Preprints
https://doi.org/10.5194/acp-2021-65
https://doi.org/10.5194/acp-2021-65

  04 Feb 2021

04 Feb 2021

Review status: this preprint is currently under review for the journal ACP.

Non-linear response of PM2.5 to changes in NOx and NH3 emissions in the Po basin (Italy): consequences for air quality plans

Philippe Thunis1, Alain Clappier2, Matthias Beekmann3, Jean Philippe Putaud1, Cornelis Cuvelier4, Jessie Madrazo5, and Alexander de Meij6 Philippe Thunis et al.
  • 1European Commission, Joint Research Centre, Ispra, Italy
  • 2Université de Strasbourg, Laboratoire Image Ville Environnement, Strasbourg, France
  • 3Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Université Paris Est Créteil et Université de Paris, Institut Pierre Simon Laplace, Créteil, France
  • 4Ex European Commission, Joint Research Centre, Ispra, Italy
  • 5Signa Terre SA, Geneva, Switzerland
  • 6MetClim, Varese, Italy

Abstract. Air pollution is one of the main causes of damages to human health in Europe with an estimate of about 380 000 premature deaths per 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, showing important non-linear processes, especially those related to interactions between NOx and NH3. We analyse the sensitivities of PM2.5 to NOx and NH3 emissions by means of a set of EMEP simulations performed with different levels of emission reductions, from 25 % up to a total switch-off of those emissions. Both single and combined precursor reduction scenarios are applied to determine the most efficient emission reduction strategies and quantify the interactions between NOx and NH3 emission reductions. The results confirmed the peculiarity of secondary PM2.5 formation in the Po basin, characterised by contrasting chemical regimes within distances of few (hundreds of) kilometres, as well as strong non-linear responses to emission reductions during wintertime. One of the striking results is the increase of the PM2.5 concentration levels when NOx emission reductions are applied in NOx-rich areas, such as the surroundings of Bergamo. The increased oxidative capacity of the atmosphere is the cause of the increase of PM2.5 induced by a reduction in NOx emission. This process can have contributed to the absence of significant PM2.5 concentration decrease during the COVID-19 lockdowns in many European cities. It is important to account for this process when designing air quality plans, since it could well lead to transitionary increases in PM2.5 at some locations in winter as NOx emission reduction measures are gradually implemented. While PM2.5 responses to NOx and NH3 emission reduction show large variations seasonally and spatially, these responses remain close to linear, i.e. proportional to the emission reduction levels, at least up to −50 % because secondary aerosol formation chemical regimes are not modified by those relatively moderate ranges.

Philippe Thunis et al.

Status: open (until 01 Apr 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Philippe Thunis et al.

Philippe Thunis et al.

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Short summary
In this work, we use modelling simulations to identify the most efficient emission reduction strategies to reduce PM2.5 concentration levels in Northern Italy. Results show contrasting chemical regimes and important non-linearities during wintertime, with the striking result that PM2.5 levels may increase when NOx reductions are applied in NOx-rich areas. A process that may have contributed to the absence of significant PM2.5 decrease during the COVID-19 lockdowns in many European cities.
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