Non-linear response of PM<sub>2.5</sub> to changes in NO<sub><i>x</i></sub> and NH<sub>3</sub> emissions in the Po basin (Italy): consequences for air quality plans

Abstract. Air pollution is one of the main causes damages to human health in Europe, with an estimate about 380 000 premature deaths per year EU28, as result exposure fine particulate matter (PM2.5) only. In this work, we focus on specific region Po basin, a where chemical regimes are most complex, showing important non-linear processes, especially those related interactions between NOx and NH3. We analyse sensitivity PM2.5 concentration NH3 emissions by means set EMEP model simulations performed different levels emission reductions, from 25 % up total switch-off emissions. Both single combined precursor reduction scenarios applied determine efficient strategies quantify reductions. The results confirmed peculiarity secondary formation characterised contrasting within distances few (hundred) kilometres, well responses reductions during wintertime. One striking slight increase when NOx-rich areas, such surroundings Bergamo. increased oxidative capacity atmosphere cause induced emission. This process could have contributed absence significant decrease COVID-19 lockdowns many European cities. It account for designing air quality plans, since it lead transitionary increases at some locations winter measures gradually implemented. While regimes, determined relative importance vs. show large variations seasonally spatially, they not very sensitive moderate (up 50 %–60 %) Beyond strength, concentrations become certain areas basin mainly