The coronavirus outbreak in 2020 had devastating impact on human life, albeit a positive effect for the environment reducing emissions of primary aerosols and trace gases and improving air quality. In this paper, we present inverse modelling estimates of ammonia emissions during the European lockdowns of 2020 based on satellite observations. Ammonia has a strong seasonal cycle and mainly originates from agriculture. We further show how changes in ammonia levels over Europe, in conjunction with decreases in traffic-related atmospheric constituents modulated PM2.5. The key result of this study is a -9.8% decrease in emissions in the first half of 2020 compared to the same period in 2016–2019 attributed to restrictions related to the global pandemic. We further calculate the delay in the evolution of the emissions in 2020 before, during and after lockdowns, by a sophisticated comparison of the evolution of ammonia emissions during the same time periods for the reference years (2016–2019). Our analysis demonstrates a clear delay in the evolution of ammonia emissions of -77 kt, that was mainly observed in the countries that suffered the strictest travel, social and working measures. Despite the general drop in emissions during the first half of 2020 and the delay in the evolution of the emissions during the lockdown period, satellite and ground-based observations showed that European levels of ammonia increased. On one hand, this was due to the reduction of and (precursors of the atmospheric acids with which ammonia reacts) that caused less binding and thus less chemical removal of ammonia (smaller loss – higher lifetime); on the other, the majority of the emissions persisted, because ammonia mainly originates from agriculture, a primary production sector that was not influenced by the lockdown restrictions, as practically agricultural activity never ceased. Despite the projected drop in various atmospheric aerosols and trace gases, PM2.5 levels stayed unchanged or even increased in Europe due to a number of reasons attributed to the complicated system. Higher water vapour during the European lockdowns favoured more sulfate production from and  (gas phase) or (aqueous phase). Ammonia first neutralised sulfuric acid (due to higher atmospheric abundance) also producing sulfate. Then, the continuously accumulating free ammonia reacted with nitric acid shifting the equilibrium reaction towards particulate nitrate. In high free ammonia atmospheric conditions such as those in Europe during the 2020 lockdowns, a small reduction of levels drives faster oxidation toward nitrate and slower deposition of total inorganic nitrate causing high secondary PM2.5 levels.

- FLEXPART v10.4 sensitivities for 6 model levels using chemistry loss after taking into account changes in emissions of NOx and SO2 in the calculations (see also associated paper): AVE_FOOTPRINT_L1_chem.nc, AVE_FOOTPRINT_L2_chem.nc, AVE_FOOTPRINT_L3_chem.nc, AVE_FOOTPRINT_L4_chem.nc, AVE_FOOTPRINT_L5_chem.nc, AVE_FOOTPRINT_L6_chem.nc

- FLEXPART v10.4 sensitivities for 6 model levels using chemistry loss after taking into account changes in emissions of NOx and SO2 in the calculations (see also associated paper): AVE_FOOTPRINT_L1_nochem.nc, AVE_FOOTPRINT_L2_nochem.nc, AVE_FOOTPRINT_L3_nochem.nc, AVE_FOOTPRINT_L4_nochem.nc, AVE_FOOTPRINT_L5_nochem.nc, AVE_FOOTPRINT_L6_nochem.nc

- Different surrounding estimates of posterior emissions covering 2°, 3° and 4° from each grid-cell: 2deg.tar.gz, 3deg.tar.gz, 4deg.tar.gz

- Performance calculations for the prior and posterior emissions against the dependent observations (CrIS): Correction_R4.tar.gz

- Forward simulations with FLEXPART v10.4 using prior and posterior emissions that were used for validation against independent (observations that were excluded from the inversion) observations from the EMEP network: FWD_pri.tar.gz, FWD_pos.tar.gz