Photochemistry and Photobiology, zyxwvuts 1997, 66(5): 683-689 zyxwvu Erythema1 UV Irradiances at Lauder, New Zealand: Relationship between Horizontal and Normal Incidence R. L. McKenzie*, K. J. Paulin and M. Kotkamp National Institute of Water & Atmospheric Research (NIWA), Lauder, Central Otago, New Zealand Received 27 May 1997; accepted 12 August 1997 ABSTRACT zyxwvutsrq Measurements from sensors designed zyxwvuts 1.0 measure ery- themal UV irradiance were used to relate the UV inci- dent on a horizontal surface to that incident on a surface maintained normal to the sun throughout the day at Lauder, New Zealand. These UV measurements were also related to variations in global radiation, total column ozone and atmospheric pressure at the surface. Strong correlations were found between these variables over the 37 day observation period in the summer of 199511996. Results from these cross-calibrated UV sensors show that the irradiance incident on a surface normal to the sun can be significantly different from that on a horizontal surface. On clear days, the normal-incidence signal can be 30-4070 greater for solar zenith angles in the range 60-70". Consequently, the risk of UV damage can be greater than reported by measurements or models that assume horizontal incidence zyxwvuts (e.g. UV index). On cloudy days the normal-incidence UV can be less than 50% of the horizontal-incidence UV. Averaged over a day, any enhancements in normal-incidence UV over horizontal- incidence UV are smaller. The effects were strongly de- pendent on cloud conditions. Under clear skies the en- hancements are generally less than 1070, and the inte- grated excess over horizontal-incidence UV is usually less than 570. However, under cloudy skies the reductions can still be large. I NTRO D U CTI 0 N In recent years there has been increased awareness of the damaging effects of UV radiation. In New Zealand, where melanoma skin cancer rates are among the highest in the world (l), interest has been further stimulated by the rela- tively intense UV levels already experienced zyxwvutsr (2), by the close proximity of the Springtime Antarctic Ozone "Hole" and by the ozone declines that have already occurred at mid- latitudes (3). Since 1989, a program to characterize the UV radiation in New Zealand has been maintained by the National Insti- *To whom correspondence should be addressed at: National Institute of Water & Atmospheric Research (NIWA), Lauder, Private Bag 50061, Omakau, Central Otago 9182, New Zealand. Fax: +64-3- 4473348; e-mail: r.mckenzie@niwa.cri.nz zyxwvutsr 0 1997 American Society for Photobiology 003 1-8655/97 $5.00+0.00 tute of Water & Atmospheric Research (NIWA)? at Lauder, Central Otago (45"S, 170"E, altitude 370 m). The main em- phasis of this program is the measurement of spectral irra- diances (4), but these measurements are supported by a wide variety of other sensors, including broadband monitors de- signed to measure erythemally weighted zyx (i. e. sunburning) UV (5). Generally, these instruments are oriented to measure the downwelling irradiance incident on a horizontal surface, although in one study these were related to upwelling irra- diances to determine the surface spectral albedo (6). In the present study we relate the measured erythemal UV irradiance incident on a horizontal surface with that incident at a surface normal to (perpendicular to) the sun direction. For detectors with a cosine-weighted field of view, the direct component measured by a horizontal detector reduces simply as the cosine of the solar zenith angle (sza). On the other hand, for diffuse radiation, the fractional sky in the detector field of view is reduced for the normal-incidence detector. The contribution from radiation reflected from the ground may be neglected in the present study, because at this site in summer the surface albedo in the UV region is less than 2% (6). There have been several theoretical studies of radiation incident on tilted surfaces reported in the literature. Rela- tionships as functions of sun angle, atmospheric turbidity and surface albedo have been derived between the radiation incident on tilted surfaces and that on a horizontal surface (7-9). However, these studies were not concerned with the UV region, and they assumed that the diffuse radiation field is isotropic More recent measurements using instruments with narrow fields of view have shown that diffuse skylight is strongly anisotropic (10-12). Although the diffuse radiation field is more uniform in the UV region, the requirements for accu- racy are more stringent because a larger fraction of the ra- diation is in the form of diffuse energy. In the case of ery- themally weighted UV incident at the surface, the diffuse beam usually dominates over the direct beam component (13). The UV component of this radiation is of particular con- cern because of its potential damaging effects on the bio- tAbbreviarions; AU, astronomical unit: DU, Dobson unit for ozone measurement (1 DU = 2.69 X 10l6 molecule cm-*); NIWA, Na- tional Institute of Water & Atmospheric Research; NZST, New Zealand Standard Time (=Universal Time + 12 h); sza, solar zenith angle. 683