Comparison of satellite and ground-based measurements of erythemal dose from two tropical sites: Belo Horizonte (20° S, 44° W, 858 m) and Saint-Denis La Reunion (21° S, 55° E, 85 m)

The ultraviolet radiation (UVR) from the Sun on the Earth surface depends on the altitude and surface albedo of the site, solar zenith angle (SZA), attenuation in the atmosphere (absorption by ozone and other trace gases, molecular scattering, scattering by clouds, and scattering and absorption by aerosols). Belo Horizonte (BH, 20° S, 44° W, 858 m a.s.l.) and Saint-Denis La Reunion (SDR, 21° S, 55° E, 85 m a.s.l.) are two very-similar-latitude tropical sites to which the incidence of solar UVR is compared. Despite the statistically significant difference in both altitude and total ozone column (TOC), where SDR has lower altitude and higher TOC, the annual averages of climatological daily erythemal dose (DED) from the Ozone Monitoring Instrument (OMI) for the sites show only -1% difference ( \(\left(SDR-BH\right)\cdot 100\%/BH\) $\left(S,D,R,-,B,H\right)·100\%/BH$ ): 4486 ± 1621 (1 standard deviation) J m⁻² for BH and 4434 ± 1687 J m⁻² for SDR. Equivalent ground-based data are 13% and 11% ( \(\left(OMI-site\right)\bullet 100\%/site\) $\left(O,M,I,-,s,i,t,e\right)\bullet 100\%/site$ ), respectively, lower: 3976 ± 1072 J m⁻² for BH and 4012 ± 1199 J m⁻² for SDR with 1% difference between them. The comparison between the sites of atmospheric geophysical parameters of TOC, cloud fraction (CF), and UV aerosol index (AI) from OMI indicates clouds and aerosols as the main culprit to such a small difference. After clouds, aerosols play an important role, as BH is a metropolis polluted mainly by by-products from fossil-fuel and ethanol combustion and smoke from surrounding biomass burning sources, while SDR is virtually a pristine seashore locality affected by marine and long-distance biomass burning aerosols.