Total ozone column, water vapour and aerosol effects on erythemal and global solar irradiance in Marsaxlokk, Malta

Abstract

Observations of erythemal (UVER; 280-400nm) and total solar shortwave irradiance (SW; 305-2800nm), total ozone column (TOC), water vapour column (w), aerosol optical depth (AOD) and Ångström exponent (α) were carried out at Marsaxlokk, in south-east Malta. These measurements were recorded during a measurement campaign between May and October 2012, aimed at studying the influence of atmospheric compounds on solar radiation transfer through the atmosphere. The effects of TOC, AOD and w on UVER and SW (global, diffuse and direct) irradiance were quantified using irradiance values under cloud-free conditions at different fixed solar zenith angles (SZA). Results show that UVER (but not SW) irradiance correlates well with TOC. UVER variations ranged between-0.24%DU-1 and-0.32%DU-1 with all changes being statistically significant. Global SW irradiance varies with water vapour column between-2.44%cm-1 and-4.53%cm-1, these results proving statistically significant and diminishing when SZA increases. The irradiance variations range between 42.15%cm-1 and 20.30%cm-1 for diffuse SW when SZA varies between 20° and 70°. The effect of aerosols on global UVER is stronger than on global SW. Aerosols cause a UVER reduction of between 28.12% and 52.41% and a global SW reduction between 13.46% and 41.41% per AOD550 unit. Empirical results show that solar position plays a determinant role, that there is a negligible effect of ozone on SW radiation, and stronger attenuation by aerosol particles in UVER radiation.