Several ground-based ultraviolet (UV) monitoring networks exist in the United States, each of which is unique in the instrumentation employed for measurements. Two of these UV networks are the U.S. Environmental Protection Agency's (EPA's) Brewer Spectrophotometer Network and the U.S. Department of Agriculture's (USDA's) UV-B monitoring network, with a combined instrument total of 52 sites, with 32 sites located in the mainland United States. The Brewer records full sky spectra from 287 to 363 nm with 0.55-nm resolution, whereas the USDA instrument is a broadband device that measures broadband erythemally weighted UV data. To date, limited comparisons of data collected from these networks have been analyzed for comparative and quality assurance (QA) purposes. The data we use is taken from sites where instruments from each program are colocated, namely, Big Bend National Park, Texas, and Everglades National Park, Florida. To reduce the contribution of errors in the Brewer-based instruments, the raw data is corrected for stray light rejection, the angular response of the full sky diffuser, the temperature dependence of the instruments, and the temporal variation. This reduces the estimated errors of the absolute irradiance values of each Brewer spectral measurement to approximately ±5%. The estimated uncertainty of the USDA instruments is approximately ±6% with a systematic bias of (–13 to 5% depending on the total ozone) and is comprised of (1) standard lamp measurement errors, (2) spectral response determination, and (3) the angular response of the diffuser. We perform comparisons between the Brewer spectrally integrated and erythemally weighted UV irradiance measurements and the data collected by the broadband erythemal UV meters at colocated sites between 1997 through to 2002.
We present data describing various techniques that operators of Brewer spectrophotometers can use to monitor and maintain the long-term performance of their instrument. The National Ultraviolet Monitoring Center (NUVMC) at the University of Georgia (UGA) operates, under contract from the U.S. Environmental Protection Agency (EPA), 21 Brewer spectrophotometer instruments. Due to the remote location of the instruments, constant monitoring of instrument performance is required to ensure that the instruments' performance does not deteriorate, hence impacting on data collection and quality. These techniques are illustrated with data from the Brewer instruments in the UGA/EPA network.
A prototype UVA dosimeter that is responsive to the UVA wavelengths only has been developed for measurement of personal exposures. The chemical phenothiazine, cast in thin film form and which is responsive to both the UVA (320-400 nm) and UVB (280-320 nm) part of the spectrum was used and filtered with mylar. This combined system responded to the UVA wavelengths only and underwent a change in optical absorbance as a result of UVA exposure. The wavelength of 370 nm was employed for quantifying the change in optical absorbance of the combined mylar/phenothiazine dosimeter and a calibration curve determined for measuring the UVA exposures. UVA exposures to approximately 50 J cm-2 may be measured prior to saturation of the response.
The assessment of human UV exposure is an important part of any public health research program investigating potential positive and negative health effects from exposure to artificial and environmental levels of UV. Human UV exposure depends on a variety of factors such as location, activity, UV protective devices used, and body posture. This paper will provide an overview of techniques that may be used for assessing UV exposures, such as UV dosimetry and UV exposure modelling. Results presented in this paper highlight the need for further research into human UV exposure assessments, in particular for population groups considered not at risk, such as homeworkers.
Hourly UV Index values at 45 sites in Canada and 52 in the USA were estimated using a statistical relationship between UV irradiance and global solar radiation, total ozone, and dew point temperature. The estimation method also takes into account the enhancement of UV irradiance by snow using an empirical correction derived from Brewer UV measurements. Different characteristics of the UV Index distribution over North America were estimated from the derived UV irradiance for the period 1979-1987 and then presented in the form of monthly maps. Brewer UV measurements at 11 Canadian and 20 US sites and erythemal UV estimates from TOMS data were used for validation. Direct comparisons with Brewer measurements at 7 Canadian sites for the period in the 1990s when both pyranometer and spectral UV data were available demonstrated agreement within 2-3 percent except for periods of melting snow when variations in snow albedo yield higher errors in the derived UV irradiance.
Several ground based ultraviolet (UV) monitoring networks exist in the United States, each of which is unique in the instrumentation employed for measurements. Two of these UV networks are the US EPA’s Brewer Spectrophotometer network and the USDA’s UVB Monitoring network, with a combined instrument total of 52 sites throughout the mainland USA, US territories and some overseas locations. The Brewer records full sky spectra from 287-363nm with 0.5nm resolution whereas the USDA instrument is a broadband device that measures broadband erythemally weighted UV data over a number of bands in the UV. To date, limited comparisons of data collected from these networks have been analyzed for comparative and quality assurance (QA) purposes. The data used in this paper is taken from sites where instruments from each program are co-located, namely, BigBend National Park, Texas and Everglades National Park, Florida. In order to reduce the contribution of errors in the Brewer-based instruments, the raw data has been corrected for stray light rejection, the angular response of the full sky diffuser, the temperature dependence of the instruments and the temporal variation. This reduces the estimated errors of the absolute irradiance values of each Brewer spectral measurement to approximately ±5%. The estimated uncertainty of the USDA instruments is also ±5%. Uncertainty is comprised of (1) standard lamp measurement errors, (2) spectral response determination and (3) the angular response of the diffuser. In this paper, we perform spectrally resolved comparisons between the Brewer UV irradiance measurements and the data collected by the broadband erythemal UV meters at co-located sites between 1997 through to 2002.
Polysulphone has been widely used as a dosimeter to quantify personal solar UV exposures to humans during normal daily activities. In summer at sub-tropical and tropical latitudes, the dynamic range for UV exposures allows the use of the polysulphone to measure solar UV over periods of approximately three to six hours. The use of mesh as a filter over the polysulphone has been previously reported to extend the dynamic range. In this paper the use of a simple filter that is incorporated as part of the dosimeter is reported. The dynamic range of the polysulphone was significantly extended with the incorporation of the filter by a factor of approximately four to five.
Most comparisons of TOMS estimates of surface UV irradiation with measured values from ground-based instruments have indicated a bias of the TOMS estimates toward larger values. A portion of this bias results from absolute uncertainties in the ground-based instruments. The comparison reported here is based on ground-based data from four
sites in the UGA/EPA Brewer network. The raw data from the ground-based instruments has been corrected for (1) stray light rejection, (2) the cosine errors associated with the full sky diffuser, (3) the temperature dependence of the response of the instruments and (4) the temporal variation in the instrument response reducing the estimated errors of the absolute irradiance values of each spectral measurement to < ±7%. Comparisons of TOMS with the surface
measurements are performed both at spectrally resolved wavelengths at the time of overpass and for erythemally-weighted daily-integrated doses. These comparisons are made for all days and for clear-sky days only. The comparisons are carried out using both linear regressions of scatter plots of the two sets of data and for mean differences with respect to both TOMS and the Brewer measurements. It is found that spectrally resolved comparisons suffer from inconsistencies at some of the sites that are believed to result from wavelength uncertainties in the Brewer; they are
therefore of more limited use than wavelength integrated data. A comparison based on daily-integrated doses shows only a small positive TOMS bias (4%) for clear-sky days with a somewhat larger bias (8%) for data taken from all days.
A network of 21 Brewer spectroradiometers, owned by the U.S. Environmental Protection Agency and operated by the University of Georgia, is measuring UV spectral irradiances throughout the United States. Corrections to the raw data have now been implemented. These corrections include (1) stray light rejection, (2) the cosine errors associated with the full sky diffuser, (3) the temperature dependence of the response of the instruments and (4) the temporal variation in the instrument response due to optical changes in the characteristics of the instruments. While for many sites the total corrections amount to less than 10%, for certain sites they are much larger, in some cases amounting to more than 25%. Application of these corrections brings the errors of the absolute irradiance values to approximately +/- 5 to 7% for all sources of error. Comparisons of corrected daily integrated erythemal UVR data (DUV) to model and TOMS- inferred values are performed for sites at Acadia National Park, Bigbend National Park, Everglades National Park and the Virgin Islands. All sites show very good agreement with the TUVSPEC model but comparison with TOMS-inferred DUV values indicate a 10-20% overestimate by TOMS for the four sites.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.