Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Reply to Comment on “Identification of Major Sources of Atmospheric NH3 In An Urban Environment In Northern China During Wintertime”
T
eng et al.1 deeply studied major sources of urban atmospheric NH3 in northern China during wintertime. Zeng and Wang2 commented on many issues related to the study. We consider that three of them are important and believe the short communication may stimulate more studies of urban atmospheric NH3. For a possible contribution of marine NH3 emissions to the atmospheric NH3 at the urban sampling site (referred to as urban site thereafter) used by Teng et al.,1 our new data and the extended analysis of original data clearly defended the argument by Zeng and Wang.2 We made measurements on 8−17 August 2016 at a resort beach site (referred to as sea-beach site thereafter) of the Yellow Sea, which were approximately 20 km from the urban sampling site. Because the world’s largest macroalgae blooms of Ulva prolifera occurred in the Yellow Sea every summer since 2008,3,4 massive Ulva prolifera gathered at the beach and degraded to some extent in the August. Hourly average mixing ratios of atmospheric NH3 measured by the AIM-IC, the same monitor as used by Teng et al.,1 largely varied from 0.8 to 75 ppb with the mean value of 16 ± 8.8 ppb at the sea-beach site. In addition, atmospheric NH3 was also monitored at the urban sampling site by the same instrument on 1−7 August 2017 when hourly average mixing ratios of atmospheric NH3 varied from 8.2 to 126 ppb with the mean value of 35 ± 23 ppb. The latter mean NH3 at the urban site doubles the former one obtained at the sea-beach site. The same spatial heterogeneity very likely happens in wintertime, although both values may largely decrease primarily due to the low ambient air temperature. Moreover, Na+ in PM2.5 was also simultaneously monitored with atmospheric NH3 at the urban site using the AIM-IC on 18−30 November 2015. Our previous study demonstrated that marine-derived sea-salt aerosols could be transported to the urban sampling site and yielded an appreciable contribution to Na+ in PM2.5.5 However, no correlation was found between atmospheric NH3 and Na+ in PM2.5 on 18−30 November 2015, i.e., r = 0.07 and P > 0.05. Considering the hills with a mean altitude of 200−500 m between the urban site and the Yellow Sea, a large amount of sea-derived atmospheric NH3 would be removed through dry deposition during the transport. All these implied that marine emissions of NH 3 should be a minor contributor to atmospheric NH3 at the urban site. In fact, atmospheric NH3 is commonly considered to be contributed from local sources in literature because of its high dry deposition rate and the fast neutralization reaction with acidic species in the transport. The latter could be particularly true in northern China in wintertime, when the mixing ratio of ambient SO2 alone is approximately 1 order of magnitude larger than that of atmospheric NH3. Regarding the emissions of NH3 from on-road traffic fleet, we have conducted several studies6,7 including Teng et al.1 For example, NH3, NO, SO2, black carbon, NH4+ in PM2.5 were © XXXX American Chemical Society
measured in summer 2007 at a roadside site near a highway with the highest traffic density in Canada. We found that emissions of NH3 from on-road traffic fleet yielded a negligible contribution to the observed atmospheric NH3.6 In addition, atmospheric NH3 and NO were also measured in winter 2008 at a roadside site with a middle traffic flow in downtown Toronto, and the same conclusion was drawn.7 However, we agree that more field measurements under various traffic environments in future will be helpful to reveal the robustness of our findings considering huge controversies on the topic. We are thereby proposing a new measurement strategy which can gain more solid evidence to diagnose this issue beyond those presented by Teng et al.1 The measurement strategy is described as below. NH3, NO, NOx, and other pollutant gases will be simultaneously monitored at two opposite sites, with each approximately 50 m away from a highway in Qingdao, China. An AIM-IC, gas analyzers including NOx, SO2, and O3, a denuder, and an automatic weather station will be set at one site of the highway. On the other site, the instruments include NH3 analyzer, a black carbon monitor, a denuder and a greenhouse gas analyzer. If the traffic fleet indeed yields an appreciable contribution to atmospheric NH3, the temporal variations of atmospheric NH3 in mixing ratio together with other data simultaneously measured at the two roadside sites will allow capturing the contribution when the wind blows nearly perpendicular to the highway. The experiment has to be performed in wintertime because of a huge contribution from other sources in warm seasons as aforementioned. However, online instruments cannot operate properly at the low ambient temperature and two mobile laboratories equipped with an airconditioning system are thereby required. We will try this in future. Teng et al.1 deeply discussed re-evaporation of predeposited NHx through wet precipitation in the morning in terms of the measured temporal profile of atmospheric NH3 in mixing ratio, the correlation between atmospheric NH3 and water vapor, as well as the findings documented in literature. However, it is vital to establish the direct link between the re-evaporation and atmospheric NH3. A total of 14 samples including 6 dew and 8 frost samples were collected from plant leaves during 8:00− 10:00 (a few dew or frost droplets had evaporated to some extent) in December-January 2017. The loadings of NH4+ in dew and frost samples were 0.75 ± 0.39 mg per m2 leaf and 0.71 ± 0.16 mg per m2 leaf, accounting for 14% ± 27% and 16% ± 11% of the total inorganic cation ions in dew and frost, respectively. The values of pH in the dew and frost samples were 6.4 ± 0.8 and 6.2 ± 0.2, respectively, favoring the reevaporation. NH4+ in the dew and frost samples had a negative correlation with the pH, i.e., r = 0.63, P < 0.05, possibly because
A
DOI: 10.1021/acs.est.7b05839 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
NH3 evaporated along with water evaporation. More studies are still on processing.
Qingjing Hu† Xiaohong Yao*,‡ †
■
Key Laboratory of Sustainable Utilization of Marine Fisheries Resources, Ministry of Agriculture, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China ‡ Key Lab of Marine Environmental Science and Ecology (MoE), Ocean University of China, Qingdao 266100, China
AUTHOR INFORMATION
Corresponding Author
*Phone: 86-532-66782565; fax: 86-532-66782810; e-mail:
[email protected]. ORCID
Xiaohong Yao: 0000-0002-2960-0793 Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Teng, X.; Hu, Q.; Zhang, L.; Qi, J.; Shi, J.; Xie, H.; Gao, H.; Yao, X. Identification of major sources of atmospheric NH3 in an urban environment in northern China during wintertime. Environ. Sci. Technol. 2017, 51 (12), 6839−6848. (2) Zeng, Y.; Wang, S. Comment on “Identification of major sources of atmospheric NH3 in an urban environment in northern China during wintertime. Environ. Sci. Technol. 2017. (3) Liu, D.; Keesing, J. K.; Xing, Q.; Shi, P. World’s largest macroalgal bloom caused by expansion of seaweed aquaculture in China. Mar. Pollut. Bull. 2009, 58 (6), 888−895. (4) Bulletin of marine disasters of China; http://www.soa.gov.cn/ zwgk/hygb/. (5) Feng, L.; Shen, H.; Zhu, Y.; Gao, H.; Yao, X. Insight into generation and evolution of sea-salt aerosols from field measurements in diversified marine and coastal atmospheres. Sci. Rep. 2017, 7, 41260. (6) Yao, X.; Hu, Q.; Zhang, L.; Evans, G. J.; Godri, K. J.; Ng, A. C. Is vehicular emission a significant contributor to ammonia in the urban atmosphere? Atmos. Environ. 2013, 80 (6), 499−506. (7) Hu, Q.; Zhang, L.; Evans, G. J.; Yao, X. Variability of atmospheric ammonia related to potential emission sources in downtown Toronto, Canada. Atmos. Environ. 2014, 99, 365−373.
B
DOI: 10.1021/acs.est.7b05839 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
