Large cities can experience high levels of fine particulate matter (PM2.5) pollution linked to ammonia (NH3) mainly emitted from agricultural activities. Using a combination of PM2.5 and NH3 measurements from in situ instruments, satellite infrared spectrometers, and atmospheric model simulations, we demonstrate the role of atmospheric NH3 and meteorological conditions in pollution events occurring in Paris, Toronto, and Mexico City.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. The three regions are subject to long range transport of NH3, as shown using HYSPLIT cluster back-trajectories. The results show that the NH3 variability is mainly driven by meteorology, and interestingly, we can detect the fertilizers application period by looking at the NH3 – temperature relationship. 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. PM2.5 is generally underestimated over Paris and Mexico, but overestimated over Toronto.We use complementary information derived from IASI, and ground-based open-path measurements over Paris. We, therefore, assess the NH3 temporal variabilities at different timescales (diurnal, seasonal, and interannual), to unravel NH3 sources (agriculture and traffic) in Paris.