Source apportionment of atmospheric ammonia before, during, and after the 2014 APEC summit in Beijing using stable nitrogen isotope signatures

. Stable nitrogen isotope composition ( δ 15 N) offers new opportunities to address the 15 long-standing and ongoing controversy regarding the origins of ambient ammonia (NH 3 ), a vital precursor of PM 2.5 inorganic components, in the urban atmosphere. In this study, the δ 15 N values of NH 3 samples collected from various sources were constrained using a novel and robust chemical method coupled with standard elemental analysis procedures. Independent of the wide variation in mass concentrations (ranging from 33 (vehicle) to over 6000 (human excreta) μg m -3 ), different NH 3 sources 20 have generally different δ 15 N values (ranging from -52.0 to -9.6‰). Significantly high δ 15 N values are seen as a characteristic feature of all vehicle-derived NH 3 samples (-14.2±2.8‰), which can be distinguished from other sources emitted at environmental temperature (-29.1±1.7, -37.8±3.6, and -50.0±1.8‰ for livestock, waste, and fertilizer, respectively). 3 model (IsoSource). The method was used to quantify the sources of ambient NH 3 before, during and after the 2014 APEC summit, when a set of stringent air quality control measures were implemented. Results show that the average NH 3 concentrations (the overall contributions of traffic, waste, livestock, and fertilizer) during the three periods were 9.1 (15.1, 31.2, 23.7, and 30.0%), 7.3 (8.8, 24.9, 14.3, and 30 52.0%), and 12.7 (29.4, 23.6, 31.7, and 15.4%) μg m -3 , respectively, representing a 20.0% decrease first and then a 74.5% increase in overall NH 3 mass concentrations. During (after) the summit, the contributions of traffic, waste, livestock, and fertilizer decreased (increased) by 58.7 (234.2), 0.9 (-5.0), 41.0 (120.8), and -87.6% (-70.5%) when compared with periods before (during) the summit, respectively, signifying that future NH 3 control efforts in megacities like Beijing should prioritize traffic 35 sector as well as livestock breeding. The results show that isotope ratio measurements of NH 3 to be a valuable tool to quantify the atmospheric sources of NH 3 in urban atmospheres. possible combination of source proportions (summing to 100%) incrementally (e.g., 1%), calculates the predicted isotope value for each combination using linear mass-balance equations. These predicted values are then examined to determine which ones fall within some tolerance range (typically 0.1‰) of the observed consumer isotope value, and all of these feasible solutions are recorded. These data suggest that of NH 3 human for an population million 1386 Mg NH 3 annually to the atmosphere within the city, which corresponds to 11.4% of the total NH 3 emissions in 280 the Shanghai urban areas (Chang 2015). The δ 15 N values of wastewater-originated NH 3 (-41.0±0.9‰; n=8) are close to that of human excreta and also show no seasonal variation. Sampling in a stable and closed physical environment may be responsible for such a small range of isotopic variation. However, although also sampled in a closed environment, the δ 15 N values of municipal solid no definitive solution exists in a linear mixing model with one isotope system tracer ( δ 15 N) in the current study. Recommended future studies should include the combinations of different types of isotope ratio measurements and the adoption of more sophisticated Bayesian mixing models. The isotopic signature of sources like on-road traffic still remains uncertain. Some may argue that since NH 3 (also NO x ) is known to be a component of vehicle-emitted exhaust,

important role of urban NH 3 emissions to form PM 2.5 , NH 3 emission reduction has been regarded as the key to curb severe haze pollution in Chinese mega-cities (Ye et al., 2011;Wang et al., 2011;Wang et al., 2013).
Although isotopic techniques have been extensively accepted as a useful tool for source apportionment of gases and PM (e.g., Cao et al., 2011;Felix et al., 2012;Liu et al., 2014b;Rudolph et al., 1997;Wang 70 et al., 2016;Xiao et al., 2012;Xiao et al., 2015;Xiao and Liu, 2002), there have been few studies to date in terms of directly observing or quantifying the contribution of non-agricultural NH 3 in the atmosphere (Felix et al., 2014;Liu et al., 2008). Greater scientific attention and regulatory efforts have been giving to nitrogen oxides (NO x =NO+NO 2 ) (Felix et al., 2012;Michalski et al., 2014;Walters et al., 2016; and sulfur dioxide (SO 2 ) (Barros et al., 2015;75 Giesemann et al., 1994;Habicht and Canfield, 1997;Zhelezinskaia et al., 2014). Also, the conventional method for analyzing δ 15 N, using elemental analyzer (EA) combustion with isotope-ratio mass spectrometry (IRMS), normally requires more than 20 µg N for a single solid sample, which poses a considerable challenge for passive sampling devices (Skinner et al., 2006). To overcome this technical restriction, a landmark paper was published by Felix et al. (2013), in which they tried to combine the 80 bromate (BrO -) oxidation of NH 4 + to NO 2 with microbial denitrifier methods (bacteria converts NO 2 to N 2 O) to permit the N isotopic analysis of low concentration NH 4 + passive samples, featuring the high throughput of sample mass and low toxicity of reagents. However, the microbial denitrifier they used needs careful cultivation and maintenance, which is time-consuming and also may present a challenge for many isotope laboratories. Recently, a novel and robust chemical method for δ 15 N-NH 4 + at natural 85 abundance has been developed (Liu et al., 2014a), which has major advantages over previous approaches: (i) substantially simplified preparation procedures and reduced preparation time particularly 3 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 13 June 2016 c Author(s) 2016. CC-BY 3.0 License. compared to the methods in which diffusion or distillation is involved since all reactions occur in the same vial and separation of NH 4 + from solution is not required; and (ii) greater suitability for low volume samples including those with low N concentration, having a blank size of 0.6 to 2 nmol. 90 The 2014 Asia-Pacific Economic Cooperation (APEC) summit, another major international event after the 2008 Olympic Games, was hosted in Beijing on 3-12, November. To ensure good air quality and traffic flow during the APEC Summit, a set of stringent measures to control atmospheric pollutants, including regulating vehicle travel (restricting traffic based on the odd and even number plate rule), delaying winter heating (for a week), suspending coal-based industries and closing construction sites, 95 were implemented in Beijing and its neighboring provinces over a month before and during the APEC summit (Chen et al., 2015;Li et al., 2015a;Tang et al., 2015;Xu et al., 2015b). This provided a unique city-wide experiment to isotopically examine the response of various NH 3 sources to such comprehensive and intensive mitigation efforts. In the present study, the isotopic signatures of various NH 3 sources in China were determined for the first time. Moreover, the ambient NH 3 concentrations and 100 their isotopic compositions were investigated before, during and after the APEC control period in Beijing. Based on the isotopic signatures of major sources we developed, a stable isotope mixing model was used to quantify the contributions of each NH 3 source so as to examine the effect of the control measures.

Ambient NH 3 Monitoring
The Adapted Low-Cost Passive High Absorption (ALPHA) samplers (Centre for Ecology and Hydrology, Edinburgh, UK), one of the most widely recognized passive sampling devices (PSDs) (Puchalski et al., 2011;Tang et al., 2001;Xu et al., 2015a), were used to collect ambient NH 3 in this study. The ALPHA sampler is a circular polyethylene vial (26 mm  Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos.

NH 3 Source Sampling
The Ogawa passive sampling device (Ogawa & Co., FL, USA) is another popular PSD that has been successfully applied in the U.S. (Butler et al., 2015;Puchalski et al., 2011) and China (Chang et al., 130 2015;Meng et al., 2011) to determine the time-integrated NH 3 concentrations. The Ogawa PSD is a double-sided passive diffusion sampler equipped with a diffusive end cap, followed by a stainless-steel screen, and a 14mm quartz filter impregnated with phosphoric acid by the manufacturer. In this study, the Ogawa PSDs were used to collect NH 3 emitted from sources for isotopic analysis. 5 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 13 June 2016 c Author(s) 2016. CC-BY 3.0 License. volatilization (laboratory simulation of NH 3 volatilization from urea-fertilized soil), human excreta (septic tanks of a residential building and a teaching building), waste water (sewage water treatment plant), solid waste (municipal waste transfer stations in a residential community and an educational area), and vehicle (a heavily used urban tunnel) ( Table 1 and SI Table S1). NH 3 emitted from fertilizer volatilization was collected in the laboratory of Fudan University. To minimize the mixture of ambient 140 atmospheric NH 3 /NH 4 + (see Fig. 2 and SI Fig. S1 for two examples) and to examine the potential influence of the macro environment, non-laboratory samples were collected within a confined space in short periods (e.g., several hours) during both the warm (summer) and cold seasons (winter) between June 2014 and January 2015. Descriptions of the sampling processes are detailed in SI Table S1.
145 Figure 2. Field photos of a solid waste container in the solid waste transfer station we used for source sampling at Fudan University. Before sampling, an Ogawa passive sampler for NH 3 is attached on a plastic strip (1.5 m in length). The sampler is fitted into the container through the holes (shown on the right panel).
The ambient average NH 3 concentration was less than 6 μg m -3 (Chang et al., 2016), which was 150 between 1 and 3 orders of magnitude smaller than all emission sources we investigated (Table 1). Therefore, the interference of ambient air to all source samples is not a major concern in our study.
HONO has been measured in significant concentrations in the UK and interfered with measurements of HNO 3 by DELTA active samplers causing the instrument now to be coated with NaCl to avoid this interference with HNO 3 measurements. The Ogawa filters used for trapping NH 3 were impregnated 155 with citric acid. Using a Discrete Auto Analyzer (Smartchem 200, AMS, Italy; the detection limit for NO 2 --N is 0.002 mg L -1 ), we recently analyzed nearly 100 passive filter samples collected from our passive ammonia network in Shanghai (Fig. 1). No detectable nitrite was found for almost all these 6 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys.
samples (including samples collected from a busy road tunnel (Chang et al., 2016), which provides compelling evidence that passive collection of ammonia don't introduce interference of nitrite in this 160 study.
In our pilot study, the sampling period of each source was optimized to avoid insufficiency of NH 3 absorption, but more importantly, to minimize potential effects of N isotope fractionation. Taking the source of human excreta for example, the concentrations of NH 3 in the exhausts of 15 ceiling ducts from collecting septic tanks in 13 buildings with 6 functions (   5803  2661 2809 + − μg m -3 ) were higher than those 165 in ambient air by 3 orders of magnitude (Chang et al., 2015). Therefore, the insufficiency of NH 3 absorption is of no significant concern. However, the δ 15 N-NH 3 values of daily samples varied widely (±10 per mil), suggesting that the isotope fractionation may occur during the process of sampling/storage. After many tests by trial and error, we found that a sampling period of 2 hours could provide sufficient N-NH 3 as well as avoid potential fractionation. Another example is the on-road 170 traffic source. The δ 15 N-NH 3 values of weekly (-11.9‰; -11.2‰), semimonthly (-11.7‰; -12.5‰) and monthly samples (-12.0±1.8‰; n=4) in the exit of Handan tunnel were almost identical, suggesting that passive NH 3 collection is an effective approach for isotopic analysis procedures.

Stable Isotope Analysis
A newly developed chemical method for δ 15 N-NH 4 + of low NH 4 + samples was used in the current study. 175 The detailed analytical procedures are given elsewhere (Liu et al., 2014a). Briefly, this method is based on the isotopic analysis of N 2 O, which is much less abundant in the atmosphere than N 2 and thus causes minimal atmospheric contamination. Filter samples were firstly soaked with ultra-pure water (18.2 MΩ.cm). Concentrations of NH 4 + were then analyzed using an ion chromatographic system ( -30.4‰ and +53.7‰, respectively) were used to correct for the reagent blank and drift during isotope analysis of the produced N 2 O. The standard deviation of δ 15 N measurements is less than 0.3‰.
Moreover, to enhance our confidence in the results determined by the PT-IRMS method, the δ 15 N values of fertilizer-, vehicle-, human excreta-, and solid waste-derived NH 3 were also examined by the EA-IRMS method at the Shanghai Institutes of Life Sciences, Chinese Academy of Sciences (SI Text 195 S1).

Isotope Mixing Model
A stable isotope mixing models offer a statistical framework to estimate the relative contributions of multiple sources to a mixture, such as food-web structure, plant water use, air pollution, and many other environments (Cole et al., 2011;Dai et al., 2015;Jautzy et al., 2015;Wang et al., 2016). A 200 common problem, however, is having too many possible sources relative to isotopes to allow unique linear mixing solutions based on mass balance equations. To this end, Phillips and Gregg (2003) developed the model IsoSource, which solves iteratively for feasible mixing solutions, and has been well tested in numerous studies (e.g., Cole et al., 2011;Dai et al., 2015;Jautzy et al., 2015;Wang et al., 2016). The model does not generate exact values for proportional contributions of each source, but 205 instead provides a range of possible contributions or feasible solutions. The IsoSource addresses every possible combination of source proportions (summing to 100%) incrementally (e.g., 1%), then calculates the predicted isotope value for each combination using linear mass-balance equations. These predicted values are then examined to determine which ones fall within some tolerance range (typically 0.1‰) of the observed consumer isotope value, and all of these feasible solutions are recorded. harvesting forests for fuelwood and timber has nearly disappeared in Beijing. Therefore, the contribution of biomass burning is considered minimal; (2) Beijing is 150 km inland from the Bohai Sea (the nearest sea) via Tianjin Municipality in the southeast. Therefore, marine source in Beijing can be neglected; and (3) previous work indicated that miscellaneous NH 3 sources like pets and household products are minor NH 3 emissions in Beijing urban areas (Chang, 2014), which thus can be largely 220 neglected.
In conclusion, ambient NH 3 in Beijing during our study period has been shown to be due to four main sources: livestock production, N-fertilizer application, on-road traffic emissions, and waste-derived emissions. The δ 15 N average values for these four NH 3 emission sources will be built and served as the baseline input to the IsoSource. For the traffic source, given that the relatively larger difference in 225 terms of their δ 15 N values in different seasons (see Table 1), the wintertime average value of δ 15 N was used in this study because the APEC summit was held during winter. In brief, the N isotopic signatures for the sources of waste, livestock, traffic, and fertilizer are set as -37.8‰, -29.1‰, -16.5‰, and -50.0‰, respectively (Table 1 and SI Table S1). The source increment and mass balance tolerance parameter values of 1% and 0.1‰, respectively, were applied. Model output files include all the 230 feasible source combinations, with histograms and descriptive statistics on the distributions for each source. Results are expressed as box-and-whisker plots for these distributions.

Isotopic Signatures of NH 3 Emission Sources
Using N isotope as a tool to discriminate the contribution of various sources to ambient NH 3 235 concentration requires (i) well-established N isotopic compositions of NH 3 emission sources and (ii) well-constrained N isotope fractionation to allow separating different sources. In total 44 NH 3 source samples in this study, δ 15 N values and NH 3 concentrations of these samples ranged from -52.0 to -9.6‰, and 33 to 6211 μg m -3 , respectively (all data are presented in SI Table S1). These NH 3 sources can be classified into four categories, i.e., fertilizer, livestock, traffic, and waste. For most sources, there was 240 9 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos.  etc.) that influence kinetic fractionation rates associated with NH 3 volatilization. Long-term (30 days) 265 monitoring of δ 15 N and NH 3 emissions of manure measured by Lee et al. (2011) indicated that the dynamics of N isotope fractionation may be complicating the usefulness of the isotope approach as a tool for estimating NH 3 emissions in field conditions. In this sense, the shorter sampling period in our work should reflect the essence of δ 15 N values of livestock-and fertilizer-derived NH 3 .
As a normal metabolic process, the release of NH 3 from human excreta has been well documented. 270 However, most emission inventories involving human excreta have focused on pit latrines in rural areas of developing and middle income countries. In urban China, human excreta are typically stored in a three-grille septic tank under the building before disposal. After a series of anaerobic decomposition processes, a substantial amount of NH 3 will be generated and emitted through a ceiling duct. In the present study, the concentrations of NH 3 in the ceiling ducts (4509.3±1248.0 μg m -3 ; n=8) outweigh 275 those in the open air by 3 orders of magnitude, and the δ 15 N-NH 3 values are seasonally consistent (-38.4±0.9‰ in summer and -38.6±1.0‰ in winter; Table 1), suggesting that human excreta may be an important and consistent source of NH 3 in urban areas. These data suggest that emissions of NH 3 from human excreta for an urban population of ~21 million people in Shanghai contribute 1386 Mg NH 3 annually to the atmosphere within the city, which corresponds to 11.4% of the total NH 3 emissions in 280 the Shanghai urban areas (Chang et al., 2015). The δ 15 N values of wastewater-originated NH 3 (-41.0±0.9‰; n=8) are close to that of human excreta and also show no seasonal variation. Sampling in a stable and closed physical environment may be responsible for such a small range of isotopic variation. However, although also sampled in a closed environment, the δ 15 N values of municipal solid 11 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 13 June 2016 c Author(s) 2016. CC-BY 3.0 License. waste demonstrate a much greater variation (-37.6 to -29.9‰), which may be due to the variable 285 composition of solid waste.

Source Apportionment of Ambient NH 3 in Beijing
Hourly observations of major air pollutants (including PM 2.5 , NO x , CO, SO 2 and O 3 ) in Beijing are show in SI Fig. S2. The meteorological differences (e.g., temperature and wind speed) for the three periods pre, during and post APEC are not significant, suggesting that emission reduction strategies 290 implemented during the APEC summit were successful (SI Fig. S2). It should be noted that several control measures, i.e., closing factories within 200 km of the city center and stopping the entrance of out-of-city vehicles, had been undertaken in Beijing and its neighboring regions before the summit. Therefore, the before-during comparison of some pollutants like SO 2 are not in stark contrast in terms of their mass concentration (SI Fig. S2). The evolution of ambient NH 3 mass concentrations measured 295 at CAU shows a similar pattern with CO and NO x (SI Fig. S3). Before the opening of the summit (from 18 th October to 3 rd November), NH 3 concentrations averaged 9.9 μg m -3 and ranged from 6.9 to 11.0 μg m -3 . During the summit session (from 3 rd to 15 th November, the ending date of the summit is 13 th November), this was reduced to 7.3 μg m -3 with a range of 5.8 to 8.6 μg m -3 . After the APEC summit (from 15 th to 29 th November), the NH 3 concentration levels rebounded to an average of 12.7 μg m -3 300 (ranging from 10.7 to 17.7 μg m -3 ). In other words, the NH 3 concentrations were reduced by 20.0% during the APEC summit compared with the period before it. Compared with the period after control, the concentrations were 74.5% lower than that during the summit (Table 2).
On the basis of the δ 15 N values of NH 3 emission sources and ambient δ 15 N-NH 3 samples (Table 2), the ranges (within 5 and 95 percentiles) of relative contribution fractions of each NH 3 source to the 305 ambient atmosphere were modeled by the IsoSource and depicted in Fig. 3a-d. Of these, sources of traffic and fertilizer are better constrained than livestock and waste. This is because the δ 15 N-NH 3 values of the ambient atmosphere are closer to the latter two. For example, after the APEC summit in our study period, the δ 15 N-NH 3 values of the ambient atmosphere averaged -30.7‰, which is very close to the isotopic signature of livestock (-29.1‰), thus leading to weaker constraint with 5 and 95 310 percentiles ranges from near 0 to 0.7 (Fig. 3).

12
Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys.  For each sample, information regarding the δ 15 N value, the NH 3 concentration and its source contributions is listed in Table 2. Fig. 4 illustrates the overall source contribution proportions before, during and after the summit, in which traffic, waste, livestock, and fertilizer comprised 18.3, 27.1, 24.0, and 30.6% of the whole period (Table 2), respectively. Specifically, the contribution of traffic initially 320 decreased by 58.7% and then doubled, and this represents the largest change among the four sources (Table 2). Considering the absolute contributions of different sources to ambient NH 3 , these results show that traffic is the most sensitive source to the emission control measures. This result was expected since over half the vehicles in Beijing were banned from entering the city during the APEC summit (Note that before the summit, out-of-city vehicles had already been banned from entering Beijing). 325 Based on the vehicle NH 3 emission factor obtained from real-world tunnel tests, Liu et al. (2014c) and Chang et al. (2016) recently reported that vehicles contribute 8.1% and 12.0% to total NH 3 emissions in the Pearl River Delta region and Shanghai urban areas, respectively. With a total of 5.5 million vehicles in 2014, the traffic fleet in Beijing was far ahead of other cities. Thus vehicle sources might contribute more than 20% of the total NH 3 to the air within Beijing. 330 13 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 13 June 2016 c Author(s) 2016. CC-BY 3.0 License. The waste-originated percentages in Fig. 3 and 4 remain fairly constant over the sampling period, appearing to be a stable and important NH 3 contributor in Beijing. Compared with wastewater and solid water, NH 3 emissions from human excreta through in situ sceptic tank system are far from quantified. 335 Based on an extensive measurement campaign, we estimated that the population of ~21 million people living in the urban areas of Shanghai annually emitted approximately 1386 Mg NH 3 , which corresponds to over 11.4% of the total NH 3 emissions in the urban areas (Chang et al., 2015).
Non-agricultural sources-merged with waste and traffic NH 3 emissions-collectively account for nearly 50% of ambient NH 3 before and after the APEC summit, which cannot be explained by previous work 340 of emission inventories (e.g., Fu et al., 2013;Huang et al., 2011;Huang et al., 2012;Kang et al., 2016;Li et al., 2015b;Zhang et al., 2009;Zhang et al., 2010). Our results do not contradict a commonly held belief that agriculture is responsible for the vast majority (normally ˃90%) of total ammonia emissions at a regional scale. However, the results show that within urban areas, non-agricultural sources are very 14 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys.  The North China Plain (NCP) is one of the most intensive agricultural regions in China, enjoying a 350 good reputation of "China's granary" (Ju et al., 2009). Situated on the northern edge of the NCP with mountains to the North and West, Beijing is a receptor of agricultural NH 3 from rural areas. During our study period, crops in the NCP had been harvested and thus fertilizer application would have been very limited. However, our results show that fertilizer is the largest contributor, accounting for 30.6% throughout the sampling periods (Table 2). One explanation might be the prevalence of intensive urban 355 agricultural production with high nitrogen fertilizer input in the suburban areas. Beijing's increase in land area from 4822 km² in 1956 to 16808 km² in 1958 led to the increased adoption of peri-urban agriculture. Such "suburban agriculture" contributed ~70% of non-staple food in Beijing, mainly consisting of vegetables and milk, to be produced by the city itself in the 1960s and 1970s (Cai, 2003).
In the late 1990s, recognizing the importance of urban agriculture to sustainable urban development, 360 Beijing's municipal government launched an official program encouraging multi-function urban agriculture in peri-urban areas by supporting the development of "agro-parks", which not only produce food but also attract tourism and are used as educational tools (Cai, 2003). One of the recent 15 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys.  (Cai, 2003). Today, Beijing is leading the way in using 365 smart-city technologies to make urban farming more sustainable. In addition to suburban agriculture, there are 17 golf courses with 2280 ha. greens in Beijing; some of them are located at the urban areas (Chang, 2014). The turf grass of golf course typically needs 200-400 kg N ha -1 yr -1 as N fertilizer to achieve high performance (Wong et al., 1998;Wong et al., 2002;Zhang, 2002), which should be considered as an overlooked NH 3 contributor in Beijing. 370

Limitations and Outlook
The dataset reported in this study represents, to the best of our knowledge, the first attempt to partition urban atmospheric NH 3 sources. Considering the current nascent stage of partitioning NH 3 sources using stable isotope approach, there are several unsolved problems that could potentially undermine the above-mentioned results. One of our fundamental assumptions in this study is that the measured NH 3 375 was directly from NH 3 emission sources. In other words, we treated the measured NH 3 as the mixture of primary NH 3 sources without "gas-aerosol conversion" fractionation. But in fact, NH 3 ↔NH 4 + equilibrium will cause 14 N to be preferentially associated with NH 3 and 15 N to be enriched in NH 4 + of PM due to the stronger associative strength of 15 N than 14 N in NH 4 + (Kawashima and Kurahashi, 2011;Yeatman et al., 2001). In Beijing, earlier studies confirmed that the atmosphere in Beijing was 380 NH 3 -limited, suggesting that acidic gases could not be fully neutralized to form ammonium salts (Ianniello et al., 2010;Wang et al., 2016). Therefore, ammonium salts had much less opportunity to volatilize to NH 3 to exert substantial isotopic effect through NH 3 ↔NH 4 + equilibrium. Still, it is critical to develop a controlled laboratory system to fundamentally understand the characteristics and mechanisms of N isotope fractionations during the process of NH 3 transformation . 385 Several additional factors could introduce uncertainty in the solutions of isotope mixing model. Given the complexity of urban NH 3 sources, no definitive solution exists in a linear mixing model with one isotope system tracer (δ 15 N) in the current study. Recommended future studies should include the combinations of different types of isotope ratio measurements and the adoption of more sophisticated Bayesian mixing models. The isotopic signature of sources like on-road traffic still remains uncertain. 390 Some may argue that since NH 3 (also NO x ) is known to be a component of vehicle-emitted exhaust, 16 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 13 June 2016 c Author(s) 2016. CC-BY 3.0 License.
why not collect vehicle-emitted NH 3 directly from the tailpipes. To our knowledge the δ 15 N of vehicle-emitted NH 3 has not previously been assessed. However, a recent research paper from  addressing the δ 15 N of vehicle-emitted NO x may shed some light on this issue. In that paper, the δ 15 N-values of NO x emitted from 26 different vehicles ranged from -19.1‰ to +9.8‰, much 395 higher than the variation of δ 15 N-NH 3 collected from the Handan tunnel in our research. In road diesel Selective Catalytic Reduction (SCR) applications, a urea-in-water solution is used as the reduction agent. Urea is injected in the exhaust line and is decomposed over a catalyst to NH 3 . In this case, the δ 15 N values of vehicle-emitted NO x and NH 3 can hardly be the same. However, as a direct product of NO reduction on the catalyst surface of TWCs (2NO+5H 2 →2NH 3 +2H 2 O and/or 400 2NO+2CO+3H 2 →2NH 3 +2CO 2 ), NH 3 emitted from light-duty vehicle exhausts can be expected to have similar δ 15 N-values to vehicle-emitted NO x . In this regard, the tunnel test has a unique advantage in measuring the overall isotopic signatures of vehicle-emitted pollutants. Therefore, we believe that the δ 15 N-NH 3 values of the samples collected from Handan tunnel in this study are representative as the isotopic signatures of vehicles in China. 405 Despite the potential limitations in this study, given the importance of NH 3 to PM 2.5 formation, this work can be expected to enrich the discussion on the methodologies (including stable isotope analysis) in terms of identifying the largest NH 3 sources in urban atmosphere where policy efforts relating to emissions abatement can be directed to deliver the largest impact.
Secondly, we demonstrated that the isotopic source signatures of NH 3 represent an emerging tool for 415 partitioning NH 3 sources. Taking advantage of the implementation of stringent air quality control measures during the APEC summit in Beijing, the IsoSource modeling results indicate that the overall contribution of traffic, waste, livestock, and fertilizer to ambient NH 3 mass concentrations is 18.3%, 17 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-432, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 13 June 2016 c Author(s) 2016. CC-BY 3.0 License. 27.1%, 24.0%, and 30.6%, respectively, in which traffic is the most sensitive to control measures. Our results clearly show that non-agricultural sources (traffic and waste) of NH 3 are of critical importance 420 in megacities like Beijing. Therefore, in addition to current SO 2 and NO x controls, China also needs to allocate more scientific, technical, and legal resources on controlling non-agricultural NH 3 emissions in the future.