Solar ultraviolet spectroradiometry in New Zealand: instrumentation and sample results from 1990

Portable device for characterizing the angular response of UV spectroradiometers

This paper introduces a device that was developed to measure the angular response of UV spectroradiometers in the field. This device is designed to be used at the operating position of spectroradiometers; thus the derived angular response also includes any effects from imperfect leveling of the diffuser and corresponds to the actual operational angular response. The design and characterization of the device and the results from its application on 11 different spectroradiometers that operate at different European UV stations are presented. Various sources of uncertainties that were identified result in a combined uncertainty in determining the angular response, which ranges between approximately 1.5% and 10%, depending on the incidence angle and the characteristics of the diffuser. For the 11 instruments, the error in reporting the diffuse irradiance ranges between 2% and - 13%, assuming isotropic distribution of the downwelling radiances.

Increasing the ultraviolet protection provided by shade structures

The side openings of a shade structure have a direct influence on where the shade is located and the level of scattered UV in the shaded area. UV exposures were assessed for the decrease in scattered UV beneath specific shade structures by the use of two types of side-on protection, namely, polycarbonate sheeting and evergreen vegetation. Dosimetric measurements conducted in the shade of a scale model shade structure during summer and winter showed significant decreases in exposure of up to 65% for summer and 57% for winter when comparing the use and non-use of polycarbonate sheeting. Measurements conducted in the shade of four shade structures with various amounts of vegetation covering different sides, showed that adequate amounts of and positioning of vegetation decreased the scattered UV in the shade by up to 87% for the larger solar zenith angles (SZA) of approximately 67 degrees and up to 30% for the smaller SZA of approximately 11 degrees when compared to the shade structure that had no surrounding vegetation.

Using a Parameterization of a Radiative Transfer Model to Build High-Resolution Maps of Typical Clear-Sky UV Index in Catalonia, Spain

To perform a climatic analysis of the annual UV index (UVI) variations in Catalonia, Spain (northeast of the Iberian Peninsula), a new simple parameterization scheme is presented based on a multilayer radiative transfer model. The parameterization performs fast UVI calculations for a wide range of cloudless and snow-free situations and can be applied anywhere. The following parameters are considered: solar zenith angle, total ozone column, altitude, aerosol optical depth, and single-scattering albedo. A sensitivity analysis is presented to justify this choice with special attention to aerosol information. Comparisons with the base model show good agreement, most of all for the most common cases, giving an absolute error within ±0.2 in the UVI for a wide range of cases considered. Two tests are done to show the performance of the parameterization against UVI measurements. One uses data from a high-quality spectroradiometer from Lauder, New Zealand [45.04°S, 169.684°E, 370 m above mean sea level (MSL)], where there is a low presence of aerosols. The other uses data from a Robertson–Berger-type meter from Girona, Spain (41.97°N, 2.82°E, 100 m MSL), where there is more aerosol load and where it has been possible to study the effect of aerosol information on the model versus measurement comparison. The parameterization is applied to a climatic analysis of the annual UVI variation in Catalonia, showing the contributions of solar zenith angle, ozone, and aerosols. High-resolution seasonal maps of typical UV index values in Catalonia are presented.

Cloud cover and horizontal plane eye damaging solar UV exposures

The spectral UV and the cloud cover were measured at intervals of 5 min with an integrated cloud and spectral UV measurement system at a sub-tropical Southern Hemisphere site for a 6-month period and solar zenith angle (SZA) range of 4.7 degrees to approximately 80 degrees . The solar UV spectra were recorded between 280 nm and 400 nm in 0.5 nm increments and weighted with the action spectra for photokeratitis and cataracts in order to investigate the effect of cloud cover on the horizontal plane biologically damaging UV irradiances for cataracts (UVBE(cat)) and photokeratitis (UVBE(pker)). Eighty five percent of the recorded spectra produced a measured irradiance to a cloud free irradiance ratio of 0.6 and higher while 76% produced a ratio of 0.8 and higher. Empirical non-linear expressions as a function of SZA have been developed for all sky conditions to allow the evaluation of the biologically damaging UV irradiances for photokeratitis and cataracts from a knowledge of the unweighted UV irradiances.

Comparison of biologically damaging spectral solar ultraviolet radiation at a southern hemisphere sub-tropical site

The first dataset of a complete year of biologically damaging spectral UV at a sub-tropical latitude in the southern hemisphere has been presented. The new data provides a baseline dataset against which comparisons can be made in the future to establish if there have been any long term trends in the biologically damaging UV. The general shape of the variation of the daily biologically damaging exposures through the year depends on the relative response of the various action spectra at the different wavelengths. The ratio of the daily erythemal to actinic exposures drops by approximately 20 to 25% from winter to summer. The ratio of the erythemal to DNA exposures drops by approximately 50% over the same period. In contrast, the ratio of the erythemal to plant damage exposures is higher in summer compared to winter. This is due to the changes in the relative proportion of UVA to UVB wavebands and relative responses of the different action spectra. The relative changes for the different action spectra show that the erythemal action spectrum cannot be used as a proxy for other biologically damaging responses.

Horizontal and sun-normal spectral biologically effective ultraviolet irradiances

The dependence of the spectral biologically effective solar UV irradiance on the orientation of the receiver with respect to the sun has been determined for relatively cloud-free days at a sub-tropical Southern Hemisphere latitude for the solar zenith angle range 35-64 degrees. For the UV and biologically effective irradiances, the sun-normal to horizontal ratio for the total UV ranges from 1.18 +/- 0.05 to 1.27 +/- 0.06. The sun-normal to horizontal ratio for biologically effective irradiance is dependent on the relative effectiveness of the relevant action spectrum in the UV-A waveband. In contrast to the total UV, the diffuse UV and diffuse biologically effective irradiances are reduced in a sun-normal compared with a horizontal orientation by a factor ranging from 0.70 +/- 0.05 to 0.76 +/- 0.03.

Increased summertime UV radiation in New Zealand in response to ozone loss

Long-term decreases in summertime ozone over Lauder, New Zealand (45 degrees S), are shown to have led to substantial increases in peak ultraviolet (UV) radiation intensities. In the summer of 1998-99, the peak sunburning UV radiation was about 12 percent more than in the first years of the decade. Larger increases were seen for DNA-damaging UV radiation and plant-damaging UV radiation, whereas UV-A (315 to 400 nanometers) radiation, which is insensitive to ozone, showed no increase, in agreement with model calculations. These results provide strong evidence of human-induced increases in UV radiation, in a region where baseline levels of UV radiation were already relatively high.

Solar actinic flux spectroradiometry: a technique for measuring photolysis frequencies in the atmosphere

A spectroradiometer has been developed for direct measurement of the solar actinic UV flux (scalar intensity) and determination of photolysis frequencies in the atmosphere. The instrument is based on a scanning double monochromator with an entrance optic that exhibits an isotropic angular response over a solid angle of 2pi sr. Actinic flux spectra are measured at a resolution of 1 nm across a range of 280-420 nm, which is relevant for most tropospheric photolysis processes. The photolysis frequencies are derived from the measured radiation spectra by use of published absorption cross sections and quantum yields. The advantage of this technique compared with the traditional chemical actinometry is its versatility. It is possible to determine the photolysis frequency for any photochemical reaction of interest provided that the respective molecular photodissociation parameters are known and the absorption cross section falls within a wavelength range that is accessible by the spectroradiometer. The instrument and the calibration procedures are described in detail, and problems specific to measurement of the actinic radiation are discussed. An error analysis is presented together with a discussion of the spectral requirements of the instrument for accurate measurements of important tropospheric photolysis frequencies (J(O(1))(D), J(NO(2)), J(HCHO)). An example of measurements from previous atmospheric chemistry field campaigns are presented and discussed.

Narrow-band, narrow-field-of-view Raman lidar with combined day and night capability for tropospheric water-vapor profile measurements

We describe a high-performance Raman lidar system with combined day and night capability for tropospheric water-vapor profile measurements. The system incorporates high-performance UV interference filters and a narrow-band, dual-field-of-view receiver for rejection of background sunlight. Daytime performance has been demonstrated up to 5 km with 150-m vertical and 5-min temporal averaging. The nighttime performance is significantly better with measurements routinely extending from 10 to 12 km with 75-m range resolution and a 5-min temporal average. We describe design issues for daytime operation and a novel daytime calibration technique.

Continuous solar UV monitoring in Germany

Early in 1993, the Federal Office for Radiation Protection (BfS, Salzgitter) together with the Federal Environmental Office (UBA, Berlin) established an overall UV monitoring network for the continuous measurement of spectrally resolved UV radiation. Every 6 min the solar UV spectrum is measured by a Bentham DM 150 double monochromator system in a wavelength ranging from 290 to 450 nm. Every night, UV data are automatically transferred via modem to the reference station in Munich where they are quality controlled and then stored in a host computer. Human health assessment of the exposure is documented in 1/2 h MED (minimal erythemal dose) values. The selected sites of Zingst (1 m, 54 degrees N, Baltic Sea), Offenbach (110 m, 50 degrees N, Rhine rift valley), Schauinsland (1205 m, 48 degrees N, Black Forest) and Neuherberg (493 m, 48 degrees N, Munich) provide a good overview of the UV radiation situation in Germany and therefore an ideal supplement to more detailed biological effect research, especially of comparison measurements with biosensors under environmental conditions. Preliminary investigations have already been started.

Erythemal UV irradiances at Lauder, New Zealand: relationship between horizontal and normal incidence

Measurements from sensors designed to measure erythemal UV irradiance were used to relate the UV incident on a horizontal surface to that incident on a surface maintained normal to the sun throughout the day at Lauder, New Zealand. These UV measurements were also related to variations in global radiation, total column ozone and atmospheric pressure at the surface. Strong correlations were found between these variables over the 37 day observation period in the summer of 1995/1996. Results from these cross-calibrated UV sensors show that the irradiance incident on a surface normal to the sun can be significantly different from that on a horizontal surface. On clear days, the normal-incidence signal can be 30-40% greater for solar zenith angles in the range 60-70 degrees. Consequently, the risk of UV damage can be greater than reported by measurements or models that assume horizontal incidence (e.g., UV index). On cloudy days the normal-incidence UV can be less than 50% of the horizontal-incidence UV. Averaged over a day, any enhancements in normal-incidence UV over horizontal-incidence UV are smaller. The effects were strongly dependent on cloud conditions. Under clear skies the enhancements are generally less than 10%, and the integrated excess over horizontal-incidence UV is usually less than 5%. However, under cloudy skies the reductions can still be large.

Ground-based differential absorption lidar system for day or night measurements of ozone throughout the free troposphere

The National Oceanic and Atmospheric Administration Aeronomy Laboratory's rapid tunable daylight differential absorption lidar system for monitoring ozone throughout the free troposphere is described. The system components are optimized to provide continuously and rapidly profiles of ozone, day or night, with a vertical resolution of 1 km and an absolute accuracy of +/-10% to the tropopause under clear sky conditions. Routine observations of ozone with frequent error assessments are made by scanning wavelengths between 286 and 292 nm.

In situ calibration technique for UV spectral radiometers

A technique for calibrating spectral radiometers measuring global (2π sr) irradiance using solar irradiance at the top of the atmosphere as the absolute irradiance reference is reported. In addition to providing a calibration at all measured wavelengths, the technique provides a direct measure of the angular response of the radiometer. For instruments that can be used to measure the ultraviolet-B region, the calibration also provides an estimate of the ozone column amount.

UV-B Robertson-Berger meter characterization and field calibration

The New Zealand Meteorological Service has used a Robertson-Berger meter (RB meter) to monitor solar UV-B radiation for the past decade. The radiometric properties of the instrument have been analyzed and agree with the standard values published by Berger. A detailed theoretical treatment of the RB meter response that fully accounts for its cosine response is presented. Field measurements made with the RB meter and with a double monochromator spectral radiometer were used to validate the theoretical model and to derive a calibration for the RB meter. Only cloud-free data were considered so that the downward radiation field at the surface could be modeled as the sum of direct solar and isotropic diffuse components. The imperfect cosine responses of both instruments were modeled. A RB meter count was found to equate to 0.25 +/- 0.02 Jm(-2) referenced to 300-nm radiation at vertical incidence.