Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Comment on “Identification of Major Sources of Atmospheric NH3 in an Urban Environment in Northern China During Wintertime” ABSTRACT: Public concerns regarding atmospheric fine particles (PM2.5) have rapidly risen in recent years because of serious haze issues in urbanized regions, especially many Chinese metropolis regions. Atmospheric ammonia (NH3) is the main precursor of atmospheric particles.1 Unfortunately, the contribution of different anthropic or natural sources to atmospheric NH3 is still unclear.2 Source apportionment of atmospheric NH3 is an essential prerequisite of successful mitigation. “Identif ication of Major Sources of Atmospheric NH3 in an Urban Environment in Northern China During Wintertime” is an inspiring work that attempts to address this topic. However, some arguments in this article do not withstand serious scrutiny:
1. THE AVAILABLE DATA WERE INSUFFICIENT TO SUPPORT THE MAIN CONCLUSION The authors observed a correlation between the mixing ratios of atmospheric NH3 and water vapor (Figures 2, 4, and 5), while there was an inconsistency between NO and NH3 profiles. The mixing ratio of NH3 was consistent with ambient temperature during three short periods of 1−2 days (Figure 6a and 7). The authors concluded that dew or rain droplets (evaporation of predeposited NHx) and green space contributed 10% and 60% to atmospheric NH3, respectively. The contribution of traffic was negligible, but there was still 30% contribution from unknown local sources. This conclusion was too easy. At first, the differentiation of green space and dew/ droplets was confusing. Dew and rain droplets were certainly an important contributor of NH3 from green space.3,4 Second, the dew served as a nighttime reservoir of atmospheric NH3.5 Source apportionment should focus on the actual original sources. For example, where did the predeposited NH3 come from? Third, winter is the dry season in Qingdao. The contribution of dew or droplets should be minor. Qingdao is the largest peninsular metropolis in northern China (urban population density:1901/km2),6 where marine and anthropic sources may be two major contributors.7 We noticed the mixing ratio of NH3 peaked once wind direction was 180−270° (Figures 4 and 5). Yellow Sea and Jiaozhou Bay are on the south and west (7−12 km) sides of the sampling site. Previous studies demonstrated marine sources as the dominant contributor to atmospheric NH3 and aerosol NH4+.8−10 In the precipitation of Jiaozhou Bay, NH4+ (40.4%) exceeded Ca2+ (29.3%) and became the dominant cation, in contrast to the situation in most areas of China.11 The correlation between NH3 and water vapor might alternatively be explained by an important contribution by a marine source in Qingdao, China. 2. The Inconsistency Between NO and NH3 Profiles Does Not Demonstrate That Traffic Emission Was a Negligible Contributor to Atmospheric NH3. The NO profile usually showed a sharp spike followed by small spikes of CO2 during the morning rush hour (Figures 4 and 5). The average diurnal profiles suggested that the NH3 spike appeared 2 h after the NO spike (Figure S4). The authors declared thereafter that traffic emission was a negligible contributor to atmospheric NH3 in this urban environment. This speculation was based on the presupposition that the vehicles emitted both NO and NH3 at the same time. In fact, the NH3 is a byproduct of three-way catalyst (TWC) in gasoline-powered vehicles once © XXXX American Chemical Society
excess hydrogen (H2) is produced and reaction temperature is low.12 The NH3 escape is the other traffic source from dieselpowered vehicles equipped with a selective catalytic reduction (SCR) system. Previous measurements reported that NH3 emission from vehicles varied in terms of vehicle model and operating condition, which did not track NOx emission.13−15 Since thousands of vehicles travel on roads, a consistency between NO and NH3 profiles might suggest a traffic source of NH3.16 However, inconsistency between them was more common, which does not negate the contribution of traffic source. In this work, the authors also observed that NH3 increased with NO (Figures 4 and 5). This delay in the NH3 spike does not demonstrate that the traffic was a negligible contributor to atmospheric NH3. In contrast, some previous studies have found vehicle emission to be an important urban NH3 source in China, especially in some developing cities.17−19 3. Equations (1) and (2) Were Very Confusing. The authors collected soil samples and measured the NH4+ content and pH of these samples. Soil emission potential (Γg, eq (1)) and canopy compensation point of NH3 (χg, eq (2)) were calculated according to Nemitz et al.20 However, Nemitz et al.20 defined Γs and χs. The Γs was in fact the ratio of [NH4+] to [NH3] in the leaf apoplast.20−22 Nemitz et al.20 calculated χs to study the gas-particle interactions between plants and the atmosphere. Furthermore, the authors forgot to define the constant (1.703 × 1010) in eq (2), which did not appear in the cited reference. Instead of using the simple single-layer canopy compensation point model, the authors should have used a two-layer model containing emission potentials of both foliage and the ground to calculate the NH3 emission from green space.23
4. FIGURE 6 WAS QUESTIONABLE Figure 6a underlined the correlation between NH3 and ambient temperature during three short periods of 1−2 days, while the whole observation period lasted one and a half months. This choice of short periods to prove their hypothesis was inappropriate. Moreover, Figure 6b is unnecessary. The Γg included three given values (1370 on 22−24 November, 2250 on 10−11 December, and 3750 on 26−27 December). The soil temperature (Tg) was replaced by the ambient temperature (eq 2). Figure 6b indicated that NH3 was correlated with the ambient temperature, as shown in Figure 6a.
A
DOI: 10.1021/acs.est.7b04986 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
source apportionment of precipitation components in the Jiaozhou Bay, North China. Atmos. Res. 2017, 190, 10−20. (12) Twigg, M. V. Progress and future challenges in controlling automotive exhaust gas emissions. Appl. Catal., B 2007, 70 (1−4), 2− 15. (13) Bishop, G. A.; Peddle, A. M.; Stedman, D. H.; Zhan, T. On-road emission measurements of reactive nitrogen compounds from three California cities. Environ. Sci. Technol. 2010, 44 (9), 3616−3620. (14) Carslaw, D. C.; Rhys-Tyler, G. New insights from comprehensive on-road measurements of NOx, NO2 and NH3 from vehicle emission remote sensing in London, UK. Atmos. Environ. 2013, 81, 339−347. (15) Kean, A. J.; Littlejohn, D.; Ban-Weiss, G. A.; Harley, R. A.; Kirchstetter, T. W.; Lunden, M. M. Trends in on-road vehicle emissions of ammonia. Atmos. Environ. 2009, 43 (8), 1565−1570. (16) Kirkby, J.; Curtius, J.; Almeida, J.; Dunne, E.; Duplissy, J.; Ehrhart, S.; Franchin, A.; Gagne, S.; Ickes, L.; Kurten, A.; et al. Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation. Nature 2011, 476 (7361), 429−U77. (17) Sun, K.; Tao, L.; Miller, D. J.; Pan, D.; Golston, L. M.; Zondlo, M. A.; Griffin, R. J.; Wallace, H. W.; Leong, Y. J.; Yang, M. M.; et al. Vehicle emissions as an important urban ammonia source in the United States and China. Environ. Sci. Technol. 2017, 51 (4), 2472− 2481. (18) Meng, Z. Y.; Lin, W. L.; Jiang, X. M.; Yan, P.; Wang, Y.; Zhang, Y. M.; Jia, X. F.; Yu, X. L. Characteristics of atmospheric ammonia over Beijing, China. Atmos. Chem. Phys. 2011, 11 (12), 6139−6151. (19) Chang, Y. H.; Zou, Z.; Deng, C. R.; Huang, K.; Collett, J. L.; Lin, J.; Zhuang, G. S. The importance of vehicle emissions as a source of atmospheric ammonia in the megacity of Shanghai. Atmos. Chem. Phys. 2016, 16 (5), 3577−3594. (20) Nemitz, E.; Sutton, M. A.; Wyers, G. P.; Jongejan, P. A. C. Gasparticle interactions above a Dutch heathland: I. Surface exchange fluxes of NH3, SO2, HNO3 and HCl. Atmos. Chem. Phys. 2004, 4, 989− 1005. (21) Husted, S.; Schjoerring, J. K.; Nielsen, K. H.; Nemitz, E.; Sutton, M. A. Stomatal compensation points for ammonia in oilseed rape plants under field conditions. Agric. For. Meteorol. 2000, 105 (4), 371− 383. (22) Nemitz, E.; Sutton, M. A.; Schjoerring, J. K.; Husted, S.; Wyers, G. P. Resistance modelling of ammonia exchange over oilseed rape. Agric. For. Meteorol. 2000, 105 (4), 405−425. (23) Nemitz, E.; Milford, C.; Sutton, M. A. A two-layer canopy compensation point model for describing bi-directional biosphereatmosphere exchange of ammonia. Q. J. R. Meteorol. Soc. 2001, 127 (573), 815−833. (24) Pan, Y. P.; Tian, S. L.; Liu, D. W.; Fang, Y. T.; Zhu, X. Y.; Zhang, Q.; Zheng, B.; Michalski, G.; Wang, Y. S. Fossil fuel combustion-related emissions dominate atmospheric ammonia sources during severe haze episodes: Evidence from 15N-stable isotope in sizeresolved aerosol ammonium. Environ. Sci. Technol. 2016, 50 (15), 8049−8056. (25) Felix, J. D.; Elliott, E. M. Isotopic composition of passively collected nitrogen dioxide emissions: Vehicle, soil and livestock source signatures. Atmos. Environ. 2014, 92, 359−366. (26) Chang, Y. H.; Liu, X. J.; Deng, C. R.; Dore, A. J.; Zhuang, G. S. Source apportionment of atmospheric ammonia before, during, and after the 2014 APEC summit in Beijing using stable nitrogen isotope signatures. Atmos. Chem. Phys. 2016, 16 (18), 11635−11647.
We believe that this work shows insights into the transport and deposition of NH3 at the urban scale. However, using just NO or water vapor as an indicator of NH3 source is not appropriate. Isotopic composition of atmospheric NH3 or aerosol NH4+ could provide rich and reliable information about its source.24−26 Further studies should focus on isotopic composition inventory of NH3 source and isotopic fractionation of atmospheric NH3 through transport and deposition.
Yang Zeng* Shuguang Wang*
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School of Environmental Science and Engineering, Shandong University, No. 72 Binhai Avenue, Jimo District, Qingdao, 266237, China
AUTHOR INFORMATION
Corresponding Authors
*(Y.Z.) E-mail:
[email protected]. *(S.G.W.) E-mail:
[email protected]. ORCID
Yang Zeng: 0000-0002-4462-9711 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge National Natural Science Foundation of China and Natural Science Foundation of Shandong Province for financial support (21607094, 21777085, ZR2016BQ29).
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REFERENCES
(1) Chang, Y. H.; Liu, X. J.; Dore, A. J.; Li, K. H. Stemming PM2.5 pollution in China: Re-evaluating the role of ammonia, aviation and non-exhaust road traffic emissions. Environ. Sci. Technol. 2012, 46 (24), 13035−13036. (2) Huang, X.; Song, Y.; Li, M.; Li, J.; Huo, Q.; Cai, X.; Zhu, T.; Hu, M.; Zhang, H. A high-resolution ammonia emission inventory in China. Global Biogeochem. Cycles 2012, 26 (1), GB1030. (3) Hansen, K.; Pryor, S. C.; Boegh, E.; Hornsby, K. E.; Jensen, B.; Sorensen, L. L. Background concentrations and fluxes of atmospheric ammonia over a deciduous forest. Agric. For. Meteorol. 2015, 214, 380− 392. (4) Li, Y.; Schichtel, B. A.; Walker, J. T.; Schwede, D. B.; Chen, X.; Lehmann, C. M. B.; Puchalski, M. A.; Gay, D. A.; Collett, J. L., Jr. Increasing importance of deposition of reduced nitrogen in the United States. Proc. Natl. Acad. Sci. U. S. A. 2016, 113 (21), 5874−5879. (5) Wentworth, G. R.; Murphy, J. G.; Benedict, K. B.; Bangs, E. J.; Collett, J. L., Jr. The role of dew as a night-time reservoir and morning source for atmospheric ammonia. Atmos. Chem. Phys. 2016, 16 (11), 7435−7449. (6) Qingdao Statistical Bureau; NBS Qingdao Survey Offic. Qingdao Statistical Yearbook; China Statistics Press: Beijing, 2017. (7) Felix, J. D.; Elliott, E. M.; Gay, D. A. Spatial and temporal patterns of nitrogen isotopic composition of ammonia at US ammonia monitoring network sites. Atmos. Environ. 2017, 150, 434−442. (8) Jickells, T. D.; Kelly, S. D.; Baker, A. R.; Biswas, K.; Dennis, P. F.; Spokes, L. J.; Witt, M.; Yeatman, S. G. Isotopic evidence for a marine ammonia source. Geophys. Res. Lett. 2003, 30 (7), 1374. (9) Altieri, K. E.; Hastings, M. G.; Peters, A. J.; Oleynik, S.; Sigman, D. M. Isotopic evidence for a marine ammonium source in rainwater at Bermuda. Global Biogeochem. Cycles 2014, 28 (10), 1066−1080. (10) Xiao, H. W.; Xiao, H. Y.; Luo, L.; Shen, C. Y.; Long, A. M.; Chen, L.; Long, Z. H.; Li, D. N. Atmospheric aerosol compositions over the South China Sea: temporal variability and source apportionment. Atmos. Chem. Phys. 2017, 17 (4), 3199−3214. (11) Xing, J.; Song, J.; Yuan, H.; Li, X.; Li, N.; Duan, L.; Qu, B.; Wang, Q.; Kang, X. Chemical characteristics, deposition fluxes and B
DOI: 10.1021/acs.est.7b04986 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
