Ground-based monitoring of UV radiation

High levels of UV radiation (UV-B) from the Sun have harmful effects on agricultural crops, forest ecosystems, humans, and livestock.¹ In recent years the level of UV-B reaching Earth's surface has increased because the protective stratospheric ozone layer has been eroded by man-made chemicals, such as chlorofluorocarbons and methyl bromide.² Although the rapid decline in ozone concentration is now showing improvement, the US Department of Agriculture (USDA) remains concerned about possible harmful effects of increased UV-B on plants and ecosystems. UV-B levels, however, are not typically monitored at weather stations in the United States.

For this reason, the USDA initiated the UV-B Monitoring and Research Program (UVMRP) in 1992.³ This program represents the only active source of complete ground-based UV measurements (since 1996) in the United States.⁴ The UVMRP network collects ambient ground-level solar radiation data, including UV-B and photosynthetically active radiation, from 37 climatological and three long-term research sites (see Figure 1). Data is downloaded from the network sites every night, and then subjected to automated quality control and calibration processes, so that it is available to researchers in several disciplines on our website the following morning.⁵

Figure 1. The location of UV-B Monitoring and Research Program (UVMRP) monitoring sites in 2013. The 37 climatological sites are intended for long-term monitoring of different ecoregions across the United States. The three research sites are used for shorter-term and specific studies.

Figure 2. Comparison of the UV index that is derived from (left) the US Department of Agriculture's (USDA) UVMRP network measurements and (right) NASA's Total Ozone Mapping Spectrometer (TOMS) data.¹Both sets of data represent the average from the summers of 2000–2004. The range of the UVMRP indices is 5.4–9.0, and 5.6–12.0 for the TOMS values.

The primary UV instruments at our network's stations use independent interference filter-photodiode detectors and automated rotating shadow bands to measure the direct-normal, total-horizontal, and diffuse-horizontal UV solar irradiance at seven wavelengths. The diffuser is an integrating cavity, with thin Teflon walls, that protrudes above the top of the detector head and is surrounded by an artificial horizon that improves the angular response of the instrument. Two diaphragms made of frosted WG-280 glass in the integrating cavity act as transmission diffusers. Light that exits the bottom of the diffuser is incident on seven photodiodes with interference filters, which are arranged in a hexagonal array, with the seventh photodiode at the center.⁶ The stations in our network are mainly located in agricultural regions, but several urban sites are also included. This coverage provides data for many different North American ecoregions (ecologically and geographically defined areas) and atmospheric conditions.

The UVMRP network provides the USDA with essential long-duration baseline data for measuring trends in UV-B radiation. Data from our network and from NASA's Total Ozone Mapping Spectrometer are compared in Figure 2. This comparison shows that there is a strong correlation between the UV index values computed from the two different data sets, which in turn indicates that the UV index can be calculated and interpolated from either ground-based sensors (which measure downwelling irradiance) or from satellite-based sensors (which measure upwelling radiation). Ground-based in situ measurements, like those from the UVMRP network, are important for monitoring atmospheric status. It is important to note that they cannot be replaced totally by space-based remote sensing retrievals. Indeed, current satellite algorithms now incorporate improved UV retrievals within the latest package for the Ozone Monitoring Instrument on the Aura Satellite.^7–9

We make several data products available from the UVMRP website. Graphs of irradiance (see, e.g., Figure 3), along with the raw and calibrated irradiance data, from each of the network's sites can be downloaded. The seven different measured UV irradiances at each station are used to generate continuous UV spectra. In addition, instantaneous cloud and aerosol optical depths, hourly and daily sums of the data, clear sky calibration checks (using the Sun as a calibration standard), and the characterization data for each instrument can all be accessed. We also provide UV index (strength of UV radiation from the Sun at a particular time and place) and synthesized continuous spectra, weighted by specific biological functions (i.e., for human or plant exposure).

Figure 3. Irradiance measured at a nominal wavelength of 332nm during 2006 at the Poker Flat Research Range climatological site (20 miles northeast of Fairbanks, Alaska).

We used data from four of our UVMRP ground stations in Wisconsin (Palouse Experimental Range), Colorado (Central Plains Experimental Range), Maryland (Wye Research and Education Center), and Arizona (Abyss Site at Grand Canyon National Park) to investigate the change in ozone on a multi-decadal timescale (1979–2005).¹⁰ We found that the UV index has increased at these four stations, although the total ozone level has decreased across the continental United States. Furthermore, the spatial distribution of ozone varies substantially from coastal zones to the Midwest. The tendency toward recovery of the ozone layer over the continental United States, however, cannot be fully confirmed by the results of our study.¹⁰

A long-term record of the actual UV-B radiation levels that reach Earth's surface is essential for scientists who are studying the effects of UV-B radiation on agricultural crops, forest ecosystems, humans, and livestock. As the only active monitoring network for UV-B radiation, UVMRP plays an essential part in this endeavor. Plans for expansion of the network and for upgrading its instrumentation are continually under review by the UVMRP and the USDA. Our ever-increasing UVMRP database provides a wealth of information for future research, and we are able to provide specialized data and technical guidance to the research community.

The UVMRP is funded by the USDA under a grant provided by USDA-NIFA (National Institute of Food and Agriculture), and is supported by Colorado State University, the Natural Resource Ecology Laboratory, and the Department of Ecosystem Science and Sustainability.