An experiment was conducted over a 6‐month period in Research Triangle Park, North Carolina, to investigate the effects of clouds and haze on ultraviolet (UV) radiation. Data were collected using a Yankee Environmental Systems UVB‐1 pyranometer, an Eppley Laboratory Precision Spectral Pyranometer, and a SCI‐TEC Brewer spectrophotometer. Hourly reports of total cloud cover and surface observations of air temperature, dew point temperature, barometric pressure, and visibility from the National Weather Service located at the nearby Raleigh‐Durham International Airport were also used in this study. An empirical relationship has been formulated for UV‐B attenuation as a function of total solar transmissivity and cloud cover. Cumulus‐type clouds were found to attenuate up to 99% of the incoming UV‐B radiation during overcast conditions. However, these same clouds were found to produce localized increases of UV‐B radiation of up to 27% over timescales less than 1 hour under partly cloudy skies when the direct solar beam was unobstructed. Summer haze was found to attenuate UV‐B radiation in the range of 5% to 23% when compared to a clear day in the autumn. In general, total radiation was attenuated more than UV‐B radiation under cloudy conditions.

A Brewer Spectrophotometer was utilized to make ground‐based measurements of solar ultraviolet (UVB) radiation received at Black Mountain, NC (35.66°N) in order to quantify the reduction of UV‐B due to cloud cover. Observations were made over the interval February–July 1995 with continuous coverage except for a three‐week period in May. A wide‐angle lens video camera at the site was used for the assessment of cloud conditions at the time of each scan. Integrated UV‐B (290–320 nm) values were evaluated and a regression through the values for clear conditions was produced for the range of zenith angles from 25°–70°. This regression effectively represents an average effect of the ambient aerosols and attenuating gases and was used to normalize the UV‐B values measured under different cloud cover amounts, providing an estimate of the reduction in irradiance due to a given cloud amount as a function of solar zenith angle. For a solar zenith angle of 50°, average UV‐B transmission was observed to be 30% for overcast skies, 61% for 8–9 tenths cloud cover, 74% for 6–7 tenths cover, and 79% for the scans during 4‐5 tenths cover. A number of fractional cloud cover cases were observed where cloud reflections resulted in measured surface irradiances exceeding the expected clear sky values by as much as 11 percent.

Abstract. A field experiment was conducted in western North Carolina to investigate the relationship between aerosol optical properties and atmospheric transmission. Two research measurement sites in close horizontal proximity but at different altitudes were established to measure the transmission of UV radiation through a slab of atmosphere. An identical set of radiation sensing instruments, including a broadband UV-B radiometer, a direct Sun pyrheliometer, a shadowband radiometer, and a spectral photometer, was placed at both sites, a mountaintop site (

For the past 30 years, the stratospheric ozone layer has decreased in the Northern Hemisphere. The main effect of this ozone decrease was an expected increase in the UV radiation at the Earth's surface, but there has been no clear evidence of an increasing urban trend in surface UV. This study shows that specific air pollutants can reduce the increased surface levels of UV radiation and offers an explanation for why the expected surface UV increases have not been observed, especially in urban regions.A U.S. Environmental Protection Agency (EPA) UV monitoring site at the University of California at Riverside combined with air pollution data from a site operated by the California Air Resources Board in Rubidoux, CA, provided the basis of this study. The 1997 South Coast Ozone Study (SCOS-97) provided three key ingredients: black carbon, PM 10 concentrations, and collocated radiometric measurements. The Total Ozone Mapping Spectrometer (TOMS) satellite data were used to provide the stratospheric ozone levels that were included in the statistical model. All of these input parameters would be used to test this study's hypothesis: the expected increase of surface UV radiation, caused by decreases in stratospheric ozone, can be masked by increases in anthropogenic emissions. The values for the pollutants were 7:00 a.m.-5:00 p.m. averages of the instrument's values taken during summer 1997.A statistical linear regression model was employed using the stratospheric ozone, black carbon, PM 10 , and surface ozone concentrations, and the sin (⌰) and cos (⌰). The angle ⌰ is defined by ⌰ ϭ 2 (Julian date/365). This model obtained a coefficient of determination of 0.94 with an uncertainty level (p value) of less than 0.3% for all of the variables in the model except ground-level ozone. The final model, regressed against a data set from a remote, western North Carolina site, resulted in a coefficient of determination of 0.92. The model shows that black carbon can reduce the Diffey-weighted UV levels that reach the surface by as much as 35%, depending on the season.

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