Effect of Aerosols on Ultraviolet and Visible Spectral Irradiance at Barrow, Alaska

Solar ultraviolet (UV) and visible irradiance has been measured with a spectroradiometer at Barrow, Alaska, (71° N) since 1991. The instrument is part of the National Science Foundation's Arctic Observing Network and measures downwelling spectral irradiance between 280 and 600 nm with a resolution of 1 nm. Data are correlated with aerosol optical depth (AOD) measurements performed with a Carter-Scott sunphotometer that is operated by NOAA's Global Monitoring Division (GMD) at Barrow. In addition, data from a Cimel sunphotometer are used. This instrument is part of NASA's AERONET network and located at Barrow's Atmospheric Radiation Measurement (ARM) Climate Research Facility operated by the U.S. Department of Energy (DOE). Intensive aerosol properties such as aerosol single scattering albedo and asymmetry parameter are used from AERONET almucantar retrievals and in-situ measurements provided by GMD. Sources of aerosols for times of high AOD are determined with back-trajectory calculations. The effect of the various aerosol parameters on cloud-free irradiance measurements is quantified with radiative transfer calculations using the libRadtran/UVSPEC model. Results indicate that aerosols reduce UV and visible irradiance by typically 5% with maximum reductions of up to 20%. The aerosol effect is more pronounced in spring when Arctic haze events are frequent.