Introduction

Determining the amount of ultraviolet (290 nm - 400 nm) and visible (VIS,

400 nm - 700 nm) radiation that reaches the earth's surface is a fundamental problem that has occupied scientists and governments from the earliest times. In highly organized major civilizations, the primary reason for the intense interest was to determine the factors (sunlight, drought, and temperature) that affected agriculture, and through agriculture, the wealth and population of nations. The quantitative interest in changing amounts of solar radiation is a more modern endeavor that still has an agricultural underpinning, but has expanded into direct human health effects, effects on land and ocean biology, materials damage, and global warming. In addition to these long-term effects (years-to-decades) from changing solar radiation, there are short-term phenomena (minutes-to-yearly), such as the effects of solar radiation on boundary layer and tropospheric photochemistry, clouds and aerosols, and local and global weather. Short-term changes in ozone and cloud cover change the amount of solar radiation reaching the earth's surface and affect each year's agriculture, the extent of human and other biological exposure, and damage to materials. The extent of human exposure and the corresponding health effects are difficult to quantify because of the mobility of people, the great variety of resistance to UV damage caused by skin type (Hemminki, 2002) and other genetic factors, in addition to the effects of culture and diet on mitigating UV exposure. An example is seen in the relative incidence of skin cancer between Japanese and Caucasians living in the U.S. For the Japanese, the non-melanoma skin cancer rate is about 1 per 100,000, while for Caucasians the rate is about 15 per 100,000 (Qiu and Marugame, 2008). Even the low cancer rate reported for the Japanese-American ethnic group is about double the rate reported in Japan.

Instrumental methods for measuring or estimating solar radiation amounts have gradually evolved from simple ground-based instrumentation that essentially measured the heating effect of all solar radiation (e.g., pyranometers and standardized water evaporation containers, Fig. 5.1) to spectrometers capable of good spectral resolution (0.5 nm) in the UV and VIS range. Before 1979, the only sources of UV and VIS data were obtained from sparsely scattered ground stations of varying accuracy and precision. The longest reconstructed time series of solar UV irradiance from measurements of ozone amounts goes back to 1928 at Arosa, Switzerland (Staehelin and Weiss, 2001; Staehelin et al., 1998), and another near Moscow, Russia goes back to 1968 (Chubarova, 2008) based on direct UV measurements. There have been calibrated measurements of total column ozone since the late 1950s at several stations from which clear-sky UVB (280 nm - 315 nm) can be derived. Attempts have been made to reconstruct the ozone time series back to the late 1600s by estimating the UV-B (280 nm - 315 nm) stress on biological systems, particularly on pine and spruce trees (Zuev and Bonderenko, 2001), and by other biological proxies (Rozema et al., 2002). Networks of standardized evaporation pans (Roderick and Farquhar, 2002), and pyranometers (Stanhill and Cohen, 2001) appear to have detected a long-term reduction in solar irradiance at the earth's surface that seems to be associated with increased cloud and aerosol amounts; a concept popularly known as "global dimming."

Figure 5.1 (a) Standardized evaporation pan for measurement of water evaporation installed on a wooden platform in a grassy location. The pan is filled with water and exposed to sunlight; (b) An example of a pyranometer used for measuring total solar irradiance

Then and now, the preponderance of UV measuring stations are located in the Northern Hemisphere, mostly in Europe and North America, with gradually increasing numbers in Asia. There are a few stations in South America (Argentina, Chile, and Brazil), New Zealand, Australia, and Africa. A well-calibrated polar network of seven stations has been maintained by the U.S. National Science Foundation in Antarctica (Palmer, McMurdo, and South Pole Stations); Ushuaia, Argentina; Barrow, Alaska; Summit, Greenland; and San Diego, California. These have been augmented in recent years by the worldwide distribution of well-calibrated sun and sky photometers as part of AErosol RObotic NETwork (AERONET) (Holben et al., 1998). Reliable satellite measurements of ozone derived from back-scattered UV radiation started in late 1978, with an earlier attempt from the BUV instrument (1970 -1977) on the Nimbus-4 satellite. Since November 1978, satellite UV data consists of daily measurements of ozone amount and cloud reflectivity from Nimbus-7/Total Ozone Mapping Spectrometer (TOMS) and the Solar Backscatter Ultra-Violet (SBUV) instrument, continued with the NOAA SBUV-2 series, Ozone Monitoring Instrument (OMI), and Global Ozone Monitoring Experiment (GOME). These data provide global coverage using an independent calibration for each instrument that can be validated with respect to existing ground stations. Global coverage has been especially important for understanding processes over the oceans, in Africa, parts of Asia, and most of the Southern Hemisphere, where ground-based data are sparse or non-existent. The availability of satellite data has intensified the efforts to develop reliable ground-based instrumentation and to perfect their calibration, for direct measurements of both UV irradiance and atmospheric composition (ozone, sulfur dioxide, aerosols, and more recently, nitrogen dioxide and formaldehyde).

Satellite measurements of ozone amount, reflectivity, and some aerosol properties have enabled good estimates of radiation reaching the ground using laboratory measured absorption and scattering coefficients in radiative transfer calculations that include polarization effects. The principal sources of error in satellite-based estimates of UV and VIS radiation are from the properties and amounts of absorbing aerosols, which are not easily measured from satellites. When satellite measurements of ozone and reflectivity are combined with ground-based

measurements of aerosols (or in very clean atmospheres), estimates of UV and VIS radiation reaching the ground compare well with ground-based data (Kalliskota et al., 2000). The best estimates of VIS wavelength aerosol properties in the atmosphere come from a large network of AERONET CIMEL sunphotometers that are strategically located in many parts of the world. The AERONET data consist of estimates of aerosol extinction at six wavelengths (340, 380, 440, 670, 870 and 1,020 nm), absorption optical depths at four wavelengths (440, 670, 870 and 1,020 nm) and particle size distribution. These aerosol parameters are essential for VIS and UV radiative transfer calculations. However, extrapolation of aerosol absorption from the VIS to the UV has proved to be incorrect (Krotkov et al., 2005; Cede et al., 2006) leading to errors in calculated UV fluxes at the surface and photolysis rates in the troposphere.

Currently, comparisons of satellite-based estimates of UV irradiance with ground-based measurements show overestimates that can range up to 40% when unmeasured aerosol loadings are large. For unpolluted sites, the agreement is within 2% or 3%, consistent with known instrument calibration errors for both satellite and ground-based instruments. For UV radiation, this is implies that the relation between ozone and absorption of UV-B is well understood, as is the relationship between UV-A (315 nm - 400 nm) and VIS irradiances and aerosols (scattering and absorbing). For a comparison of ground-based measurements with satellite estimates of solar radiation, the major outstanding problems are outlined as follows:

(1) Accounting for the aerosol absorption optical depth from 300 nm to 1,000 nm.

(2) Satellite estimation of UV and VIS radiation over snow and ice.

(3) Measurements of ozone, reflectivity, and UV+VIS radiation at high latitudes and solar zenith angles (SZAs) (latitudes of 65° and SZA > 70°)both from the ground and from satellites.

(4) Establishment of international transfer standards and transfer methods that are better than the current limit of about 3%.

(5) Creating common stable calibrations between all types of ground-based instruments at different locations.

(6) Accounting for the differences between ground-based and satellite measurements caused by their different fields of view (FOV).

(7) Improved understanding of radiance and irradiance transmission by clouds, which becomes increasingly important as the FOV becomes smaller.

The amount of solar ultraviolet UV and VIS radiation reaching the earth's surface, and the fraction reflected back to space, is primarily governed by the amount of cloud cover, and to a lesser extent, by Rayleigh scattering, aerosols, and various absorbing gases (e.g., O3, NO2, H2O). Unlike the weakly surface reflected UV and blue radiation, most of the VIS solar radiation is moderately reflected by the ground and vegetation. For UV-B irradiance, ozone, aerosols, and cloud cover are the most important atmospheric components limiting the amount radiation able to reach the ground. Ultraviolet-A irradiances reaching the ground are limited by aerosols and cloud cover.

Figure 5.2 shows an example of solar flux measured above the atmosphere by the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) (McClintock et al., 2000) and the space shuttle borne Atlas-3/Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) (Brueckner et al., 1994) spectrometers, and the calculated solar irradiance reaching the ground for overhead sun and 300 Dobson Units (DU) of ozone. Atlas-3/SUSIM was a high spectral resolution instrument (0.15 nm), which clearly shows the solar Fraunhofer line structure, especially the calcium K and H lines at 393.3 nm and at 396.9 nm (see inset in Fig. 5.2). SOLSTICE was an independent lower resolution instrument that agrees quite well with Atlas-3/ SUSIM. These data are a good starting point for UV radiative transfer calculations in the atmosphere to provide estimates of UV irradiance reaching the ground.

Extraterrestrial Radiation

Figure 5.2 The extraterrestrial solar UV irradiance measured by SOLSTICE and Atlas-3/SUSIM instruments above the atmosphere normalized to a sun-earth distance of 1 astronomical unit, and the calculated irradiance at the ground for 300 DU of ozone and overhead sun. Only the inset, 340 nm - 410 nm is shown at full resolution of 0.15 nm. 1 DU = 2.67 x 1016 molecules/cm2

Figure 5.2 The extraterrestrial solar UV irradiance measured by SOLSTICE and Atlas-3/SUSIM instruments above the atmosphere normalized to a sun-earth distance of 1 astronomical unit, and the calculated irradiance at the ground for 300 DU of ozone and overhead sun. Only the inset, 340 nm - 410 nm is shown at full resolution of 0.15 nm. 1 DU = 2.67 x 1016 molecules/cm2

Ozone absorbs a wavelength-dependent fraction of the solar UV radiation in the 200 nm to 340 nm range, with peak absorption at 255.4 nm. Almost no solar photons with wavelengths shorter than 280 nm reach the earth's surface because of ozone (Hartley and Huggins bands) and molecular oxygen absorption (SchumannRunge band and continuum absorption). There is still strong absorption at 310 nm, where a 1% change in ozone leads to an approximate 1% change in irradiance for SZA = 45°. At wavelengths less than 305 nm, the increasingly large ozone absorption coefficient leads to a proportionally large increase in irradiance for a small decrease in ozone (e.g., at SZA = 45°,a 1% decrease in ozone can produce a 2.2% increase in 305 nm irradiance).

Clouds and scattering aerosols reduce solar radiation at all wavelengths A by reflecting a fraction of the energy back to space, which depends on the aerosol's refractive index and particle size. Except in cases of dense smoke or dust, scattering aerosols (e.g., sulfates) usually reduce radiation reaching the surface by less than 10%. Under special conditions, clouds can locally increase UV from 1% to 10% by cloud edge reflections. Extremely heavy cloud cover (black thunderstorm) can decrease UV and VIS by almost 100%, while even moderate cloud thickness can change the UV-B spectral distribution because of the additional ozone absorption caused by multiple scattering.

In addition to the above effects, VIS wavelengths are modulated by H2O, and those in the Near-IR (NIR) (700 nm - 3,000 nm) are modulated by changes in CO2, CH4, and H2O, within their respective absorption bands. There is an additional small amount of absorption for UV-A and blue wavelengths from NO2 and HCHO, and for UV-B from SO2, in addition to a stronger absorption for UV and VIS from smog in urban or industrial areas.

Ultraviolet and VIS radiation at the surface are generally highest near the equator following the seasonally changing sub-solar point (latitude between ± 23.3°), where ozone amounts are low and the SZA is the smallest. For any specific latitude, larger amounts of solar radiation are seen at high altitude sites because of reduced Rayleigh scattering, especially those with predominantly dry and clear weather and large surface reflectivity (e.g., from snow or ice cover).

Understanding, modeling, and measuring the factors that affect the amount of UV and VIS radiation reaching the earth's surface are important, since changes in these radiation amounts impact agricultural and ocean productivity, global energy balance, and human health. Changes in UV radiation can affect human health adversely through skin cancer (Diffey, 1991), eye cataracts (Taylor, 1990), and suppression of the immune system (Vermeer et al., 1991), yet positively through increased vitamin D production (Grant, 2002; Holick, 2004). Changes in UV radiation can also significantly affect ecosystem biology (Smith et al., 1992; Ghetti et al., 2006).

The focus of this chapter is the estimation or detection of UV irradiance changes that can be measured directly from the ground, or estimated using satellite-based or ground-based measurements that characterize the optical properties of the atmosphere. Section 5.2 describes a subset of the instrumentation widely deployed today, as well as two new instruments recently deployed and validated for measurements of atmospheric optical properties. Section 5.3 is devoted to a discussion of estimating long-term trends in UV irradiance based on satellite derived ozone and reflectivity amounts. The concept of radiation amplification factor (RAF) is reviewed for the purpose of estimating monochromatic irradiance changes from Beer's Law. A second, apparently empirical, power law form is introduced for deriving irradiance trends when changes are estimated for action spectra weighted wavelength integrated irradiances. In a later section, it is shown that the empirical power law form can be numerically derived from the Beer's Law form. Section 5.4 presents a brief discussion of UV irradiance in the Polar Regions. Section 5.5 briefly discusses the effects of UV irradiance on human health. Section 5.6 discusses the UV Index and commonly used units for irradiance and exposure (time integral of irradiance). Section 5.7 introduces the concept of action spectra and discusses their application, gives accurate fitting functions for four of these spectra, derives the power law RAF for each of these spectra, and shows a comparison of theory with data obtained for the erythemal action spectrum. Estimated annual zonal average irradiance changes are presented for monochromatic irradiances (305 nm - 325 nm), erythemal irradiances, and monthly zonal average irradiance (305 nm), as well as the DNA damage action spectrum.

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