Wet Inorganic Nitrogen Deposition at the Daheitin Reservoir in North China: Temporal Variation, Sources, and Biomass Burning Inﬂuences

: Atmospheric nitrogen deposition is of great concern to both air quality and the ecosystem, particularly in northern China, which covers one-quarter of China’s cultivated land and has many heavily air polluted cities. To understand the characteristics of wet N deposition at rural sites in northern China, one-year wet deposition samples were collected in the Daheitin reservoir region. Due to the intense emissions of gaseous nitrogen compounds from heating activities during cold seasons and distinct dilution e ﬀ ects under di ﬀ erent rainfall intensities and frequencies, the volume weighted mean concentrations of wet N deposition showed higher levels in dry seasons but lower levels in wet seasons. In contrast, the wet N deposition rates varied consistently with precipitation, i.e., high during the wet season and lower during the dry season. The annual wet deposition rate of total inorganic ions (the sum of NO 3 − –N and NH 4 + –N) at the rural site in North China from July 2019 to June 2020 was observed at 18.9 kg N ha − 1 yr − 1 , still remained at a relatively high level. In addition, biomass burning activities are ubiquitous in China, especially in northern China; however, studies on its impact on wet N deposition are limited. Non-sea salt potassium ion (nss-K + ) was employed as a molecular tracer to investigate the characteristics of biomass burning activities as well as their impact on the chemical properties of wet N deposition. Three precipitation events with high nss-K + levels were captured during the harvest season (June to July). The variations in the patterns of nss-K + , deposited N species, and ratios of nss-K + to nitrogen species as well as their relationships all indicated that biomass burning emissions contributed remarkably to NO 3 − –N but had a minor inﬂuence on NH 4 + –N. the patterns of nss-K + and deposition of N species, the ratios of nss-K + to N species as well as their relationships, it was indicated that biomass burning emissions contributed remarkably to NO 3 − –N but had a minor influence on NH 4+ –N. These results highlight that biomass burning activity had a substantial influence on the composition of wet nitrogen deposition at the rural site in North China. Understanding the variations in trends and the influences of wet nitrogen deposition is important for effective air pollution control and for gaining insight into the atmospheric nitrogen cycle.


Introduction
With the rapid development of agriculture, industry, and the expansion of the urban population over the last several decades, anthropogenic activities, such as fossil fuel combustion, chemical fertilizer, etc., have greatly increased the levels of reactive nitrogen (N) in the atmosphere [1][2][3][4]. Reactive N in the atmosphere not only leads to air quality degradation by contributing to the production of ozone and PM 2.5 [5][6][7][8][9][10][11] but also disturbs the N cycle through excess deposition of N, leading to aquatic eutrophication, soil acidification, and impacting on ecosystem functions [2,[12][13][14][15].
building, approximately 3.0 m above ground, surrounded by a small village with approximately 500 inhabitants, roughly 1.0 km to the DHT Reservoir. Precipitation samples were collected using an automatic wet deposition collector that opens its lid automatically to collect a rainwater sample when it rains and closes automatically when the rain stops. The collected rainwater samples were transferred to pre-cleaned brown polyethylene bottles (200 mL) and stored in a refrigerator at −20 • C until chemical analysis. A total of 37 precipitation samples were collected from June 2019 to June 2020, covering the whole year, including the wet season (June-September) as well as the dry season (October-May).

Chemical Analysis
The rain samples were filtered with 0.45 µm filters to separate insoluble fractions before chemical analysis of the ions. The water-soluble inorganic ions in precipitation sample, including three anions (i.e., SO 4 2− , NO 3 − , Cl − ) and five cations (i.e., NH 4 + , Ca 2+ , Na + , K + , and Mg 2+ ) were analyzed by a Dionex ICS-3000 ion chromatograph (Thermo Fisher Scientific. Ltd., Waltham, MA, USA). The potassium concentrations presented here were corrected by a sea salt indicator (i.e., sodium; C nss-K + = C K + − 0.0355× C Na + ) that has been widely used for various field observations [47][48][49]. More details about the water-soluble inorganic ion analysis method can be found elsewhere [50].

Calculations
2.3.1. The Annual Volume-Weighted Mean Concentrations (C vwm , mg N L −1 ) The annual volume-weighted mean (VWM) concentrations of inorganic nitrogen species (C vwm , mg N L −1 ) were calculated using Equation (1). C i is the mass concentration of inorganic nitrogen species in the ith sample (mg N L −1 ), P i is the precipitation amount collected within the ith sampling period (mm), and n was the total number of precipitation samples during the whole year.
2.3.2. Flux of Wet N Deposition (F wd, kg N ha −1 ) F wd represents the monthly, seasonal, and annual wet deposition N flux, calculated by Equation (2). C i is the mass concentration of NH 4 + -N and NO 3 − -N (mg N L −1 ), and P i (in mm) is the total amount of precipitation over a month, season or year.

Concentrations of Inorganic Nitrogen Species in Rainwater
The total amount of precipitation for the whole year in the DHT reservoir area from July 2019 to June 2020 was 595 mm, similar to what was observed at other sites in North China [34] but obviously lower than in South China [29], East China [51], and Southeast China [42,52]. NO [22][23][24]. Ca 2+ was the second most abundant cation with an annual VWM concentration of 1.54 mg L −1 , exhibiting a non-negligible contribution of soil dust sources in the atmosphere at the rural site in North China (Table 1). Most of the rain events occurred during the summer from June-August ( Figure 1; Table 2). During the entire sampling period, precipitation in the DHT region was concentrated in the summer season (416 mm), 70% of the whole year's precipitation (595 mm). The annual VWM concentrations of deposition of inorganic N species, i.e., NH 4 + -N, NO 3 − -N, and TIN (the total sum VWM concentrations of NH 4 + -N and NO 3 − -N) were 2.11 mg N L −1 , 1.06 mg N L −1 , and 3.17 mg N L −1 , respectively.
Notably, seasonal VWM concentrations of NH 4 + -N and NO 3 − -N showed higher levels in autumn and winter, and the lowest in summer ( Figure 1). In late autumn and winter, more gaseous nitrogen compounds (i.e., NO x ) are accumulated in the atmosphere due to the increased emissions from coal combustion for heating and biomass combustion activities, as well as rare precipitation wash out, subsequently resulting in higher concentrations of N deposition. In contrast, there was no coal combustion for heating over the summer season, the emission of precursor gases were relatively lower, and coupled with the dilution of frequent rainfall events. The VWM concentration of NO 3 − -N was observed at its lowest level in summer ( Figure 1) and showed a negative relationship with precipitation amount (R 2 = 0.22, p < 0.05, Figure S2).
Atmosphere 2020, 11, x FOR PEER REVIEW 4 of 14 Most of the rain events occurred during the summer from June-August ( Figure 1; Table 2). During the entire sampling period, precipitation in the DHT region was concentrated in the summer season (416 mm), 70% of the whole year's precipitation (595 mm). The annual VWM concentrations of deposition of inorganic N species, i.e., NH4 + -N, NO3 − -N, and TIN (the total sum VWM concentrations of NH4 + -N and NO3 − -N) were 2.11 mg N L −1 , 1.06 mg N L −1 , and 3.17 mg N L −1 , respectively. Notably, seasonal VWM concentrations of NH4 + -N and NO3 − -N showed higher levels in autumn and winter, and the lowest in summer ( Figure 1). In late autumn and winter, more gaseous nitrogen compounds (i.e., NOx) are accumulated in the atmosphere due to the increased emissions from coal combustion for heating and biomass combustion activities, as well as rare precipitation wash out, subsequently resulting in higher concentrations of N deposition. In contrast, there was no coal combustion for heating over the summer season, the emission of precursor gases were relatively lower, and coupled with the dilution of frequent rainfall events. The VWM concentration of NO3 − -N was observed at its lowest level in summer ( Figure 1) and showed a negative relationship with precipitation amount (R 2 = 0.22, p < 0.05, Figure S2).  Compared to summer months, the gap between NH 4 + -N and NO 3 − -N was smaller in the autumn and winter months ( Figure 1a). This may have been due to the large amount of NO x emissions during heating-related coal combustion in November to March in northern China and coupled with the decreased NH 3 emissions from agriculturally associated sources; the gap between NH 4 + -N and

Wet Inorganic Nitrogen Species Deposition Flux
The monthly variations in the amounts of precipitation, wet deposition rates of N species, as well as their proportions of the total deposition rate of wet inorganic nitrogen (DIN, sum of wet deposition rates of NH 4 + -N and NO 3 − -N) are shown in Figure 2. The deposition rates of NH 4 and DIN were generally higher in the wet season than those in the dry season. The wet deposition flux of NH 4  Figure 2b). The monthly varied patterns of wet N species deposition rates were consistent with that of precipitation as shown in Figure 2. Furthermore, the relationship between monthly deposition rates of NH 4 + -N and NO 3 − -N were positively correlated with the amount of monthly rainfall (R 2 = 0.89, p < 0.01; R 2 = 0.92, p < 0.01, respectively, Figure S3) indicating that the amount of precipitation is crucial in controlling wet N deposition. The annual total wet N deposition flux in this study was 18.9 kg N ha −1 yr −1 ; a comparison to previous studies is shown in Table S1. The level of annual wet deposition flux in this study was lower prior to the implementation of the Action Plan, when anthropogenic impact was even stronger, for example, in northern China from 2007-2010 (16.3-28.2 kg N ha −1 yr −1 ) [32], urban sites in East China from 2003-2005 (26.8 kg N ha −1 yr −1 ) [43], and urban sites, for example, Zhengzhou, China, during 2011 (33.3 kg N ha −1 ) [28]. However, these results were still higher than those observed at a rural site of Beijing from 2017-2018 (8.0 kg N ha −1 yr −1 ) [34], the Three Gorges Reservoir Region in the Southwest China in 2015 (7.1-16.8 kg N ha −1 yr −1 ) [44], and rural areas in South China from 2017-2018 (13.2) [29]. Moreover, this level was also obviously higher than the wet DIN deposition rate in the USA (0.5-3.5 kg N ha −1 yr −1 ) (Table S1) [3,16,53]. This indicates that although the wet N deposition flux at the rural site in North China decreased due to the Action Plan's implementation; it still remained at a relatively high level.

Sources of Wet Inorganic N Deposition
In general, NH4 + in rainwater is formed from the precursor gas NH3, the main anthropogenic source of which is the volatilization of fertilizer and livestock manure; while NO3 − in rainwater is produced by the precursor gases NOx (i.e., NO and NO2), which are mainly emitted from fossil fuel combustion, such as from power plants, vehicles, and heating activities. Moreover, NH3 is readily converted to NH4 + , having a relatively shorter transportation distance, indicating NH4 + in rainwater The annual total wet N deposition flux in this study was 18.9 kg N ha −1 yr −1 ; a comparison to previous studies is shown in Table S1. The level of annual wet deposition flux in this study was lower prior to the implementation of the Action Plan, when anthropogenic impact was even stronger,  [29]. Moreover, this level was also obviously higher than the wet DIN deposition rate in the USA (0.5-3.5 kg N ha −1 yr −1 ) (Table S1) [3,16,53]. This indicates that although the wet N deposition flux at the rural site in North China decreased due to the Action Plan's implementation; it still remained at a relatively high level.

Sources of Wet Inorganic N Deposition
In general, NH 4 + in rainwater is formed from the precursor gas NH 3 , the main anthropogenic source of which is the volatilization of fertilizer and livestock manure; while NO 3 − in rainwater is produced by the precursor gases NO x (i.e., NO and NO 2 ), which are mainly emitted from fossil fuel combustion, such as from power plants, vehicles, and heating activities. Moreover, NH 3 is readily converted to NH 4 + , having a relatively shorter transportation distance, indicating NH 4 + in rainwater is subjected to local sources [42]. Thus, the NH 4 + /NO 3 − ratio is a good indicator of the relative contributions of agricultural sources (i.e., fertilizer and livestock manure) and non-agricultural sources (e.g., industrial fossil fuel combustion and vehicle emissions) [42,43,54,55]. Generally, in areas with advanced industrialization, the NH 4 + -N/NO 3 − -N ratio is usually smaller than 1.0, e.g., at an urban site in Nanjing (0.94), China [43], Pearl River Delta (PRD) (0.33-4.0, averaged at 1.1), China [56], and New York (0.76-0.99) [57]. On the contrary, areas with intensive agriculture, such as rural or agricultural sites, are usually characterized by wet N deposition with NH 4 + /NO 3 − ratios greater than 1.0. In China, is subjected to local sources [42]. Thus, the NH4 + /NO3 − ratio is a good indicator of the relative contributions of agricultural sources (i.e., fertilizer and livestock manure) and non-agricultural sources (e.g., industrial fossil fuel combustion and vehicle emissions) [42,43,54,55]. Generally, in areas with advanced industrialization, the NH4 + -N/NO3 − -N ratio is usually smaller than 1.0, e.g., at an urban site in Nanjing (0.94), China [43], Pearl River Delta (PRD) (0.33-4.0, averaged at 1.1), China [56], and New York (0.76-0.99) [57]. On the contrary, areas with intensive agriculture, such as rural or agricultural sites, are usually characterized by wet N deposition with NH4 + /NO3 − ratios greater than 1.0. In China, annual average NH4 + -N/NO3 − -N ratio was reported to be 1.76 for an agricultural site and 1.14 for a rural site in South China [29] [60]. The monthly variation of NH4 + -N/NO3 − -N ratio at the DHT reservoir is shown in Figure 3. In this study, the annual average NH4 + -N/NO3 − -N ratio was 1.97 with all monthly average values exceeding 1.0 (in the range of 1.08 to 2.26). Thus, compared to NO3 − from fossil fuel combustion in industry and transportation, NH4 + deposition from agriculture and excrement from humans and animals still occupied the larger portion in the DHT region. This was mainly because the sampling site was located in a rural area, surrounded by scattered rice paddy fields and villages, where N fertilization had a great effect on NH4 + deposition. NH4-N was the dominant form of N deposition; however, the contribution from NO3 − significantly increased over the past decades in China [2]. The ratio of NH4-N to NO3-N in wet precipitation decreased significantly by approximately five to two over time from 1980 to 2010 in China [2]. Similarly, the NH4-N/NO3-N ratio in wet deposition sharply decreased from 5.8 to 1.2 in Yangtze River Delta Region from 1980 to 2005 [43]. This indicates a great enhancement in NO3-N emissions over the past few decades, corresponding to the rapid increase in fossil fuel consumption in the industry and transportation sectors.  China, e.g., high values, particularly, in the wet season and low values in the dry season were observed in South China [29,56]. At the same, the annual average NH 4 + -N/NO 3 − -N ratio was 2.3 in the western part of South Korea from 2007-2008, but the lowest NH 4 + -N/NO 3 − -N ratio was observed during the winter season (0.63) [14]. This seasonal variation of wet N deposition is in accordance with the nitrogen-fertilizing period during the crop growing season in China. Meanwhile, temperatures over the wet season were higher than in the winter season, and hot temperature conditions were much easier for agriculturally associated NH 3 volatilization from fertilizer (urea, di-ammonium phosphate, etc.) and livestock manure applied in agricultural soils. The contribution of agricultural activity to atmospheric N deposition was substantially weakened during the winter season. Moreover, when entering into late autumn, the weather becomes cold, and the coal and biomass burning combustion activities for heating become abundant in North China, resulting in obvious elevated concentrations of NO 3 − in ambient particles, and thus decreased the NH 4 + -N/NO 3 − -N ration during winter season.

Biomass Burning Influences on Wet Inorganic N Deposition
Potassium is normally regarded as a tracer of biomass burning [5,47,61]. Non-sea-salt potassium concentrations were used to exclude the influence of potassium derived from sea-salt. Therefore, the ratio of non-sea salt (nss)-K + to NH 4 + -N and NO 3 − -N in the atmosphere can applied to estimate the effect of biomass burning to wet N deposition. Three precipitation events occurred during the harvest season on 6 June, 29 June and 30 July, 2019 ( Figure 4). The nss-K + concentrations in those rainwater samples were significantly higher than in other precipitation samples, indicating the field biomass combustion was active during these periods. Fire pixel counts observed from MODIS (Moderate Resolution Imaging Spectroradiometer) Terra and Aqua also identified many fire spots in North China during June and July 2019, when it is time to harvest wheat and corn along with the common practice of straw burning (Figure 5a,b).
Atmosphere 2020, 11, x FOR PEER REVIEW 8 of 14 studies in China, e.g., high values, particularly, in the wet season and low values in the dry season were observed in South China [29,56]. At the same, the annual average NH4 + -N/NO3 − -N ratio was 2.3 in the western part of South Korea from 2007-2008, but the lowest NH4 + -N/NO3 − -N ratio was observed during the winter season (0.63) [14]. This seasonal variation of wet N deposition is in accordance with the nitrogen-fertilizing period during the crop growing season in China. Meanwhile, temperatures over the wet season were higher than in the winter season, and hot temperature conditions were much easier for agriculturally associated NH3 volatilization from fertilizer (urea, di-ammonium phosphate, etc.) and livestock manure applied in agricultural soils. The contribution of agricultural activity to atmospheric N deposition was substantially weakened during the winter season. Moreover, when entering into late autumn, the weather becomes cold, and the coal and biomass burning combustion activities for heating become abundant in North China, resulting in obvious elevated concentrations of NO3 − in ambient particles, and thus decreased the NH4 + -N/NO3 − -N ration during winter season.

Biomass Burning Influences on Wet Inorganic N Deposition
Potassium is normally regarded as a tracer of biomass burning [5,47,61]. Non-sea-salt potassium concentrations were used to exclude the influence of potassium derived from sea-salt. Therefore, the ratio of non-sea salt (nss)-K + to NH4 + -N and NO3 − -N in the atmosphere can applied to estimate the effect of biomass burning to wet N deposition. Three precipitation events occurred during the harvest season on 6 June, 29 June and 30 July, 2019 ( Figure 4). The nss-K + concentrations in those rainwater samples were significantly higher than in other precipitation samples, indicating the field biomass combustion was active during these periods. Fire pixel counts observed from MODIS (Moderate Resolution Imaging Spectroradiometer) Terra and Aqua also identified many fire spots in North China during June and July 2019, when it is time to harvest wheat and corn along with the common practice of straw burning (Figure 5a,b).   Based on the 72 h backward trajectories for the three precipitation events during the summer season (6 June, 29 June, and 30 July 2019), most of the air masses arriving in the DHT region were restricted to the polluted areas of North China with dense fire spots (Figure S4a-c). Abundant combustion aerosols were transported from these source areas to the DHT region, associated with significantly increased levels of biomass burning tracers, i.e., nss-K + , as well as Cl − (Figure 4). Cl − is regarded as another common tracer for biomass burning [63], which synchronously changed with nss-K + in the wet deposition over the entire year, except for the precipitation event on 21 November 2019. Based on fuel combustion studies, Cl − in aerosols can also be contributed by coal combustion [64,65] and chlorinated plastics waste burning [66]. Entering into middle November, the weather became cold, thus, coal combustion activities commenced in rural areas and central heating systems started operating. Subsequently, the level of Cl − sharply increased during the precipitation event on 21 November 2019 at the start of the heating period. There was almost no influence on NH3 emissions from biomass burning; hence, the ratio of nss-K + /NH4 + -N in the atmosphere was expected to be higher when biomass burning occurred. Correspondently, the ratio of nss-K + /NH4 + -N obviously increased during the summer harvest biomass burning episode but displayed no variation over the winter heating season (Figure 4c), which further confirmed that the high levels of Cl − during the precipitation event on 21 November 2019 was emitted from coal combustion or chlorinated plastics waste burning but not biomass burning.
The concentration of nss-K + was positively correlated with NO3 − -N (R 2 = 0.98, p < 0.01), but revealed no relationship to NH4 + -N (p > 0.5) (Figure 6). In general, NH4 + -N/TIN ratios were higher than NO3 − -N/TIN ratios throughout the observations. However, due to the substantial contribution of biomass burning to atmospheric NOx emissions during the harvest season or intense fossil fuel combustion during the start of the heating season, the ratios of NO3 − -N/TIN were obviously elevated, and surpassed the ratios of NH4 + -N/TIN (Figure 4d) during those intense air pollution periods. The change in the N species in the atmosphere exhibited the influences of air pollution on wet N deposition. Several studies speculate that biomass burning might be a source of water-soluble  (Figure S4a-c). Abundant combustion aerosols were transported from these source areas to the DHT region, associated with significantly increased levels of biomass burning tracers, i.e., nss-K + , as well as Cl − (Figure 4). Cl − is regarded as another common tracer for biomass burning [63], which synchronously changed with nss-K + in the wet deposition over the entire year, except for the precipitation event on 21 November 2019. Based on fuel combustion studies, Cl − in aerosols can also be contributed by coal combustion [64,65] and chlorinated plastics waste burning [66]. Entering into middle November, the weather became cold, thus, coal combustion activities commenced in rural areas and central heating systems started operating. Subsequently, the level of Cl − sharply increased during the precipitation event on 21 November 2019 at the start of the heating period. There was almost no influence on NH 3 emissions from biomass burning; hence, the ratio of nss-K + /NH 4 + -N in the atmosphere was expected to be higher when biomass burning occurred. Correspondently, the ratio of nss-K + /NH 4 + -N obviously increased during the summer harvest biomass burning episode but displayed no variation over the winter heating season (Figure 4c), which further confirmed that the high levels of Cl − during the precipitation event on 21 November 2019 was emitted from coal combustion or chlorinated plastics waste burning but not biomass burning. The concentration of nss-K + was positively correlated with NO 3 − -N (R 2 = 0.98, p < 0.01), but revealed no relationship to NH4 + -N (p > 0.5) ( Figure 6). In general, NH 4 + -N/TIN ratios were higher than NO 3 − -N/TIN ratios throughout the observations. However, due to the substantial contribution of biomass burning to atmospheric NO x emissions during the harvest season or intense fossil fuel combustion during the start of the heating season, the ratios of NO 3 − -N/TIN were obviously elevated, and surpassed the ratios of NH 4 + -N/TIN (Figure 4d) during those intense air pollution periods. The change in the N species in the atmosphere exhibited the influences of air pollution on wet N deposition. Several studies speculate that biomass burning might be a source of water-soluble organic N in wet deposition [29]. In this study, we provided evidence for the fact that biomass burning evidently contributed to the wet deposition of some inorganic N (e.g., NO 3 − -N); however, it had a minor influence on others (such as NH 4 + -N).
Atmosphere 2020, 11, x FOR PEER REVIEW 10 of 14 organic N in wet deposition [29]. In this study, we provided evidence for the fact that biomass burning evidently contributed to the wet deposition of some inorganic N (e.g., NO3 − -N); however, it had a minor influence on others (such as NH4 + -N).

Conclusions
One-year wet deposition samples were collected at the Daheitin reservoir in North China. The annual VWM concentrations of NH4 + -N and NO3 − -N were 2.11 mg N L −1 and 1.06 mg N L −1 , respectively. Due to the intense emissions of more gaseous nitrogen compounds (i.e., NOx and NH3) in heating activities in the cold seasons and the dilution effect by different rainfall intensity and frequency, the seasonal VWM concentrations of NH4 + -N and NO3 − -N showed higher levels during the dry season (i.e., autumn and winter) and lower levels during the wet season, i.e., summer. In contrast, the wet N deposition rates varied consistently with precipitation (i.e., high during the wet season and low during the dry season), indicating that the amount of precipitation was crucial in controlling wet N deposition. Compared to the years prior to the implementation of the Action Plan in 2013, the wet inorganic nitrogen deposition rate at the rural site in North China decreased to 18.9 kg N ha −1 yr −1 but still maintained a relatively high level.
Non-sea salt K + was employed as a molecular tracer of biomass burning emission, and three precipitation events with high nss-K + levels were captured during the harvest season (June to July). Based on the variations in the patterns of nss-K + and deposition of N species, the ratios of nss-K + to N species as well as their relationships, it was indicated that biomass burning emissions contributed remarkably to NO3 − -N but had a minor influence on NH4 + -N. These results highlight that biomass burning activity had a substantial influence on the composition of wet nitrogen deposition at the rural site in North China. Understanding the variations in trends and the influences of wet nitrogen deposition is important for effective air pollution control and for gaining insight into the atmospheric nitrogen cycle.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1: Wet N deposition rates (kg N ha −1 yr −1 ) in DHT region compared to those reported in other studies, Figure S1: Location

Conclusions
One-year wet deposition samples were collected at the Daheitin reservoir in North China. The annual VWM concentrations of NH 4 + -N and NO 3 − -N were 2.11 mg N L −1 and 1.06 mg N L −1 , respectively. Due to the intense emissions of more gaseous nitrogen compounds (i.e., NO x and NH 3 ) in heating activities in the cold seasons and the dilution effect by different rainfall intensity and frequency, the seasonal VWM concentrations of NH 4 + -N and NO 3 − -N showed higher levels during the dry season (i.e., autumn and winter) and lower levels during the wet season, i.e., summer. In contrast, the wet N deposition rates varied consistently with precipitation (i.e., high during the wet season and low during the dry season), indicating that the amount of precipitation was crucial in controlling wet N deposition. Compared to the years prior to the implementation of the Action Plan in 2013, the wet inorganic nitrogen deposition rate at the rural site in North China decreased to 18.9 kg N ha −1 yr −1 but still maintained a relatively high level. Non-sea salt K + was employed as a molecular tracer of biomass burning emission, and three precipitation events with high nss-K + levels were captured during the harvest season (June to July). Based on the variations in the patterns of nss-K + and deposition of N species, the ratios of nss-K + to N species as well as their relationships, it was indicated that biomass burning emissions contributed remarkably to NO 3 − -N but had a minor influence on NH 4 + -N. These results highlight that biomass burning activity had a substantial influence on the composition of wet nitrogen deposition at the rural site in North China. Understanding the variations in trends and the influences of wet nitrogen deposition is important for effective air pollution control and for gaining insight into the atmospheric nitrogen cycle.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4433/11/11/1260/s1, Table S1: Wet N deposition rates (kg N ha −1 yr −1 ) in DHT region compared to those reported in other studies, Figure S1: Location of Daheitin Reservoir station (red star) and the surrounding cities, Figure S2: Relationship between the concentrations of NO 3 − -N in rainwater and rainfall amounts during each precipitation event at the DHT reservoir, a rural site in North China from July 2019 to June 2020. Statistical analysis was conducted with logarithmic fitting method, Figure S3: Relationship between monthly deposition rates of NH 4 + -N and NO 3 − -N and rainfall amounts at the DHT reservoir, a rural site in North China from July 2019 to June 2020. Statistical analysis was conducted with linear fitting method, Figure