资源环境科技发展态势分析平台

Abstract. Megacities can experience high levels of fine particulate matter (PM_2.5) pollution linked to ammonia (NH₃) mainly emitted from agricultural activities. Here, we investigate such pollution in the cities of Paris, Mexico and Toronto, each of which have distinct emission sources, agricultural regulations, and topography. Ten years of measurements from the Infrared Atmospheric Sounding Interferometer (IASI) are used to assess the spatio-temporal NH₃ variability over and around the three cities.

In Europe and North America, we determine that temperature is associated with the increase in NH₃ atmospheric concentrations with coefficient of determination (r²) of 0.8 over agricultural areas. The variety of the NH₃ sources (industry and agricultural) and the weaker temperature seasonal cycle in southern North America induce a lower correlation factor (r² = 0.5). The three regions are subject to long range transport of NH₃, as shown using HYSPLIT cluster back-trajectories. The highest NH₃ concentrations measured at the city scales are associated with air masses coming from the surrounding and north-northeast regions of Paris, the south-southwest areas of Toronto, and the southeast/southwest zones of Mexico City.

Using NH₃ and PM_2.5 measurements derived from IASI and surface observations from 2008 to 2017, annually frequent pollution events are identified in the 3 cities. Wind roses reveal statistical patterns during these pollution events with dominant northeast-southwest directions in Paris and Mexico cities, and the transboundary transport of pollutants from the United-States in Toronto. To check how well chemistry transport models perform during pollution events, we evaluate simulations made using the GEOS-Chem model for March 2011. In these simulations we find that NH₃ concentrations are overall underestimated, though day-to-day variability is well represented. PM_2.5 is generally underestimated over Paris and Mexico, but overestimated over Toronto.