DIFFUSE COMPONENT OF THE SOLAR ULTRAVIOLET RADIATION IN TREE SHADE A.V. Parisi1,*, M.G. Kimlin1, J.C.F. Wong2, M. Wilson1
1Centre for Astronomy and Atmospheric Research, University of Southern Queensland, Toowoomba, 4350, Australia. Ph: 61 7 46 312226. Fax: 61 7 46 312721. Email: firstname.lastname@example.org
2Centre for Medical and Health Physics, Queensland University of Technology, GPO Box 2434, Brisbane 4001, Australia. Ph: 61 7 38642585. Fax: 61 7 38641521.
*To whom correspondence should be addressed
This modelling and measurement by previous researchers of the diffuse UV irradiances has been in unshaded sunlight. The UV irradiances in tree shade have been measured (Parisi et al., 1999, Parsons et al., 1998 and Grant, 1997) and under crop canopies (Grant, 1991) and forest canopies (Flint and Caldwell, 1998). In tree shade, a larger proportion of the UV compared to that in full sun may be as a result of the diffuse component due to attenuation and filtering of the direct component. However, no previous research has considered the diffuse UV irradiances in tree shade. This paper presents the results of quantitative measurements of the diffuse erythemal and diffuse UVA (320 to 400 nm) at ground level on a horizontal plane during a Southern Hemisphere sub-tropical summer.
2. MATERIALS AND METHODS
2.1 Irradiance Measurements
The irradiance measurements were at ground level on a horizontal plane over a Southern Hemisphere summer in the tree shade on the side of the trunk furthermost from the sun between 1 December 1998 and 28 February 1999. The trees employed in the research were forty-two isolated trees in the grounds of the University of Southern Queensland campus, Toowoomba, Australia at a latitude of 27.5o S. The width of the tree canopies ranged from 2.2 to 13 m, the tree heights ranged from 6.4 to 25 m and the height above the ground to the first branches ranged from 0.4 to 10 m. The cloud cover varied between zero cloud and seven eights of the sky dome covered in cloud as determined by an observer. The cloud cover and atmospheric conditions vary with time and from day to day. The measurements over the summer were designed to take into account the typical cloud and other atmospheric conditions that would be encountered over a Southern Hemisphere summer.
2.2 Diffuse Irradiances
The diffuse irradiances were measured for each of the trees between 8:30 Australian Eastern Standard Time (EST) and 9:30 EST, 11:30 EST and 12:30 EST and 14:30 EST and 15:30 EST. For the remainder of this paper, these time periods are referred to as morning, noon and afternoon.
Figure 1 The percentage diffuse erythemal UV in the (a) morning, (b) noon and (c) afternoon.
Figure 2 The percentage diffuse UVA radiation in the (a) morning, (b) noon and
For the majority of the cases, the percentage diffuse UVery and UVA are higher in the tree shade compared to the respective diffuse irradiance percentages in the full sun. At noon, there is a better defined separation between the diffuse erythemal UV in the tree shade compared to the diffuse erythemal UV in the full sun. In comparison, for the morning and afternoon, the diffuse erythemal UV in the full sun is in general higher with less difference between the diffuse UV in the tree shade and the full sun. This is most likely due to the higher atmospheric pathlength in the morning and afternoon resulting in higher percentage diffuse UV in the full sun.
In Figure 1 there are two data points in the morning and one data point in the afternoon with more than 70% diffuse erythemal UV in full sun. Similarly, in Figure 2 there is one data point in the morning and one data point in the afternoon with more than 70% diffuse UVA in full sun. These high diffuse irradiances in full sun were most likely due to the seven eights cloud at the time on these specific days. The percentage diffuse UVA in Figure 2 is different to the percentage diffuse UVery in Figure 1, possibly due to the differences in Rayleigh scattering for the two wavebands.
3.2 Average Diffuse UV
The diffuse UVery and UVA irradiances in the tree shade are higher at noon by 48% and 20% respectively compared to the average of the morning and afternoon irradiances, most likely due to the higher noon irradiances. Employing linear interpolation between the three measurement times provides an exposure due to the diffuse UV to a horizontal plane in tree shade of 4 MED and 14 J cm-2 between 9:00 EST and 15:00 EST for erythemal UV and UVA respectively. For comparison, the erythemal UV irradiances on a horizontal plane in full sun for a relatively cloud free day (4 December 1998) and a cloudy day (17 December 1998) are provided in Figure 3. The units are in MED/15 minutes as 15 minutes was the recording period of the datalogger on the Biometer. The erythemal irradiances summed between 9:00 EST and 15:00 EST are 29.1 MED and 25.4 MED for the cloud free and cloudy days respectively.
Table 1 The diffuse UVery and UVA irradiances averaged over all of the trees in tree shade and in full sun.
Figure 3 Erythemal UV data for a relatively cloud free day and a cloudy day in summer.
The averages over all the trees of the percentage diffuse UV radiation in the tree shade compared to the UV in the tree shade and the percentage diffuse UV radiation in the full sun compared to the UV in full sun are provided in Table 2. Additionally, the percentage diffuse visible radiation in tree shade and full sun are provided for comparison. The error is one standard deviation of the mean.
For the summer measurements in this research, the percentage diffuse radiation of UVery in the tree shade is relatively constant at about 60% from the morning period to the afternoon period. In comparison, as expected due to the longer atmospheric pathlength causing additional scattering in the morning and afternoon, the percentage diffuse radiation in the full sun is higher in the morning and afternoon compared to that at noon. The differences between the visible waveband and the erythemal UV of the percentage diffuse radiation in the tree shade was approximately the same in the morning, noon and afternoon time periods. In comparison, the same difference in the full sun varies over the three periods.
Table 2 Average over all the trees of the percentage diffuse UV and visible irradiances in tree shade and in full sun.
This high percentage of diffuse radiation has a consequence that the UV protective capability of hats for the relatively high UV irradiances in tree shade (Parisi et al., 1999, Parsons et al., 1998), while useful, is not as high as in full sun. The exposures to diffuse UV on a horizontal plane in the tree shade between 9:00 EST to 15:00 EST were of the order of 4 MED and 14 J cm-2 for erythemal UV and UVA respectively. This high diffuse UV component in the shade may result in high UV exposures to not only parts of the body on a horizontal plane that are not protected, but also, equally high UV irradiances to parts of the body, including the eyes and face, that are not UV protected.
Acknowledgments This research was partially funded by Queensland Department of Health. Two of the authors (MK and MW) were employed through the funding. The authors also acknowledge Ken Mottram, Oliver Kinder and Graeme Holmes in Physics, USQ for their technical assistance in this project.
Blumthaler, M., Ambach, W. & Salzgeber, M. 1994, "Effects of cloudiness on global and diffuse UV irradiance in a high-mountain area," Theoretical and Applied Climatology, vol.50, pp.23-30.
Blumthaler, M. & Ambach, W. 1991, "Spectral measurements of global and diffuse solar ultraviolet-B radiant exposure and ozone variations," Photochemistry and Photobiology, vol.54, pp.429-432.
CIE (International Commission on Illumination) Research Note 1987, A reference action spectrum for ultraviolet induced erythema in human skin, CIE J. 6, 17-22.
Diffey, B.L. 1992, "Stratospheric ozone depletion and the risk of non-melanoma skin cancer in a British population," Physics in Medicine and Biology, vol.37(12), pp.2267-2279.
Flint, S.D. & Caldwell, M.M. 1998, "Solar UV-B and visible radiation in tropical forest gaps: measurements partitioning direct and diffuse radiation," Global Change Biology, vol.4(8), pp.863-870.
Grant, R.H. 1991, "Ultraviolet and photosynthetically active bands: Plane surface irradiance at corn canopy base," Agronomy Journal, vol.83(2), pp.391-396.
Grant, R.H. 1997, "Biologically active radiation in the vicinity of a single tree," Photochemistry and Photobiology, vol.65(6), pp.974-982.
Ireland, W. & Sacher, R. 1996, "The angular distribution of solar ultraviolet, visible and near-infrared radiation from cloudless skies," Photochemistry and Photobiology, vol.63(4), pp.483-486.
Parisi, A.V., Willey, A., Kimlin, M. & Wong, J.C.F. 1999, "Penetration of solar erythemal UV radiation in the shade of two common Australian trees," Health Physics, vol.76(6), pp.682-686.
Parsons, P., Neale, R., Wolski, P. & Green, A. 1998, "The shady side of solar protection," Medical Journal of Australia, vol.168, pp.327-330.
Wong, C.F., Toomey, S., Fleming, R.A. & Thomas, B.W. 1995, "UV-B radiometry
and dosimetry for solar measurements," Health Physics, vol.68(2), pp.175-184.