Annual and Interannual Behavior of Solar Ultraviolet Irradiance Revealed by Broadband Measurements(para)

ABSTRACT

This research examines the behavior of ground-level solar UV radiation as measured by eight broadband meters in the continental United States during the period from late 1994 to late 1998. The goal is to define the variability that occurs in UV irradiance over time scales ranging from one to several years. The monthly integrated irradiances, from latitude 32N to 47N, contain large annual cycles and latitudinal gradients which depend on season. Seven of the eight sites show a maximum in July, a behavior related to proximity to the summer solstice, with modifications associated with the annual cycle in column ozone. A large interannual variability in monthly integrated irradiance appears over the 4 year period studied. A comparison of corresponding months during different years shows differences in irradiance of 20% or more in one-third of the cases analyzed. When the solar zenith angle (SZA) is held fixed in the range 60-65, a substantial annual cycle in UV irradiance remains where the maximum monthly mean irradiance is 1.4-1.9 times the minimum, depending on location. Furthermore, the annual cycle at fixed SZA is not in phase with the normal seasonal cycle. Maximum irradiances at fixed SZA tend to occur in the October to December period, while minima cluster in April through July. The annual cycle in ozone, with maximum column values in spring and minima in autumn, explains the general character of the fixed-SZA data, although changes in cloudiness are significant contributors to interannual variability.

Abbreviations: CUCF, Central UV Calibration Facility; DU, Dobson unit; SRF, spectral response function; SZA, solar zenith angle; USDA, United States Department of Agriculture; YES, Yankee Environmental Systems.
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INTRODUCTION

Solar UV radiation at or near the Earth's surface drives air chemistry in clean and polluted environments (1), in addition to being a contributor to a variety of negative biological effects (2-). The variation of ground-level LTV irradiance with local time, latitude and season arises primarily from the changing path length taken by sunlight through an absorbing and scattering atmosphere as solar elevation varies over time scales ranging from hours to 1 year. Finally, changing optical properties of the local atmosphere, including ozone abundance, clouds, haze and particulates lead to additional variations in UV irradiance at a fixed site. These localized effects can be major factors in determining the radiation doses received by local biota and the vigor of chemical processes in a particular geographic region, such as an urban area.

This research examines the behavior of ground-level solar UV radiation measured from several locations during a 4 year period of the 1990s. Although considerable attention has focused on systematic changes in UV radiation in association with long-term trends in the ozone layer (5), it is apparent that regional meteorological conditions can lead to a large variability over a variety of time scales (6,7). This work seeks to define the variability that takes place in UV irradiance over time periods ranging from months up to several years. The objectives are (1) to define the nature and magnitude of interannual variability; (2) to examine the dependence of UV irradiance on location; and (3) to define the magnitude and character of the annual cycle in UV irradiance when the solar zenith angle (SZA)t is held fixed. This final objective addresses the role of atmospheric composition in driving an annual cycle in UV irradiance at the ground, when the large influence of a changing solar elevation is absent.

MATERIALS AND METHODS

Broadband meters. The data sets used in this research come from several broadband UV sensors operating in the continental United States as part of the United States Department of Agriculture (USDA) UV-B Monitoring Program (8). These meters have the advantages of mechanical simplicity and a high measurement frequency, and the resulting data set allows detailed studies of the temporal behavior in UV irradiance. The limitation is that the broadband information alone does not allow unambiguous attribution of observed variability to a specific cause, such as a change in ozone or in cloudiness.

and the integral extends from zero to infinity. Note that fiX) has the units of inverse wavelength. The structure of the meter determines f(a), while the convention adopted in the calibration fixes the value of Ro. For the UVB-1 meters, Ro is chosen such that the daily maximum irradiance at local noon in summer for a clear middle latitude location is typically near 4 W m-z.

Biases and uncertainties in measured irradiances. Possible biases and uncertainties in the broadband measurements may be divided into three areas: angular or cosine response, radiometric sensitivity or gain and the spectral response function (SRF). Any of these factors could vary in a single instrument over the time period between successive calibrations. There could also be instrument-to-instrument differences that influence the data set at a fixed location since a new detector is placed at each site as part of the yearly calibration process. The following paragraphs summarize the calibration procedure, followed by estimates of systematic biases and random errors in the measured irradiances.