Ultraviolet Radiation and its Influencing Factors at Barrow, Alaska, and Summit, Greenland

The National Science Foundation's Ultraviolet Spectral Irradiance Monitoring Network has measured ultraviolet (UV) and visible solar irradiance at Barrow, Alaska, and Summit, Greenland since 1991 and 2004, respectively. Instruments at both locations are now part of the U.S. Arctic Observing Network. Data have been used to establish the relationship of UV irradiance on its influencing factors, namely solar elevation, total ozone, clouds, Rayleigh scattering, surface albedo (e.g., snow cover, sea ice extent), and aerosols. At Barrow, the annual cycle in total ozone leads to a ±25% variation in the UV Index. Spikes in UV are frequently being observed in spring and are linked to episodes of photochemically-induced ozone depletion. High albedo in spring due to snow and sea ice cover increases the UV Index by 40-55% compared to summer when albedo is very low. The albedo in May and June is greatly affected by the date of snow melt and ranges between 0.2 and 0.8. Aerosols reduce the UV Index by typically 3-5%. Aerosol effects are largest during the spring due to 'Arctic haze' episodes. The effect of clouds is much larger during the summer and fall, partly because of the increased frequency of storms. Results indicate that the annual ozone cycle is overcompensated by the effects of albedo and clouds, highlighting that changes in snow fall, sea ice extent, and atmospheric circulation patterns may have a profound effect on the future Arctic UV climate. Measurements at Barrow were also compared with UV levels of the 1970s and 1980s, estimated from historical measurements of total ozone using model calculations. For the months February through April, the average increase over the last 35 years is 4-7%. Measurements in spring at Barrow are on average 14% smaller than at Summit. The disparity is mostly caused by differences in altitude and albedo of the two sites, but model studies indicate that differences in atmospheric aerosol loading are also a contributing factor. During summer, when the surface at Barrow is snow-free and the effect of clouds is more pronounced, UV measurements at Summit exceed measurements at Barrow by about 56%.