From January 2018 to January 2019, continuous and simultaneous observations of atmospheric NH3 were conducted at an urban site and a suburban site in Beijing. The urban site was located on the roof of the Science and Technology Building of Minzu University of China (39.95∘ N, 116.32∘ E, altitude: 102 m) and the suburban site was in the Changping Meteorological Station (40∘13′ N, 116∘13′ E; 77 m altitude). The suburban site is in the northwestern direction relative to the urban site, and the shortest distance between these two sites is approximately 32 km (Fig. 1). More farmland and grassland is located around the suburban site than the urban site.

NH3 concentrations were measured using two NH3 analyzers (Economical ammonia analyzer, Los Gatos Research Inc., USA), which have a minimum detection limit of <0.2 ppb and a maximum drift of 0.2 ppb over 24 h. The NH3 analyzers were deployed in air-conditioned rooms. These analyzers use off-axis integrated cavity output spectroscopy (OA-ICOS) technology, which is a fourth-generation cavity-enhanced absorption technique, to simultaneously measure NH3 and water vapor (H2O) in the atmosphere. The incident laser beam of the OA-ICOS technology deviates from the optical axis, which differs from the traditional coaxial incidence mode. The axial incidence mode of the OA-ICOS technology can increase the optical path, stimulate additional high-order transverse modes, effectively suppress the noise of the cavity mode, reduce the cross interferences and errors due to contaminants existing in the cavity, and improve the detection sensitivity (Baer et al., 2002, 2012). The analyzer method is a quasi-absolute measurement, which theoretically does not require calibration. However, to ensure the comparability of the obtained data with other monitoring data, NH3 standard gas (Beijing AP BAIF Gases Industry Co., Ltd.) was used for a comparison measurement before the observation. The recorded concentrations were revised with respect to the reference NH3 concentration in the standard gas mixture.

Ambient air was drained at >0.4 Lmin-1 through Teflon lines (1/4′ o.d.) from a manifold. The lengths of the Teflon lines were designed to be as short as possible (less than 2 m from the manifold). Particulate matter was filtered by Teflon membranes with a pore size of less than 5 µm. As NH3 easily “sticks” to surfaces (such as the inside walls of tubes), heated sample lines have been suggested by many measurement studies. However, according to our laboratory test (Fig. S1), when the heating (70 ∘C) was on, there was a peak lasting 5–6 min that then deceased to normal levels in ambient air, which means that a new balancing process was established in less than 10 min. This suggests that heating is not necessarily a solution for NH3 sticking. Maintaining the relatively stable balance between adsorption and desorption of NH3 in the sampling system is important. When tested using air with different humidity levels, only very sharp changes in humidity obviously influenced and altered the balance, and a new balance required tens of minutes to be reestablished (Fig. S2). Under normal weather conditions, humidity changes in a relatively smooth way unless a quickly changing weather system, like rain, is approaching. The minute-level data were converted into hourly averages during the data analysis process, and the hourly resolution can smooth the effect caused by variations in humidity and temperature during the observation to some extent.

The balancing idea was also used to carry out multi-point calibrations on the NH3 analyzers (Fig. S3). A high mixing ratio (e.g., 400 ppb or higher) of NH3 mixing gases was firstly produced by a dynamic diluter and measured by the NH3 analyzers overnight. After the signals reached a stable level, other lower span values were switched in turn. At each span point, the measurement time was at least 40 min or longer. A linear regression function was then obtained with an R2 higher than 0.999. Nowadays, NH3 in a compressed gas cylinder is also trustworthy, as confirmed by the comparison with the NH3 standard in a permeation tube (Fig. S4).

In total, 7645 and 8342 valid hourly mean observations were obtained for the urban (Haidian) and suburban (Changping) sites, respectively. In addition, the urban and suburban meteorological data (temperature, relative humidity, wind direction, and wind speed) during the sampling period were obtained from the Haidian Meteorological Observation Station and Changping Meteorological Station, respectively.

Figure 2 displays the time series variations in the NH3 mixing ratios, temperatures, and relative humidity at the urban and suburban sites in Beijing. At the urban site, the mean ±1σ, median, maximum, and minimum values of the hourly average NH3 mixing ratio during the observation period were 21±14, 17, 133 and 1.6 ppb, respectively. At the suburban site, the corresponding values were 22±15, 18, 199, and 0.8 ppb, respectively. The annual average and range of the NH3 mixing ratio at the suburban site were marginally higher than those at the urban site. The characteristics of the weekly smoothed data indicate that the NH3 variations and temperature and humidity fluctuations at the two sites were practically consistent, which suggests that both sites were under the influence of similar weather systems. The hourly mean NH3 concentrations at the urban site were significantly correlated (R=0.849, P<0.01) with those at the suburban site.

Table 1 shows the comparison of the atmospheric NH3 concentrations (ppb) observed in different areas. Meng et al. (2011) obtained an average NH3 mixing ratio of 22.8±16.3 ppb for the period from 2008 to 2010 in the Beijing urban area, which is very close to our result (21±14 ppb) for 2018–2019. Therefore, the annual average NH3 mixing ratio in urban Beijing did not change significantly from 2008 to 2019. Moreover, results from this study and Meng et al. (2011) indicate that the NH3 concentrations at the urban and suburban sites were higher than those in the background areas. The observed NH3 concentrations in Beijing were higher than those in northwestern China (Meng et al., 2010) and the Yangtze River Delta region (Chang et al., 2019). The average annual NH3 concentration in the urban area of Shanghai, a megacity in Southeast China (31∘ N), was approximately 50 % lower than that in urban Beijing. This might be related to the fact that the North China Plain, in which Beijing is located, is one of the most intensive agricultural production regions in China. The differences in the soil properties of Beijing and Shanghai may be another reason for this difference, as the loss of soil NH3 can increase with an increase in the soil pH (Ju et al., 2009). Shanghai and its surrounding areas are dominated by the acidic soil of paddy fields (Zhao et al., 2009), whereas Beijing is dominated by the alkaline soils of dry lands (Wei et al., 2013). In addition, the climate in Beijing is much drier than in Shanghai; thus, less atmospheric NH3 can be removed by wet deposition in Beijing compared with Shanghai.

Table 1 also shows observational results of atmospheric NH3 from some other countries. The NH3 concentrations in the USA (Edgerton et al., 2007; Nowak et al., 2006; Zhou et al., 2019), Scotland (Burkhardt et al., 1998), Canada (Hu et al., 2014), Japan (Osada et al., 2019), and Germany (Vogt et al., 2005) were 0.23–13, 1.6–2.3, 0.1–4, 4.1, and 5.2 ppb, respectively. These values are considerably lower than those in Beijing. However, Delhi, India (Saraswati et al., 2019), exhibited a higher NH3 concentration (53.4±14.9 ppb) than Beijing did. This result might be attributed to the well-developed livestock breeding activities in Delhi. This comparison indicates that the NH3 concentration in Beijing did not change considerably in the decade before 2019; however, the NH3 concentration in Beijing is the highest of the big cities in China and much higher than those observed in developed countries in America, Europe, and Asia.

Figure 3 displays the monthly statistics for the NH3 mixing ratios at the urban and suburban sites in Beijing. The seasonal variations in the NH3 mixing ratios were very similar at the urban and suburban sites, with higher mixing ratios in the spring and summer and low values in the autumn and winter (Table 2). The daily mean concentrations fluctuated considerably in the spring, particularly in April. The highest mean NH3 concentrations at the urban and suburban sites were 42±17 and 42±8.2 ppb, respectively. Both occurred in July, when the NH3 concentrations also fluctuated considerably. On average, the seasonal NH3 mixing ratios at the urban and suburban sites can be arranged as follows: summer > spring > autumn > winter. The main grain crops in the rural area of Beijing are corn and wheat. Corn is categorized as spring corn and summer corn, which are sown in April and June, respectively. Usually, a large amount of base fertilizer is applied when planting corn, and the topdressing is applied after 2 months. Wheat is sown from September to October, and the topdressing is applied in the following spring. The volatilization of nitrogen fertilizers can cause an increase in atmospheric NH3 mixing ratios and their fluctuations in fertilization seasons (Zhang et al., 2016). In addition, the high temperature in summer should also be responsible for the high NH3 mixing ratios in this season. An increase in the temperature can increase the biological activity and, thus, enhance the NH3 production and emission. High temperature is also conducive for the volatilization of the urea and diammonium phosphate applied to crops. Moreover, the equilibrium among ammonium nitrate particles, gaseous NH3, and nitric acid is transferred to the gas phase at high temperature, which increases the NH3 concentration (Behera et al., 2013). Sewage treatment, household garbage, golf courses, and human excreta are crucial NH3 sources that are easily neglected (Pu et al., 2020). The relatively low NH3 concentrations in the autumn and winter might be caused by the decrease in NH3 emission in the soil and vegetation, the decrease in the NH4NO3 decomposition capacity at low temperatures, and the reduced human activities caused by a large floating population returning to their native locations outside of Beijing during the Spring Festival (Liao et al., 2014). In spring and summer, the NH3 mixing ratios at the suburban site were higher than those at the urban site, which might be related to the higher agricultural activity around the suburban site. In the autumn and winter, the NH3 mixing ratios at the urban site were marginally higher than those at the suburban site. In the autumn and winter seasons, the influences of agricultural activities on the NH3 concentration were weakened, whereas the influences of other sources (such as traffic sources) were enhanced. According to Wang et al. (2019), the traffic NH3 emissions per unit area in Haidian (urban site) were 3 times higher than in Changping (suburban site). This difference in traffic source emissions might have resulted in higher NH3 concentrations at the urban site than at the suburban site in the autumn and winter.

Figure 4 displays the average diurnal variations in the NH3 and H2O mixing ratios in different seasons at the urban and suburban sites in Beijing. Ambient NH3 exhibited different diurnal behaviors in different seasons.

In spring, the average diurnal variations in the NH3 mixing ratio were similar at the urban and suburban sites. The diurnal variations exhibited a single-peak pattern with high values in the daytime and low values at night. The NH3 mixing ratio began to increase in the morning, reached its maximum value at 16:00 Beijing time, UTC+8, and then decreased gradually. The lowest mixing ratios at the urban and suburban sites occurred at 03:00 and 09:00, respectively. The NH3 mixing ratio began to increase earlier at the urban site than at the suburban site. A plausible explanation for the earlier increase in NH3 emissions at the urban site is traffic emissions during the morning rush hours. In spring, the mixing ratio of NH3 was higher at the suburban site than that at the urban site, with an average difference of 4.1 ppb and a maximum difference of 6.1 ppb. The average diurnal amplitude of the NH3 mixing ratio at the suburban site was 5.3 ppb, which was higher than that (2.6 ppb) at the urban site. At the urban site, the average diurnal variations in the NH3 and H2O mixing ratios exhibited nearly opposite trends. The H2O mixing ratio had high values at night and low values during the day. At the suburban site, the variation characteristics of NH3 and H2O were very similar; however, the peak NH3 concentration occurred 5 h earlier than the peak H2O concentration. In spring, in contrast to the NH3 mixing ratio, the H2O mixing ratio at the urban site was 1279 ppm higher than that at the suburban site.

The diurnal variation in the NH3 mixing ratio at the suburban site in summer was similar to that in spring. This phenomenon was also observed in the rural areas of Shanghai by Chang et al. (2019). The diurnal variations in NH3 at the suburban site were considerably affected by the temperature and the contribution from volatile NH3 sources. However, the diurnal summer variation of NH3 at the urban site was completely different from that at the suburban site. The summer level of NH3 at the urban site was obviously lower during the daytime and evening than that at the suburban site, increased gradually from 21:00 to levels higher its suburban counterpart, dropped after reaching its peak value at 07:00, and then reached its lowest value at 14:00. This diurnal pattern (with a peak in early morning) has been observed in other areas, such as rural (Ellis et al., 2011), urban (Gong et al., 2011), and steppe areas located far away from human activity (Wentworth et al., 2016). Kuang et al. (2020) believed that such a diurnal pattern of NH3 was caused by the evaporation of dew in the morning, which resulted in the release of NH3 originally stored in the droplets. A lag was observed between the changes in the NH3 and H2O concentrations in the early morning, which supported the hypothesis of Kuang et al. (2020). In addition, the increase in the NH3 concentration in the morning might have been caused by the breakup of the boundary layer formed at night. The downward mixing of air with a higher NH3 concentration in the residual layer led to a morning increase in the NH3 concentration on the ground (Bash et al., 2010). In summer, the NH3 concentrations at the suburban site were significantly higher than those at the urban site during the daytime and first half of the night. The average diurnal amplitude of the NH3 concentration was 7.5 and 3.7 ppb at the urban and suburban sites, respectively. Similar to the situation in spring, the H2O concentrations at the urban site were significantly higher than those at the suburban site in the summer.

In autumn, the NH3 concentration at the suburban site was relatively stable and remained almost consistently lower than that at the urban site, which showed low values during the day and high values during the night, with a peak at midnight and a minimum (about 2.0 ppb lower than the peak) at 17:00. The H2O concentration was marginally lower (250 ppm) at the urban site than at the suburban site. The diurnal profiles of H2O at both sites resemble that of NH3 at the urban site, but the lowest values of H2O occurred earlier than the lowest value of NH3 at the urban site.

The diurnal patterns of NH3 and H2O in winter were similar to those in autumn, although the mixing ratios of NH3 and H2O were lower than their autumn counterparts. There were two slight differences: (1) the mixing ratios of NH3 at both sites exhibited lower fluctuations than those in autumn, and (2) the mixing ratio of NH3 at the urban site reached its minimum in winter earlier than that in autumn.

The above results indicate that although the two sites were under the influence of similar weather systems, the diurnal variations in the NH3 mixing ratios at the two sites were different in different seasons. This finding suggests that different NH3 sources and possibly sinks had different contributions to the NH3 concentrations at the urban and suburban sites. Additional studies should be conducted to better understand the behaviors of atmospheric NH3 and its influencing factors.

Table 3 presents the annual and seasonal correlation coefficients between the daily means of NH3 mixing ratios and those of the temperature, relative humidity, and wind speed at the two sites. Annually, the NH3 mixing ratios at both sites were positively correlated with temperature and relative humidity and negatively correlated with wind speed, and the correlations are all highly significant. However, the correlations deteriorated somewhat in warm seasons. In summer and autumn, no significant correlations were noted between ambient NH3 and temperature at the two sites. The correlation between NH3 and wind speed in summer was much weaker than in the other seasons. The relative humidity was more strongly correlated with the NH3 concentration at the two sites than temperature, which can be perceived in Fig. 2. Also, the correlation between NH3 and relative humidity did not vary much from season to season. This implies the possibility that relative humidity exerts a certain influence on the variation of the NH3 level in the surface layer.

Figure 5 displays the seasonal mean diurnal variations in the NH3 mixing ratio, temperature, and relative humidity in different seasons at the urban and suburban sites, with their correlation coefficients shown in Fig. S5. At the urban site, the seasonal hourly means of the NH3 mixing ratio were positively (negatively) correlated with those of temperature (relative humidity) in spring, but the correlations were reversed in the other seasons. At the suburban site, the seasonal hourly means of the NH3 mixing ratio were positively (negatively) correlated with those of temperature (relative humidity) in the spring and summer, but they were less correlated in autumn and winter. Similar correlation behaviors (diurnal variations) were found at both sites in spring, but the correlations (diurnal variations) at the urban site behaved differently from those at the suburban site in other seasons. The inconsistent behavior in summer, autumn, and winter caused urban–suburban differences in the annual diurnal patterns of NH3, temperature, and relative humidity as well as the NH3–temperature (relative humidity) correlations, as can be seen from Fig. S6. Figure 6 displays the contour maps of the NH3 mixing ratio, temperature, and relative humidity in different seasons at the urban and suburban sites. The annual contour maps are shown in Fig. S7. As shown in these contour maps, the NH3 mixing ratios at both sites increased with relative humidity at the same temperature and increased with temperature at the same relative humidity. Although there is some scatter in the contour maps, high NH3 levels are generally associated with high temperature and humidity. In winter, when the air temperature was low (<0 ∘C), the NH3 mixing ratios at both sites often had low values except at high humidity (>60 %). An increase in temperature caused higher NH3 mixing ratios at both sites; however, the NH3 concentration at the suburban site was more significantly correlated with temperature than that at the urban site (Table 3), suggesting that volatile NH3 sources might have a higher contribution to the NH3 concentration in suburban than in urban area. A higher amount of NH3 removal via chemical transformation is expected during the day at the urban site than at the suburban site, as the urban area had a higher relative humidity, higher particulate matter concentrations, and higher acid gas emissions (particularly NOx) than the suburban area. In 2018, the concentrations of PM2.5, SO2, and NO2 were 50, 5, and 43 µgm-3 in Haidian and 46, 6, and 35 µgm-3 in Changping, respectively, as reported by the Beijing Ecology and Environment Statement.

To explore the influence of wind on the NH3 mixing ratios, wind rose diagrams were drawn for the hourly mean concentration of NH3, wind direction frequency, and wind speed during the observation period (Fig. 7). The large-scale wind circulation in the North China Plain is often influenced by the mountain–plain topography; therefore, the dominant winds in this region are southerly (from noon to midnight) and northerly (from midnight to noon) (Lin et al., 2009, 2011). As displayed in Fig. 7, some differences existed in the distributions of the surface wind between the urban and suburban sites. The prevailing surface winds were northeasterly and southwesterly at the urban site and northwesterly and easterly at the suburban site. At the urban site, the NH3 mixing ratios were relatively high when the winds originated from the southern sectors and relatively low when the winds originated from the northwestern sectors. Therefore, under southwesterly wind conditions, air masses from the south of Beijing carry not only air pollutants but also higher levels of NH3 to the urban site. Meng et al. (2017) examined the effect of long-range air transport on the urban NH3 levels in Beijing during the summer using trajectory analysis. They concluded that the air mass from the southeast has a cumulative effect on the NH3 concentration. Although the dominant wind direction at the suburban site was different from that at the urban site, the NH3 mixing ratios were also relatively high in the southern sectors. Thus, winds from the southeast, south, and southwest can elevate levels of atmospheric NH3 at both the urban and suburban sites. The NH3 mixing ratios were relatively low when air masses originated from the northwestern sector at the urban site and from the western sector at the suburban site. The western and northwestern winds were stronger and promoted the dilution and diffusion of NH3 emitted into the boundary layer.

As a water-soluble gas, NH3 can be impacted by precipitation. Heavy rainfall occurred on 18 August 2018 (Fig. 8). Before the rainfall, the NH3 concentration at the urban site was higher than the average level in August. After the rainfall, the NH3 concentration decreased rapidly, and it was significantly lower than the mean value in August. However, the diurnal pattern of NH3 on that day did not differ considerably from the average diurnal pattern in August. On the same day, the NH3 mixing ratio at the suburban site remained at a low level during the rainfall period, which was considerably lower than the August mean NH3 concentration during the same time of day. However, the NH3 mixing ratio increased rapidly after the precipitation and reached the mean level at 17:00. The rainfall might have had an obvious clearing effect on NH3, but more case studies are needed to reach a robust conclusion.