A case study of the impact of boundary layer aerosol size distribution on the surface UV irradiance

The relationship between scattering characteristics of surface aerosol and surface UV irradiance was examined on the basis of the measurements carried out in June–August 1999 in Pärnu, Estonia on the Eastern coast of the Baltic Sea (58°22′27″N, 24°30′43″E). The UV radiation spectra (300–340 nm) were measured with the Ocean Optics Inc. UV spectrometer PC1000, the aerosol size distributions (3–10 000 nm) were measured with the electric aerosol spectrometer EAS. A case study was conducted for six sequential cloudless days, when the decrease of the surface UV irradiance was seemingly influenced by atmospheric aerosol. Aerosol radiative properties were calculated from the measured size distributions that represented the maritime polar (North Atlantic) and mixed maritime–continental air. The aerosol optical depths at 500 nm for the North Atlantic air were estimated to be from 0.08 to 0.13. The spectral aerosol optical depth agreed well with the Ångström law, the Ångström exponent α varied from day to day between values of 0.52–0.90. Aerosol asymmetry factor at 300 nm changed between values of 0.76 and 0.80, and was highly correlated with the mean radius of aerosol number distribution. The total aerosol UV scattering was mostly influenced by changes in aerosol with a diameter of 100–560 nm. The aerosol scattering coefficients were positively correlated with the relative humidity of air. The ground aerosol properties were used for calculating the surface UV irradiance from the radiative transfer model of Bird and Riordan (J. Climate Appl. Meteorol. 25 (1986)). The calculated UV irradiances correlated quite well with the measured ones, showing that the use of ground aerosol data for radiative transfer calculations turned out reasonable results. However, ignoring the changes in the aerosol vertical distribution resulted in overestimation of aerosol optical depth on hazy days.