Two decades of ozone standard exceedances in Santiago de Chile

A drastic decline of 2.4 ppbv decade(-1) in the ozone mixing ratio has been measured in Santiago de Chile during the 2000s. Subsequently, in the 2010s, ozone trends stabilized in downtown and showed upward trends in eastern Santiago. The number of days with an 8-h average ozone mixing ratio above 61 ppbv, deemed harmful to health according to Chilean legislation, has declined significantly both in western and central Santiago. However, in eastern Santiago, one finds a 2010-2018 decade average of 43 days per year above recommended levels. Also, at a Receptor Site located ~ 70 km downwind from Santiago, this number rose to up to 3 months per year. A common denominator for the last two decades has been a steady increase in both gasoline and diesel-powered private cars. In the 2010s, the ozone weekend effect was frequently noted, providing evidence that the ozone formation regime in Santiago is VOC-limited. Nitrogen oxides and carbon monoxide (a proxy of anthropogenic VOCs) have increased steadily since 2014 in a relatively constant CO-to-NOx ratio. Therefore, we propose that primary emissions of NOX and VOCs from motor vehicle exhaust have remained as the main driver of the photochemical air pollution in Santiago as well as explaining the weekly variation. Santiago, like other megacities in the world, faces several challenges associated with increasing urbanization as well as the effects of climate change. An increasing population, growth in private car use, and urban sprawl have contributed to maintain high levels of ozone. New threats such as increasing temperatures observed in the central valleys of Chile, along with more frequent occurrences of heat waves, whose number has doubled in the last decade, will require a different approach to manage ozone pollution during the next decade. Santiago will not meet its own goals in the upcoming years without implementing robust, scientifically sound, and cost-effective strategies designed specifically to tackle photochemical pollution