Urban Air Quality: Revisiting the Case of Mexico City

Mexico-City has been prone to severe air pollution episodes in the late 20^th century. Since then air quality conditions improved significantly. However, in March 2016 Mexico-City experienced its most severe smog episode for almost a decade. The Metropolitan Index of Air Quality (IMECA) for Mexico-City surpassed the value of 200, indicating an extremely bad situation. Hourly peak values for both, NO₂ and O₃, exceeded 200 ppb, while for CO more than 2 ppm were observed. Restrictions on traffic and industrial activities, among other emergency measures, were imposed. What conditions led to this unexpected pollution event? This presentation will provide a historical review of past air quality conditions in Mexico-City and will then describe the anatomy of the 2016 severe air quality episode, its specific synoptic and boundary layer environment favoring this episode as well as the photochemical conditions in the basin of Mexico-City. This study presents temporally highly resolved boundary layer heights (CBL, RL, SBL) at a monitoring site in Mexico City in conjunction with air quality data within the basin and surrounding elevated sites. Those elevated sites are a recent addition to the air quality network in Mexico City. They are able to measure continuously the atmospheric conditions representative for inflow/background conditions, but also nighttime conditions representative for the RL environment. The partitioning of O_x indicates significant photochemical production within the basin with O_x increasing about 100 ppb from the surrounding areas toward the inner area of the basin. Background and RL variations of O_x vary little during this episode (increase by about 15-20 ppb). On top of this photochemical environment strong boundary layer variations in combination with recirculation critically influence the air quality in Mexico City.