Maïdo observatory (21.0792°S, 55.38°E) lies at an altitude of 2160 masl on the western flank of La Réunion Island (see Figure ), a relatively small tropical island in the Southern Hemisphere Indian ocean. GEM concentrations at Maïdo are mostly below 1 ng m -3 , but show a marked and regular diurnal variation that has been attributed to mixing processes and surface photo-reemission .