This study examines three UV-B data sets: groundbased long-term spectral records at Thessaloniki,... more This study examines three UV-B data sets: groundbased long-term spectral records at Thessaloniki, Greece (40.5 • N, 22.9 • E) and San Diego, California, USA (32.7 • N, 117.2 • W) as well as a global data set of daily erythemal dose obtained from the Total Ozone Mapping Spectrometer (TOMS) onboard the Nimbus-7 satellite. Both ground-based stations have long enough records of spectral UV-B measurements to allow independent time series analyses. For 63 • solar zenith angle (SZA) and clear sky conditions the quasi biennial oscillation (QBO) effect in solar irradiance at 305nm E 305 is about 32% of the annual cycle for both San Diego and Thessaloniki. The effect slightly increases with cloud cover of up to 4/8, and decreases thereafter for cloud cover greater than 4/8. The data reveal that cloudiness cannot offset interannual signals in UV-B records. The observations at San Diego provide an independent confirmation of the widespread nature of the QBO in UV-B, which about coincides in amplitude at the two station studies, both located in the latitude zone 30 •-40 • N. The peak-to-peak amplitude of the QBO in erythemal dose derived from TOMS/Nimbus-7 data is 6.5% at Thessaloniki. This is similar to the values calculated from ground-based measurements from this station. Based on satellite data, we find that the amplitude of the QBO in the erythemal dose is almost 40% of the amplitude of the annual cycle only in the tropics. The ratio of the amplitudes of the QBO over the annual cycle in erythemal dose decreases towards the extratropics, becoming less than 5% over middle latitudes.
The EPIC instrument onboard the DSCOVR spacecraft, located near the Earth-Sun gravitational plus ... more The EPIC instrument onboard the DSCOVR spacecraft, located near the Earth-Sun gravitational plus centrifugal force balance point, Lagrange-1, measures Earth reflected radiances in 10 wavelength channels ranging from 317.5 nm to 779.5 nm. Of these channels, four are in the UV range 317.5, 325, 340, and 388 nm, which are used to retrieve O<sub>3</sub>, 388 nm scene reflectivity (LER Lambert Equivalent Reflectivity), SO<sub>2</sub>, and aerosol properties. These quantities are derived synoptically for the entire sunlit globe from sunrise to sunset every 68 minutes or 110 minutes for summer or winter at the receiving antenna in Wallops Island, Virginia, respectively. Depending on solar zenith angle, either 317.5 or 325 nm channels are combined with 340 and 388 nm to derive ozone amounts. As part of the ozone algorithm, the 388 nm channel is used to derive LER. The retrieved ozone amounts and LER are combined to der...
The field of atmospheric ultraviolet radiation (UV) research is plagued with difficulties in nome... more The field of atmospheric ultraviolet radiation (UV) research is plagued with difficulties in nomenclature. The problems arise from (a) the strong wavelength dependence of UV radiation received at the Earth’s surface, (b) the ad-hoc way disparate groups have approached the subject and (c) the incorrect use of units when action spectra for different UV effects are incorporated. This report highlights some of the issues, taking vitamin-D synthesis, a beneficial effect, as a specific example. Standard vitamin-D dose (SDD) and minimum vitamin-D dose (MDD) are proposed, analogous to the standard erythema dose (SED) and minimum erythema dose (MED) that are in common use for erythema. In recognition of the fact that currently accepted action spectra may be revised in future if new data become available, it is recommended that spectrally resolved irradiance measurements are maintained and continued so that biologically effective irradiances and doses may be reprocessed.
Seasonal variability of solar UV radiation in ocean waters is estimated on a global scale by comb... more Seasonal variability of solar UV radiation in ocean waters is estimated on a global scale by combining satellite measurements of scene reflectivity (TOMS), column ozone (TOMS) and chlorophyll concentration (SeaWiFS) with radiative transfer calculations for an ocean-atmosphere system. The new features are an extension of underwater radiative transfer (scattering and absorption) into the UV, inclusion of polarization in the above water diffuse radiances, the proper treatment of Fresnel reflection, and first order atmospheric backscatter of water-leaving radiance to the oceans. Maps of downwelling diffuse irradiances (E d) at ocean surface and at different depths in the ocean, diffuse attenuation coefficient (K d), and ten percent penetration depth (Z 10) of solar irradiation are computed for open ocean waters. Results on spectral irradiances at 310 nm in UV-B and at 380 nm in UV-A part of the spectrum are presented with particular emphasis on the role of aerosols, clouds, and ozone in the atmosphere and chlorophyll concentrations in the ocean.
Ultraviolet Ground- and Space-based Measurements, Models, and Effects, 2002
Satellite instruments currently provide global maps of surface UV irradiance by combining backsca... more Satellite instruments currently provide global maps of surface UV irradiance by combining backscattered radiance data with radiative transfer models. The models are often limited by uncertainties in physical input parameters of the atmosphere and surface.
Satellite-derived ultraviolet (UV) irradiances may form the basis for establishing a global UV cl... more Satellite-derived ultraviolet (UV) irradiances may form the basis for establishing a global UV climatology, provided that their accuracy is confirmed against ground-based measurements of known quality. In this study, quality-checked spectral UV irradiance measurements from four European stations
During the last decade, methods have been developed for estimating ultraviolet (UV) irradiance re... more During the last decade, methods have been developed for estimating ultraviolet (UV) irradiance reaching the Earth&amp;#x27;s surface using satellite-measured backscattered UV radiances. The aim of this work is to compare UV products (version 8), noon erythemal ...
The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board the NASA EOS Aura spacecraft is a na... more The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board the NASA EOS Aura spacecraft is a nadir viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600 km wide viewing swath and it is capable of daily, global contiguous mapping. The Finnish Meteorological Institute and NASA Goddard Space Flight Center have developed a surface ultraviolet irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths, noontime erythemal dose rate (UV index), and the erythemal daily dose. The overpass erythemal daily doses derived from OMI data were compared with the daily doses calculated from the ground-based spectral UV measurements from 18 reference instruments. Two alternative methods for the OMI UV algorithm cloud correction were compared: the plane-parallel cloud model method and the method based on Lambertian equivalent reflectivity. The validation results for the two methods showed some differences, but the results do not imply that one method is categorically superior to the other. For flat, snow-free regions with modest loadings of absorbing aerosols or trace gases, the OMI-derived daily erythemal doses have a median overestimation of 0-10%, and some 60 to 80% of the doses are within ±20% from the ground reference. For sites significantly affected by absorbing aerosols or trace gases one expects, and observes, bigger positive bias up to 50%. For high-latitude sites the satellite-derived doses are occasionally up to 50% too small because of unrealistically small climatological surface albedo.
The global stratospheric ozone-layer depletion results in an increase in biologically harmful ult... more The global stratospheric ozone-layer depletion results in an increase in biologically harmful ultraviolet (UV) radiation reaching the surface and penetrating to ecologically significant depths in natural waters. Such an increase can be estimated on a global scale by combining satellite estimates of UV irradiance at the ocean surface from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument with the SeaWiFS satellite ocean-color measurements in the visible spectral region. In this paper we propose a model of seawater optical properties in the UV spectral region based on the Case 1 water model in the visible range. The inputs to the model are standard monthly SeaWiFS products: chlorophyll concentration and the diffuse attenuation coefficient at 490nm. Penetration of solar UV radiation to different depths in open ocean waters is calculated using the RT (radiative transfer) quasi-single scattering approximation (QSSA). The accuracy of the QSSA approximation in the water is tested using more accurate codes. The sensitivity study of the underwater UV irradiance to atmospheric and oceanic optical properties have shown that the main environmental parameters controlling the absolute levels of the UVB (280-320nm) and DNA-weighted irradiance underwater are: solarzenith angle, cloud transmittance, water optical properties, and total ozone. Weekly maps of underwater UV irradiance and DNA-weighted exposure are calculated using monthlymean SeaWiFS chlorophyll and diffuse attenuation coefficient products, daily SeaWiFS cloud fraction data, and the TOMS-derived surface UV irradiance daily maps. The final products include global maps of week1 y-average UVB irradiance and DNA-weighted daily exposures at 3m and lOm, and depths where the UVB irradiance and DNAweighted dose rate at local noon are equal to 10% of their surface values.
The major factors causing differences between satellite-derived and ground-based ultraviolet (UV)... more The major factors causing differences between satellite-derived and ground-based ultraviolet (UV) erythemal irradiances and doses are discussed. Measurements totaling more than 4700 days during 1997-1999 were obtained at 8 stations (22°S-64°S) of the Argentine UV Monitoring Network. The satellite retrieval uses radiative transfer calculations for cloud-and aerosol-free conditions multiplied by correction factors for clouds and aerosols. Key parameters are total ozone, cloud optical depth, and surface albedo derived from Total Ozone Mapping Spectrometer (TOMS). When no aerosol correction is applied, systematic differences of satellite-derived erythemal irradiance relative to ground-based measurements amount to +1% at a tropical high-altitude Andean location, +10% at stations in the central Pampas, +5% at southern Patagonian sites, and À7% at the southernmost continental and Antarctic stations with varying snow cover. When an aerosol correction is applied by estimating ''minimum'' and ''maximum'' aerosol loading, the systematic differences are within ±10% for all ''snow-free'' stations. To reduce the differences at places with varying snow conditions, an ''average surfacealbedo climatology'' must be used instead the TOMS climatology of minimum albedo. Although the statistical uncertainty of the differences increases with TOMS reflectivity, the systematic difference is independent of TOMS reflectivity for most of the stations, so on average the comparison for cloudy situations is as good as for clear-sky conditions. The comparison for daily erythemal doses gives similar results with smaller statistical uncertainty. Measured uncertainties are in agreement with a theoretical analysis. For most locations, well-characterized ground-based instruments should agree with TOMS satellite estimations within 10% if aerosol corrections are known.
The Photochemical Activity and Ultraviolet Radiation (PAUR) II project (a continuation of an earl... more The Photochemical Activity and Ultraviolet Radiation (PAUR) II project (a continuation of an earlier PAUR I project) had the purpose of studying the interrelationships between changes in total ozone, tropospheric aerosols, UV radiation and photochemical activity. As part of PAUR II project, a campaign took place in Greece and Italy during May-June 1999, with the participation of 15 European and 3 American research institutions. A variety of radiation and gaseous and aerosol atmospheric composition and optical characteristics measurements were made during the campaign. Radiative transfer models and three-dimensional (3-D) regional chemistry transport models (CTM) were applied and compared to the available data set of PAUR II. The present overview paper gives an introduction to the project and to the meteorological and environmental conditions that prevailed and outlines some results that are extensively described in the subsequent papers which form this special section. The modulation of the UVB field in the presence of different types of aerosols, its transmittance and role in the photochemistry of the particular eastern Mediterranean environment is overviewed. Using a 3-D CTM, it is shown that even a 50% reduction in Greek anthropogenic emissions has only a small effect in reducing the ozone levels over the eastern Mediterranean in summer. The environmental conditions, which prevailed during the PAUR I and PAUR II campaigns, offered cases of background conditions over the Aegean Sea as well as conditions with Saharan dust episodes and extremes in total ozone.
Journal of Geophysical Research: Atmospheres, 2000
This study examines the distribution of long-term trends in ground level erythemally weighted ult... more This study examines the distribution of long-term trends in ground level erythemally weighted ultraviolet (UV) exposures in the northern latitudes for the period 1979-1991 using measurements from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument. A new erythemal UV data set (now available to the public via World Wide Web) was produced recently by NASA and has been tested
Journal of Geophysical Research: Atmospheres, 1998
We discuss the theoretical basis of a recently developed technique to characterize aerosols from ... more We discuss the theoretical basis of a recently developed technique to characterize aerosols from space. We show that the interaction between aerosols and the strong molecular scattering in the near ultraviolet produces spectral variations of the backscattered radiances that can be used to separate aerosol absorption from scattering effects. This capability allows identification of several aerosol types, ranging from nonabsorbing sulfates to highly UV-absorbing mineral dust, over both land and water surfaces. Two ways of using the information contained in the near-UV radiances are discussed. In the first method, a residual quantity, which measures the departure of the observed spectral contrast from .that of a molecular atmosphere, is computed. Since clouds yield nearly zero residues, this method is a useful way of separately mapping the spatial distribution of UV-absorbing and nonabsorbing particles. To convert the residue to optical depth, the aerosol type must be known. The second method is an inversion procedure that uses forward calculations of backscattered radiances for an ensemble of aerosol models. Using a look-up table approach, a set of measurements given by the ratio of backscattered radiance at 340-380 nm and the 380 nm radiance are associated, within the domain of the candidate aerosol models, to values of optical depth and single-scattering albedo. No previous knowledge of aerosol type is required. We present a sensitivity analysis of various error sources contributing to the estimation of aerosol properties by the two.methods. 1.
IEEE Transactions on Geoscience and Remote Sensing, 2006
The Ozone Monitoring Instrument (OMI) onboard the NASA Earth Observing System (EOS) Aura spacecra... more The Ozone Monitoring Instrument (OMI) onboard the NASA Earth Observing System (EOS) Aura spacecraft is a nadir-viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600-km-wide viewing swath, and it is capable of daily, global contiguous mapping. We developed and implemented a surface ultraviolet (UV) irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths (305, 310, 324, and 380 nm). Additionally, noontime erythemal dose rate and the erythemal daily dose are estimated. The OMI surface UV algorithm inherits from the surface UV algorithm developed by NASA Goddard Space Flight Center for the Total Ozone Mapping Spectrometer (TOMS). The OMI surface UV irradiance products are produced and archived in HDF5-EOS format by Finnish Meteorological Institute. The accuracy of the surface UV estimates depend on UV wavelength and atmospheric and other geolocation specific conditions ranging from 7%to 30%. A postprocessing aerosol correction can be applied at sites with additional ground-based measurements of the aerosol absorption optical thickness. The current OMI surface UV product validation plan is presented.
Due to the combined effect of two polarization sensitive elements, the entrance window and gratin... more Due to the combined effect of two polarization sensitive elements, the entrance window and grating, the sensitivity of Brewer spectrophotometers for direct-sun measurements changes with solar zenith angle (SZA). We determined this SZA-polarization dependence with four independent methods, which agreed within ±1.5%. For SZA < 50°this effect is negligible. At SZA = 60°, 70°, and 80°the Brewer's sensitivity is reduced by 1%, 4%, and 10%, relative to SZA = 35°, when the direct solar irradiance is perpendicular to the entrance window. Differential absorption algorithms for retrieving trace gases (e.g., ozone) are unaffected since the polarization effect is almost wavelength independent. However, systematic errors are introduced in Langley extrapolations (2-4% overestimation of the zero air mass factor), retrievals of aerosol optical depth (overestimation of 0.01-0.04), and aerosol single scattering albedo. Therefore, Brewer direct irradiance measurements should be corrected for the SZA-polarization dependence. The effect in sky-radiance measurements can be removed only by hardware modifications.
... PK Bhartia. Code 916, NASA/Goddard Space Flight Center, Greenbelt, MD. J. Ziemke. NRC, NASA/G... more ... PK Bhartia. Code 916, NASA/Goddard Space Flight Center, Greenbelt, MD. J. Ziemke. NRC, NASA/Goddard Space Flight Center, Greenbelt, MD. Z. Ahmad. Software Corporation of America, Greenbelt, MD. D. Larko. Hughes‐STX Corporation, Greenbelt, MD. ...
Measurements of UV and Visible irradiances have proven valuable for assessing the health effects ... more Measurements of UV and Visible irradiances have proven valuable for assessing the health effects (human, animal, and plants) of radiation that reaches the Earth's surface as well as penetrating the.
This study examines three UV-B data sets: groundbased long-term spectral records at Thessaloniki,... more This study examines three UV-B data sets: groundbased long-term spectral records at Thessaloniki, Greece (40.5 • N, 22.9 • E) and San Diego, California, USA (32.7 • N, 117.2 • W) as well as a global data set of daily erythemal dose obtained from the Total Ozone Mapping Spectrometer (TOMS) onboard the Nimbus-7 satellite. Both ground-based stations have long enough records of spectral UV-B measurements to allow independent time series analyses. For 63 • solar zenith angle (SZA) and clear sky conditions the quasi biennial oscillation (QBO) effect in solar irradiance at 305nm E 305 is about 32% of the annual cycle for both San Diego and Thessaloniki. The effect slightly increases with cloud cover of up to 4/8, and decreases thereafter for cloud cover greater than 4/8. The data reveal that cloudiness cannot offset interannual signals in UV-B records. The observations at San Diego provide an independent confirmation of the widespread nature of the QBO in UV-B, which about coincides in amplitude at the two station studies, both located in the latitude zone 30 •-40 • N. The peak-to-peak amplitude of the QBO in erythemal dose derived from TOMS/Nimbus-7 data is 6.5% at Thessaloniki. This is similar to the values calculated from ground-based measurements from this station. Based on satellite data, we find that the amplitude of the QBO in the erythemal dose is almost 40% of the amplitude of the annual cycle only in the tropics. The ratio of the amplitudes of the QBO over the annual cycle in erythemal dose decreases towards the extratropics, becoming less than 5% over middle latitudes.
The EPIC instrument onboard the DSCOVR spacecraft, located near the Earth-Sun gravitational plus ... more The EPIC instrument onboard the DSCOVR spacecraft, located near the Earth-Sun gravitational plus centrifugal force balance point, Lagrange-1, measures Earth reflected radiances in 10 wavelength channels ranging from 317.5 nm to 779.5 nm. Of these channels, four are in the UV range 317.5, 325, 340, and 388 nm, which are used to retrieve O<sub>3</sub>, 388 nm scene reflectivity (LER Lambert Equivalent Reflectivity), SO<sub>2</sub>, and aerosol properties. These quantities are derived synoptically for the entire sunlit globe from sunrise to sunset every 68 minutes or 110 minutes for summer or winter at the receiving antenna in Wallops Island, Virginia, respectively. Depending on solar zenith angle, either 317.5 or 325 nm channels are combined with 340 and 388 nm to derive ozone amounts. As part of the ozone algorithm, the 388 nm channel is used to derive LER. The retrieved ozone amounts and LER are combined to der...
The field of atmospheric ultraviolet radiation (UV) research is plagued with difficulties in nome... more The field of atmospheric ultraviolet radiation (UV) research is plagued with difficulties in nomenclature. The problems arise from (a) the strong wavelength dependence of UV radiation received at the Earth’s surface, (b) the ad-hoc way disparate groups have approached the subject and (c) the incorrect use of units when action spectra for different UV effects are incorporated. This report highlights some of the issues, taking vitamin-D synthesis, a beneficial effect, as a specific example. Standard vitamin-D dose (SDD) and minimum vitamin-D dose (MDD) are proposed, analogous to the standard erythema dose (SED) and minimum erythema dose (MED) that are in common use for erythema. In recognition of the fact that currently accepted action spectra may be revised in future if new data become available, it is recommended that spectrally resolved irradiance measurements are maintained and continued so that biologically effective irradiances and doses may be reprocessed.
Seasonal variability of solar UV radiation in ocean waters is estimated on a global scale by comb... more Seasonal variability of solar UV radiation in ocean waters is estimated on a global scale by combining satellite measurements of scene reflectivity (TOMS), column ozone (TOMS) and chlorophyll concentration (SeaWiFS) with radiative transfer calculations for an ocean-atmosphere system. The new features are an extension of underwater radiative transfer (scattering and absorption) into the UV, inclusion of polarization in the above water diffuse radiances, the proper treatment of Fresnel reflection, and first order atmospheric backscatter of water-leaving radiance to the oceans. Maps of downwelling diffuse irradiances (E d) at ocean surface and at different depths in the ocean, diffuse attenuation coefficient (K d), and ten percent penetration depth (Z 10) of solar irradiation are computed for open ocean waters. Results on spectral irradiances at 310 nm in UV-B and at 380 nm in UV-A part of the spectrum are presented with particular emphasis on the role of aerosols, clouds, and ozone in the atmosphere and chlorophyll concentrations in the ocean.
Ultraviolet Ground- and Space-based Measurements, Models, and Effects, 2002
Satellite instruments currently provide global maps of surface UV irradiance by combining backsca... more Satellite instruments currently provide global maps of surface UV irradiance by combining backscattered radiance data with radiative transfer models. The models are often limited by uncertainties in physical input parameters of the atmosphere and surface.
Satellite-derived ultraviolet (UV) irradiances may form the basis for establishing a global UV cl... more Satellite-derived ultraviolet (UV) irradiances may form the basis for establishing a global UV climatology, provided that their accuracy is confirmed against ground-based measurements of known quality. In this study, quality-checked spectral UV irradiance measurements from four European stations
During the last decade, methods have been developed for estimating ultraviolet (UV) irradiance re... more During the last decade, methods have been developed for estimating ultraviolet (UV) irradiance reaching the Earth&amp;#x27;s surface using satellite-measured backscattered UV radiances. The aim of this work is to compare UV products (version 8), noon erythemal ...
The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board the NASA EOS Aura spacecraft is a na... more The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board the NASA EOS Aura spacecraft is a nadir viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600 km wide viewing swath and it is capable of daily, global contiguous mapping. The Finnish Meteorological Institute and NASA Goddard Space Flight Center have developed a surface ultraviolet irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths, noontime erythemal dose rate (UV index), and the erythemal daily dose. The overpass erythemal daily doses derived from OMI data were compared with the daily doses calculated from the ground-based spectral UV measurements from 18 reference instruments. Two alternative methods for the OMI UV algorithm cloud correction were compared: the plane-parallel cloud model method and the method based on Lambertian equivalent reflectivity. The validation results for the two methods showed some differences, but the results do not imply that one method is categorically superior to the other. For flat, snow-free regions with modest loadings of absorbing aerosols or trace gases, the OMI-derived daily erythemal doses have a median overestimation of 0-10%, and some 60 to 80% of the doses are within ±20% from the ground reference. For sites significantly affected by absorbing aerosols or trace gases one expects, and observes, bigger positive bias up to 50%. For high-latitude sites the satellite-derived doses are occasionally up to 50% too small because of unrealistically small climatological surface albedo.
The global stratospheric ozone-layer depletion results in an increase in biologically harmful ult... more The global stratospheric ozone-layer depletion results in an increase in biologically harmful ultraviolet (UV) radiation reaching the surface and penetrating to ecologically significant depths in natural waters. Such an increase can be estimated on a global scale by combining satellite estimates of UV irradiance at the ocean surface from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument with the SeaWiFS satellite ocean-color measurements in the visible spectral region. In this paper we propose a model of seawater optical properties in the UV spectral region based on the Case 1 water model in the visible range. The inputs to the model are standard monthly SeaWiFS products: chlorophyll concentration and the diffuse attenuation coefficient at 490nm. Penetration of solar UV radiation to different depths in open ocean waters is calculated using the RT (radiative transfer) quasi-single scattering approximation (QSSA). The accuracy of the QSSA approximation in the water is tested using more accurate codes. The sensitivity study of the underwater UV irradiance to atmospheric and oceanic optical properties have shown that the main environmental parameters controlling the absolute levels of the UVB (280-320nm) and DNA-weighted irradiance underwater are: solarzenith angle, cloud transmittance, water optical properties, and total ozone. Weekly maps of underwater UV irradiance and DNA-weighted exposure are calculated using monthlymean SeaWiFS chlorophyll and diffuse attenuation coefficient products, daily SeaWiFS cloud fraction data, and the TOMS-derived surface UV irradiance daily maps. The final products include global maps of week1 y-average UVB irradiance and DNA-weighted daily exposures at 3m and lOm, and depths where the UVB irradiance and DNAweighted dose rate at local noon are equal to 10% of their surface values.
The major factors causing differences between satellite-derived and ground-based ultraviolet (UV)... more The major factors causing differences between satellite-derived and ground-based ultraviolet (UV) erythemal irradiances and doses are discussed. Measurements totaling more than 4700 days during 1997-1999 were obtained at 8 stations (22°S-64°S) of the Argentine UV Monitoring Network. The satellite retrieval uses radiative transfer calculations for cloud-and aerosol-free conditions multiplied by correction factors for clouds and aerosols. Key parameters are total ozone, cloud optical depth, and surface albedo derived from Total Ozone Mapping Spectrometer (TOMS). When no aerosol correction is applied, systematic differences of satellite-derived erythemal irradiance relative to ground-based measurements amount to +1% at a tropical high-altitude Andean location, +10% at stations in the central Pampas, +5% at southern Patagonian sites, and À7% at the southernmost continental and Antarctic stations with varying snow cover. When an aerosol correction is applied by estimating ''minimum'' and ''maximum'' aerosol loading, the systematic differences are within ±10% for all ''snow-free'' stations. To reduce the differences at places with varying snow conditions, an ''average surfacealbedo climatology'' must be used instead the TOMS climatology of minimum albedo. Although the statistical uncertainty of the differences increases with TOMS reflectivity, the systematic difference is independent of TOMS reflectivity for most of the stations, so on average the comparison for cloudy situations is as good as for clear-sky conditions. The comparison for daily erythemal doses gives similar results with smaller statistical uncertainty. Measured uncertainties are in agreement with a theoretical analysis. For most locations, well-characterized ground-based instruments should agree with TOMS satellite estimations within 10% if aerosol corrections are known.
The Photochemical Activity and Ultraviolet Radiation (PAUR) II project (a continuation of an earl... more The Photochemical Activity and Ultraviolet Radiation (PAUR) II project (a continuation of an earlier PAUR I project) had the purpose of studying the interrelationships between changes in total ozone, tropospheric aerosols, UV radiation and photochemical activity. As part of PAUR II project, a campaign took place in Greece and Italy during May-June 1999, with the participation of 15 European and 3 American research institutions. A variety of radiation and gaseous and aerosol atmospheric composition and optical characteristics measurements were made during the campaign. Radiative transfer models and three-dimensional (3-D) regional chemistry transport models (CTM) were applied and compared to the available data set of PAUR II. The present overview paper gives an introduction to the project and to the meteorological and environmental conditions that prevailed and outlines some results that are extensively described in the subsequent papers which form this special section. The modulation of the UVB field in the presence of different types of aerosols, its transmittance and role in the photochemistry of the particular eastern Mediterranean environment is overviewed. Using a 3-D CTM, it is shown that even a 50% reduction in Greek anthropogenic emissions has only a small effect in reducing the ozone levels over the eastern Mediterranean in summer. The environmental conditions, which prevailed during the PAUR I and PAUR II campaigns, offered cases of background conditions over the Aegean Sea as well as conditions with Saharan dust episodes and extremes in total ozone.
Journal of Geophysical Research: Atmospheres, 2000
This study examines the distribution of long-term trends in ground level erythemally weighted ult... more This study examines the distribution of long-term trends in ground level erythemally weighted ultraviolet (UV) exposures in the northern latitudes for the period 1979-1991 using measurements from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument. A new erythemal UV data set (now available to the public via World Wide Web) was produced recently by NASA and has been tested
Journal of Geophysical Research: Atmospheres, 1998
We discuss the theoretical basis of a recently developed technique to characterize aerosols from ... more We discuss the theoretical basis of a recently developed technique to characterize aerosols from space. We show that the interaction between aerosols and the strong molecular scattering in the near ultraviolet produces spectral variations of the backscattered radiances that can be used to separate aerosol absorption from scattering effects. This capability allows identification of several aerosol types, ranging from nonabsorbing sulfates to highly UV-absorbing mineral dust, over both land and water surfaces. Two ways of using the information contained in the near-UV radiances are discussed. In the first method, a residual quantity, which measures the departure of the observed spectral contrast from .that of a molecular atmosphere, is computed. Since clouds yield nearly zero residues, this method is a useful way of separately mapping the spatial distribution of UV-absorbing and nonabsorbing particles. To convert the residue to optical depth, the aerosol type must be known. The second method is an inversion procedure that uses forward calculations of backscattered radiances for an ensemble of aerosol models. Using a look-up table approach, a set of measurements given by the ratio of backscattered radiance at 340-380 nm and the 380 nm radiance are associated, within the domain of the candidate aerosol models, to values of optical depth and single-scattering albedo. No previous knowledge of aerosol type is required. We present a sensitivity analysis of various error sources contributing to the estimation of aerosol properties by the two.methods. 1.
IEEE Transactions on Geoscience and Remote Sensing, 2006
The Ozone Monitoring Instrument (OMI) onboard the NASA Earth Observing System (EOS) Aura spacecra... more The Ozone Monitoring Instrument (OMI) onboard the NASA Earth Observing System (EOS) Aura spacecraft is a nadir-viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600-km-wide viewing swath, and it is capable of daily, global contiguous mapping. We developed and implemented a surface ultraviolet (UV) irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths (305, 310, 324, and 380 nm). Additionally, noontime erythemal dose rate and the erythemal daily dose are estimated. The OMI surface UV algorithm inherits from the surface UV algorithm developed by NASA Goddard Space Flight Center for the Total Ozone Mapping Spectrometer (TOMS). The OMI surface UV irradiance products are produced and archived in HDF5-EOS format by Finnish Meteorological Institute. The accuracy of the surface UV estimates depend on UV wavelength and atmospheric and other geolocation specific conditions ranging from 7%to 30%. A postprocessing aerosol correction can be applied at sites with additional ground-based measurements of the aerosol absorption optical thickness. The current OMI surface UV product validation plan is presented.
Due to the combined effect of two polarization sensitive elements, the entrance window and gratin... more Due to the combined effect of two polarization sensitive elements, the entrance window and grating, the sensitivity of Brewer spectrophotometers for direct-sun measurements changes with solar zenith angle (SZA). We determined this SZA-polarization dependence with four independent methods, which agreed within ±1.5%. For SZA < 50°this effect is negligible. At SZA = 60°, 70°, and 80°the Brewer's sensitivity is reduced by 1%, 4%, and 10%, relative to SZA = 35°, when the direct solar irradiance is perpendicular to the entrance window. Differential absorption algorithms for retrieving trace gases (e.g., ozone) are unaffected since the polarization effect is almost wavelength independent. However, systematic errors are introduced in Langley extrapolations (2-4% overestimation of the zero air mass factor), retrievals of aerosol optical depth (overestimation of 0.01-0.04), and aerosol single scattering albedo. Therefore, Brewer direct irradiance measurements should be corrected for the SZA-polarization dependence. The effect in sky-radiance measurements can be removed only by hardware modifications.
... PK Bhartia. Code 916, NASA/Goddard Space Flight Center, Greenbelt, MD. J. Ziemke. NRC, NASA/G... more ... PK Bhartia. Code 916, NASA/Goddard Space Flight Center, Greenbelt, MD. J. Ziemke. NRC, NASA/Goddard Space Flight Center, Greenbelt, MD. Z. Ahmad. Software Corporation of America, Greenbelt, MD. D. Larko. Hughes‐STX Corporation, Greenbelt, MD. ...
Measurements of UV and Visible irradiances have proven valuable for assessing the health effects ... more Measurements of UV and Visible irradiances have proven valuable for assessing the health effects (human, animal, and plants) of radiation that reaches the Earth's surface as well as penetrating the.
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Papers by Jay Herman