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Identification of major sources of atmospheric NH in an urban environment in northern China during wintertime Xiaolin Teng, Qingjing Hu, Leiming Zhang, Jiajia Qi, Jinhui Shi, Huan Xie, Huiwang Gao, and Xiaohong Yao Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017
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Environmental Science & Technology
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Identification of major sources of atmospheric NH3 in an urban
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environment in northern China during wintertime
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Xiaolin Teng1#, Qingjing Hu2#, Leiming Zhang3*, Jiajia Qi1, Jinhui Shi1, Huan Xie1,
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Huiwang Gao1,4 & Xiaohong Yao1,4*
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1
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Ocean University of China, Qingdao 266100, China
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2
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Agriculture, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery
Key Lab of Marine Environmental Science and Ecology, Ministry of Education,
Key Laboratory of Sustainable Utilization of Marine Fisheries Resources, Ministry of
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Sciences, Qingdao 266071, China
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3
12
Climate Change Canada, Toronto, Canada
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4
14
China
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# contributed equally to the work; * corresponding authors:
[email protected] 16
and
[email protected] (phone number: 86-532-66782565, fax number:
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86-532-66782810)
Air quality Research Division, Science and Technology Branch, Environment and
Qingdao Collaborative Center of Marine Science and Technology, Qingdao 266100,
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Abstract
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To assess the relative contributions of traffic emission and other potential sources to
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high-levels of atmospheric ammonia (NH3) in urban areas in wintertime, atmospheric
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NH3 and related pollutants were measured at an urban site, ~300 m from a major
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traffic road, in northern China in November and December 2015. Hourly average NH3
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varied from 0.3 to 10.8 ppb with an average of 2.4 ppb during the campaign. Contrary
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to the common perspective in literature, traffic emission was demonstrated to be a
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negligible contributor to atmospheric NH3. Atmospheric NH3 correlated well with
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ambient water vapor during many time periods lasting from tens of hours to several
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days, implying NH3 released from water evaporation being an importance source.
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Emissions from local green space inside the urban areas were identified to
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significantly contribute to the observed atmospheric NH3 during ~60% of the
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sampling times. Evaporation of pre-deposited NHx through wet precipitation
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combined with emissions from local green space likely caused the spikes of
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atmospheric NH3 mostly occurring 1-4 hours after morning rush hours or after and
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during slight shower events. There are still ~30% of the data samples with appreciable
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NH3 level for which major contributors are yet to be identified.
Winter urban atmospheric NH3
～30%
Unidentified local emission sources
～60%
Traffic
Green space
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Dew or rain droplets Evaporation of pre-deposited NHx
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Keywords: Atmospheric NH3, emission potentials of NH3, NHx deposition, vehicle
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emission, ion chromatograph
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Introduction
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Atmospheric ammonia (NH3) and acidic gases such as SO2 and NOx are important
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gaseous precursors forming secondary inorganic ammonium salts, e.g., NH4NO3,
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(NH4)2SO4, NH4HSO4, etc., in PM2.5 (atmospheric particles with an aerodynamic
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diameter smaller than 2.5 µm)1-6. It was reported that these ammonium salts
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accounted for 40-60% of PM2.5 mass under extremely polluted conditions in China,
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e.g., when PM2.5 mass concentrations exceeded 300 µg m-3 7-9. Large-scale severe
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PM2.5 pollution events occurred frequently during wintertime in the last few years in
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northern China. A thorough knowledge on the sources of these gaseous precursors is
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needed for making effective emission control policies to alleviate PM2.5 pollution.
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With decreasing emissions of SO2 and NOx in recent years in China, more attention
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needs to be paid to NH310-13. Major contributors responsible for increasing NH3
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mixing ratios in urban environments in China are yet to be identified14-15.
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Agriculture emissions of NH3 are at low levels in winter due to limited agriculture
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activities13. Thus, the relative contributions of non-agriculture emissions to
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atmospheric NH3 become more important13-19. Three-way catalytic converters
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equipped on on-road vehicles are commonly believed to generate NH310-29. In addition,
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the use of urea to remove NOx in diesel plumes can be another emission source of
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NH3 in urban atmospheres26. Some studies suggested traffic emissions as important
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sources of NH3 in different urban atmospheres in China14-15, 25. However, our previous
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studies reported negligible contributions from traffic emissions to atmospheric NH3 at
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a sampling site ~190 m from a highway with the highest traffic flow (~4.0*105
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vehicles/day) in Toronto and an urban site in downtown Toronto, Canada18, 31. Our
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previous findings motivate us to design a study to examine the contribution of traffic
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emissions to urban atmospheric NH3 in winter in China. To date, there is still no
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consensus whether traffic emissions are among the major sources of urban
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atmospheric NH3 in winter season.
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In this study, a winter campaign was launched to measure hourly NH3 and other
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supporting data in an urban environment in northern China. The objectives of the
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present study are to 1) examine whether traffic emission is a significant contributor to
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urban atmospheric NH3 in northern China; 2) explore major contributors of
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atmospheric NH3 during wintertime; and 3) present implications for mitigation of
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urban atmospheric NH3.
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Experimental
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The sampling site is located in an urban area in Qingdao, a city with 9 million
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population in northern China (Fig. 1). From mid-November to the end of December
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2015, a suite of semi-continuous and real-time instruments were used to monitor
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atmospheric NH3, H2O vapor, and other pollutant gases such as CO2, NO, NO2, O3
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and SO2. All instruments were housed in an air-conditioned lab at the second floor of
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an academic building. The lab is approximately 300 m away from a major traffic road
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with moderately heavy traffic flow, for which the counted traffic density on weekdays
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and weekends are shown in Fig. S1. On weekday morning (07:30-08:00) and
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afternoon (16:30-17:00) rush hours, the maximum traffic flows reached ~3300-3500
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vehicles per hour, which were about three times of the minimum traffic from midnight
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to early morning. On weekends, the maximum traffic decreased by approximately
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30%. The on-site counting also showed that ~90% of the vehicle fleet consisted of
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passenger cars and light-duty vehicles. In China, passenger cars and light-duty
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vehicles are overwhelmingly powered by gasoline and they are enforced to equip with
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3-way catalytic converters. When winds blow from the northwest, west and southwest,
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the major traffic road is situated upwind of the lab.
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An URG-9000D Ambient Ion Monitor - Ion Chromatograph (AIM-IC) was operating
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at a flow rate of 3 L min-1 to semi-continuously measure atmospheric NH3 together
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with acidic gases, anion and cation in PM2.5 at 1-hr resolution. The AIM-IC is
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equipped with a wet denuder to absorb NH3 and acidic gases and the denuder solution
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is injected to IC for chemical analysis. Details about AIM-IC instrument can be found
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in Markovic et al.31 In this study, the ambient air was drawn through a stainless steel
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tube in 1.2 m length and then passed through a PM2.5 sharp-cut cyclone on the
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AIM-IC. The tube’s inlet was ~5 m above the ground level and the surrounding
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buildings are ~15 m height. As reported32, the length of sampling line had a negligible
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influence on the low resolution measurement of atmospheric NH3, e.g., time
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resolution > 30 minutes. The hourly average mixing ratios of atmospheric NH3
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measured by the AIM-IC were available only on 18-30 November, and 10-12 and
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18-30 December 2015 while the system experienced maintenance or calibration at
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other times.
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An NH3 analyzer (NH3-H2O, Model 912-0016, LGR), a greenhouse gas analyzer
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(CO2-CH4-CO2-H2O, Model 911-0011, LGR), and gas analyzers including NOx
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(TECO 42C), SO2 (TECO 43CTL) and O3 (TECO 49C) were also used to
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simultaneously measure related gases. The former two analyzers operating at 1-sec
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resolution at a flow rate of 0.3 L min-1 were calibrated by a commercial company
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before use. The latter three analyzers operating at 1-min resolution had a regular
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calibration following the standard protocol (US EPA, Quality Assurance Guidance
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Document 2.3). Teflon tubes in length of 2-4.5 m were used to connect the analyzers
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with ambient air. Teflon filters were used to remove particles in the air upstream of the
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analyzers. In addition, a PM2.5 sampler (Thermo Scientific™) equipped with two
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denuders was used to measure NH3 and acidic gases and chemical species including
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NH4+ in PM2.5 during a few days. The sampler stood at ~1.5 m above the green space
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and was ~30 m away from the lab. The collected samples were extracted and the
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extracts were determined by the AIM-IC operating offline. The measured mixing
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ratios of atmospheric NH3 reflected the 24-hr average values.
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During the campaign, a total of 120 soil samples at 0-1 cm and 1-2 cm depths were
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also collected from green spaces in surrounding areas (Fig. 1), i.e., Site A was situated
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at the roadside green space of the major traffic road, Site B at the green space where
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the denuder sampler stood, and Site C at the green space outside the lab (Table 1). The
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morning and afternoon soil samples were collected at 08:30-09:30 and 13:00-14:00,
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respectively, on each day. For each soil sample, 50 g soil was added to 50 ml
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deionized water and then mixed for one minute. The solution was filtered into two
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vials for measuring pH immediately and storing at 4C. After the campaign, the stored
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solution was diluted by 10 times and the diluted solution was injected to the AIM-IC
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operating offline to determine ionic species. In addition, more soil samples were
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collected at two other urban green spaces and three agriculture fields across northern
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China in January and February 2016. These samples were stored at -4C and then
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moved to the lab using an ice box.
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Soil emission potential (Γg) and canopy compensation point of NH3 (χg) of the urban
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green space was calculated according to Nemitz et al.33: [NH+ 4 ]𝑔
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𝛤𝑔 =
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𝜒𝑔 = 〈
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where [NH4+]g and [H+]g were the concentrations of NH4+ and H+ (moles per liter),
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respectively, in the soil. Tg was the temperature of the ground soil (K). In-situ Tg was
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not measured, and surface soil T (in the top 1-2 cm depth) usually responses rapidly to
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changes in ambient T, i.e., within a few hours34. The ambient temperature was thus
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used as a surrogate for Tg, knowing that this could introduce small uncertainties to the
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calculated χg.
[H+ ]𝑔
=
[NH+ 4 ]𝑔
(1)
10−PH
161,500
10,378
𝛵𝑔
𝛵𝑔
〉 exp (−
) 𝛤𝑔 × (1.703 × 1010 )
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Meteorological data were obtained from an automatic weather station set up at the
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building roof nearby the major traffic road (Fig. 1). During the measurement period in
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November 2015, the automatic weather station didn’t operate properly. In the
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November, ambient temperature recorded from a station ~2 km away from the
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sampling site was used. The recorded ambient temperature was highly consistent with
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the local ambient temperature (T) based on a comparison using the data collected in
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the December, which shows Tlocal = 1.01* Tstation, and with Pearson correlation
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coefficient (r)=0.96. Wind speed (WS) at the two locations also correlated well if
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excluding a few outlier (accounting for 0.5% of the total data points), WSlocal = 0.66*
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WSstation and r=0.84.
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equation for the period of November 22-24 when measurement was not available.
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Wind direction between the two locations was generally inconsistent under WS
