Introduction

Nitrogen deposition, an important component of the global nitrogen cycle, has increased twofold over the past 100 years globally1 and by 60% from 1980 to 2010 in China2, as a result of reactive nitrogen emissions associated with rapid expansion in agriculture and increased fossil fuel combustion3. These reactive nitrogen species include reduced (NHx = NH3 + NH4+) and oxidized (NOy = NOx (NO + NO2) + its oxidation products) compounds. Given the continual reductions in NOx emissions, NHx deposition has become increasingly important, with its contribution currently reaching 60–85% of the total nitrogen deposition in the United States and Europe4,5,6. Enhanced NHx deposition is favorable for crop production and plant growth in nitrogen-limited regions7, whereas excess NHx deposition causes greater damage to nitrogen-sensitive ecosystems, including global biodiversity loss and eutrophication, compared with NOy deposition8.

The removal of NHx from the atmosphere can occur by dry or wet processes1,9,10. Wet deposition occurs mainly through scavenging by rainfall, sleet, fog, snow, and hail2,11, while dry deposition occurs by vertical turbulent transfer to the earth’s surface8,9. Dry NHx deposition (often dominated by gaseous NH3) is a continuous process that drives plant species composition change and reduces species cover and diversity much faster, and inflicts greater threats to plant growth compared to the same unit of wet NHx deposition12,13. Previous studies have characterized the spatiotemporal variation and ecological effects of NHx deposition; however, the main focus of these studies has been on wet NHx deposition because it is relatively easy to measure in existing networks2,11, whereas long-term and large-scale dry NHx deposition has been less studied due to the technical difficulty in directly measuring and monitoring dry deposition. It has been reported that dry NHx deposition has been a critical component of total NHx deposition in recent years14,15. Based on a combination of historical measurement-based wet NHx deposition and model-based dry NHx deposition data, Yu et al.16 suggested that there has been a shift in the relative contributions of wet and dry deposition to total NHx deposition in China from 1980 to 2015; wet deposition previously dominated total NHx deposition, but the modeled ratio of dry to wet NHx deposition (\({R}_{{\rm{NH}}x({\rm{dry/wet}})}\)) gradually rose from 1980 to 2015, to the point where wet and dry deposition now have an approximately equal contribution to total NHx deposition (Fig. 1). Thus, there is an urgent need to better understand why dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) have increased in China during the past decades according to the modeled deposition, especially when NH3 emissions have remained stable since 200516.

Fig. 1: Temporal trends in NHx deposition and precursor emissions over China from 1980 to 2015.
figure 1

a Deposition flux (kg N ha−1 yr−1) and the ratio of dry to wet NHx deposition (\({R}_{{\rm{NH}}x({\rm{dry/wet}})}\)). b Anthropogenic emissions of SO2, NOx, and NH3 (Tg yr−1) (data obtained from Yu et al.16); circles represent the average modeled dry deposition, measured wet deposition, total deposition (dry + wet), and emissions for a given time period; the dashed lines are a linear fit to the points; gray bars are \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) and are plotted on the right-hand axis; error bars indicate standard deviation (s.d.).

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The major factors that control the variation of NHx deposition include emissions, meteorology, and land cover9,17,18. Chemical transport models (CTMs) have been widely used to investigate the relative impact of these factors on nitrogen deposition. The results suggest that the long-term trends of NHx deposition depend to a large extent on emissions and meteorology on regional and global scales, while land use and land cover are important for the spatial distribution of NHx deposition19,20,21,22. Specifically, variations in meteorological variables (e.g., precipitation, surface air temperature, wind speed, solar radiation, and atmospheric stability) driven by global climate change are closely related to the partitioning of dry and wet NHx deposition, although they play only a minor role (less than 20%) in controlling the long-term trends of total nitrogen deposition19,20,21,22. Precipitation, in particular, controls the partitioning of dry and wet NHx deposition due to its role in the wet scavenging process. Regardless of the importance of precipitation amount in the partitioning of dry and wet NHx deposition18,23, regional and global annual precipitation amounts have not shown a statistically significant trend in the past decades24 and accordingly cannot explain the decadal-scale variation of the modeled partitioning of dry and wet NHx deposition. Precipitation patterns (e.g., occurrence, intensity, and duration), however, have changed significantly around the world. Specifically, extreme and consecutive rain events have intensified in the past 100 years25,26 and are projected to intensify further in the future around the world27,28. The change in the precipitation patterns can be mainly attributed to the increased moisture in the atmosphere due to global warming and persistent low pressure due to the slowing of atmospheric circulation29,30. Nevertheless, the influence of changing precipitation patterns on the partitioning of dry and wet NHx deposition remains poorly understood19,20,23,31,32,33.

In this work, we address the following questions: how do the variations in climate change-induced precipitation patterns contribute to the continual increase in dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) in China15 ? Is the importance of climate change-induced precipitation patterns for the partitioning of dry and wet NHx deposition specific to China, or is it a ubiquitous phenomenon that also occurs in other regions worldwide? We first establish a linear emission-deposition response relationship based on CTM (WRF-EMEP) simulations to estimate the contribution of precursor emission variation to the change in dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\). The change in dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), which cannot be explained by precursor emissions, suggests the effect of the interdecadal variability of precipitation patterns driven by climate change. We further explore the potential impact of future climate change on the partitioning of dry and wet NHx deposition by relying on prediction equations established through historical data and predicted data under two future climate change scenarios, representative concentration pathway (RCP) 4.5 and 8.5, for the period of 2030–2100. Finally, we broaden our analysis to include the United States (USA) and Europe, the other two hotspots of NHx deposition, to assess the universality of the increasing importance of dry NHx deposition. The purpose of this study is to advance our understanding of the partitioning of dry and wet NHx deposition driven by global climate change to date and in the future, which will have important implications for assessing ecological impacts and formulating policy to reduce NH3 emissions in the face of climate change.

Results

Historical perspective of NHx deposition

Historical NHx deposition data based on a combination of measured wet NHx deposition and modeled dry NHx deposition15 (see “Methods” section for detailed information regarding data sources and methodology) for the period of 1980–2015 indicate that the total NHx deposition in China increased considerably at the rate of 0.188 kg N ha−1 yr−2 (1.77% yr−1, p < 0.05) before 2005 and showed a slight decreasing trend (0.112 kg N ha−1 yr−2, 0.80% yr−1) after 2005 (Fig. 1). On average, the total NHx deposition was estimated at 12.7 ± 1.1 kg N ha−1 yr−1 in China from 1980 to 2015. In general, the variation in total NHx deposition was consistent and in line with the trends of NH3 emissions, with a significant positive correlation observed between them (r = 0.95, p < 0.05).

With respect to the forms of NHx deposition, variation in wet NHx deposition was consistent with total NHx deposition, but the growth rate for wet NHx deposition (0.055 kg N ha−1 yr−2, 0.74% yr−1) was lower than that of total NHx deposition before 2005, while the corresponding rate of decrease (0.258 kg N ha−1 yr−2, 3.07% yr−1) was relatively higher after 2005. In contrast, modeled dry NHx deposition was on the rise during the study period, at the rate of 0.143 kg N ha−1 yr−2 (4.50% yr−1, p < 0.05), leading to an upward trend for modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) (ratio of dry to wet NHx deposition) at the rate of 0.026 per year (6.21% yr−1, p < 0.05). These results indicate a shift in the relative contributions of wet and dry deposition to total NHx deposition, with modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) increasing from 0.42 (1980s) to 1.20 (2010s), implying that China has gone from having wet deposition as the dominant contributor to total NHx deposition to having approximately equal dry and wet NHx deposition for the last two decades.

Variation in deposition velocity is not responsible for the partitioning of NHx deposition

The dry deposition flux is calculated as the product of ambient concentration and vertical deposition velocity (\({V}_{d}\))5 (see detailed information in Supplementary Note 1); ambient concentration depends heavily on precursor emissions while \({V}_{d}\) relies heavily on meteorological conditions, surface characteristics, and the pollution regime34,35,36. The parameterization of gaseous \({V}_{d}\) utilizes a resistance approach analogous to Ohm’s law in electrical circuits34,35,36 (see detailed information in Supplementary Note 1). The resistance includes aerodynamic resistance (\({R}_{a}\)), quasi-laminar sublayer resistance (\({R}_{b}\)) and canopy resistance (\({R}_{c}\)), among which, \({R}_{c}\) is generally the most dynamic and difficult to estimate37,38,39,40. Furthermore, \({R}_{c}\) is dominant in the deposition process as it is typically the largest in magnitude among the three resistances40. Non-stomatal resistance (\({R}_{{ns}}\)) is one of the most important terms of \({R}_{c}\) for gaseous NH3 \({V}_{d}\) since non-stomatal uptake is the dominant pathway for highly water-soluble gaseous NH337,38. \({R}_{{ns}}\), is mostly influenced by surface temperature, relative humidity, and the presence of other trace substances (i.e., SO2 and NH3)38,39. According to the work of Simpson et al.39, \({R}_{{ns}}\) was calculated in this study (see Supplementary Eqs. (3 and 4)). Stomatal resistance, \({R}_{{ns}}\), did not change significantly during the study period (Supplementary Fig. 1), although surface temperature (Supplementary Fig. 2) and precursor emissions (Fig. 1b) changed substantially in the past few decades. Further evidence provided by the WRF-EMEP model also revealed that the annual average \({V}_{d}\) for gaseous NH3 and particulate NH4+ had no obvious change between 2010 and 2017 (Supplementary Fig. 3, see detailed information in Supplementary Note 5). The results suggest that \({V}_{d}\) affected by anthropogenic emissions and meteorology was not responsible for the partitioning of dry and wet NHx deposition.

Variation in emissions is not a dominant factor affecting the partitioning of NHx deposition

Among the precursor emissions, NH3 emissions are the primary driver determining the magnitude and trends of NHx deposition. On the other hand, emissions of the acidic gases SO2 and NOx can affect the equilibrium between gaseous NH3 and particulate NH4+ through the variation of aerosol acidity. The variation in aerosol acidity can, in turn, influence the partitioning of dry and wet NHx deposition11,14. In order to quantify the contribution of variations in precursor emissions to the change in modeled dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), we first establish a linear emission–deposition response relationship based on CTM (WRF-EMEP) simulations (see “Methods” section for detailed information). As a result, we estimated that the changes in precursor emission amounts during 1985–2015, with values of +5.4 (+73% yr−1), +10.3 (+62% yr−1), and +13.2 (298% yr−1) Tg yr−1 for NH3, SO2, and NOx, respectively, resulted in changes of +1.00 kg N ha−1 yr−1 and +0.03 for the variation in modeled dry NHx deposition (ΔDry) and modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) (ΔRatio), respectively. To summarize, changes in precursor emissions explained 26% of the increase in modeled dry NHx deposition and 4% of the increase in modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), with an overall variation of +3.85 kg N ha−1 yr−1 and +0.78 for ΔDry and ΔRatio, respectively, suggesting that variation in anthropogenic emissions is not a dominant factor affecting the partition of dry and wet NHx deposition from the perspective of long-term trends. This finding is also confirmed by the spatial analysis of NHx deposition (Supplementary Fig. 4) and anthropogenic emissions (Supplementary Fig. 5), which corroborate the finding from the temporal analysis that variation in anthropogenic emission is not a dominant factor affecting the partition of dry and wet NHx deposition (Supplementary Note 3).

Effect of climate change-induced precipitation patterns on the partitioning of NHx deposition

It is widely accepted that precipitation amount is the most important meteorological factor affecting the magnitude of NHx deposition flux18,41. However, there has been no significant variation in the long-term annual accumulated precipitation amount in China during the past few decades, with an average value of 1009–1047 mm yr−1 (Supplementary Fig. 6a). In addition, there has been no statistically significant correlation between precipitation amount and modeled dry NHx deposition, modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), suggesting that interdecadal variability in precipitation amount is not sufficient to explain the upward trends in modeled dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\). However, precipitation patterns (e.g., intensity and duration) have changed significantly around the world despite stable precipitation amounts. In order to characterize rain intensity and rain event duration quantitatively, rain intensity was classified as light rain (0.1–10 mm d−1) and moderate rain or above (≥10 mm d−1). Based on rainfall duration, a rain event was classified as a consecutive rain event (with two or more consecutive rainy days) or a single rain event.

As shown in Supplementary Fig. 6b, c, the frequency of light rain decreased significantly while upward trends were observed for consecutive rain events in China during 1985–2015 under the effect of climate change. Moreover, the frequency of light rain was negatively correlated with modeled dry NHx deposition (r = −0.73) and modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) (r = −0.90), and the occurrence of consecutive rain events was positively correlated with modeled dry NHx deposition (r = 0.55) and modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) (r = 0.70), indicating that rain intensity and rain event duration may be responsible for the upward trends in modeled dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) in China.

Previous studies have revealed that owing to global climate change, variation in precipitation patterns plays a more important role than the amount of rain in the removal of air pollutants42,43. In terms of rain intensity, it has been theoretically and empirically proven that pollutant concentration in rainwater has a negative correlation with rainfall intensity and that light rain is the most effective at removing water-soluble pollutants through below-cloud scavenging processes44,45,46,47,48,49. Wang et al.50 further demonstrated that aerosol wet scavenging on a global scale is predominantly constrained by light rain and that the magnitude of scavenging varies approximately exponentially with precipitation intensity. To verify the relationship between the wet removal of NH4+ and rain intensity, the concentration of NH4+ in rainfall and the rain intensity was analyzed with data across China, the USA, and Europe. Results showed an exponential relationship between the two parameters, with r-squared values above 0.80 and p-values less than 0.001 (Supplementary Fig. 7, see Supplementary Note 2.1 for data sources), providing additional evidence that light rain has a stronger washout capacity for NH4+ globally. With respect to rain event duration, the washout amount of NH4+ from the atmosphere by consecutive rain events is less than that by an equivalent amount of inconsecutive rain events. This is because as a rain event continues, less NHx content is removed from the atmosphere due to less available NH3 in the atmosphere. NH3 is less available because of suppressed NH3 emissions during the rainfall process and less accumulated NH3 due to washout in the previous time period51. As an example, the dynamics of NH4+ concentrations recorded during two consecutive rain events on April 11th, 2009, in Guiyang, China, revealed that NH4+ was scavenged quickly at the start of each precipitation event, followed by a relatively low level of scavenging until the end of the event41 (Supplementary Fig. 8).

In order to characterize the precipitation pattern-induced effect on the below-cloud wet removal of NHx content with the co-influence of rain intensity and rain event duration, we have defined a new indicator, the precipitation scavenging index (PSI), in this study. The PSI is calculated as follows:

$${\rm{PSI}}=\frac{\rm{LR}}{\rm{CR}}$$
(1)

Where LR represents the frequency of days with light rain each year, and CR represents the occurrence of consecutive rain events each year. PSI is, therefore, a dimensionless indicator. A lower value of PSI suggests that less frequent but increasingly intense rainfall events are less efficient at “cleaning” the atmosphere of gaseous NH3 and particulate NH4+. This implies that more NHx remains in the atmosphere for dry deposition by turbulent diffusion to vegetation and other surfaces. This indicator, despite not having a specific physical meaning, provides a simple way to reflect the negative correlation between LR and dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), as well as the positive correlation between CR and dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), and comprehensively represents the contrasting effect of LR and CR on the below-cloud wet removal of NHx.

An overall negative trend was found for PSI in China during 1980–2015 (Fig. 2). The negative correlations were found between PSI and modeled dry NHx deposition (r = −0.79) as well as modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) (r = −0.63), indicating that variation in climate change-induced precipitation patterns favor a shift in NHx deposition from the wet to dry form. Spatial analysis of NHx deposition (Supplementary Fig. 4) and precipitation patterns (Supplementary Fig. 9) in China further confirmed that changing precipitation patterns contributed to the increase in dry NHx deposition in China (Supplementary Note 3). Our finding is supported by studies conducted in the USA, which also revealed that most of the increases in ambient NH3 concentration are a manifestation of climate change52,53.

Fig. 2: Long-term trends of the historical (1980–2015) and predicted (2030–2100) precipitation scavenging index (PSI) in China.
figure 2

Predicted PSI was calculated by using the meteorological parameters from the two global future climate change scenarios RCP 4.5 and RCP 8.5. Error bars indicate 1/3 standard deviation (s.d.).

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Implications for future partitioning of NHx deposition

As global climate change is accelerating, partitioning of dry and wet NHx deposition driven by precipitation pattern changes is expected to continue in the future. The results from the two future global climate scenarios (RCP4.5 and RCP8.5) show that less frequent light rain and more consecutive rain events are expected for the period of 2030–2100 (Supplementary Fig. 6b, c). Therefore, the PSI will continue to decline in the future (Fig. 2), implying that more NHx remains in the atmosphere and can subsequently be removed from the atmosphere via dry deposition processes. Further reductions in anthropogenic SO2 and NOx emissions, and a steady increase in NH3 emissions, are projected from 2030 to 2100 under these global climate scenarios (Supplementary Fig. 10). Hence, the projected \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), as a function of the emission index (EI) and PSI (see detailed information in Supplementary Note 6), will continue to increase between 2030 and 2100 at a rate of 0.007 per year (0.59% yr−1, p < 0.05) and 0.010 per year (1.13% yr−1, p < 0.05) for the RCP4.5 and RCP8.5 scenarios, respectively (Fig. 3a). As a result, modeled dry NHx deposition shows increasing trends at a rate of 0.011 kg N ha−1 yr−2 (0.16% yr−1, p < 0.05) for RCP4.5 and 0.020 kg N ha−1 yr−2 (0.30% yr−1, p < 0.05) for RCP8.5, respectively (Fig. 3b). Accordingly, based on the projected dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\), we extrapolated that wet NHx deposition shows a continuing downward trend (Fig. 3c), while the total NHx deposition remains stable since NH3 emissions remain constant (Fig. 3d). Even though the total NHx deposition remains almost stable at around 12–13 kg N ha−1 yr−1 in the future, the threats to natural ecosystems from NHx deposition are expected to increase in the future since the projected increase in dry NHx deposition could lead to greater damage to plant growth as compared to a similar amount of wet NHx deposition12,13. Meanwhile, the shifting of the partitioning of NHx deposition will also affect the air quality16,54,55. This is because the shifting of the partitioning of NHx deposition from the particulate phase to the gaseous phase would decrease particle pollution to some extent54. However, the shifting of the partitioning of NHx deposition would increase the lifetime of NHx, and this can facilitate long-range transportation of NHx, which may affect other regions55.

Fig. 3: Temporal variations of NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) in China over the historical (1980–2015) and future period (2030–2100).
figure 3

a \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\); b dry deposition; c wet deposition; d total deposition.

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Discussion

In light of the importance of dry NHx deposition to the nitrogen cycle and the associated ecological effects, we broaden our analysis to historical NHx deposition data in the USA and Europe, which are recognized as additional hotspots of NHx deposition (see more details in Supplementary Note 2.1 and Supplementary Note 4). Consistent upward trends for modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) also occur in the USA and Europe, irrespective of changes in total, wet, and dry deposition of NHx (Supplementary Fig. 11). Specifically, modeled \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) increased at a rate of 0.010 per year (2.05% yr−1, p < 0.05) in the USA and 0.005 per year (1.19% yr−1, p < 0.05) in Europe. Although wet deposition remained the dominant mechanism for NHx removal in the USA and Europe, the contribution of modeled dry NHx deposition to total NHx deposition increased in the past two decades, from ~30% to ~35% for both the USA and Europe. Similar to the estimation in China, we extrapolate that precipitation pattern variations driven by climate change offset the effect of stable or decreasing NH3 emissions and increase the contribution of modeled dry NHx deposition to total NHx deposition in the USA and Europe (see detailed information in Supplementary Note 4).

The results from the two additional continents further suggest the widespread and growing importance of dry NHx deposition driven by precipitation patterns resulting from global climate change. This is in contrast with previous studies that suggest relatively little influence of climate change, which may partly be because they only considered changes in precipitation amount, whereas we show that precipitation patterns (intensity and duration) play a far more important role in the shifting of NHx deposition partitioning19,20,21,22. Figure 4 schematically illustrates how NHx deposition partitioning shifts under the influence of climate change-induced variations in precipitation patterns. In view of the acceleration of global climate change, less frequent light rain and more frequent consecutive rain events are expected at present and in the coming decades. In particular, extreme rain events are projected to become more frequent in the future, which will further strengthen the shifting of NHx deposition from the wet to the dry form. Consequently, relatively less NHx content will be removed from the atmosphere via wet deposition, resulting in a continued increase in the contribution of dry NHx deposition, which will have a series of side effects on natural ecosystems and human health and air quality12,54,56. More importantly, other extreme weather events (e.g., drought and heatwave) due to global climate change will weaken the growth of plants and increase their susceptibility to increased dry NHx deposition in the future57.

Fig. 4: Schematic representation of the shift in NHx deposition partitioning driven by climate change-induced variations in precipitation patterns.
figure 4

Atmospheric reduced nitrogen (NHx) is emitted in the form of NH3 to a large extent by agricultural activities (nitrogen fertilizer application and livestock manure) and then deposited back to the earth’s surface via dry and wet removal pathways. NH3 emissions, in combination with emissions of the acidic gases SO2 and NOx, can influence the partitioning of dry and wet NHx deposition by affecting the equilibrium between gaseous NH3 and particulate NH4+11,18. More importantly, as a consequence of climate change, global trends in precipitation patterns, in particular trends toward less frequent light rain and more frequent consecutive rain events, shift the partitioning of the dry and wet deposited NHx fractions. Specifically, this leads to a decrease in wet deposition and an increase in dry deposition, leading to negative ecological consequences since the continued increasing dry NHx deposition causes greater damage to plant growth as compared to the same unit of wet NHx deposition12,13.

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Note that changes in NHx deposition can reflect the tendency of total nitrogen deposition to a certain extent, although we focus only on the partitioning of dry and wet NHx deposition in this study without analyzing NOy deposition. For example, the increasing importance of NHx deposition compared with NOy deposition can be inferred based on the combination of historically measured wet nitrogen deposition and modeled dry nitrogen deposition data shown in Yu et al.16. Looking to the future, NHx deposition will dominate total nitrogen deposition after 2030, as NH3 emissions are expected to surpass NOx emissions at that time (Supplementary Fig. 10), suggesting that the role of NHx deposition in nitrogen deposition will become more prominent, especially with the continued strict control of NOx emissions. Lamarque et al.58 also reported that nitrogen deposition was projected to increase over certain regions around the world by the end of the 21st century, especially in Asia, owing to the projected increase in NH3 emissions. Consequently, NH3 emission control is still the most effective and fundamental way to reduce NHx deposition and to curb its damage to natural ecosystems and human health, although NH3 emission abatement cannot fully control NHx deposition as previous studies documented that per-unit reduction in NH3 emissions led to only a 60–80% mitigation of NHx deposition10,59. Nevertheless, the analysis in the USA and Europe also highlights that the expected benefits from NH3 emission reductions on dry NHx deposition are diminished by the effects of climate change. Hence, more stringent NH3 emission controls are required to achieve the desired reductions in NHx deposition levels in the future20. A significant proportion of the NH3 emissions in China can be attributed to its agricultural sources, and improving agricultural nitrogen management (e.g., reduced fertilizer application, covered manure storage, and implementation of the deep application of fertilizers during growing seasons) could be the most effective approach to reduce NH3 emissions from agricultural sources in China to date and in the future54,60,61.

This study prompts our re-examination of the significance of dry NHx deposition in the global nitrogen cycle and ecosystem assessment and poses an urgent need to mitigate NHx deposition in the face of global climate change. Nevertheless, this study is still subject to four major uncertainties and limitations (see more information in Supplementary Note 7): (1) Long-term trends of dry NHx deposition are modeled results, which entail a higher degree of uncertainty than the observed wet NHx deposition11,62,63. Deposition velocity (\({V}_{d}\)) is the dominant contributor to the uncertainty of dry deposition simulations and could potentially affect the estimation of dry NHx deposition flux34,35,36. However, year-specific instead of time-varying \({V}_{d}\) was used to estimate long-term trends of dry NHx deposition in this study, which could introduce additional uncertainties. (2) The historical NH3 emissions data were calculated based on bottom-up activity data and static emissions factors, which did not consider the effect of interannual meteorological variability64,65,66,67. Whereas NH3 emissions were extremely weather/climate-sensitive68, the influence of climate change-induced precipitation patterns and other meteorological factors (e.g., temperature and wind speed) are supposed to affect the emission processes of NH3, which might also introduce additional uncertainties. (3) The land/atmosphere exchange of gaseous NH3 is, in fact, bi-directional and controlled by a range of environmental factors69,70,71. Nonetheless, owing to the difficulty of obtaining detailed input parameters and empirical tuning at a large scale, NH3 bi-directional exchange process has been incorporated into only a few of the regional- and global-scale CTMs (i.e., CMAQ, CAMX, GEM-MACH, and LOTOS-EUROS)35,36,72,73. Therefore, the lack of consideration of bi-directional parameterization schemes simultaneously affects the estimation of NH3 emissions and NHx deposition74,75,76 and introduces additional uncertainties. (4) Apart from precipitation patterns, global climate change can also affect NHx deposition partitioning through other aspects such as surface temperature, solar radiation, relative humidity, soil temperature, and natural surface properties68,77,78. However, the complex interactions of multiple meteorological parameters, as opposed to their individual effects on NHx deposition partitioning, make it difficult to isolate and quantify the relative importance of these meteorological parameters. Hence, the quantification of global climate change-induced NHx deposition partitioning is still subject to large uncertainty and remains a great challenge for the whole community. Development of climate-dependent models for better quantification of the dynamic relationship between NH3 emissions, climate/meteorology, and deposition under global environmental change in the future is therefore needed77.

Methods

Historical dry and wet NHx deposition data in China

Both the annual modeled dry NHx deposition and measured wet NHx deposition data in China for the period of 1980–2015 were obtained from Yu et al.16, which is the most comprehensive and current data on long-term NHx deposition available in China. General information regarding the sources of the NHx deposition data collected in China has been summarized in Supplementary Table 1. Briefly, observed wet NHx deposition data were obtained from four sources, including (1) peer-reviewed published papers screened by Yu et al.16, (2) the Acid Deposition Monitoring Network in East Asia (https://www.eanet.asia/, last access: 3 January 2023), (3) the National Ecosystem Research Network of China (http://www.cnern.ac.cn/index.action, last access: 3 January 2023), and (4) the Nationwide Nitrogen Deposition Monitoring Network established by China Agricultural University72, with a total of 956 monitoring sites.

Modeled dry NHx deposition fluxes were obtained from inferential methods, which combine ambient concentrations and dry deposition velocities (see Supplementary Note 1 for the parameterization of dry NH3 deposition)5,79,80. The average land use specific \({V}_{d}\) collected from previous studies was used to calculate annual dry NHx deposition (Supplementary Fig. 12, see Extended Data Table 4 in Yu et al.16). Specifically, modeled \({V}_{d}\) for 9 land use types was derived from Xu et al.11, who applied the GEOS-Chem model coupled with the scheme of Wesely81 and Zhang et al.82 to calculate \({V}_{d}\) for gaseous NH3 and particulate NH4+, respectively. Modeled \({V}_{d}\) in the rest of 5 land use types (i.e., desert, tundra, water, wetland, and ice) was estimated by using similar parameterization schemes used as in Xu et al.11 but with different CTMs (CMAQ, RegAMDS)83,84,85,86,87,88. It is worth noting that the modeled \({V}_{d}\) in the 14 land use types derived from Yu et al.16 was originally based on a unidirectional method, except for Su et al.83, who used a bi-directional NH3 exchange method to calculate \({V}_{d}\) for gaseous NH3 over deserts.

Yu et al.16 averaged the annual NHx deposition during 1980–2015 for 6 periods, including 1980–1990, 1991–1995, 1996–2000, 2001–2005, 2006–2010, and 2011–201515. Hence, we also averaged precursor emissions for the corresponding six periods (see Supplementary Note 2.3 for detailed information regarding emissions data). The data in each period were fitted with the middle year of the corresponding period in order to obtain the linear regressions. Subsequently, the slope of the linear regression was used to represent the average annual rate of change. The relative annual rate of change was then calculated by normalizing the annual rate of change with the value for the first time period in each region.

Quantification of the contribution of emissions to NHx deposition partitioning

The response relationship between NHx deposition and precursor emissions was developed based on the WRF-EMEP model (the Weather Research and Forecasting model coupled with the European Monitoring and Evaluation Program)32. Model configuration and evaluation are presented in Supplementary Note 5. A series of sensitivity studies were conducted to quantify the response of NHx deposition to reductions in SO2, NOx, and NH3 separately, as well as SO2 + NOx + NH3 together. The reduction values for individual emissions were set to be 10%, 30%, 50%, 70%, and 90%, respectively. Sensitivity results show that the response of regional NHx deposition to individual emission reductions is nearly linear. This is consistent with the study by Civerolo et al.19, which also revealed that the response of global nitrogen deposition to emissions was nearly linear based on six different tropospheric chemistry models. Furthermore, sensitivity results also revealed that the effect of NH3 emission variation is far more important than that of SO2 and NOx emissions, which has also been found by previous studies74,89. Last but not least, the response to the co-reduction of SO2 + NOx + NH3 was nearly equal to the sum of the individual reductions of SO2, NOx, and NH3. Therefore, the total variations in dry NHx deposition (ΔDry) and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) (ΔRatio) caused by precursor emissions during 1980–2015 can be obtained using Eq. (2):

$$\Delta {\rm{Dry}}\,{\rm{or}}\,\Delta {\rm{Ratio}}={a}_{1}\times {\Delta {\rm{SO}}}_{2}+{a}_{2}\times {\Delta {\rm{NO}}}_{x}+{a}_{3}\times \Delta {\rm{NH}}_{3}$$
(2)

Where \({a}_{1}\), \({a}_{2}\), and \({a}_{3}\) are constants, indicating the variations in dry NHx deposition and \({R}_{{\rm{NH}}x({\rm{dry/wet}})}\) caused by per-unit reduction in SO2, NOx, and NH3 emissions, respectively, as listed in Supplementary Table 2. \({\Delta {\rm{SO}}}_{2}\), \({\Delta {\rm{NO}}}_{x}\), and \({\Delta {\rm{NH}}}_{3}\) are the changes in SO2, NOx, and NH3 emissions between 1980 and 2015, respectively.