Ultraviolet Radiation Environment of a Tropical Megacity in Transition: Mexico City 2000-2019.

Tropical regions experience naturally high levels of UV radiation, but urban pollution can reduce these levels substantially. We analyzed 20 years of measurements of the UV index (UVI) at several ground-level locations in the Mexico City Metropolitan Area and compared these data with the UVI values derived from the satellite observations of ozone and clouds (but not local pollution). The ground-based measurements were systematically lower than the satellite-based estimates by ca. 40% in 2000 and 25% in 2019. Calculations with a radiative transfer model using observed concentrations of air pollutants explained well the difference between satellite and ground-based UVI and showed specific contributions from aerosols, O