Daily Maximum Erythemal Dose Rates in the Tropics

The maximum erythemal dose rate (EDR_max ) at the Earth's surface tends to occur at local noon. However, clouds can make the timing of EDR_max significantly away from local noon. In fact, EDR_max and its time of occurrence depend mainly on the solar zenith angle at noon (SZA_n ), site's altitude, the total ozone column (TOC), cloud cover, cloud genera and aerosols. This work depicts the daily incidence of EDR_max for Belo Horizonte (19.92°S, 43.94°W, 858 m a.s.l.) in the Southern Hemisphere tropics for a period of five years (2005-2010). Daily values of EDR_max ranged from 0.063 W m^-2 (1σ > 6.9%, Moderate UV-Index of 3, winter) to 0.486 W m^-2 (Extreme UVI of 19, summer). Indicative values of EDR_max for cloudless days were 0.336 W m^-2 (summer, TOC = 258 Dobson Units), 0.311 W m^-2 (fall, 260 DU), 0.253 W m^-2 (spring, 274 DU) and 0.143 W m^-2 (winter, 246 DU). Radiation enhancement events by clouds made EDR_max up to 45% higher than the reference EDR_max for cloudless summer skies at a time resolution of a few minutes. The main cloud genera to be associated with such events are Cumulus, Altocumulus, Altostratus and Stratocumulus. The EDR can also be significantly affected by aerosols, which attenuated on average 0.031 W m^-2 (22%) of the erythemal UV in a case study at the site.