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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Differences between global radiation UVER (erythemal ultraviolet solar radiation) received under full sun and diffuse radiation received under the shadow of two types of tree are analyzed to check the importance of these components on human exposure to UV radiation. Blue Line spores dosimeters of VioSpor were used for measurement of erythemal dose of UV radiation (able to produce erythema in human skin.) The response profile of these devices is extremely similar to human skin, thus they are suitable to determine and predict the interactions between UV erythema and human skin. Measurements were obtained in relatively clear days from February to December 2009 between 9:30 and 15:30 h. Three dosimeters were placed on a horizontal surface: one in full sun and the other two under the shadow of each tree. Values of UVER in both cases, in full sun and under the shadow of pine and Sauce, were obtained. In addition, the comparison was made between values of dose received in each case and the exposure limits recommended by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). Finally, average daily irradiance received under the shadow of each tree in comparison with those received in full sun, was also analyzed using two PMA2100 radiometers situated on a horizontal surface.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Solar ultraviolet radiation reaching the earth's surface has many implications for human health. Part of these implications depends on environmental conditions as well as humans habits [1].

The existence of life on earth depends on solar radiation and, particularly, on the incident ultraviolet radiation. Although it only represents 8.3% of the extraterrestrial solar spectrum [2] and 5% of the radiation reaching the Earth's surface, the ultraviolet component plays an important role in different processes of biosphere because it is biologically active and has not only several beneficial effects but also it can be very harmful to animals, plants and humans [3].

Specifically, in the case of humans, continuous exposure to high levels of ultraviolet radiation can lead into premature skin aging, wrinkling, cataracts or skin cancer. In addition, the materials used in construction like paints, packaging and many other substances, especially polymers, are degraded by ultraviolet radiation.

One of the strategies recommended by the public authorities to reduce exposure of people to UV radiation is the use of tree shadows while being outdoors. For example, while doing some sport, the presence of tree shadows can reduce solar UV exposure received by people. However, we still are receiving some UV radiation and therefore it is important to know how solar UV radiation is influenced by the presence of trees to know the real UV exposure we are receiving. This can be solved by UV modeling or by direct measurement of UV radiation in the shadow of a tree. UV modeling under the tree shadow has occasionally been described [3, 4].

Some articles have studied measurements of spectral UV irradiance at the center of a typical Australian shadow tree compared with full sun in an adjacent area on a cloudless day [5].

An important parameter in the study of the influence of trees on UV radiation is called protection factor, and it is defined as the ratio of UV radiation on a horizontal plane in full sun in comparison with the one under the tree shadow. In this study, a horizontal surface containing the dosimeter and situated in a central position under the shade is considered.

In connection with this parameter, important results have been achieved [6] in Queensland in which horizontal protection factors obtained for Eucalyptus were of about 3.5, for Norfolk Island pines were of 3.7 and for trees with high leaf density (Shanks and Chinese elms) a factor of 5.5. Grant et al. [4] measured a protection factor of 6–10 for forests and found a factor from 2 to 5 for single trees. Parisi and Kimlin [7] measured spectral ratios obtaining protection factors from 3 to 6. Also Parisi et al. [8] calculated spectral ratios from 0.16 to 0.49 and protection factors from 2 to 6. Finally, Diffey & Diffey [9] measured protection factors from 8 to 50 for single trees and about 100 for forests.

The differences between the spectral irradiance biologically active in the tree shadow in comparison with that obtained in the sun is lower in the UVB band. This implies that the relative proportion of UVA to UVB irradiance varies when changing the position from full sun to shadow tree.

Spectral ratio, defined as the ratio of spectral irradiance measured in the tree shadow in comparison with spectral irradiance measured under the full sun, has been analyzed both horizontally and perpendicular to the sun [10] and in different positions of the tree shadow. It can be concluded that spectral ratio of shadow in both positions, horizontal and perpendicular to the sun, decreases when wavelength increases. This is because as the wavelength increases, the amount of scattered radiation decreases.

The reason that the higher proportion of UVA to UVB under shadow is because of Rayleigh (proportional to 1/λ4) and Mie (proportional to 1/λ, being λ the wavelength) scattering.

The International Radiation Protection Association established exposure limits in its Occupational UV Esposure Standard [11] in 1985. These were later adopted and reissued by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) in 1989 [12] and 1996 [13] respectively.

As a result of research, the guidelines for protection were updated in 2004 [14], although no significant changes were made. The ICNIRP recommends a maximum personal exposure of 30 Jm−2 effective UV dose per 8-h period for a sensitive unprotected skin using the American Conference of Governmental Industrial Hygienists (ACGIH) action spectrum [12-15], which differs from the International Commission on Illumination (CIE) action spectrum [16], to which the spectral sensitivity of the spore film dosimeter used in our study corresponds. The relationship between CIE and ACGIH weighted exposures depends on the time of day, changing position of the sun, ozone concentrations, etc. Gies and Wright [17] researched how CIE and occupational weighted exposures can vary by factors of 3.5–4.5 at mid latitudes under different conditions, with the lower limit of 3.5 being more applicable near noon due to increases in the relative proportion of UVB and differences in the two weighted spectra. The relation applied here between the two action spectra as shown in Moehrle et al. [18] is: effective exposure CIE = 3.63 × effective exposure ACGIH, so that the effective ACGIH exposure (30 Jm−2) is equal to an effective CIE exposure of 109 Jm−2 per workshift.

The goal of this study is to analyze the differences between the overall UVER radiation received on a horizontal surface under the full sun and UVER diffuse radiation received under the shadow of two kinds of tree, to determine the importance that these components have on the human exposure to UV radiation.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Measurements were carried out at the Institute of Electrical Technology located in the Technological Park of Paterna (Valencia) (39°32′46.72″N, 0°27′12″W).

Trees description

To analyze the effect of the tree shadow on UVER radiation received by people, two kinds of tree have been studied: the first one (A tree) is a common pine, evergreen, with a total height of 11.7 m, a height to the first branch of 2 m, a width of 9 m, a trunk diameter of 1.3 m and an irregular distribution of leaves, so we can conclude that tree A is a low-density leaf tree.

The second tree (B tree) is a Sauce, evergreen, with a height of 5.4 m, a height to the first branch of 1.6 m, a width of 7.8 m, a trunk diameter of 0.84 m and it can be characterized by abundant and elastic branches filled by numerous and small golden leaves. Therefore, it can be said that B tree is a high-density leaf tree.

Situation of dosimeters for measuring the UVER

A UV-sensitive spore-film filter system (VioSpor Blue Line Type II Dosimeter; Bio-Sense, Bornheim, Germany) [19] was used as the UV dosimeter. Spore-film production (DNA repair-deficient strain of Bacilus subtilis) and the development of the films are described elsewhere [20, 21]. The spore films are covered by a filter system with optical properties simulating the erythemal response of human skin in accordance with the CIE reference spectrum and mounted in waterproof casings with a diameter of 32 mm. The units of solar erythemal exposure are given by the manufacturer as J m−2 and minimal erythema dose (MED) for skin Type II. One MED corresponds to 250 J m−2 normalized to 298 nm, the dose which causes erythema in nontanned Caucasian skin (skin Type II) with sharply defined edges 24 h after sun exposure. The measurement range of the dosimeter is from 10−3 to 101 WCIE m−2, corresponding to 0.05 and 1000 MED h−1, respectively, where WCIE m−2 corresponds to the erythemal irradiance in accordance with the CIE reference spectrum. The working range used is, according to the manufacturer, 0.4–22 MED (Type II) and measurement error is ±10%. The response is independent of humidity and temperature from −20 to 50°C.

Two dosimeters were used for each kind of tree: one placed in the middle of the shadow and another one under full sun. Both of them are placed horizontally.

For measurements, two supports of 47 cm of height over ground level were employed. They had a small horizontal platform situated in the upper part of the support. The platform has a length of 4 cm and is where the corresponding dosimeters were placed: (1) Dosimeters in A tree. The location of dosimeters to make measurements on the A tree is as follows: Dosimeter placed in the shadow: the dosimeter was placed horizontally on the support described above at a distance of 4.30 m from the trunk of the tree and in the middle of the tree shadow.

Dosimeter placed in the sun: the dosimeter was placed horizontally on the support described above and at a distance of 19.30 m from the dosimeter placed in the shadow, so it was placed far enough from tree shadow so that the incident radiation were not affected by the shadow; (2) Dosimeters in B tree. The situation of dosimeters to make measurements on the B tree is as follows: Dosimeter placed in the shadow: the dosimeter was placed horizontally on the support described above at a distance of 1.80 m from the trunk of the tree and in the middle of the tree shadow. Dosimeter placed in the sun: the same dosimeter of A tree is used.

Measurements of erythemal dose received

Measurements were made on relatively clear days. The sky conditions were always lower than 1 okta, being okta the unit of measurement used to describe the amount of cloud cover ranging from 0 (completely clear sky) to 8 (completely overcast). The measurements were made from February to December 2009 on the A and B trees.

Measurements were made from 9:30 to about 15:30 h (local time). The methodology was as explained above. Three dosimeters are placed: one in full sun, another one in the A tree shadow and the last one in the B tree shadow.

In Table 1, a list of measurement days and atmospheric conditions is shown. The average daily temperature, average relative humidity, average wind speed and average global solar radiation were measured by the weather station of (CEAM, personal communication), located about 500 m away from the place where tree measurements were carried out:

Table 1. Measurement days in the tree zone (climate data provided by CEAM). Values averaged over the time period listed in Column 1
Date (day/month/year)Air temperature (°C)Relative humidity (%)Wind speed (m s−1)Global radiation (W m²)Ozone (DU)Θ, Zenith angle (°)
18-19-23-26-27/02/200913.553.32.1510.3352.860.5
5-6-9-10-13-24/03/20091642.34.8608.6342.354.6
21-27-29/04/200920.430.83.4793.539140.6
4-7-15-18-26-27-28-29/05/200923.338.13.5815.3344.835.7
1-10-17-26/06/200926.841.63.9807.131933.3
2-7-17-20-24-30/07/200928.355.63.7729.930834.7
9-11-16-21-22-24-25/09/200923.849.62.6575.5312.950.6
1-19-21-22-23-26-27/10/200920.355.63.8488.2282.662.1
2-9-10-12-26/11/200918.843.22.7383.7272.767.3
1-3-9/12/200913.845.84.5317.1287.570.9

where the values are averaged during the interval corresponding to that listed in Column 1 and the value of θ, zenith angle, is the sun incidence average angle on a horizontal surface in a day between 9 and 15 h.

Measurement of erythemal daily UVER irradiance with radiometers

For this part of the study, two PMA2100 sensors have been employed: one placed in the center of the shadow and another one placed in full sun and both horizontally. The sensors are connected to a data recorder which receives data from both sensors.

The PMA 2102 UVB radiometer was calibrated by comparison, under solar radiation, with a reference detector, the UVB-1. In a UVB-1 radiometer (Yankee Environment System-YES [22, 23]), the calibration uncertainty is approximately 10%. The UVB-1 was installed on the flat roof, without obstructions or shade, of the building of the Earth Physics Department of the University of Valencia in the Burjassot Campus, 60 m above sea level. The UVB-1 has a spectral range between 280 and 400 nm and a spectral sensitivity close to the erythemal action spectrum. The cosine response error is less than 4% for solar zenith angles below 55° according to the manufacturer. This instrument was calibrated in the National Institute for Aerospace Technology. This calibration consists of a measurement of the spectral response of the radiometer indoor and a comparison with a Brewer MKIII spectroradiometer outdoor.

It should be noted that the YES UVB-1 presents nonnegligible errors for high zenith angles unless a double entry zenith angle-ozone calibration matrix is used. For a constant ozone value of 300 DU, the error given by the calibration matrix remained below 9% for zenith angles below 70°.

To perform these measurements, the same support devices as used for dosimeters case were used. The sensors are placed horizontally. Methodology and situation of irradiance sensors are the same as dosimeters measurements.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Erythemal dose

The measurements took place in 2009, and were divided into four periods: (1) February–March–April; (2) May–June; (3) July–September; and (4) October–November–December.

Average values of incidence angle of sunlight rays on a horizontal surface have been calculated for measurements days, thus these values can be compared with dose values obtained for each season.

Comparison with exposure limits

In the following tables, erythemal dose received on dosimeters over a period of 8 h, under the shadow of the A tree, under the shadow of the B tree and in full sun, is shown.

People usually have only part of their body exposed to sun, therefore, Tables 2–5 show the results of dose received in the following cases: (1) 100% of the radiation arriving is received; (2) 10% of the incident radiation is received, which could be the case in which only face and hands were exposed, i.e. 10% of the body surface; (3) 27% of the incident radiation is received, which could be the case in which the face, the arms and hands were exposed, i.e. 27% of the body surface; and (4) 63% of the incident radiation received, which could be the case in which the face, hands, arms and legs were exposed, i.e. 63% of body surface [24].

Table 2. UVER dose received over a period of 8 h under the shadow of the A tree, under the shadow of B tree and in full sun depending on the different percentages of body surface exposed in February–March–April period
Corporal surface exposed (%)8-h UVER dose under the A tree (J m−2)8-h UVER dose under the B tree (J m−2)8-h UVER dose under the sun (J m−2)
10 (face and hands)50.941.5131.8
27 (face, hands and arms)137.3112.1355.7
63 (face, hands, arms and legs)320.4261.5830
100 (complete body)508.5415.11317.5
Table 3. UVER dose received over a period of 8 h under the shadow of the A tree, under the shadow of B tree and in full sun depending on the different percentages of body surface exposed in May–June period
Corporal surface exposed (%)8-h UVER dose under the A tree (J m−2)8-h UVER dose under the B tree (J m−2)8-h UVER dose under the sun (J m−2)
10 (face and hands)90.541.8358.7
27 (face, hands and arms)244.2112.8968.4
63 (face, hands, arms and legs)569.8263.22259.6
100 (complete body)904.5417.83586.6
Table 4. UVER dose received over a period of 8 h under the shadow of the A tree, under the shadow of B tree and in full sun depending on the different percentages of body surface exposed in July–September period
Corporal surface exposed (%)8-h UVER dose under the A tree (J m−2)8-h UVER dose under the B tree (J m−2)8-h UVER dose under the sun (J m−2)
10 (face and hands)9241.6314.4
27 (face, hands and arms)248.5112.4848.9
63 (face, hands, arms and legs)579.9262.11980.7
100 (complete body)920.4416.13144
Table 5. UVER dose received over a period of 8 h under the shadow of the A tree, under the shadow of B tree and in full sun depending on the different percentages of body surface exposed in October–November–December period
Corporal surface exposed (%)8-h UVER dose under the A tree (J m−2)8-h UVER dose under the B tree (J m−2)8-h UVER dose under the sun (J m−2)
10 (face and hands)4730.6125.2
27 (face, hands and arms)126.882.7338.1
63 (face, hands, arms and legs)295.8193.1788.8
100 (complete body)469.6306.41252.1

Dose data in different cases were compared with Exposure Limit.

Taking a look at results shown in Tables 2–5, it can be concluded that both A and B trees can be considered as effective barriers against UVER dose received in case only hands and face are exposed. In fact, for any time of the year, UVER dose values are lower than Exposure Limit (109 J m−1).

In the case of B tree, in October–November–December period, it can be considered an efficient barrier even if the arms were also exposed. However, apart from this specific case, the dose received under the A and B trees, when body surface exposed is greater or equal to 27% (face, hands and arms exposed) is higher than Exposure Limit value for any season, reaching values in July–September period that are higher in a factor of 5.32 in the case of A tree and of 2.40 in the case of B tree.

Exposure ratio

The Exposure ratio (ER) is defined as the ratio between the UVER radiation dose received under the shadow of the tree in J m² and the UVER radiation dose received in full sun in J m². This parameter is analyzed for each season and for each type of tree.

Figure 1 shows the value of the ER for each period and for the two trees and the average value of the solar zenith angle for the measurement days.

image

Figure 1. Shadow Exposure ratio throughout the year for A and B trees.

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The average value of the ER in winter-spring and autumn (February–March–April and October–November–December) months was 0.38 for A tree and 0.27 for B tree. However, for the summer months (May–June and July–September) was 0.27 for A tree and 0.12 for B tree.

In winter period, the solar zenith angle has a higher value. As a consequence, solar radiation has to cover a longer path to reach the earth's surface and therefore Rayleigh scattering phenomena is more evident causing a greater presence of diffuse radiation over the global.

In contrast, as the solar zenith angle decreases, the effect mentioned above is inverted because the path for solar radiation is reduced and also the air dispersion phenomena, thus a greater proportion of direct radiation reaches the surface. Therefore, the percentage of diffuse radiation over the global radiation will decrease.

Average daily irradiance

From the irradiance values measured with PMA 2102 radiometers, ER has been calculated in every period of the year and for each kind of tree showing the variation of the diffuse component in each position.

Results of these calculations are shown in Figs. 2 and 3.

image

Figure 2. Exposure ratio under the shadow of A tree for different periods of year measured in averaged days.

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image

Figure 3. Exposure ratio under the shadow of B tree for different periods of year measured in averaged days.

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It should be first noted that in both types of tree, values of ER, which is the ratio of diffuse to global radiation, are higher for winter, specifically in October–November–December period. In this period, the value of averaged zenith angle is the highest.

On the other hand, for A tree, there is a clear change in the ratio between the higher values early in the morning, when the influence of diffuse irradiance is more important due to high zenith angle, and the lower values around noon when the diffuse percentage decreases.

In the case of the B tree, this trend is not so clear to see. It can be concluded that the high density of B tree implies a more homogeneous behavior during the day.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Erythemal doses received under the shadow of two kinds of trees (A, with low density and B with high density) has been compared in 8-h periods. These values have also been compared with values of Exposure Limits recommended by the ICNIRP, concluding that both the A and B trees can be considered as effective barriers against UVER dose received, despite the fact that when more than 27% of the incident radiation reaches the person under the trees, the dose values received exceed, for any time of year, the corresponding value of the Exposure Limit.

Exposure ratio is also analyzed and it is shown that, for both types of tree, this ratio is higher for periods of winter-spring-autumn (February–March–April and October–November–December) than for the summer periods (May–June and July–September).

It is observed that there is a reduction in the proportion of diffuse UV from winter to summer of 28.9% for A tree and 55.5% for the B tree.

This trend can be explained with the variation of sun position throughout the year. In winter, solar zenith angle has a higher value. The consequence is the longer path that solar radiation has to cover to reach the earth's surface and therefore Rayleigh scattering phenomena is more intense causing a greater presence of diffuse radiation over the global. However, in summer the zenith angle is lower and therefore the path for solar radiation is reduced, and also the air dispersion phenomenon is less important, so that a higher proportion of direct radiation over diffuse reaches the surface.

It is observed that values of ER are, for any time of the year, higher in the case of A tree than in the case of B tree, reaching, in winter months, a value of 0.38 for A tree and 0.27 for B tree and in summer months a value of 0.27 for A tree and 0.12 for B tree. This is due to the fact that in a dense tree, like B, the amount of global radiation (sum of diffuse plus direct radiation) reaching the surface is lower, and therefore the diffuse component and also the Exposure Ratio finally decreases.

In case of averaged daily irradiance, for both trees, values of ER are higher for winter periods when the averaged zenith angle is higher. This is due to the longer path that solar radiation has to cover to reach the surface in the case in which the value of zenith angle is high. This causes a greater influence of atmospheric scattering phenomena and therefore an increased amount of diffuse radiation received on the surface.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This work was carried out with financing from the Ministry of Economy and Competitiveness Project CGL2010-15 931/CLI and Generalitat Valenciana within PROMETEO/2010/064 project, and the Ministry of Science and Innovation in the CGL200761813 project.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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