparison with a global model

The tropospheric ozone residual method is used to derive zonal maps of tropospheric column ozone using concurrent measurements of total column ozone from Nimbus 7 and Earth Probe (EP) Total Ozone Mapping Spectrometer (TOMS) and stratospheric column ozone from the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS). Our study shows that the zonal variability in TOMS total column ozone at tropical and subtropical latitudes is mostly of tropospheric origin. The seasonal and zonal variability in tropospheric column ozone (TCO), derived from the TOMS/MLS residual, is consistent with that derived from the convective cloud differential method and ozonesonde measurements in regions where these data overlap. A comparison of TCO derived from the TOMS/MLS residual and a global three‐dimensional model of tropospheric chemistry (GEOS‐CHEM) for 1996–1997 shows good agreement in the tropics south of the equator. Both the model and observations show similar zonal and seasonal characteristics including an enhancement of TCO in the Indonesian region associated with the 1997 El Niño. Both show the decline of the wave‐1 pattern from the tropics to the extratropics as lightning activity and the Walker circulation decline. Both show enhanced ozone in the downwelling branches of the Hadley Circulation near ±30o. Model and observational differences increase with latitude during winter and spring.

A model to extend spectral and multiwavelength UV irradiances time series: Model development and validation

Measurements of spectral and multichannel UV radiation became more common in the middle and late eighties after the discovery of the “ozone hole,” but time series for these measurements are still relatively short for the determination of trends to be applied in geophysical and biological studies. However, systematic measurements of total column ozone have been performed since the late 1950s at several stations, and global coverage has been available since the late 1970s. Also, long‐term time series of broadband instruments (Pyranometers, UV and erythemally weighted) are available from many sites around the world. In this paper a multiregressive model that enables the inference of spectral or narrow band UV irradiances from total ozone column and broadband irradiance, in places where relatively short time series of UV spectral irradiances are available, is proposed. To test the model, measurements of irradiances performed at three of the stations in the NSF UV Radiation Monitoring Network, under all weather, solar zenith angle and surface conditions, were used. The model generated very good results over a wide variety of situations. Pyranometer data from the NOAA/CMDL surface radiation budget database for South Pole and Barrow Stations were used to estimate daily integrated narrow band irradiances. A time series of monthly means for the narrowband (303.030–307.692 nm) were then computed, dating back to the late 1970s.

Assimilation of ozone profiles and total column measurements into a global general circulation model

Ozone profiles from the Microwave Limb Sounder (MLS) instrument flown on board the Upper Atmosphere Research Satellite (UARS) and total ozone columns measured by the Global Ozone Monitoring Experiment (GOME) on board the Second European Remote Sensing Satellite (ERS‐2) have been assimilated using a troposphere‐stratosphere data assimilation system. The analysis system is based on the global analysis system used for operational analysis of the stratosphere at the Meteorological Office from 1991 to 2000. Three assimilation runs have been completed for a three‐week period in April 1997 to test the advantage of using a combination of MLS and GOME observations, compared with the assimilation of each observation data set separately. The statistical information produced by the assimilation system shows that the combination of MLS and GOME observations via the assimilation process produces ozone fields that show improvement compared with analysis fields produced by the assimilation of either MLS or GOME separately. Comparison of the analyzed ozone fields with independent observations (ozonesondes, Halogen Occultation Experiment (HALOE) profiles and Total Ozone Mapping Spectrometer (TOMS) total ozone column measurements) corroborates these results and shows that the combined MLS and GOME ozone analyses provide a realistic representation of the atmospheric ozone distribution. The global root‐mean‐square residual (difference between the analyses and independent observations) against HALOE and TOMS observations is comparable to the quoted errors in the HALOE and TOMS instruments (5% in each case).

Validation of version 5.20 ILAS HNO3, CH4, N2O, O3, and NO2 using ground‐based measurements at Arrival Heights and Kiruna

Ground‐based measurements of O3 and NO2 columns at Kiruna (68°N) and Arrival Heights (78°S) were used to validate stratospheric profile measurements of the same species by the satellite‐based Improved Limb Atmospheric Spectrometer (ILAS) flown between November 1996 and June 1997. In addition, measurements of HNO3, CH4, and N2O made at Arrival Heights are compared with measurements of these species by ILAS. Even though there were no temporal coincidences in the latter comparison, the ground‐based data provide a climatological context for the ILAS measurements. ILAS profiles measured within 10° of longitude and 2° of latitude of each site were used. For the species with large tropospheric components (CH4 and N2O), ILAS profiles were integrated upward from 12 km and were compared with ground‐based retrievals of partial columns above 12 km. For O3, which is primarily stratospheric but has an appreciable tropospheric component, tropospheric columns (typically to 8–10 km altitude) calculated from ozonesonde climatologies were added to the integrated ILAS profiles to allow comparison with ground‐based total column ozone measurements. Tropospheric columns of HNO3 and NO2 were assumed to be sufficiently small so that ILAS profiles integrated above 8 km could be compared directly with ground‐based total column measurements.

Comparison of total ozone overburden from handheld photometers with the Wallops Island Dobson Spectrophotometer

Total ozone column measurements are compared between the Dobson spectrophotometer located at Wallops Flight Facility, Wallops Island, VA, and two Microtops II photometers. The purpose of these comparisons is to determine whether the Microtops II ozone measurements are comparable to those obtained with the Dobson spectrophotometer. Results have shown that the handheld photometer can provide measurements within about 2% of the Dobson's. A comparison between the two Microtops showed approximately a 3% difference in the derived ozone overburdens. Better results with the two Microtops were obtained using the 305.5‐nm and the 312.5‐nm filters only.

The global mass of ozone: 1978–1998

Global measurements of total column ozone from five inter‐compared satellite‐based instruments were used to calculate the daily global mass of ozone from November 1978 to December 1998. Missing data in polar darkness were estimated by interpolation across the polar caps. Global ozone mass and ozone mass totalled over 30° latitude zones were examined for the mean annual cycle, its evolution over the 20 year period, the impacts of volcanic eruptions and hemispheric differences in these impacts. Ozone mass for the globe or within a latitude zone is more suitable than mean column amount (e.g. in Dobson Units) because it is invariant under transport within zones and is a direct measure of exchange between zones. Furthermore, the relative contribution of impacts at different latitudes to global changes in ozone can be more easily quantified. While global ozone mass is significantly perturbed by high latitude wintertime ozone depletion, the impact of the Mt. Pinatubo eruption on northern midlatitude ozone mass was larger than any single Antarctic ozone hole event.

“Cloud slicing”: A new technique to derive upper tropospheric ozone from satellite measurements

A new technique called cloud slicing has been developed for measuring upper tropospheric O3. Cloud slicing takes advantage of the opaque property of water vapor clouds to ultraviolet wavelength radiation. Measurements of above‐cloud column O3 from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument are combined together with Nimbus 7 temperature‐humidity and infrared radiometer (THIR) cloud‐top pressure data to derive O3 column amounts in the upper troposphere. In this study, tropical TOMS and THIR data for the period 1979–1984 are analyzed. By combining total tropospheric column ozone (referred to as TCO) measurements from the convective cloud differential (CCD) method with 100‐ to 400‐hPa upper tropospheric column O3 amounts from cloud slicing, it is possible to estimate 400‐ to 1000‐hPa lower tropospheric column O3 and evaluate its spatial and temporal variability. Results for both the upper and lower tropical troposphere show a year‐round zonal wave number 1 pattern in column O3 with the largest amounts in the Atlantic region (up to ∼15 DU in the 100‐ to 400‐hPa pressure band and ∼25–30 DU in the 400‐ to 1000‐hPa pressure band). Upper tropospheric O3 derived from cloud slicing shows maximum column amounts in the Atlantic region in the June‐August and September‐November seasons which are similar to the seasonal variability of CCD‐derived TCO in the region. For the lower troposphere, the largest column amounts occur in the September‐November season over Brazil in South America and also southern Africa. Localized increases in the tropics in the lower tropospheric O3 are found over the northern region of South America around August and off the west coast of equatorial Africa in the March‐May season. Time series analysis for several regions in South America and Africa show an anomalous increase in O3 in the lower troposphere around the month of March which is not observed in the upper troposphere. The eastern Pacific indicates weak seasonal variability of upper, lower, and total tropospheric O3 compared with the western Pacific, which shows the largest TCO amounts in both hemispheres around spring months. O3 variability in the western Pacific is expected to have greater variability caused by strong convection, pollution and biomass burning, land‐sea contrast, and monsoon developments.

European light dosimeter network (ELDONET): 1998 data

The European light dosimeter network of over 40 stations has been established in Europe and other continents equipped with three-channel filter dosimeters to measure solar radiation in three channels, UV-B (280–315 nm), UV-A (315–400 nm) and photosynthetically active radiation (PAR). The recorded data have been evaluated, and the monthly doses in all three channels show a strong latitudinal dependence from northern Sweden to the Canary Islands. There are a few remarkable exceptions such as the data recorded at the high mountain station on the Zugspitze (German Alps) and unequal doses at stations at comparable latitudes which indicate the impact of local weather conditions and mean sunshine hours. While generally peak values are recorded in the months of June and July, the UV-B maxima are shifted later into the year, which is due to the antagonistic functions of decreasing solar angles and increasing transparency of the atmosphere as the total column ozone decreases in the second half of the year for the Northern Hemisphere. This is supported by comparison with modelled total column ozone and satellite-based measurements. Also the ratios of UV-B:UV-A and UV-B:PAR as well as UV-A:PAR peak during the summer months, with the exception of the northernmost station at Abisko (north Sweden) where the UV-A:PAR ratio peaks in the winter months which is due to the specific photoclimatic conditions north of the polar circle. The penetration of solar radiation into the water column was found to strongly depend on the transparency of the water column. In Gran Canaria more than 10% of the surface UV-B penetrated to 4–5 m depth. The path of the solar eclipse on 11 August 1999 could be followed in several stations with different degrees of occlusion of the sun disk.