Concentrations of particles with a diameter of less than 2.5 µm (PM2.5) have been decreasing in most of Europe since the turn of the century as a combined result of reductions in anthropogenic emissions of primary particles and gaseous PM2.5 precursors. Emissions of NH3 play a central role in the secondary particle formation and are also major contributors to the exceedances of critical loads for eutrophication . In most parts of Europe emissions of in particular SOx (emitted predominantly as SO2 but also as SO42-), and NOx (NO+NO2), have been steadily decreasing in the past decades. At the same time, emissions of NH3 have changed much less, decreasing in some European countries and increasing in others (see Sect. 3 and Appendix B in ). Further reductions of SOx, NOx, and NH3 emissions are required by the year 2030 according to the EU NEC2030 directive (https://www.eea.europa.eu/themes/air/air-pollution-sources-1/national-emission-ceilings, last access: 14 December 2021), but the projected percentage reductions in NH3 emissions in NEC2030 are smaller than for SOx and NOx. In the atmosphere SO2 is oxidised to SO42- and NOx to HNO3. Contrary to SOx and NOx, more than 90 % of the NH3 emissions are from agriculture, with only minor contributions from industry and traffic . As a result these emissions are in general not co-located with the SOx and NOx emissions. In addition the temporal distribution of the emissions differ, with NH3 emissions peaking in spring and summer, whereas anthropogenic SOx and NOx emissions in general peak in winter. In the fine mode, ammonium sulfate ((NH4)2SO4) particles are first formed from NH3 and SO42-. Any excess NH3 can then form ammonium nitrate (NH4NO3) in thermodynamic equilibrium with HNO3 (see e.g. ). When NH3 is in excess relative to both SO42- concentrations and the equilibrium with HNO3, the formation of NH4+ salts will slow down at some point (when there is less acid available to react with NH3), and free NH3 (NH3 in excess of SO42-) will be present. With NH3 emissions greatly exceeding SOx and NOx emissions already before 2005, one could question the effects of small or moderate reductions in NH3 emissions on secondary inorganic aerosols (SIA25), a major component in PM2.5.

Using emissions as described in , showed that the fraction of NH4+ in SIA25 was similar when calculated with 1990 versus 2030 emissions. With 1990 versus 2030 emissions, the fraction of SO42- in SIA25 dropped significantly, whereas the NO3- fraction increased, compensating for the reduction in SO42-. In many air pollution episodes in Europe involving PM2.5, NH4NO3 has accounted for a large portion of the aerosol mass . With a large surplus of NH3 relative to HNO3 it could be that NH4NO3 formation will be virtually unaffected by changes in NH3 emissions. Both NOx and NH3 are relatively short-lived, with a lifetime in the atmosphere of about 1 d . Given the difference in both spatial and temporal distribution in the sources of NH3, NOx, and SOx, substantial local variability in the ratio between NH3 on one hand and SO42- and/or HNO3 on the other hand can be expected. Thus, locally the formation of SIA25 may be limited by the availability of either NH3 or HNO3 and SO42- due to the lack of co-location in both space and time of the sources of these species.

Here we apply the EMEP MSC-W model to investigate how PM2.5 concentrations, and deposition of reduced nitrogen (NH3+NH4+), have changed from 2005 to 2017. But the main focus is on model calculations for 2030, assuming that the NEC2030 requirements will be met. Given that NH3 concentrations in Europe are generally in substantial excess of HNO3 concentrations, we explore to what extent additional reductions in NH3 emissions will contribute to further reductions in NH4+ and subsequently to reductions in PM2.5 levels, and to what extent the response to further NH3 emissions is linear. We try to answer this with a sensitivity study for PM2.5 for post NEC2030, applying additional step-wise NH3 emission reductions on top of the NEC2030 requirements holding all other emissions constant. At the same time we also investigate to what extent reductions in NH3 emissions may affect deposition of reduced nitrogen and the exceedance of critical loads for nitrogen deposition.

The model calculations have been made with the EMEP MSC-W model (hereafter “EMEP model”), version rv4.34, on 0.1×0.1∘ resolution for the domain between 30∘ W and 45∘ E and between 30 and 75∘ N. A detailed description of the EMEP model can be found in , with later model updates described in and references therein. In the EMEP model the composition of the metastable aqueous aerosols of the inorganic system containing NO3- and NH4+ and water and the system containing NH3 and HNO3 in the gas phase is calculated using the MARS equilibrium model . In the EMEP model results using the MARS model are compared to model calculations with EQUSAM4clim , giving very similar results.

The EMEP model is available as open-source code (see code availability) and is under continuous development, receiving feedback from a host of users. It is regularly evaluated against measurements (see ) for the most recent evaluations. Scatter plots of model versus measurements for the concentrations of several key species, as well as for the wet depositions of reduced and oxidised nitrogen, are shown in Appendix . The model performance is comparable for both 2005 and 2017, even though the selection of measurement sites differs for the two years. Measurements are also available for subsets of common sites for the two years, in general showing comparable biases between the model and measurements for the concentrations of key species for the years 2005 and 2017. The EMEP model has also participated in model intercomparisons and model evaluations in a number of peer-reviewed publications . In depositions of sulfur and nitrogen species in Europe have been calculated by 14 regional models and compared to measurements, and in the model-calculated trends in the wet deposition of SO42- as well as reduced and oxidised nitrogen from six models, including the EMEP model, are compared to measurements from 1990 to 2010. These two studies showed good results for the EMEP model. Out of the 14 models included in the study by , the EMEP model was one of very few with low fractional biases compared to measurements for the wet depositions of reduced nitrogen (-0.01), oxidised nitrogen (-0.05), and SO42- (-0.11). For the trend studies presented in , the fractional bias for the years 1990 to 2010 was -0.18, -0.02, and 0.22 for the wet deposition of reduced nitrogen, oxidised nitrogen, and SO42- respectively, but the overall overestimation of SO42- was mainly caused by an overestimation in the first years of the period. Running the EMEP model in global mode, showed that the model captures the overall spatial and seasonal variations well for the major inorganic pollutants NH3, NO2, SO2, HNO3, NH4+, NO3-, and SO42- and wet depositions in East Asia, Southeast Asia, Europe, and North America.

The EMEP model runs have been performed with 2017 meteorological conditions. In these model runs, emissions estimated for the years 2005 and 2017 and projected for 2030 have been used. The EMEP emissions are used as far as possible and supplemented by ECLIPSE (see below) emissions where needed. The model run with 2017 emissions is previously reported in the 2019 EMEP report . For the EU28 countries (EU28 includes the current EU27 countries and United Kingdom), the official EMEP emissions have been used for both the 2005 and 2017 model runs. For the model run with 2017 emissions, the EMEP emissions are also used for other countries and regions. The 2005 and 2017 EMEP emissions are listed in appendix B in the . For the 2030 model runs the emissions for the individual EU28 countries are scaled from the 2005 emissions according to the NEC2030 obligations. The total emissions of NH3, SOx, and NOx in the EU28 countries in 2005, 2017, and 2030 are illustrated in Fig. a. For all other countries and regions the 2005 and 2030 emissions have been provided by the International Institute for Applied Systems Analysis (IIASA) within the European FP7 project ECLIPSE (http://www.iiasa.ac.at/web/home/research/researchPrograms/air/ECLIPSEv5.html, last access: 14 December 2021). In this study we use ECLIPSE version 6a (hereafter referred to as “ECLIPSEv6a”), which is a global emission dataset widely used by the scientific community. Some of the methods used in ECLIPSEv6a are described in the recent publication of . NH3 and NOx emissions from all EU28 countries and selected non-EU European countries are listed in Table . For year 2005 non-EU28 EMEP and ECLIPSEv6a, emissions are very similar for NOx, but for NH3 the EMEP emissions are in general higher.

In order to explore the effects that further emission reductions of NH3 in 2030 may have on PM2.5 concentrations and nitrogen depositions, additional model sensitivity runs have been made. The 2030 NH3 emissions have been reduced by up to 50 % in steps of 10 %. In addition the 2030 emissions of SOx and NOx have been reduced by 10 % and the 2005 NH3 emissions by 10 %. All model runs are listed in Table .

Focusing on the effects of NH3 emissions, we have investigated how PM2.5 concentrations and depositions of reduced nitrogen will change from 2005 to 2030, assuming that the NEC2030 emission targets will be met. In addition, we have made a sensitivity study for PM2.5 for post NEC2030, assuming additional emission reductions on top of the NEC2030 requirements.

Emissions of SOx and NOx have decreased in Europe from the year 2005 to present, and further emissions reductions are expected by the year 2030. However, NH3 emissions have so far remained high, and projected NEC2030 emission reductions of NH3 are much smaller than for SOx and NOx. Our model calculations show that these differences in emission trends lead to a smaller fraction of the emitted NH3 being converted to NH4+ and an increasingly larger portion of free NH3 versus NH4+ in the atmosphere in Europe. Based on 10 % emission reductions of NH3, NOx, and SOx, we calculate that the potential for PM2.5 formation per gigagram of NH3 emitted is expected to drop by a factor of about 2.6 as an annual average between 2005 and 2030. Over the same time span the potential for forming PM2.5 from NOx per gigagram emitted has increased by a factor of 1.8 and from SOx by a factor of 1.6 per gigagram emitted.

In winter, with low NH3 emissions and relatively higher NOx and SOx emissions, the ratio of NH3 to HNO3 and SO42- is higher, and a larger portion of the emitted NH3 will form particulate NH4+. Also, the formation of NH4NO3 in equilibrium with HNO3 and NH3 is favoured by low temperatures. As a result we find that in winter the effects of further reductions in NH3 emissions are larger than in other seasons and comparable to additional reductions in SOx and NOx emissions. This is in agreement with the findings in , pointing out that even though the NH3 emissions are highest in spring and summer due to the application of manure on the fields, emission reductions in winter have a stronger impact on the formation of secondary aerosols than in any other season. Furthermore they stated that the potential of reducing NH3 emissions in winter is highest through the reduction of animal farming, as this source accounts for about 80 % of the NH3 emissions in the autumn and winter months.

Following the emission reductions of NH3, deposition of reduced nitrogen is decreasing in Europe. However, the reductions in NOx emissions are much larger than for NH3, resulting in a much faster decline in oxidised nitrogen deposition compared to reduced nitrogen. Thus the fraction of reduced over total deposition of nitrogen is increasing and is expected to reach more than 60 % in large parts of Europe by the year 2030. Our calculations show that with the existing emission projections the CL for nitrogen will also be exceeded in large parts of Europe in 2030. There are also indications that reduced nitrogen inputs are more effective in decreasing biodiversity than oxidised nitrogen, as reduced nitrogen is more readily available, stimulating growth of specific plants at the expense others (see , and , and references therein). Furthermore they also suggest that increased levels of NH4+ can be toxic to plants (see also ).

Reducing, and preferably removing, these exceedances will require larger reduction in nitrogen emissions than currently projected. Given that reduced nitrogen is responsible for the major fraction of nitrogen depositions, the largest cuts should be made in the NH3 emissions.

As discussed in , gas–aerosol partitioning of total reduced and oxidised nitrogen is affected by aerosol pH level and water content, so that low (high) pH is favourable for NH4+ (NO3-) formation. The increase in the aerosol fraction in total reduced and oxidised nitrogen would lead to changes in their dry deposition and subsequently their residence times and transport distances. This effect has not been accounted for in the EMEP model. Thus some limited local effects might have been missed in our model simulations. For instance, based on the results, there may be additional NO3- formation in areas with low acidity, such as coastal or dusty regions. Potentially this may reduce the deposition of total nitrate near these local sources, somewhat enhancing the accumulation of particles. Furthermore, as future emissions of SOx and NOx are expected to decrease, the pH of the particles is likely to increase, potentially favouring NO3- formation and thus decreasing dry deposition and increasing the transport distances of oxidised and thereby total nitrogen in some regions. On the other hand, our results show that overall, the fraction of reduced nitrogen in the total nitrogen has been increasing, and this increase is expected to continue until 2030. Assuming that the deposition rates for total nitrogen are mostly driven by those of reduced nitrogen (following ), the local effects of NO3- formation bursts would probably not play a major role across the regions in different present and future chemical regimes. Therefore we believe that overall the main conclusions presented in our paper remain valid.

For many countries the latest source-oriented legislation may potentially reduce the emissions of SOx and NOx below their emission reduction requirements, and as a result the EU28 as a whole could be on track to overshoot the reduction requirements for these species by 2030. But for NH3 further efforts are needed in order to meet the 2030 commitments for many countries in Europe . Cost-effective measures to further reduce NH3 emissions differ among various parts of Europe. According to the damage cost estimate of EUR 17.50 per kilogram of NH3 emitted is much higher than the average abatement costs. Also, find that much more ambitious commitments for NH3 emission reductions could be applied by EU28 countries with relatively minimal costs. According to , low-emission animal housing would be the least cost-effective measure, but still a quarter of the costs of the avoided damage. However, this measure could be the most effective one to reduce emissions in winter, when PM2.5 health-related limits are the most likely to be exceeded in Europe.