The vertical variability of ammonia in urban Beijing, China

Weekly vertical profiles of ammonia (NH₃) were measured at 16 heights on the Beijing 325 m meteorological tower for 1 year from March 2016 to March 2017. The average NH₃ concentrations exceeded 4 µg m⁻³ at all heights with an overall average (±1σ) value of 13.3 (±4.8) µg m⁻³. The highest NH₃ concentrations along the vertical profiles mostly occurred from 32 to 63 m, decreasing both towards the surface and at higher altitudes. Significant decreases in NH₃ concentrations were only found at the top two heights (280 and 320 m). These results suggest an NH₃ rich atmosphere during all seasons in urban Beijing, from the ground to at least 320 m. The highest seasonal NH₃ concentrations across the profile were observed in summer (18.2 µg m⁻³) with high temperature, followed by spring (13.4 µg m⁻³), autumn (12.1 µg m⁻³) and winter (8.3 µg m⁻³). A significant vertical variation in the NH₃ concentration was only found in summer. Source region analyses suggest that air masses from intensive agricultural regions to the south contribute most to the high NH₃ concentrations in Beijing. Local sources such as traffic emissions also appear to be important contributors to atmospheric NH₃ in this urban environment.

1 Introduction

Ammonia (NH₃) has long been recognized as an important form of reactive nitrogen (Nr) in the atmospheric environment, playing a key role in biogeochemical cycles from atmospheric chemical processes to deposition and in subsequent environmental impacts (e.g., air pollution, reduced biodiversity, acidification and eutrophication) (Fowler et al., 2009; Sutton et al., 2008). NH₃ reacts with nitric and sulfuric acids in air, forming secondary inorganic aerosols (e.g., NH₄NO₃, (NH₄)₂SO₄) with long atmospheric lifetimes that can transport these species far from sources and contribute 40 %–57 % of the fine particle matter in megacities (Fowler et al., 2009; Huang et al., 2014; Yang et al., 2011). Therefore, NH₃ has received increasing attention in air pollution research (Wang et al., 2015). In addition to agriculture, which is considered the largest global NH₃ source, emissions from biomass burning, industry, vehicles and other sources (Galloway et al., 2003; Sutton et al., 2008; Erisman et al., 2008; Sun et al., 2016, 2017) can also be significant.

In China, annual NH₃ emissions were approximately 2 and 3 times higher than European and US emissions, respectively, over the period from 1990 to 2005 (Reis et al., 2009; Kang et al., 2016; Zhao and Wang, 1994; Klimont, 2001; EMEP, 2018; USEPA, 2018), and were estimated to be 14.6 Tg N yr⁻¹ in 2010 (Liu et al., 2013) and 15.6 Tg N yr⁻¹ in 2015 (Zhang et al., 2017). Such high emissions, in addition to the important role NH₃ plays in degrading air quality, makes NH₃ a key target to curb serious air pollution in Chinese urban areas (Fu et al., 2017; Chang et al., 2016; Ye et al., 2011; Wang et al., 2011). Some studies have indicated that reducing NH₃ concentrations could be an effective method for alleviating secondary inorganic PM_2.5 pollution in China (Gu et al., 2014; Wang et al., 2015; Wu et al., 2016; Xu et al., 2017). However, NH₃ has received less attention from the government than SO₂ and NO_x, which have been controlled since 2005 and were effectively reduced during the 12th Five-Year Plan period (2011–2015) (Fu et al., 2017). Currently there are strong arguments regarding the role of regional transport in contributing to haze pollution in China (Guo et al., 2014; Li et al., 2015), especially for severe haze episodes occurring during stagnant meteorological conditions with a shallow boundary layer (Sun et al., 2014; Zheng et al., 2015; Quan et al., 2013). The vertical characterization of air pollutant concentration profiles may be helpful for elucidating factors contributing to the formation and transport of regional haze events (Quan et al., 2013; Tang et al., 2015; Wiegner et al., 2006). Many studies have been conducted to improve our understanding of temporal and spatial concentration dynamics of atmospheric NH₃ and how they relate to underlying factors (e.g., emission intensity and meteorological conditions) and air quality (Yamamoto et al., 1988, 1995; Bari et al., 2003; Vogt et al., 2005; Lee et al., 1999). However, such studies in China have generally focused on the spatial distribution of NH₃ near the ground (Ianniello et al., 2010; Wu et al., 2009; Meng et al., 2011; Xu et al., 2015), whereas the vertical characterization of NH₃ concentrations has been very limited.

NH₃ mixing ratios may vary significantly as a function of height, as NH₃ is a trace gas with both point and non-point sources, and it also has a tendency to deposit rapidly to surfaces. In urban locations, like Beijing, where NH₃ is a key contributor to fine particle formation, local sources (e.g., traffic) emit at the surface and are then mixed through the boundary layer, while NH₃ transported from agricultural sources outside the city is presumably already mixed through the boundary layer. The influence of these behaviors may be reflected in the vertical NH₃ concentration gradients measured within the city. For example, dominant local surface traffic emissions might give rise to a profile that peaks near the surface, while NH₃ transported into the urban area may be uniformly mixed in the vertical or even decline near the surface due to loss by dry deposition. Of course these patterns are expected to be further affected by sinks, including surface deposition as well as by the fine particle formation of ammonium salts. NH₃ vertical distribution measurements are also useful for advancing satellite retrievals, which offer a great potential for understanding the global distribution of gaseous NH₃ (Shephard and Cady-Pereira, 2015; Sun et al., 2015; Van Damme et al., 2015).

To our knowledge there are few studies reporting long-term observations of the vertical distributions of NH₃ in the lowest few hundred meters of the atmosphere, including measurements at the BAO tower in the USA (Li et al., 2017; Tevlin et al., 2017) and the CESAR site in the Netherlands (Dammers et al., 2017). Li et al. (2017) analyzed vertical NH₃ concentration profiles at the BAO tower in Colorado, USA, reporting the minimum concentration at the top of the tower, which slowly increased towards a peak concentration at ∼10 m before a large reduction in concentration was found at 1 m. The site was influenced by the transport of high NH₃ concentrations from large animal feeding operations to the northeast. Using higher time resolution measurements at the BAO tower, Tevlin et al. (2017) pointed out that the surface can act as an occasional NH₃ sink as well as a source. The CESAR study in the Netherlands showed that vertical profile differences were mainly due to local and regional transport influences (Dammers et al., 2017). Because the BAO and CESAR tower sites are both located in suburban areas with low aerosol mass loadings, observed vertical profiles of aerosol and gas species (Öztürk et al., 2013; VandenBoer et al., 2013; Riedel et al., 2013) could be substantially different from those in megacities in China. Zhou et al. (2017) measured vertical concentration profiles of NH₃ and 7 other air pollutants at 10 heights (8, 15, 47, 80, 120, 160, 200, 240, 280 and 320 m) in urban Beijing, finding that NH₃ concentrations peaked at 160 m. However, only one vertical profile was measured and may not adequately represent typical conditions. Until now, long-term monitoring of vertical NH₃ concentration profiles has not been carried out in China.

Here, we report a 1-year field campaign on the Beijing 325 m meteorological tower to investigate vertical NH₃ concentration profiles and consider how temporal variations may relate to urban emission sources, meteorological factors and air transport from more distant sources. Study findings are relevant for our understanding of precursor NH₃ distributions and the role of NH₃ in the formation of severe aerosol pollution in China; furthermore, they will provide benchmarks to assist in meeting air quality goals and policy needs in future.

Figure 1 (a) Modeled NH₃ emissions distribution (0.1^∘, ∼10 km) over the North China Plain in 2015 including the location of the monitoring site shown as a black dot. NH₃ emission estimates are from the inventory of Zhang et al. (2018) at a 0.1^∘ horizontal resolution. (b) Map of Beijing showing the location of the monitoring tower. (c) The 325 m meteorological tower and ALPHA passive samplers.

2 Materials and methods

2.1 Site description

The sampling site is located at the State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS) in urban Beijing (39^∘58^′ N, 116^∘22^′ E; Fig. 1). The site is approximately 0.8 km north of the Third Ring Road, 1.3 km south of the Fourth Ring Road and 0.2 km west of the Beijing–Tibet expressway, which are three transport arteries encircling Beijing, each with average traffic volumes of over 200 000 vehicles day per day in 2016 (Beijing Transport Institute, 2017); therefore, this site represents a typical urban site that is mainly surrounded by residential areas.

2.2 NH₃ measurement

From 16 March 2016 to 16 March 2017, weekly atmospheric NH₃ samples were collected at 16 heights on the 325 m meteorological tower using ALPHA passive samplers (adapted low-cost high absorption, Centre for Ecology and Hydrology, Edinburgh, UK) except for a few samples with slightly different durations due to tower maintenance schedules. The samplers operate on the principle of diffusion using an acid-coated filter to capture the NH₃. A PTFE (Teflon) membrane is placed directly at the mouth of the sampler, forming a quiescent boundary layer in front of the sample membrane. A stable, turbulent-free diffusion path length is achieved behind the membrane, whilst allowing gaseous NH₃ to diffuse through for capture and minimizing the sampling of ${\mathrm{NH}}_{\mathrm{4}}^{+}$ aerosol (Tang et al., 2014). NH₃ was sampled at 2, 8, 15, 32, 47, 63, 80, 102, 120, 140, 160, 180, 200, 240, 280 and 320 m a.g.l. (above ground level). At each height, three ALPHA samplers were deployed under a PVC shelter to protect the samplers from rain and direct sunlight (shown in Fig. 1). NH₃ samples were extracted with 10 mL high-purity water (18.2 MΩ-cm) and analyzed using a continuous-flow analyzer (Seal AA3, Germany). Three field (travel) blanks were prepared for each batch of samples, which were analyzed together with the abovementioned samples, and used to blank correct sample results and determine the method detection limit (MDL) values. MDL was calculated using the following equation: MDL $\ge t\times S_{\mathrm{b}}\times \sqrt{\frac{N_{\mathrm{1}}+N_{\mathrm{2}}}{N_{\mathrm{1}}\times N_{\mathrm{2}}}}$, where the t value is given at the 95 % confidence level for the appropriate of degrees of freedom, S_b is the blank standard deviation, N₁ and N₂ are the number of sample measurements (single measurement, N₁=1) and the number of analyzed blanks, respectively. From the field blanks, the MDL was calculated to be 0.31 µg m⁻³ for a 1-week ALPHA passive NH₃ sample. All lab measurements were conducted in the Key Laboratory of Plant-Soil Interactions, Chinese Ministry of Education, China Agricultural University. More details regarding the passive samplers and the related laboratory preparation and analysis can be found in Xu et al. (2015).

2.3 Meteorological data

Meteorological parameters, including wind speed (WS), wind direction (WD), relative humidity (RH) and temperature (T), were obtained at all sampling heights except 2 m; the temperature was also not available at 8 m. WS and WD were measured using four-cup anemometers (model O1OC, Met One Instruments), and RH and T were measured using a T/RH sensor (model HC2-S3, ROTRONIC).

Figure 2 Time series of the vertical distribution of weekly atmospheric NH₃ concentrations (±1σ) in Beijing urban (16 March 2016–16 March 2017).

2.4 Data analysis

Repeated-measures analysis of variance (ANOVA) was used to test changes in the NH₃ concentration along vertical profiles. When the ANOVA results were significant, the Tukey's honest significant difference (HSD) test was used to determine the significance of the difference between means with a significance level of P<0.05. The coefficient of determination was used to test the linear correlations with a significance level of P<0.05. All of the statistical analyses were conducted using SPSS version 23.0 (IBM Corp., Armonk, NY, USA).

Potential source contribution function analysis (PSCF) (Ashbaugh et al., 1985) of atmospheric NH₃ was performed using MeteoInfo (TrajStat package) (Wang, 2014), where 72 h back trajectories arriving at the monitoring site (IAP tower) at each height were calculated every 3 h for the entire study period. The average NH₃ concentration for each cluster was computed using the cluster statistics function. NH₃ pathways could then be associated with the high concentration clusters. The number of trajectory segment endpoints falling in a grid cell (i, j) is n_ij. The number of trajectory endpoints associated with the data with NH₃ concentrations higher than an arbitrarily set criterion for each height during the four seasons (75th percentile for NH₃ was set here) is m_ij (Table S1 in the Supplement). The PSCF value for the ijth cell is then calculated as m_ij∕n_ij. A weighting function W_ij was applied to reduce the uncertainties of small values of n_ij (Polissar et al., 1999). Weighted PSCF values (WPSCF) were calculated by multiplying a particular W_ij (≤1.00) if the total number of the endpoints for one grid cell was lower than 3 times the average of the endpoints per each cell. Higher WPSCF values indicate higher potential contributions of NH₃ to the receptor site (IAP tower).

$$\begin{array}{}\text{(1)}& {\displaystyle }{\displaystyle }{W}_{ij}=\left\{\begin{array}{ll}\mathrm{1.00}& \mathrm{80}<n_{ij}\\ \mathrm{0.70}& \mathrm{20}<n_{ij}\le \mathrm{80}\\ \mathrm{0.42}& \mathrm{10}<n_{ij}\le \mathrm{20}\\ \mathrm{0.05}& n_{ij}\le \mathrm{10}\end{array}\right\}\end{array}$$

3 Results

3.1 Vertical profiles of NH₃ concentrations

The time series of weekly averages of NH₃ concentrations from 16 March 2016 to 16 March 2017 are shown in Fig. 2. The weekly NH₃ concentration across all heights averaged 13.3±4.8 µg m⁻³ during the year-long study period. Individual weekly concentrations ranged from 4.4 µg m⁻³ at 2 m to 25.3 µg m⁻³ at 32 m. Nearly all (99.6 %) of the weekly NH₃ concentrations along the profile exceeded 5 µg m⁻³. Summer concentrations were generally the highest. Maximum NH₃ concentrations mostly occurred between 32 and 63 m, decreasing both towards the surface and the top of the tower. Minimum concentrations mostly occurred at 2 and 320 m (Fig. S1 in the Supplement). Significant differences of annual average NH₃ concentrations across the vertical profile were only found between the “maximum concentration” height and the top two heights, i.e., 280 and 320 m (Fig. 3i). Even at 320 m, the annual average NH₃ concentration was still relatively high at 11.3 µg m⁻³ (Fig. 3i). During the whole observation period, the daily average boundary layer height was generally above 320 m, indicating that a good portion of the sampling occurred within a well-mixed boundary layer (Fig. S2).

Seasonal vertical concentration profiles exhibited fairly similar shapes to the annual average profile, although there were some important differences in absolute concentration values and the magnitude of vertical gradients within the profiles (Fig. 3). The average NH₃ concentration across the profile from high to low was observed in summer (18.2 µg m⁻³), spring (13.4 µg m⁻³), autumn (12.1 µg m⁻³) and winter (8.3 µg m⁻³). Proportional declines of the NH₃ concentration from the peak to higher and lower elevations differed between seasons: the greatest proportional decline was seen in autumn (28.1 % decrease from 63 to 320 m), followed by winter (23.8 %), summer (20.5 %) and spring (15.8 %) (Fig. S3).

Figure 3 Comparison of seasonal vertical NH₃ concentrations with the mean (dots), median, 10th, 25th, 75th and 90th percentiles of the NH₃ concentrations of each height for the IAP tower (Beijing, this study; a, c, e, g, i) and BAO tower (USA, Li et al., 2017; b, d, f, h, j). The lowercase letters next to the boxes denote the statistical difference in the NH₃ concentration between all heights, where a one-way ANOVA was used, at the p<0.05 level.

3.2 Meteorological variability

Vertical NH₃ concentration profiles varied substantially during the sampling period, along with vertical changes in meteorological parameters. Bivariate polar plots (Fig. 4) show that high NH₃ concentrations below 47 m were mostly observed during periods with low wind speeds (<4 m s⁻¹). As heights and associated wind speeds increased, the relationship between NH₃ concentrations and wind speed weakened. For example, at 280 m, the highest concentration was observed when the wind speed was also high (up to an average of ∼15 m s⁻¹).

Wind direction also plays an important role in air pollution transport. Transport from the northwest was typically associated with low NH₃ concentrations at all heights, consistent with the absence of large emissions sources in the mountains northwest of Beijing. It is noteworthy that high NH₃ concentrations at near-surface heights (8 and 15 m) always coincide with winds from the south, including the southeast and southwest directions. High NH₃ concentrations appear to be associated with winds from the northeast from 32 m to 80 m. Above 80 m, winds from the south contribute more to high NH₃ concentrations. Major regions of agricultural NH₃ emissions are located south and east of Beijing.

Figure 4 The frequency distributions of wind directions and NH₃ concentration for all height during the observation period. Radial data are WS (m s⁻¹) as a function of WD (^∘). The colors denote the NH₃ concentrations (µg m⁻³).

Figure 5 Probability density of NH₃ concentrations (µg m⁻³) at different ranges of temperature¹ (^∘C) and relative humidity² (%) for 14 heights. ¹ Temperature includes four subsets: <4, 4–12, 12–20 and >20^∘C; ² relative humidity includes four subsets: <25 %, 25–50 %, 50–75 % and >75 %.

To further investigate observed variability, we show the probability density function of NH₃ concentrations in relation to the relative humidity (RH) and temperature (T) (Fig. 5). Clear positive relationships between T and NH₃ concentrations were found at all heights from low RH to high RH. When T was low (T<12 ^∘C), the NH₃ concentration mostly fell below 10 µg m⁻³ under all RH conditions. The occurrence of high NH₃ concentrations increased with T>12 ^∘C, which is not surprising given that agricultural NH₃ emissions increase with T; furthermore, higher T and lower RH also shift the equilibrium of the NH₃(gas) + HNO₃(gas) ↔ NH₄NO₃(particulate) system toward the gas phase. Statistically, a strong positive relationship was found between NH₃ and T at all heights from the surface to the top of the tower (R²∼0.6; Fig. S4); both the slope and the correlation coefficients were similar across all heights. Although, a positive correlation between NH₃ and RH and a negative correlation between NH₃ and WS were found, the correlation coefficients were quite low.

Figure 6 Weighted potential source contribution analysis (WPSCF) of atmospheric NH₃ in Beijing from 16 March 2016 to 16 March 2017.

3.3 Potential source analysis

Analysis of the relationship between local wind direction and NH₃ concentrations does not fully clarify the potential source regions contributing to observed NH₃ at the sampling site (Fig. S6). Some seasonal variations were observed, i.e., the frequency of high NH₃ concentrations were greater under southerly winds than northwesterly winds in the spring, the increased frequency of high NH₃ concentrations were associated with southerly and easterly winds in the summer and autumn, and NH₃ concentrations still exceeded 5 µg m⁻³ during winter with relatively frequent winds from the northwest.

To examine the relationship between air transport and NH₃ concentrations more rigorously, weighted PSCF (WPSCF) during the four seasons were calculated for several measurement heights (2, 63, 180 and 320 m) (Fig. 6). In summer, from the surface to the tower top, a strong influence from source areas to the south of Beijing was seen, coinciding with regions (e.g., Tianjin, Henan, Hebei and Shandong provinces) characterized by elevated anthropogenic emissions of NH₃ (Fig. 1), largely from agricultural activities (Zhang et al., 2009; Gu et al., 2012). During summer, regions to the north and west of the monitoring site had low WPSCF values, whereas high WPSCF values to the south and southeast were common during spring. High WPSCF values were mainly located northwest and southeast of Beijing in autumn, while their WPSCF values were typically lower in winter than during other seasons.

It is important to remember that aerosol–gas partitioning can also strongly influence measured NH₃ concentrations. To investigate seasonal phase changes between NH₃ and ${\mathrm{NH}}_{\mathrm{4}}^{+}$, we define the NH₃ gas fraction ($F_{{\mathrm{NH}}_{\mathrm{3}}}$ = the gaseous NH₃ concentration divided by the sum of the gaseous NH₃ and fine particulate ${\mathrm{NH}}_{\mathrm{4}}^{+}$ concentrations), where the concentrations are expressed in molar units. Monthly average partitioning for these reduced inorganic nitrogen forms from a nearby urban monitoring site, 10 km from the IAP tower, is plotted in Fig. S8. The NH₃ gas fraction ($F_{{\mathrm{NH}}_{\mathrm{3}}}$) was found to be the highest in summer (0.83 in August) and the lowest in winter (0.36 in February). As expected, gas phase NH₃ is favored in the warmer months, while particle phase ${\mathrm{NH}}_{\mathrm{4}}^{+}$ is favored in the cooler months, with a gradual transition. Weekly ${\mathrm{NH}}_{\mathrm{4}}^{+}$ concentrations at the tower were estimated using weekly NH₃ concentrations divided by monthly $F_{{\mathrm{NH}}_{\mathrm{3}}}$, and WPSCF analysis of the sum of NH₃ + ${\mathrm{NH}}_{\mathrm{4}}^{+}$ was then performed (see results in Fig. S9). Results of this total WPSCF (NH₃ + ${\mathrm{NH}}_{\mathrm{4}}^{+}$) analysis yielded similar patterns to the NH₃ WPSCF analysis for all heights and seasons, indicating the importance of the identified source regions for both the gaseous and particulate atmospheric forms of emitted NH₃.

Table 1 Overview of measured vertical NH₃ concentrations (µg m⁻³) in previous studies and in this study.

4 Discussion

4.1 Vertical NH₃ concentration profiles

The North China Plain is a well-known “hotspot” for NH₃ emissions due to the rapid development of industrialization, urbanization and intensive agriculture (Kang et al., 2016; Y. Zhang et al., 2010). In our study, high atmospheric NH₃ concentrations (13.3±4.8 µg m⁻³) were found up to 320 m a.g.l. in urban Beijing (16 March 2016–16 March 2017), and were much higher than the average annual NH₃ concentration (3.3±1.4 µg m⁻³) observed across a vertical profile at the 300 m rural BAO tower, USA (Li et al., 2017). Some studies of NH₃ vertical distribution found that the NH₃ concentration decreased significantly with height. For example, Tevlin et al. (2017) reported an overall increase in summertime NH₃ mixing ratios toward the surface of 6.7 ppb or 5.1 µg m⁻³ (89 %) during the day and 3.9 ppb or 3.0 µg m⁻³ (141 %) at night. In the BAO tower study (Li et al., 2017), which also measured concentrations using passive (Radiello) samplers deployed for 1- to 2-week sample periods, the concentration profiles showed a similar overall vertical distribution: the minimum NH₃ concentration was observed at the top of the tower, it slowly increased towards a peak concentration at ∼10 m, and a sharp reduction was then seen near the surface. By contrast, our results showed much smaller decreases in NH₃ concentrations in the upper air in urban Beijing (Table 1), with only a 1.18 µg m⁻³ (9.5 %) average decrease from the surface to the top of the tower (Fig. 3i). The flatter shape of the Beijing vertical profile may reflect a combination of strong local (e.g., vehicle) and regional (e.g., industrial and agricultural emissions) sources (Figs. 2 and 6) in our study, the fact that deep mixing layers regularly enveloped the full height of the tower within the surface boundary layer so that all sources influencing the tower measurements were vertically well mixed (Fig. S2), and/or the averaging of more distinct profiles over the week-long sample periods. In contrast to the “rural” boundary layer above the fields surrounding the BAO tower, the mixing in the Beijing urban area could be greatly enhanced by larger surface roughness (e.g., the average urban building height is ∼50 m) and surface heating (Baklanov and Kuchin, 2004). Higher time resolution vertical profile measurements are needed in the future to untangle the influence of these potential factors.

Distinct seasonal variations in NH₃ concentrations were found (Fig. 2), which were statistically most strongly associated with temperature rather than relative humidity or wind speed (Fig. S4). High temperatures enhance NH₃ emissions from soil, applied fertilizers, animal waste, vertical mixing and increase volatilization of NH₃ from NH₄NO₃ particulate matter (Bari et al., 2003; Ianniello et al., 2010; Li et al., 2014; Lin et al., 2006; Meng et al., 2011; Plessow et al., 2005; Walker et al., 2004; Zbieranowski and Aherne, 2012). While high (low) mixed-layer heights in spring and summer (autumn and winter) could dilute (concentrate) NH₃ in the surface boundary layer (Fig. S3), average NH₃ concentrations across the profile were actually high in summer/spring and low in winter/autumn, consistent with the strong temperature-driven seasonal variation of the NH₃ concentration and the greater NH₄NO₃ particle formation during cold periods in autumn and winter. Conducting simultaneous measurements of fine particle composition at different heights in future studies would be valuable for more closely evaluating the influence of changes in phase-partitioning.

Li et al. (2017) found a vertical difference of approximately 75 % from the concentration peak near the surface to the top of the BAO tower in winter (Fig. 3j), and attributed this strong vertical gradient to the occurrence of low level temperature inversions which trapped emissions closer to the surface during this period. During our study in Beijing, the vertical gradient was only 28 % in winter (maximum concentration found at 32 m), consistent with a deeper average boundary layer. However, inversions did limit the vertical mixing of NH₃ during some periods in Beijing. Examination of the thermal inversion layer probability at 06:00 and 15:00 LT (Fig. S7b and c) revealed that T inversions (0.22±0.26 ^∘C) frequently occurred between 102 and 160 m. Consequently, persistent higher NH₃ concentrations begin at a lower altitude (Fig. S7a) as also observed by Tevlin et al. (2017). Because the time resolution of our Beijing study was one sample per week, we could not catch the changes between the daytime and nighttime NH₃ vertical mixing. Compared to NH₃ monitoring in real time (Tevlin et al., 2017), weekly sampling smooths diurnal vertical distributions and makes it harder to identify the influence of local surface sources or sinks.

Surfaces can act either as sources or sinks of NH₃, depending on the surface NH₃ content, ambient NH₃ concentrations, and local meteorology and surface type (Tevlin et al., 2017; L. Zhang et al., 2010 ). The maximum NH₃ concentration occurrence at 2 m in Beijing and the concentration decrease with increased height may reflect an important surface source of NH₃, although our limited time resolution makes such conclusions tentative. The influence of the evaporation of dew/precipitation may also be important. Some studies found that dew is both a significant nighttime reservoir/sink and strong morning source of NH₃ (Wentworth et al., 2016; Teng et al., 2017).

4.2 Potential source analysis

Areas south of Beijing with high WPSCF values appear to be important NH₃ source regions (Fig. 6), suggesting regional transport from high agricultural NH₃ emission areas (e.g., Hebei, Henan, Shandong provinces) contributed significantly to atmospheric NH₃ in the Beijing urban region. Consistently higher NH₃ concentrations were observed during periods with winds from the southeast, south and southwest at all heights, especially in summer (Fig. S6). Although NH₃ has a limited atmospheric lifetime with respect to dry deposition, concentrations in these agricultural NH₃ source regions can be extremely high (Shen et al., 2011) while significant NH₃ can be tied up in longer-lived ammonium nitrate particles that partially dissociate to release NH₃ back to the gas phase in response to NH₃ loss by dry deposition (Ianniello et al., 2011; Kang et al., 2016; Xu et al., 2017). The WPSCF (Fig. 6) and NH₃ emissions distribution (Fig. 1a) both suggest the importance not only of regional transport from nearby areas, but also the potential for local emissions to play an important role in sustaining the high NH₃ level in Beijing, e.g., vehicular traffic (Chang et al., 2016; Pan et al., 2018a). As discussed above, stagnant meteorological conditions with low WS and T inversions allow local emissions, such as those from urban traffic, to accumulate. Additionally, the topography of the mountains to the west and north of Beijing effectively traps polluted air over Beijing during southerly airflow, an effect reported in many Beijing particulate matter studies (Xia et al., 2016; Wu et al., 2009; Zhao et al., 2009).

Generally, NH₃ source regions identified in the WPSCF analysis (Fig. 6) suggest that regional transport from the south exerts an important influence on Beijing NH₃ concentrations throughout the year. The area south of Beijing (e.g., Hebei, Henan and Shandong provinces) is a hotspot of NH₃ emission (Zhang et al., 2018), and half of NH₃ emissions have been estimated to deposit as NH₃ at urban sites in the North China Plain (Pan et al., 2018b). In addition, seasonal patterns of NH₃ potential sources (Fig. 6) matched well with the seasonal surface NH₃ concentrations in China (Zhang et al., 2018). In detail, NH₃ concentrations were typically highest in summer, and south winds produced higher NH₃ concentrations than other wind directions (Fig. S6). Spring and summer had a similar wind direction distribution (Fig. S6) and wind speeds (Fig. S5), but corresponding NH₃ concentrations were lower in spring. This may reflect decreased emissions in regions to the south during cooler spring temperatures and the increased partitioning of NH₃ into fine particles during this cooler season. As shown above aerosol–gas partitioning strongly influences NH₃ concentrations; high $F_{{\mathrm{NH}}_{\mathrm{3}}}$ during warm periods, especially summer, favored greater NH₃ gas concentrations due to the thermodynamic tendency for NH₄NO₃ to dissociate to NH₃ and HNO₃ at high temperatures. Although $F_{{\mathrm{NH}}_{\mathrm{3}}}$ was low in winter, indicating that ${\mathrm{NH}}_{\mathrm{4}}^{+}$ is the dominant NH_x form in this cold season, winter NH₃ concentrations across all heights still averaged 8.3±2.6 µg m⁻³, with a similar wind direction distribution as other seasons, except at high altitudes (i.e., 240 and 320 m; Fig. S6).

5 Conclusions and implications

Our study is the first to continually monitor the vertical concentration profile of NH₃ in urban Beijing. Weekly concentrations were measured for 1 year at 16 heights on the Beijing 325 m meteorological tower. The NH₃ concentration averaged 13.3±4.8 µg m⁻³. The highest NH₃ concentrations were always observed between heights of 32 and 63 m, decreasing toward the surface and toward higher altitudes.

NH₃ concentrations at all heights increased during warmer periods, consistent with increased NH₃ emissions under warm conditions and the tendency for semivolatile ammonium nitrate to release NH₃ to the gas phase. An analysis of the relationship between NH₃ concentrations and local wind direction showed a tendency for higher concentrations during transport from regions to the south of Beijing; this was consistent with findings from the WPSCF analysis which showed that important source areas were mainly located to the south of Beijing: an area comprised of large agricultural regions and high NH₃ emissions in the North China Plain. Local NH₃ sources, such as urban traffic emissions, may also help account for the elevated NH₃ concentrations (>5 µg m⁻³) observed even in periods when transport mostly came from the low NH₃ mountainous regions to Beijing's north/northwest.

High NH₃ concentrations in urban Beijing, from the surface up to 320 m, the important role that NH₃ plays in PM_2.5 and haze formation, and the importance of regional transport of NH₃ emissions from agricultural regions in neighboring provinces, suggest that future air quality improvement efforts should consider NH₃ emission reductions and that the pollution controls should be jointly practiced at regional scales (e.g., the whole North China Plain) rather than only controlling local Beijing sources.