A Multiscale Analysis of the Tropospheric and Stratospheric Mechanisms Leading to the March 2016 Extreme Surface Ozone Event in Mexico City

This study analyzed the physical mechanisms behind a multiday high surface ozone (O-3) event in Mexico City in March 2016. In early March, a strong zonal jet stream over the Pacific Ocean amplified and underwent wave breaking. An unusual cutoff low pressure system then migrated across central Mexico, with 200-hPa geopotential heights among the lowest in the reanalysis historical record (1948 to present). A tropopause fold on the west side of the cutoff low transported O-3-rich air from the stratosphere into the troposphere and resulted in high O-3 concentrations all the way to the surface. The stratospheric intrusion began on 09 March and ended on 12 March, but surface O-3 concentrations continued to rise, peaking on 14 March. Given the deep boundary layer observed on 12 March, it is very likely that remnants of the stratospheric O-3 in the midtroposphere at the regional level could have been entrained into the boundary layer. Furthermore, our analysis indicates that as the cutoff low progressed eastward, the pronounced low-level thermal inversion and reduced ventilation observed on subsequent days (13-15 March) contributed to elevated concentrations of O-3 precursors, which together with strong UV radiation, led to efficient photochemical production of O-3 and to the very large values observed in several of the monitoring stations (up to 210 parts per billion on 14 March). Finally, mitigation measures put into place by authorities reduced both the number of vehicles on the road and the resulting nighttime titration, possibly extending the duration of dangerous O-3 levels in the city.