This paper explores the literature on ultraviolet irradiance (UV) in urban ecosystems with respect to the likely effects on human health. The focus was the question of whether the health effects of UV radiation should be included in planning of landscape elements such as trees and shading structures. In examining the literature, special attention was given to seeking information on the question of whether it is important that shade be provided for elementary school play areas, and if so, how should it be accomplished? Before such practical questions could be dealt with, it became obvious that answers to several pertinent secondary questions had to be sought. Foremost of these was, what are the negative and positive health effects of UV exposure? Recent epidemiological findings of apparent benefits of sunlight because of vitamin-D photosynthesis and resulting anti-cancer effects make this highly relevant. Another basic question is that of trends in ozone depletion, which leads to interesting questions of long-term trends, short-term extremes, and urban influences on UV irradiance. A host of these and other pertinent questions, such as, "What is the relationship between climate of a location and dress," i.e., "How much exposure will people receive during time spent outdoors?" require much more study. Judging from current knowledge of typical spectra of solar radiation in tree shade and the difference between the action spectra for vitamin D synthesis and erythema in human skin, exposure to solar radiation in tree shade for a short period of time can be somewhat more beneficial for vitamin D synthesis and regulation than detrimental in producing sunburn.
The effect of cloud cover on the amount of solar UV radiation that reaches pedestrians under tree cover was evaluated using a three-dimensional canopy radiation transport model. The UVB irradiance across a horizontal domain at the base of a regular array of spherical tree crowns of varying radius was modeled under the full range of sky conditions: clear, few clouds, scattered clouds, broken clouds, and overcast. Differences in crown radius created differences in crown cover (m) with resulting differences in portions of the domain in direct beam shade. The spatial mean relative irradiance and erythemal irradiance of the domain and the spatial mean relative irradiance (Ir) and erythemal irradiance in the shaded regions of the domain were determined for solar zenith angles of 15°, 30°, 45°, and 60°. The mean Ir and erythemal UV irradiance under skies with 4 octas or less was not remarkably different from that under clear skies. Broken cloud cover reduces the spatial mean irradiance by approximately 20% to 30% across the 15o to 60o solar zenith range. In the shade, the irradiance was greater under partly cloudy than under clear skies. Partial cloud cover did not greatly influence the irradiance in the shade of the canopies. Significant changes in erythemal irradiance in the shade did not occur except with cloud cover of 8 octas (overcast) with solar zenith angles less than 45°. Consequently the mean ultraviolet protection factor for vegetation canopies under partly cloudy skies (50% or less cloud cover) is nearly equivalent to that for clear sky days. Regression equations were developed to estimate the areally averaged relative irradiances across the entire domain and only the shaded regions of the domain for each cloud cover fraction as functions of the solar zenith angle and canopy cover. These equations were then used to predict the variation in erythemal irradiance received across a region of suburban Baltimore, Maryland.
KEYWORDS: Ultraviolet radiation, 3D modeling, Buildings, Solar radiation models, Data modeling, Climatology, Vegetation, Meteorology, Geographic information systems, Cancer
Evaluating the impact of ultraviolet-B radiation (UVB) on urban populations would be enhanced by improved predictions of the UVB radiation at the level of human activity. This paper reports the status of plans for incorporating a UVB prediction module into an existing Urban Forest Effects (UFORE) model. UFORE currently has modules to quantify urban forest structure, urban tree volatile organic compound emissions, carbon storage and sequestration in urban vegetation, dry deposition of air pollutants on trees, tree influences on energy use for heating and cooling buildings, tree pollen allergenicity index, and replacement cost of trees. These modeled effects are based upon field sampling to characterize land use, vegetation cover, and building features. The field sampling includes recording of tree species, total height, height to base of live crown, and crown width on randomly selected 0.04-ha (0.1 acre) plots. Distance and direction from sampled trees to buildings are also measured. The input for UFORE modeling of effects includes hourly meteorological data and pollution-concentration data. UFORE has already been used in assessing the urban forest functionof 13 cities in the United States and 5 cities in other countries. The objective of the present work is to enable UFORE to predict the effect of different urban tree densities on potential average human exposure to UVB. The current version of UFORE is written using the Statistical Analysis System (SAS); a new version will be a user-friendly Windows application and will be available for wide distribution. Progress to date on the UVB module consists primarily of examining available modeling and data collection tools. Two methods are proposed for the UVB module. In Method 1, we will derive predicted UVB irradiance <Ib> at person height, that is, below the urban tree and building canopy, using gap fractions (sky view portions) measured from digitized fisheye photos taken from each of the UFORE plot centers during a UFORE field survey. A promising method for analyzing the photos is the use of Gap Light Analyzer (GLA). A human thermal comfort model will be used to determine the times when people would be comfortable outdoors in light attire, and UVB <Ib> will be determined for those times. Method 2 will be applied in cases where hemispherical photos cannot be made available, and for making predictions for cities where surveys have already been done. Method 2 will use a 3D canopy UV radiation transfer model to derive <Ib> based on tree canopy cover maps from GIS analysis of aerial color IR photographs or Landsat TM images. The UV module addition to UFORE will make it useful in epidemiology of UV-related human disease and assessment of UV benefits, such as in vitamin D production, and it will also facilitate consideration of UV exposure in urban forest management.
The depth of light penetration from the adaxial surfaces of the mature leaves of pecan (Carya illinoensis) was measured using a fiber optic microprobe system at four wavelengths: UV-B (310nm), UV-A (360 nm), blue light (430nm), and red light (680nm). The average thickness of the leaf adaxial epidermal layer was 15um and the total leaf thickness was 219um. The patterns of the light attenuation by the leaf tissues exhibited strong wavelength dependence. The leaf adaxial epidermal layer was chiefly responsible for absorbing the UV-A UV-B radiation. About 98% of 310 nm light was steeply attenuated within the first 5 um of the adaxial epidermis; thus, very little UV-B radiation was transmitted to the mesophyll tissues where contain photosynthetically sensitive sites. The adaxial epidermis also attenuated 96% of the UV-A radiation. In contrast, the blue and red light penetrated much deeper and was gradually attenutated by the leaves. The mesophyll tissues attenuated 17% of the blue light and 42% of the red light, which were available for photosynthesis use. Since the epidermal layer absorbed nearly all UV-B light, it acted as an effective filter screening out the harmful radiation and protecting photosynthetically sensitive tissues from the UV-B damage. Therefore, the epidermal function of the UV-B screening effectiveness can be regarded as one of the UV-B protection mechanisms in pecan.
Stratospheric ozone depletion has caused an increase in the amount of UV-B radiation reaching the earth’s surface. Numerous investigations have demonstrated that the effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, susceptibility to disease, and changes in plant structure and pigmentation. Many experiments examining UV-B radiation effects on plants were conducted in growth chambers or greenhouses. It has been questioned if the effect of UV-B radiation on plants can be extrapolated to field responses from indoor studies because of the unnaturally high ratios of UV-B/UV-A and UV-B/PAR in many indoor studies. Field studies on UV-B radiation effect on plants has been recommended in order to use the UV and PAR irradiance provided by natural light. This study reported the growth and yield responses of a maize crop exposed to enhanced UV-B radiation and the UV-B effects on maize seed qualities under field conditions. Enhanced UV-B radiation caused a significant reduction of the dry matter accumulation, and the maize yield in turn was affected. With increased UV-B radiation the flavonoid accumulation in maize leaves increased, and the contents of chlorophyll a, b, and (a+b) of maize leaves were reduced. The levels of protein, sugar, and starch of maize seed decreased with enhanced UV-B radiation, while the level of lysine increased with enhanced UV-B radiation.
Ultraviolet radiation from the sun, especially the UVB (280 to 320 nm), has important roles in urban ecosystems, including effects on human health. Broadband UVB radiation is being continuously monitored in the city of Baltimore, MD as part of a long-term ecological research program, the Baltimore Ecosystem Study. This paper compares above-canopy broadband UVB irradiance at the Baltimore station to broadband UVB irradiance at a more-rural station 64 km SE (at Wye Research Center in Queenstown, MD) and a station characterized as suburban within the Baltimore-Washington metropolitan area, 42 km SW (at Beltsville Agricultural Experiment Station). The Baltimore data are from the initial 14 months of measurements there. The solar radiation monitoring station in Baltimore is located on a 33-m-tall building on a high point with no significant obstructions to sky view. The broadband instruments, all of which were provided by the USDA UVB Monitoring and Research Program, were calibrated in the same facility, the NOAA Central UV Calibration Facility in Colorado. In general, UVB irradiances at the three sites were similar. Over all conditions, Baltimore and the suburban site measured 3.4% less irradiance than the rural site. This difference is within the anticipated ±3% calibration uncertainty of the broadband pyranometers. On the 59 days with cloud-free conditions at all three sites, the average differences between measured UVB at the three sites was even smaller; Baltimore measured 1.2% less irradiance than the rural site. On the clear days, differences between total daily irradiance and the trend of daily irradiance through the year were clearly related to total column ozone as indicated by the EPTOMS satellite. High aerosol optical thickness strongly reduced daily UVB dose; whereas [SO2] had no influence. Surface O3 increased with increasing UVB dose when [NO2] exceeded 10 ppb.
Increased concentration of leaf UV-B absorbing-compounds due to exposure to UV-B radiation is widely accepted as one of the plant adaptations to resist enhanced UV-B radiation. This paper reports a field comparative study of dynamics and temporal changes of UV-B absorbing-compound concentration in 35 southern broadleaf trees over a growing season. Leaf UV-B absorbing-compound, chlorophyll concentration, and leaf thickness were measured from the sun-exposed leaves of 35 tree species collected monthly from individual trees growing within the city of Baton Rouge, Louisiana from April to October in 2000. The USDA UV-B Monitoring Network Baton Rouge Station provided the ambient UV-B radiation data. Leaf UV-B absorbing-compound concentration varied significantly with leaf age and species. Intra-specifically, leaf UV-B absorbing-compound concentration exhibited a generally increasing trend during leaf growth and development in response to the increased exposure to natural UV-B/solar radiation during the growing season. Inter-specifically, significant differences existed in leaf UV-B absorbing-compound concentration. The species were compared and ranked based on the growing season averages of the leaf total UV-B absorbing-compound concentration. The species were further classified into three levels (high, medium, and low) based on the magnitude of UV-B absorbing-compound concentration.
Decreasing stratospheric ozone concentrations have led to concern for an enhanced UVB radiation (wavelength range of 280-315 nm) reaching the Earth's surface. An understanding of the mechanism of UVB radiation tolerance and sensitivity may improve our ability to assess the potential effects of changes in solar UVB radiation on plant species. The purpose of this research was to conduct an assessment of tolerance of southern trees to future enhanced UVB radiation. This paper describes the systematic approach developed for such an assessment and the preliminary results associated with thirty-five broadleaf tree species in the South studied during two growing seasons. Results indicated that the diverse trees in the South possessed various biological characteristics in defense of UVB damage. It is suggested that the approach to comprehensive evaluation of UVB tolerance in diverse trees should focus on leaf optical properties, mainly depth of light penetration into leaves, leaf anatomical and morphological changes, and concentration of leaf UVB absorbing compounds during leaf development.
Many of the effects of ultraviolet radiation (UVR) on people and their environment--damage to various materials, survival of insects and microbial pathogens, growth of vegetation, and adverse or beneficial effects on human health--are modified by the presence of trees. Human epidemiological investigations generally consider exposure as given by indices of UVR irradiance on horizontal surfaces in the open. Though many people are exposed to UVR while reclining at a beach or swimming pool, thus experiencing irradiance on essentially horizontal surfaces in the open, exposure to UVR during daily routines in urban areas may also be important in affecting human health. Tree influences on UVR irradiance, particularly in the UVB, can differ substantially from influences on the visible portion of the solar spectrum. Trees greatly reduce UVB irradiance in their shade when they obscure both the sun and sky. Where trees obscure the sun but leave much of the sky in view, UVB irradiance will be greater than suggested by the visible shadow. In small sunny areas near trees that block much of the sky from view, UVB irradiance is reduced substantially, whereas visible irradiance may be nearly as great or slightly greater than in the open.
The stratospheric ozone depletion and enhanced solar ultraviolet-B (UVB) irradiance may have adverse impacts on the productivity of agricultural crops. The effect of UVB enhancements on agricultural crops includes reduction in yield, alteration in species competition, decrease in photosynthetic activity, susceptibility to disease, and changes in structure and pigmentation. Many studies have examined the influence of supplementing UVB irradiance on different crops, but the effect of UVB irradiance on cotton crops has received little attention. The cotton crop is one of the most versatile of all the crops. It is a major fiber crop of the world and a major source of trade and economy in many countries. In this study we provide quantitative examination of the effects of elevated UVB irradiance on cotton. The tested cotton crop was grown under natural and supplemental levels of UVB irradiance in the cotton field. The results in this study showed that the supplemental UVB irradiance has negative impacts on cotton growth, development, yield, and fiber quality. A brief discussion is included on how the impacts on cotton contrast with impacts that have been observed in other studies on other plants, including trees.
A decrease in stratospheric ozone may result in a serious threat to plants, since biologically active short-wavelength ultraviolet-B (UVB 280-320 nm) radiation will increase even with a relatively small decrease in ozone. Experimental work has shown that various cultivars and species respond to UVB in different ways. To determine the physiological effects on plants of any increases in UVB radiation, the irradiances at the potential sensitive plant surface need to be known. Numerical models are needed to calculate UVB irradiance. This paper compares spatially and temporally averaged measurements of UVB canopy transmittance (Tcanopy, irradiance below canopy/irradiance above canopy) to that predicted by three models. Maize was selected as the canopy for the study because direct measurements of leaf area and leaf angle distribution are manageable. The models can be applied to other plants including urban trees, though other methods of characterizing leaf area and angle distributions generally would be used. Using measurements of canopy parameters as inputs to the numerical scheme, the models attempt to simulate the UVB Tcanopy that the UVB sensors measure. The purpose of this paper is: (1) to describe the models developed for calculating UVB irradiances (as measured by UVB Tcanopy) at given positions in maize canopies; and (2) to report the results of experimental tests of the models.
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