Measurement report: Exploring the NH3 behaviours at urban and suburban Beijing: Comparison and implications

Ammonia (NH3) plays an important role in particulate matter formation; however, few long-term observations with a high temporal resolution have been conducted on the NH3 concentrations in Beijing. In this study, online ammonia analyzers were used to observe continuously the atmospheric NH3 concentrations at an urban site and a suburban site in Beijing from 10 January 13, 2018, to January 13, 2019. The average mixing ratio of NH3 at the urban site was 21 ± 14 ppb (range: 1.6–133 ppb) and that at the suburban site was 22 ± 15 ppb (range: 0.8–199 ppb). The NH3 mixing ratios at the urban and suburban sites exhibited similar seasonal variations, with high values being observed in the summer and spring and low values being observed in the autumn and winter. The hourly mean NH3 mixing ratios at the urban site were highly correlated (R = 0.849, P < 0.01) with those at the suburban site. However, the average diurnal variations in the NH3 mixing ratios at the urban and 15 suburban sites differed significantly, which indicated the different contributions of NH3 sources and sinks at the urban and suburban sites. In addition to the emission sources, meteorological factors were closely related to the changes in the NH3 concentrations. For the same temperature (relative humidity) at the urban and suburban sites, the NH3 mixing ratios increased with the relative humidity (temperature). The relative humidity was the factor with the strongest influence on the NH3 mixing ratio in different seasons at the two sites. In general, a high wind speed promoted a reduction in the NH3 mixing ratio. Similar 20 with other primary pollutants in Beijing, the NH3 mixing ratios were high when winds originated from the south and low when winds originated from the north and northwest.

Sample gases were drained through Teflon lines (1/4'OD), which lengths were designed as short as possible (less than 2 m).
Particulate matters were filtered by Teflon membranes with a pore size less than 5 μm. Since NH3 easily "sticks" to surfaces (like inside walls of tubes), heated sample lines were suggested by many measurements. However, according our test using certain concentrations of NH3 in the lab, when heating (70℃) was on, there did have a peak lasting several minutes and then deceasing to the normal level in when heating was off. This tells us that heating is not a solution for NH3 sticking. Keeping the 85 relatively stable balance between adsorption and desorption of NH3 in the sampling system are the most important. When tested by different humidity air, only very sharply change of humidity obviously influenced and changed the balance, and a new balance needed tens of minutes to reestablished. In the routine weather conditions, humidity changed in a relatively smoothing way except in a quickly changing weather system, like rainy days. The minute-level data were converted into hourly averages in the data analysis process and the hourly resolution can smoothing the effect to some extent caused by variations 90 in humidity and temperature during the sampling time.
The balancing idea was also used to carry out multiple calibrations on NH3 analyzers. A high mixing ratio (e.g. 400 ppb) of NH3 mixing gases were firstly produced by a dynamic diluter and measured by the NH3 analyzer overnight. After the signals were keeping in stable level, other lower span values were switched in turn. At each span point, the measurement time was lasting at least two hours. Then a linear regression function was obtained with R 2 higher than 0.999. Nowadays, NH3 in 95 compressed gas cylinder is also trustworthy, which result is concluded by the comparison with NH3 permeation tube.
Finally, 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. 100

Overall variations in the NH3 mixing ratios
Fig. 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 105 corresponding values were 22 ± 15, 18, 199, and 0.8 ppb, respectively. The annual average NH3 mixing ratio and range of the NH3 mixing ratio at the suburban site were marginally higher than those at the urban site. The variation characteristics of the weekly smoothing data indicated that the NH3 variations and temperature/humidity fluctuations at the two sites were practically consistent, which suggested 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. 110 https://doi.org/10.5194/acp-2020-1047 Preprint. Discussion started: 2 November 2020 c Author(s) 2020. CC BY 4.0 License. 115 Table 1 showed the comparison of the atmospheric NH3 concentrations (ppb) in different areas. Meng et al. (2011) obtained an average NH3 mixing ratio of 22.8 ± 16.3 ppb for the period 2008-2020 in Beijing urban area, which means there was no significant change in the annual average NH3 mixing ratio from 2018 to 2019 compared with the change in the average NH3 mixing ratio over the past decade. Moreover, 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 northwest China (Meng et al. 120 2010) and the Yangtze River Delta region (Chang et al. 2019). For example, the average annual NH3 concentration in the urban area of Shanghai, a mega city in the south of China (31° N), was approximately 50% lower than that in Beijing. This result 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 because 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 125 by acidic soil of paddy fields (Zhao et al., 2009), whereas Beijing is dominated by the alkaline soils of dry land (Wei et al., 2013).  The NH3 mixing ratios in the United States (Edgerton et al., 2007;Nowak et al., 2006;Zhou et al. 2019), Great Britain (Burkhardt et al., 1998), Canada (Hu et al., 2014), and Japan (Osada et al., 2019) were 0.23-13, 1.6-2.3, 0.1-4, and 4.1 ppb, respectively. These NH3 mixing ratios are considerably lower than that in Beijing. However, Delhi, India (Saraswati et al., 2019), exhibited a higher NH3 mixing ratio (53.4±14.9 ppb) than Beijing did. This result might be attributed to the welldeveloped livestock breeding activities in Delhi. The comparisons indicate that in the past decade, NH3 concentration in Beijing 135 has not changed considerably, but that it is higher than in large cities in other developed countries. The highest mean NH3 concentrations at the urban and suburban sites were 42± 17 ppb and 42 ± 8.2 ppb, respectively. The NH3 concentrations fluctuated considerably in July. On average, the NH3 mixing ratios at the urban and suburban sites can be 145 arranged according to season as follows: summer > spring > autumn > winter. The main grain crops in Beijing are corn and wheat. Corn is categorized as spring corn and summer corn, which are sown in April and June, respectively. 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 the NH3 mixing ratios and fluctuations in fertilization seasons . In addition, the NH3 mixing ratios are 150 relatively high in the summer season due to the relatively high temperature in this season. An increase in the temperature can increase the biological activity and thus enhance the NH3 emission. A 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 NH3 concentration (Behera et al., 2013). Sewage treatment, household garbage, golf courses, and human excreta are crucial NH3 sources that are easily 155 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 Beijing during the Spring Festival (Liao et al., 2014). In the 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, 160 the NH3 mixing ratios at the urban site were marginally higher than those at the suburban site. This result was obtained possibly because in the autumn and winter seasons, the influence of agricultural activities on the NH3 concentration weakened, whereas the influences of other sources (such as traffic sources) on the NH3 concentration were enhanced. According to Wang et al. (2019), the traffic NH3 emission per unit area in Haidian was three times higher than that in Changping. This difference in traffic source emissions might have resulted in higher NH3 concentrations at the urban site than at the suburban site in the 165 autumn and winter.

Seasonal variations
https://doi.org/10.5194/acp-2020-1047 Preprint. Discussion started: 2 November 2020 c Author(s) 2020. CC BY 4.0 License.  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, 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. This result was obtained possibly due to the increased NH3 emission at the urban site due to traffic in the morning rush hours. On average, the mixing ratio of NH3 was considerably higher at the suburban site than that 185 at the urban site, with an average difference of 4.1 ppb and a maximum difference of 6.1 ppb in the NH3 mixing ratios of the sites. The average fluctuation in 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 opposite trends.

Diurnal variations
The H2O mixing ratio had high values in the night and low values in the day. At the suburban site, the variation characteristics of NH3 and H2O were very similar; however, the peak NH3 concentration occurred 5 hours earlier than the peak H2O 190 concentration. In the 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 variations in the NH3 mixing ratio at the suburban site in the summer were similar with those in the spring. This phenomenon was also observed in the rural areas of Shanghai by Chang et al. (2019). The diurnal variations were considerably affected by the temperature and the contribution from volatile NH3 sources. However, at the urban site, the diurnal variations 195 in the NH3 mixing ratio were different. In the summer, the NH3 mixing ratio increased gradually from 21:00, decreased after reaching its peak value at 7:00, and then reached its lowest value at 14:00. The diurnal variability pattern (with a peak value in the 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 the diurnal variability pattern was caused by the evaporation of dew in the morning, which results in the release of NH3 that was originally stored in the 200 droplets. A lag was observed between the changes in the NH3 with 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 high NH3 concentration in the residual layer led to an increase in the NH3 concentration on the ground in the morning (Bash et al., 2010). The NH3 and H2O concentrations at the urban and suburban sites exhibited opposite diurnal variations patterns in the spring. In the summer, the 205 NH3 concentrations at the suburban site were significantly higher than those at the urban site during the daytime and first half of the night. However, the NH3 concentrations at the urban site were significantly higher than those at the suburban site during the second half of the night. The average fluctuation in the NH3 concentration was 7.5 and 37 ppb at the urban and suburban site, respectively. Similar with the situation in the spring, the H2O concentrations at the urban site were significantly higher than those at the suburban site in the summer. 210 In the autumn, the NH3 concentration at the urban site had low values during the day and high values during the night. The peak NH3 concentration occurred at midnight, and the lowest NH3 concentration occurred at 17:00. There was essentially no diurnal variation in the NH3 concentration at the suburban site, but obvious at the urban site with a fluctuation of 2.0 ppb. The concentration of NH3 at the urban site was 1.2 ppb higher than that at the suburban site. The H2O concentration was marginally lower (250 ppm) at the urban site than at the suburban site. The correlation between the diurnal variations in the NH3 and H2O 215 concentrations was strong; however, the lowest value of NH3 occurred later than the lowest value of H2O at the urban site. The correlation between the diurnal variations in the NH3 and H2O concentrations was poor at the suburban site.
In the winter, the NH3 mixing ratios at the urban and suburban sites exhibited similar diurnal variation patterns. The mixing ratios exhibited high values in the night and low values in the day. However, the NH3 mixing ratio at the urban site was higher than that at the suburban site. This result was related to the decrease in the boundary layer height and temperature as well as 220 the slow conversion and easy accumulation of pollutants in the night. In the daytime, increases in the temperature and boundary layer height enhanced the diffusion of pollutants, and the NH3 mixing ratio decreased. The H2O mixing ratio at the suburban site was close to that at the urban site, but higher in the morning. In the winter, the average diurnal variation in the NH3 concentration was well correlated with that in the H2O concentration at the urban site (R = 0.89). The correlation was close to that (R = 0.93) obtained by Teng et al. (2017) between the NH3 and H2O concentrations in the winter. However, at the suburban 225 site, the mean diurnal variation in the NH3 mixing ratios had a poor correlation with that in the H2O mixing ratios. https://doi.org/10.5194/acp-2020-1047 Preprint. Discussion started: 2 November 2020 c Author(s) 2020. CC BY 4.0 License.
The results indicated 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 suggested 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 on the behaviors of NH3. 230 Table 3 presents the correlations between the daily mean NH3 mixing ratios and the diurnal mean values of the temperature, relative humidity, and wind speed at the two sites. Annually, the correlations were highly significant as the NH3 mixing ratios at both sites were significantly and positively correlated with the temperature and relative humidity and negatively correlated with the wind speed. However, in the summer and autumn, no significant correlations were noted between the NH3 and 235 temperature at the two sites. The relative humidity had the stronger influence on the NH3 concentration at the two sites than temperature, which phenomenon also can be found in Fig 2. The fluctuations between NH3 and relative humidity were much more consistent.  summer, but less correlated in the fall and winter. Similar behaviors were found in spring, but different in other seasons. In general, the annual diurnal behaviors of NH3 with temperature and relative humidity were different at the urban and suburban 250 sites (see Figure S1). Fig. 6 displays the contour maps of the NH3 mixing ratio, temperature, and relative humidity in different seasons at the urban and suburban sites. As displayed in Fig. 6 and Fig. S2, the NH3 mixing ratios at both sites increased with the relative humidity at the same temperature and increased with the temperature at the same relative humidity. In winter, when the temperature was low (< 0 °C), the NH3 mixing ratios at both sites often had low values except in high humidity (>60%).

Table 3. Correlations between the daily mean values of NH3 and meteorological elements (Spearman's rank correlation coefficient)
An increase in the temperature increased the NH3 mixing ratios; however, the NH3 concentration at the suburban site was more 255 significantly affected by the temperature than that at the urban site (Table 3), indicating that volatile NH3 sources might have a higher contribution to the NH3 concentration at the suburban site than at the urban site. A higher amount of NH3 removal through chemical transformation was expected during the day at the urban site than at the suburban site because the urban site had a higher relative humidity and higher amounts of primary particulate matter, NOx, and SO2 acid gas emissions than the suburban site did. In 2018, the concentrations of PM2.5, SO2 and NO2 were 50μg/m 3 , 5μg/m 3 , 43μg/m 3 in Haidian, and 46μg/m 3 , 260 6μg/m 3 , 35μg/m 3 in Changping, respectively, which were reported by Beijing Ecology and Environment Statement, 2018.

270
To explore the influence of wind on the NH3 mixing ratios, rose charts 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 area originate from the south (often in the day) and north (often at night). As displayed in Fig. 6, some differences existed in the distributions of the surface wind between the urban and suburban sites. The dominant surface winds originated from the northeast and 275 southwest at the urban site and from the northwest and east 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 northwest sectors. Therefore, under the action of the southwest wind, a polluted air mass from the south of Beijing can be easily transported 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 through trajectory analysis. The authors concluded that the air mass from the southeast has a 280 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 relatively high in the south sectors for both sites. Thus, winds from the southeast, south, and southwest had a cumulative effect on the NH3 mixing ratios at both the urban and suburban sites. The NH3 mixing ratios were relatively low when the wind originated from the northwest sector at urban site and from the west sector at the suburban site, in which the wind speed was strong, which indicated that the northwest/west wind could promote NH3 dilution 285 and diffusion.

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Heavy rainfall occurred on a few days in Beijing in 2018. Heavy precipitation occurred for a long duration on August 18, 2018 ( Fig. 8). Before the rainfall, the NH3 concentration at the urban site was higher than the average level in August. After the rainfall occurred, the NH3 concentration decreased rapidly, and it was significantly lower than the mean value in August.
However, the diurnal variation of NH3 on the rain day did not differ considerably from the average diurnal variation in August.
On August 18, 2018, the NH3 mixing ratio at the suburban site remained at a low level during the rainfall period and was 295 considerably lower than the mean NH3 concentration in August. However, the NH3 mixing ratio increased rapidly after the precipitation and reached the mean level at 17:00. The rainfall might have an obvious clearing effect on NH3 but needed more cases to support.

Conclusion
In this study, the atmospheric NH3 concentrations at an urban site and a suburban site in Beijing were continuously and simultaneously observed from January 2018 to January 2019. The mean NH3 mixing ratios were 21 ± 14 and 22 ± 15 ppb at 305 the urban and suburban sites, respectively. The annual average NH3 mixing ratio at the suburban site was higher than that at the urban site. Moreover, the variation range of the NH3 mixing ratio was larger at the suburban site than at the urban site. In the summer and spring, the NH3 mixing ratio at the suburban site was higher than that at the urban site. In the autumn and winter, the NH3 mixing ratio at the suburban site was lower than that at the urban site. The highest NH3 mixing ratios at the urban and suburban sites were observed in July. The lowest NH3 mixing ratio at the urban site occurred in February, and the 310 lowest NH3 mixing ratio at the suburban site occurred in January. In the past decade, the concentration of NH3 in Beijing did not change considerably, and the NH3 levels in Beijing were higher than those in other large cities.
The hourly mean NH3 mixing ratios at the urban site were highly correlated (R = 0.849, P < 0.01) with those at the suburban site. However, the mean diurnal variations in the NH3 mixing ratios at the urban and suburban sites were different. At the urban site, low NH3 mixing ratios were observed in the day and high NH3 mixing ratios were observed in the night. The opposite 315 trend was observed at the suburban site. Although both sites were under the influence of similar weather systems, the seasonal diurnal variations in the NH3 mixing ratio were different at the urban and suburban sites. This result indicated that NH3 sources had different contributions to the NH3 levels at the urban and suburban sites.
The influence of meteorological factors on the NH3 mixing ratio was complex. At the same temperature, the NH3 mixing ratios increased with the relative humidity at the urban and suburban sites. At the same relative humidity, the NH3 mixing ratios 320 increased with the temperature at both sites. The relative humidity had the strongest influence on the NH3 mixing ratio in different seasons at the two sites. No strong correlation was observed between the NH3 concentration and the temperature in the summer and autumn at the two sites. A high wind speed promoted a decrease in the NH3 concentration. The NH3 mixing ratios were higher when the winds originated from the south than when the winds originated from the north and northwest.
Rainfall had a certain scavenging effect on NH3; however, it had little effect on the diurnal variations in the NH3 concentration. 325 Data availability. The data of stationary measurements are available upon request to the contact author Weili Lin (linwl@muc.edu.cn).
Author contributions. ZL and WL developed the idea for this paper, formulated the research goals, and carried out the 330 measurement at urban site. WP and ZM carried out the NH3 field observations at the suburban site.
Competing interests. The authors declare that they have no conflict of interest.