LAUSR

Abstract The closure of pipelines to tackle fuel-theft in central Mexico caused an unexpected fuel shortage that disrupted transport systems in Mexico City in January of 2019. Fuel sales in the Metropolitan Area and C O emissions from reanalysis showed a significant decrease during the fuel shortage of 7% and 6%, respectively. This study analyses the air quality and meteorological conditions during this period to evaluate whether these measures indirectly affected air quality in Mexico City. During the shortage, mean-ambient concentrations of nitric oxide (NO), nitrogen dioxide (NO2) and carbon monoxide ( C O ) were significantly lower than normal whereas levels of particulate matter (PM) were only modestly lower than usual. Daily-mean N O and C O had record-low anomalies of −10 ppb and −0.5 ppm from typical days, respectively. In contrast, ozone mean-levels were not significantly different than average. The percentage of days with PM mean concentrations above the World Health Organisation guidelines (5 and 19% for particles smaller than 2.5 and 10 μ m , respectively) and the percentage of ozone 8-h rolling means above Mexican law (0.5%) were record lows. Meteorological factors, such as wind speed or the mixed-layer height were not significantly different than average. The anomalously low pollution levels were accentuated when each day was compared to days of similar flow patterns. This episode of better than usual air quality showcases how strategies addressing transport emissions could control air quality in Mexico City and highlights that improving ozone mean levels require comprehensive strategies that reduce emissions from all sectors.