Aerosol Research
Aerosol Research
Aerosol Research
Articles | Volume 3, issue 1
Articles | Volume 3, issue 1
https://doi.org/10.5194/ar-3-155-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/ar-3-155-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Articles | Volume 3, issue 1
Research article
 | 
28 Mar 2025
Research article |  | 28 Mar 2025

Unchanged PM2.5 levels over Europe during COVID-19 were buffered by ammonia

Nikolaos Evangeliou, Ondřej Tichý, Marit Svendby Otervik, Sabine Eckhardt, Yves Balkanski, and Didier A. Hauglustaine

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Latest update: 28 Aug 2025
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Short summary
The COVID-19 lockdown measures in 2020 reduced emissions of various substances, improving air quality. However, PM2.5 stayed unchanged due to NH3 and related chemical transformations. Higher humidity favoured more SO42- production, as did the accumulated NH3. Excess NH3 reacted with HNO3 to make NO3-. In high-NH3 conditions such as those in 2020, a small reduction in NOx levels drove faster oxidation of NO3- and slower deposition of total inorganic NO3-, causing high secondary PM2.5.
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