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. Air pollution is one of the main causes of damages tohuman health in Europe, with an estimate of about 380 000 premature deathsper year in the EU28, as the result of exposure to fine particulate matter(PM2.5) only. In this work, we focus on one specific region in Europe,the Po basin, a region where chemical regimes are the most complex, showingimportant non-linear processes, especially those related to interactionsbetween NOx and NH3. We analyse the sensitivity of PM2.5concentration to NOx and NH3 emissions by means of a set of EMEPmodel simulations performed with different levels of emission reductions,from 25 % up to a total switch-off of those emissions. Both single andcombined precursor reduction scenarios are applied to determine the mostefficient emission reduction strategies and quantify the interactionsbetween NOx and NH3 emission reductions. The results confirmed thepeculiarity of secondary PM2.5 formation in the Po basin, characterisedby contrasting chemical regimes within distances of a few (hundred)kilometres, as well as non-linear responses to emission reductions duringwintertime. One of the striking results is the slight increase in thePM2.5 concentration levels when NOx emission reductions areapplied in NOx-rich areas, such as the surroundings of Bergamo. Theincreased oxidative capacity of the atmosphere is the cause of the increasein PM2.5 induced by a reduction in NOx emission. This process couldhave contributed to the absence of a significant PM2.5 concentrationdecrease during the COVID-19 lockdowns in many European cities. It isimportant to account for this process when designing air quality plans,since it could well lead to transitionary increases in PM2.5 at somelocations in winter as NOx emission reduction measures are graduallyimplemented. While PM2.5 chemical regimes, determined by the relativeimportance of the NOx vs. NH3 responses to emission reductions,show large variations seasonally and spatially, they are not very sensitiveto moderate (up to 50 %–60 %) emission reductions. Beyond 25 % emissionreduction strength, responses of PM2.5 concentrations to NOx emissionreductions become non-linear in certain areas of the Po basin mainly duringwintertime.

atmospheric chemistry and physics

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