Fossil Fuel Combustion-Related Emissions Dominate Atmospheric

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Fossil fuel combustion-related emissions dominate atmospheric ammonia sources during severe haze episodes: Evidence from 15N-stable isotope in size-resolved aerosol ammonium Yuepeng Pan, Shili Tian, Dongwei Liu, Yunting Fang, Xiaying Zhu, Qiang Zhang, Bo Zheng, Greg Michalski, and Yuesi Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00634 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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Environmental Science & Technology

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Fossil fuel combustion-related emissions dominate atmospheric ammonia sources

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during severe haze episodes: Evidence from

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aerosol ammonium

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N-stable isotope in size-resolved

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Yuepeng Pan*†, Shili Tian†, Dongwei Liu‡, Yunting Fang*‡, Xiaying Zhu§, Qiang Zhangǁ, Bo

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Zhengǁ, Greg Michalski⊥, Yuesi Wang†

7 8

†

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(LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry

10

‡

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Academy of Sciences, Shenyang, Liaoning 110164, China

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§ National climate center, China Meteorological Administration, Beijing 100081, China

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ǁ

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Science, Tsinghua University, Beijing 100084, China

15 16

⊥

Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese

Ministry of Education Key Laboratory for Earth System Modeling, Center for Earth System

Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907,

United States

17 18

Contact author: Yuepeng Pan ([email protected])

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TOC/Abstract art

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ACS Paragon Plus Environment


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Abstract

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The reduction of ammonia (NH3) emissions is urgently needed due to its role in

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aerosol nucleation and growth causing haze formation during its conversion into

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ammonium (NH4+). However, the relative contributions of individual NH3 sources are

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unclear, and debate remains over whether agricultural emissions dominate

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atmospheric NH3 in urban areas. Based on the chemical and isotopic measurements of

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size-resolved aerosols in urban Beijing, China, we find that the natural abundance of

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15

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development of haze episodes, ranging from -37.1‰ to -21.7‰ during clean/dusty

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days (relative humidity: ~40%), to -13.1‰ to +5.8‰ during hazy days (relative

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humidity: 70-90%). After accounting for the isotope exchange between NH3 gas and

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aerosol NH4+, the δ15N value of the initial NH3 during hazy days is found to be -14.5‰

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to -1.6‰, which indicates fossil fuel-based emissions. These emissions contribute 90%

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of the total NH3 during hazy days in urban Beijing. This work demonstrates the

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analysis of δ15N values of aerosol NH4+ to be a promising new tool for partitioning

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atmospheric NH3 sources, providing policy makers with insights into NH3 emissions

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and secondary aerosols for regulation in urban environments.

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Keywords: Isotope; Ammonia; Ammonium; Haze; Size distribution; Secondary

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aerosols; Urban Beijing

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1. Introduction

N (expressed using δ15N values) of NH4+ in fine particles varies with the

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Severe and persistent haze episodes in China, which are characterized by

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extremely high concentrations of particulate matter (PM), are of concern due to

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regional and global impacts PM has on human health, the environment and climate

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change.

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secondary aerosols, and their chemical composition is similar to the compositions

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typically observed in other urban areas.

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either secondary organic aerosols (SOAs) or secondary inorganic aerosols (SIAs), and

1, 2

The severe PM concentrations in China’s urban centers are dominated by

3

Secondary aerosols can be categorized as


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both of them are important sources of fine (< 2.5 µm diameter) particles.

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urban areas of China, such as Beijing, a higher proportion of SIAs relative to SOAs

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has been observed during haze episodes, suggesting a greater importance of SIAs in

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haze pollution chemistry. 4 SIAs are mainly formed when the dominant alkaline gas in

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the lower atmosphere, ammonia (NH3), reacts with acidic compounds such as sulfuric

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acid (H2SO4) and nitric acid (HNO3) to form particulate ammonium (NH4+).

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addition to acid gas to particle conversion, NH3 can also play a role in aerosol

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nucleation enhancing SIAs formation and growth by neutralizing H2SO4 droplets.

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However, despite the importance of NH3 in urban haze chemistry, this pollutant is

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currently not included in emission control policies in China or in other regions

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worldwide that also experience severe particulate pollution.

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emission strength of individual NH3 sources remains unclear, and a debate continues

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over whether NH3 transported from agricultural regions are the major contributors to

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atmospheric NH3 in urban areas as opposed to local NH3 sources. Thus, identifying

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NH3 emission sources and quantifying the relative contributions of different sources

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to the NH3 budget is important for management strategies designed to reduce the

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adverse impacts of atmospheric NH3. 7

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6

In many

5

In

2

Besids, the relative

The application of stable nitrogen (N) isotopes as potential tracers of the origin of 7

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NH3 has been proposed,

which might help constrain regional NH3 budgets. N

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isotopic signatures in the U.S. revealed that the NH3 emitted from volatilized

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livestock waste and fertilizer has relatively low δ15N values (-56‰ to -23‰, relative

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to air N2) that could be used to differentiate it from NH3 emitted from fossil fuel

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sources, which have higher δ15N values (-15‰ to +2‰). 7 This approach was applied

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in Pittsburgh, an urban center in the U.S., and revealed high δ15N values (-8.5 ± 7.2‰)

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of NH3, which were suggested to mainly originate from fossil fuel combustion-related

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emissions, 8 despite the fact that global emission inventories indicate that agricultural

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NH3 emissions are responsible for 80% to 93% of NH3 emissions.

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atmospheric NH3 from fossil fuel sources in China has not been accurately quantified,

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although non-agricultural sources of NH3 in Beijing have been elucidated during the

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winter based on correlations between NH3 and the traffic-related pollutants NOx and


9

To date,


10, 11

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CO.

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concentrations, and the potential complexity of determining NH3 source attribution in

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a system with a mixture of multiple sources,

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challenge. Therefore, quantitative estimates based on the δ15N technique and other

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top-down constraints are needed to further quantify the NH3 sources in large cities.

In large urban centers, such as Beijing, the high spatial variability of NH3

10-12

makes closing the NH3 budget a

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Here, we present stable N isotopic composition results for NH4+ that was sampled

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using a size-resolved aerosol sampler in urban Beijing during extreme haze episodes

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in early 2013. These haze pollution events were persistent, and they covered ~1.3

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million km2 and affected ~800 million people.

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showed that the daily average concentrations of PM10 and PM2.5 exceeded the Chinese

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National Ambient Air Quality Standard (GB3095-2012, Grade I) of 50 and 35 µg m–3

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for 88% and 81% of the days, respectively, between January 1 and February 28, 2013,

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with record-breaking instantaneous concentrations of 995 and 772 µg m–3,

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respectively, on January 12.

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500 µg m–3 occurred in January 2013. 15 These five episodes were cyclic with a period

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ranging from 4-7 days. Such strong haze cycles are regional in nature and are

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controlled by synoptic scale weather systems, specifically the passage of cold fronts,

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which induce inversion layers and stagnate air flow. 16 In this study, we focused on the

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samples collected during the period from January 24 to February 1, which represents

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an extreme haze pollution episode with a clear periodic cycle of 7 days.

14

13

PM measurements in urban Beijing

Five episodes with mean PM2.5 concentrations above

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The objective of this study was to identify the major sources of NH3 in urban

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regions during haze pollution based on the N isotope fractionation that occurs during

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the conversion of NH3 gas to NH4+ in fine particles. To our knowledge, this study

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represents the first isotopic interpretation of size-resolved aerosol NH4+ in China and

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is likely the first such interpretation in a highly urbanized environment in the world.

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The size-resolved isotopic data collected here are unique and may be useful for

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characterizing NH3 sources and N chemistry during haze formation. This information

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is critical for atmospheric chemistry and transport modeling and for the assessment of

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haze mitigation policy options in the regions with severe NH3-related air pollution.


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2. Materials and methods

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2.1 Aerosol sampling

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The PM sampling was performed on the roof of a building (15 m height) at the

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State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric

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Chemistry (LAPC), Institute of Atmospheric Physics (IAP), Chinese Academy of

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Sciences (CAS) in urban Beijing (39°58′ N, 116°22′ E).

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Yuandadu Park and is surrounded by a dense residential area (ca. 7500 persons km–2).

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The site is approximately 0.8 km north of the 3rd Ring Road and 1.3 km south of the

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4th Ring Road, with daily average traffic volumes of approximately 10 million day–1

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for both ring roads. No major industrial emissions from coal combustion are present

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in urban Beijing, but residential coal burning during the cold observation period was

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expected for home heating.

12

The site is located in

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Size-segregated aerosols were collected using a 9-stage impactor sampler

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(Anderson Series 20-800, USA) at a flow rate of 28.3 L min–1. The 50% cutoff size

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bins of the sampler are >9.0, 9.0-5.8, 5.8-4.7, 4.7-3.3, 3.3-2.1, 2.1-1.1, 1.1-0.65,

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0.65-0.43 and 2.1 µm), causing the NH3 to

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dissolve on the surface of wet particles. During January 31 to February 1, when the air

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mass shifted from the SE to the NW with increased WS, the NH4+ concentrations

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decreased to 15.8 µg m–3 and peaked at 1.1-2.1 µm (Figures 1-2).

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3.2 Isotopic features of size-resolved NH4+

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Figure 3 plots the δ15N-NH4+ values as a function of particle size during the

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observation period. Since the formation pathway of fine and coarse particles is

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different, the isotopic features of size-resolved NH4+ were discussed separately below.

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For fine particles, we observed relatively low δ15N-NH4+ values on the clean

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days of January 24-25, with mean values of -26.6‰ (range: -31.9‰ to -21.7‰).


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Because the air mass was from the NW during this period, this signature may indicate

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a regional transport of NH4+. This pattern is evident from another dusty day on

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February 28, with the comparable range of δ15N-NH4+ values (-37.1‰ to -26.2‰)

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found in fine particles. The clean and dusty days are also characterized by low RH

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(~40%), which could indicate that the particles are less deliquesced; thus the NH4+ in

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transported particles is much slower to equilibrate with local NH3 sources because it

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is mostly in the solid phase. In contrast, the increase in RH and NH4+ concentrations

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during the haze development stage suggests that the equilibration of NH3 with fine

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particle NH4+ became favored under these conditions because the particles are

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deliquesced and liquid-gas exchange is expected to be rapid. This interpretation is

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supported by the increase in δ15N-NH4+ values from -2.7‰ on January 25-27 and to

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+1‰ on January 27-28. During the subsequent days, the δ15N-NH4+ value was stable

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at approximately -10‰, although the NH4+ concentrations reached their peak on

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January 29-30 and then decreased from January 30-31 to January 31-February 1.

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For coarse particles, the δ15N-NH4+ values showed an irregular pattern, which is

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also different from that of the fine mode. In general, the value of δ15N-NH4+ in the

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coarse mode (except for 2.1-4.7 µm) on January 24-25 was higher than that on

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January 25-28 but lower than that on January 28-February 1, which is in profound

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contrast to the temporal pattern of the NH4+ concentrations. During the January

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28-February 1 period, however, the δ15N-NH4+ values in coarse particles (except for

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4.7-5.8 µm) tended to increase and showed values similar to those of fine particles.

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In addition, the size distribution and isotopic features of NH4+ on January 7-9

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(hazy days) and February 28 (dusty day) from other pollution episodes exhibited

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similar patterns to the hazy days and clean days during January 24-February 1

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(Figures 2-3). Moreover, prior to the arrival at Beijing during the sampling days, the

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air parcels during this study overlapped with the source regions of NH4+ on an annual

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basis (Figure S5). The findings indicate that the results presented are representative

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and reproducible on a regular basis.



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3.3 Tracing NH3 emissions using the calculated initial δ15N-NH3 values

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The δ15N-NH4+ values of fine particles observed in this study span a wide range

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from -37.1‰ to +5.8‰ (Figure 3). This range is similar to that reported at coastal

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sites, with values of -40‰ to +15‰,

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of 4‰ to +32‰ in marine aerosols. 29 However, these δ15N values of NH4+ may differ

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from the isotopic composition of the precursor NH3 due to isotopic fractionation

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during the conversion from NH3 gas to aerosol NH4+, highlighting the difficulties in

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tracing the sources of NH3 using aerosol δ15N-NH4+ values. Currently, it is also

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difficult to perform source apportionment of NH4+ in different particle sizes due to the

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lack of size-resolved isotopic signatures of aerosol NH4+. Fortunately, the difference

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in δ15N-NH4+ values among the different size bins in the fine mode was small for a

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given sampling day, indicating the similarity of their sources. Thus, the

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concentration-weighted mean of δ15N-NH4+ values for fine particles with sizes less

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than 2.1 µm was used to calculate the initial δ15N-NH3 values, after accounting for the

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isotope exchange between NH3 gas and aerosol NH4+.

28

but is more depleted than the reported values

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Figure 3 also shows the calculated results of the initial δ15N-NH3 value against

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various NH3 emission signatures. For clean days, the initial δ15N-NH3 value was

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-48‰, which is close to the value of NH3 from agricultural sources. During the

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remaining haze days, the initial δ15N-NH3 value was estimated to be approximately

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-10‰ (range: -1.6‰ to -14.5‰), which differs from the values of agricultural sources

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but falls within the range of NH3 related to fossil fuel sources (e.g., vehicles, power

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plants and coal combustion).

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fossil fuel combustion rather than agricultural sources might be the major source of

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NH3 in urban Beijing.

7

This result is the first isotopic evidence showing that

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Interestingly, on hazy days, the initial δ15N-NH3 value fell within the range for

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vehicle exhaust and power plant slip sources and was comparable to that of aerosol

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δ15N-NH4+ (Figure 3, right). However, on clean days, the difference between aerosol

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δ15N-NH4+ and initial δ15N-NH3 for the agricultural emissions is substantially larger

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than that for the other two sources. The lower degree of modification of the original


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δ15N value of NH3 gas for these two sources compared with that for agricultural

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emissions results from the lower gas/aerosol phase distribution of N (hence, higher f)

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in the air during haze periods. This effect is expected because the concentration of

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NH3 is significantly lower than that of NH4+ during cold seasons. 11 This is especially

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true under conditions with lower temperature and higher humidity, which favor the

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conversion of gaseous NH3 to particulate NH4+.

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higher than particulate NH4+ under warm weather conditions (Figure S2-b). In

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addition, the equilibrium fractionation factor may be greater when temperatures

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decrease,

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gas δ15N-NH3 values under various conditions.

30

6

In contrast, NH3 gas is usually

and this should considered when comparing aerosol δ15N-NH4+ values to

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To further quantify the contribution of NH3 sources to the initial air

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concentrations of NH3, the isotope model “IsoSources” was run. The source

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apportionment results shown in Figure 4 indicate that 84% and 16% of ambient NH3

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during the clean days of January 24-25 can be attributed to agricultural emissions and

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non-agricultural sources, respectively. The reverse was true during hazy days, when

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non-agricultural sources dominated the ambient NH3, with a mean contribution of

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approximately 90%. During the haze development stage, the contribution from fossil

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fuels increased from 34% (January 25-26) to 59% (January 26-27) and to 81%

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(January 27-28), whereas the contribution of NH3 slip (power plants) decreased from

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48% (January 25-26) to 34% (January 26-27) and to 18% (January 27-28). However,

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the percent contributions of fossil fuels and NH3 slip to each sampling day were

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comparable during January 28- February 1 (Figure 4). These results suggest that fossil

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fuel combustion can be a substantial contributor to ambient NH3 in urban

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environments during haze pollution, whereas that from agricultural sources can be

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minor.

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4. Discussion

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Previous studies showed that the major aerosol compounds, such as nitrate,

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sulfate and carbon, in the extreme haze events in early 2013 in China were due to



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fossil fuel combustion and biomass burning, 1, 4, 13 but the source of aerosol NH4+ was

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not quantified. This study presents the first isotopic evidence that fossil fuel emissions

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and NH3 slip from power plants are more important than agricultural sources during

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haze pollution periods in urban Beijing. In contrast, agricultural sources are the major

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sources of NH3 in the air during clean days when the WS is higher and the air

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transport is from the SW of Beijing.

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The present interpretation is reasonable because agricultural and fossil fuel

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emissions are usually associated with rural and urban areas, respectively. During haze

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periods, the atmospheric transport velocity was 50-100 km d–1, as estimated both from

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the measured surface WS (Figure 1) and by a back-trajectory analysis (Figure S4).

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Under such stagnant weather conditions, local emissions from fossil combustion are

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expected to be important. In contrast, during clean days, strong winds brought cleaner

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air from agricultural areas to Beijing. Hence, the ambient NH3 that formed NH4+

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aerosols during haze periods in Beijing was sourced at the local, urban scale rather

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than at the regional scale.

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In urban environments, vehicles equipped with three-way catalytic converters 31

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can be an important NH3 source.

Previous estimates indicate that in 2005 the total

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emission of NH3 from traffic sources was 1.8 kt NH3-N yr–1, which is half of the

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amount from agricultural sources in Beijing.

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registered in Beijing, however, has increased by 10% per year, suggesting that the

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emission of NH3 by traffic in Beijing during 2013 may have become comparable to

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that of agricultural emissions.

10

The total number of vehicles

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In addition to being emitted form automobiles, NH3 is emitted as “fuel NH3” from

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electrical generating units (EGUs) and as “NH3 slip” from EGUs equipped with SCR

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and selective non-catalytic NOx reduction (SNCR) technologies.

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recent report from China’s Ministry of Environmental Protection, 25 coal-fired power

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utilities in Beijing have installed NOx removal systems. Of these utilities, 16 are

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controlled through an SCR system (with a total capacity of approximately 3400 MW),

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and the remaining utilities (with a total capacity of approximately 2000 MW) combine

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SCR and SNCR. Although it is currently difficult to determine the NH3 emissions


8

According to a

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from these sources in Beijing, the emissions are expected to be enormous due to the

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electricity demand and the fact that extra coal is consumed for residential heating in

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the winter. A recent finding in Shanghai also highlights the importance of industrial

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NH3 emissions, rather than those from vehicles. 6

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The above fossil fuel-based NH3 emissions are important in urban areas where

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the abundant acidic compounds (HNO3 and H2SO4) can react with NH3 to form fine

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particles, promoting serious haze pollution. Evidence can be found in the positive

394

relationship between NH4+ and other fossil fuel-related species (SO42–, NO3–, OC, SO2,

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NOx and CO) in this study (Figure S6) and elsewhere. 10, 11 Although NH3 is currently

396

not included in emission control policies in China and is also unregulated in most

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other regions, the abatement of NH3 emissions has become an increasing concern to

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air quality managers, especially during periods of haze pollution.

399

The results reported here provide the first real isotopic evidence for fossil fuel

400

sources of aerosol NH4+ during extreme haze episodes in urban Beijing. The use of

401

stable isotopes of NH4+ provides a powerful method to evaluate the relative

402

contribution of fossil fuel-related and agricultural sources, allowing for better

403

targeting of NH3 reduction. Other non-agricultural area sources of NH3 emissions

404

include oceans, human waste, vegetation and soils,

405

contributors during the winter at this location. Additional emissions from

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biomass/biofuel burning could also contribute, but intense biomass burning was not

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found based on moderate resolution imaging spectroradiometer (MODIS) fire/hotspot

408

observation (Figure S7). We did not include indoor biofuel burning in this study due

409

to the paucity of signature data, a point that should be considered in the interpretation

410

of our results. Other potential limitations in this study include the assumptions that the

411

δ15N values of the endmembers in China are similar to those in the U.S. and that the

412

parameters used in N fractionation were well constrained. To further reduce the

413

uncertainty in the calculation of initial δ15N-NH3 values, additional field and lab

414

experiments are required to adequately characterize the endmember signatures and the

415

subsequent fractionation process under different air pollution and meteorological

416

conditions.

32

which are unlikely major



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Associated content

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Supporting Information

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Figures S1-S7 illustrating haze pollution events, modeling validation, source

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apportionment, potential source regions, backward trajectory, various component

421

correlations and fire/hotspot archive, and text details the N fractionation process, with

422

accompanying references. This information is available free of charge via the Internet

423

at http://pubs.acs.org.

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Author information

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Corresponding Authors

426 427 428 429

* Y.P. e-mail: [email protected]. Tel.: +86 01082020530; Fax: +86 01062362389. * Y.F. e-mail: [email protected]. Author Contributions

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Y.P. and Y.W. conceived and designed the project; Y.P. and S.T. conducted the

431

field work; S.T. and D.L. performed the laboratory experiments; Q.Z. and B.Z.

432

performed the modeling experiments; Y.P., Y.F., X.Z. and G.M. analyzed the data; and

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Y.P. wrote the paper with comments from the coauthors.

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Notes

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436

The authors declare no competing financial interest. Acknowledgements

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This work was supported by the CAS Strategic Priority Research Program (No.:

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XDA05100100 and XDB05020000) and the National Natural Science Foundation of

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China (No.: 31370464, 41405144, 41321064 and 41230642). We are indebted to the

440

staff who collected and analyzed the samples during the study. The authors would like

441

to thank three anonymous reviewers for stimulating a fruitful discussion, due to which

442

some results presented in the original version of the paper could be revised and


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expanded.

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Figures

0

60

-5

40

T

-10

-1

WS (m s )

-3

PM Conc. (µg m )

-3 +

20

0 -15 Jan 24 Jan 25 Jan 26 Jan 27 Jan 28 Jan 29 Jan 30 Jan 31 Feb 1 8 360 (b) WD 6 270 WS 4

180

2

90

0 0 Jan 24 Jan 25 Jan 26 Jan 27 Jan 28 Jan 29 Jan 30 Jan 31 Feb 1 800 1050 (c) PM10 Air pressure 600 1040

PM2.5

400

1030

PM1

200

NH4 Conc. (µg m )

559

RH

RH (%)

80

o

T ( C)

5

100

(a)

WD (degree)

10

P (hPa)

558

1020

0 1010 Jan 24 Jan 25 Jan 26 Jan 27 Jan 28 Jan 29 Jan 30 Jan 31 Feb 1 70 60 50 40 30 20 10 0

(d) Coarse mode Fine mode >9.0 µm 5.8-9.0 4.7-5.8 3.3-4.7 2.1-3.3

1.1-2.1 0.65-1.1 0.43-0.65 9

Fossil Fuel Combustion-Related Emissions Dominate Atmospheric

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