Quantitative UV measurements have been made for the past 50 years with gradually growing sophistication in the instruments. Requirements for determining UV irradiance changes at the earth's surface are currently well understood and have been implemented at a number of sites. Measurements from ground-based instruments and estimations from satellite instruments around the globe show a mixture of UV-B increases and decreases that depend on changes in local cloud cover, and ozone and aerosol amounts.
Measurements of ozone and cloud plus aerosol reflectivity from satellites have been used to estimate the changes in UV-B for the past 30 years using data collected since 1979. The estimation of irradiance change can be obtained using radiative transfer calculations, or from a simplified, yet accurate, approach using the "Radiation Amplification Factor." Based on the satellite ozone record, the summer average, clear-sky UV erythemal irradiance PERY, averaged over the 32.5°N latitude band, increased by about 8% from 1979 to the mid-1990s. Since the mid-1990s, PERY has decreased so that the current level is about 7% higher than it was at the start of the record in 1979. Similar, changes have occurred in the 32.5°S latitude band where the increase was about 7% in 2008 (Fig. 5.9). At higher latitudes, the annual average increase has been about 8% near the tip of South America (50°S), and about 4.5% at 50°N (Fig. 5.20).
Because of increased sensitivity to ozone changes at 305 nm (near the peak of the DNA damage weighted irradiance /DNA), the increases have been substantially larger. The change in annual average 305 nm at 32.5 °S is about 8%, rising to about 23% at 50°S. In the Northern Hemisphere, the changes have been somewhat smaller, about 6% at 32.5°N and 8.5% at 50°N. These changes are large enough to cause concern for an increase in diseases related to sun exposure. The larger changes for 305 nm irradiance and /DNA, which occurred at higher latitudes, were only partially moderated by a strongly latitude-dependent apparent increase in cloud and aerosol cover (preliminary estimate). When change in cloud cover is included, average annual increase in 305 nm irradiance at 50°S was about 17% over 30 years, while the increase at 50°N was about 11%. Clouds will have approximately the same effect on all UV wavelengths. Since, the change caused by just ozone was larger in the Southern Hemisphere than in the Northern Hemisphere, it implies that danger from human exposure on clear days has increased more in the Southern Hemisphere than in the Northern Hemisphere. The increases for DNA damage irradiance PDNA have been substantially larger than for PERY, since the weighting is more towards the shorter wavelengths. The effect is much larger in the Southern Hemisphere during spring and summer than in the Northern Hemisphere at higher latitudes, 30° to 50° (Fig. 5.22).
Ground-based measurements of surface UV trends present a challenge that can be overcome with proper filtering of the data for cloud-free conditions along with simultaneous aerosol measurements. Ultraviolet estimates from satellite measurements of ozone, aerosols, and cloud reflectivity are averages over large areas on the order of 25 km to 100 km, which minimizes many problems with local variability of cloud and aerosol amounts. Both ground and satellite UV estimates are critically dependent on establishing and maintaining an accurate calibration over the lifetime of an instrument and between successive instruments. Ground-based measurements are essential to provide validation of satellite calibration and as a bridge between successive satellite instruments.
While the Northern Hemisphere UV irradiance maximum in 1993 was associated with the massive equatorial Mt. Pinatubo eruption in 1991 (Kerr and McElroy, 1993, Bhartia et al., 1993), a portion of the total increase occurred before 1991 and was associated with ozone destruction from chlorine loading in the atmosphere before being limited by the Montreal Protocol. Though there have been significant zonal average ozone decreases over 30 years at most latitudes (except the equator), even larger chlorine-driven ozone decreases and UV-B increases were prevented by this and subsequent agreements that were effective for limiting releases of chloroflourocarbons (CFCs) and other chlorine bearing compounds, with CFCs being almost completely phased out by 1995.
Because of the large observed changes in UV-B irradiance at mid- and high-latitudes, it is very important that we continue monitoring from both space and the ground. For space, it is essential that we have continuous well-calibrated data sets from successive near-noon, sun-synchronous satellites, with at least a one-year overlap. The calibration of satellite instruments is an extended effort that must be performed over the life of the satellite using in-flight data leading to occasional reprocessing (perhaps once every two years) of the entire dataset. Comparisons with ground-based UV irradiance and radiance data play an essential role in traditional validation and identification of problems with either satellite or ground-based instruments. For this purpose, a few well-characterized and maintained spectrometers are preferable to large networks of lesser instruments. The few instruments should be located in both clean and moderately polluted sites, and in key locations (e.g., mountains) where satellite estimations may be intrinsically in error. More effort should be put into instruments and procedures that characterize the atmosphere in terms of absorption, scattering, and composition rather than just measuring irradiance.
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