- Email: [email protected]
Ice nucleation, shape, and composition of aerosol particles in one of the most polluted cities in the world: Ulaanbaatar, Mongolia
Accepted Manuscript Ice nucleation, shape, and composition of aerosol particles in one of the most polluted cities in the world: Ulaanbaatar, Mongolia Christa A. Hasenkopf, Daniel P. Veghte, Gregory P. Schill, Sereeter Lodoysamba, Miriam Arak Freedman, Margaret A. Tolbert PII:
S1352-2310(16)30384-3
DOI:
10.1016/j.atmosenv.2016.05.037
Reference:
AEA 14625
To appear in:
Atmospheric Environment
Received Date: 28 January 2016 Revised Date:
18 May 2016
Accepted Date: 21 May 2016
Please cite this article as: Hasenkopf, C.A., Veghte, D.P., Schill, G.P., Lodoysamba, S., Freedman, M.A., Tolbert, M.A., Ice nucleation, shape, and composition of aerosol particles in one of the most polluted cities in the world: Ulaanbaatar, Mongolia, Atmospheric Environment (2016), doi: 10.1016/ j.atmosenv.2016.05.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Ice Nucleation, Shape, and Composition of Aerosol Particles in One of the Most Polluted Cities in the World: Ulaanbaatar, Mongolia
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Christa A. Hasenkopf a,*, Daniel P. Veghteb, Gregory P. Schillc, Sereeter Lodoysambad, Miriam Arak Freedmanb,*, Margaret A. Tolbert c,*
Addresses: a) OpenAQ, Washington, DC 20009
b) Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
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c) CIRES & Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO 80309
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d) German-Mongolian Institute for Resources and Technology, Nalaikh, 2 Khoroo, Ulaanbaatar, Mongolia and National University of Mongolia, Ikh Surguuliin Gudamj–1, Ulaanbaatar, Mongolia
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* To whom all correspondence should be addressed: Christa Hasenkopf OpenAQ Washington, D.C. 20009 Phone: (507) 246-2097 Fax: Not Available [email protected]
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Miriam Freedman Department of Chemistry The Pennsylvania State University University Park, PA 16802 Phone: (814) 867-4267 Fax: (814) 865-3314 [email protected]
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Margaret Tolbert CIRES & Department of Chemistry and Biochemistry University of Colorado at Boulder Boulder, CO 80309 Phone: (303) 492-3179 Fax: (303) 492-1149 [email protected]
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Abstract Air pollution is attributable to 7 million deaths per year, or one out of every eight deaths globally. In particular, high concentrations of particulate matter (PM), a major air pollutant, have significant impacts on health and regional climate in urban centers. Many of the most polluted places, largely in developing countries, go severely understudied. Additionally, high particulate matter levels can have an impact on the microphysical properties of clouds, impacting precipitation and regional climate. Semi-arid regions can be especially affected by small changes in precipitation. Here we characterize the physical and chemical properties of PM in one of the most PM-polluted cities in the world: Ulaanbaatar, Mongolia, a semi-arid region in central Asia. Twice monthly aerosol samples were collected over 10 months from a central location and analyzed for composition and ice nucleation activity. Almost all particles collected were inhalable, consisting primarily of mineral dust, soot, and sulfate-organic. In winter, all classes of PM increase in concentration, with increased sulfur concentrations, and the particles are less active towards heterogeneous ice nucleation. In addition, concurrent monthly average PM10, SO2, NOx, and O3 levels and meteorological data at a nearby location are reported and made publicly available. These measurements provide an unprecedented seasonal characterization of the size, shape, chemical structure, and ice nucleating activity of PM data from Ulaanbaatar. This 10month field study, exploring a variety of aerosol properties in Ulaanbaatar, Mongolia, is one of very few such studies conducted in the region or in such a highly polluted environment. The results of this study may inform work done in other similarly situated and polluted cities in Asia and elsewhere.
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Keywords: Air pollution; climate; ice nucleation; electron microscopy; particulate matter; sulfur
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Introduction Driven by urbanization, overall increasing populations, and improved economic
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development, developing countries must often meet higher energy demands with fuels that
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produce airborne-pollutants as byproducts. In addition to affecting climate, these pollutants
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compromise the health of billions, especially those in developing countries, which account for
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more than 80% of the world’s population (Central Intelligence Agency, 2013). The World Health
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Organization (WHO) deems air pollution the most lethal environmental hazard on the planet,
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responsible for one of every eight deaths (WHO, 2014).
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Despite the threats air pollution poses, the most polluted places are vastly understudied.
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Fig. 1 compares the annual average concentration of PM10 (particulate matter, PM, 10µm or less
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in diameter) in cities around the world with the number of scientific papers that have been
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published about air pollution in a given city through 2011 (WHO, 2011). As well as showing
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well-researched cities, Fig. 1 shows the ten most PM10-polluted cities from the 2011 WHO
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Outdoor PM10 Survey of over 1000 cities (WHO, 2011).
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According to Fig. 1, Ulaanbaatar, Mongolia is the city with the second highest annual
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average PM10 levels. Additionally, a 2011 World Bank study reports monthly Ulaanbaatar
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average PM2.5 (particles with diameters less than 2.5µm) concentrations in December 2008 and
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January 2009 exceeding 1000 µg m-3 – more than 40 times the PM2.5 daily average WHO
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guidelines (World Bank, 2011). A study by Allen et al. conservatively estimates that up to 25%
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of deaths annually in Ulaanbaatar are caused by PM (Allen et al., 2013). In addition, Asian air
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pollution can intensify storms over the Pacific (Zhang et al., 2007; Wang et al., 2014a & 2014b).
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The majority of Ulaanbaatar’s 1.3 million people live in the ger district, comprised of traditional
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Mongolian homes (gers) and wooden buildings, typically heated by coal-burning stoves that emit
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incompletely combusted fuel as soot from chimneys 3-4m high (Zagdjav, 2003). Coal-fired heat-
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only boilers (HOBs) are also prevalent in the ger district for heating in larger buildings. With
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60% of the city’s residents living in the ger district, it is unsurprising that the PM emissions from
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stoves and HOBs have been found to be main sources of PM-pollution (Guttikunda, 2008; Davy
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et al., 2011). Ulaanbaatar is situated at an altitude of 1,350m in a valley. Like many higher
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altitude valleys, the city experiences temperature inversions and horizontal airflow is blocked to
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the north and south due to surrounding mountains. Additionally, Ulaanbaatar’s sub-arctic,
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semiarid climate influenced by the Siberian High exacerbates the effects of pollution from the
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ger district. Its semi-arid climate also leaves Ulaanbaatar susceptible to potential changes in the
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regional climate, such as precipitation, due to the presence of high PM concentrations.
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To date, there have been few studies spanning all four seasons on ambient PM levels in
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Ulaanbaatar (See Table 1 for full comparison). Except for Davy et al. (2011), previous studies
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have explored the concentration and chemical composition of PM over limited time periods or
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have relied city air quality monitoring for longer-term measurements.
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sources, chemistry, and consequences of PM for air quality and regional climate, however, long-
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term data sets are needed. Recently, donor organizations such as the U.S.’s Millennium
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Challenge Corporation (MCC) and World Bank have, along with the Mongolian Government,
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taken several PM-pollution mitigation measures. These measures included a 41 million USD
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effort by MCC to sell over 100,000 efficient stoves and Mongolian government-subsidized
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alternative fuels (United States Department of State, 2013). The stove subsidy started in 2011,
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and use of these stoves was evaluated during three survey periods spanning October 2012
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through March 2013 (MCC, 2014a & 2014b). The MCC stove program found that while ambient
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PM2.5 was estimated to have been reduced by 30%, no statistically significant reduction in fuel
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consumption resulted, perhaps due to households heating to warmer indoor temperatures than
To understand the
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with less efficient stoves (MCC Summary: Measuring Results of the Mongolia Energy and
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Environment Project Stove Subsides Component, 2014). Further, no data for the chemical
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composition of the PM2.5 were reported. There are no other published peer-reviewed studies
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that characterize and report PM during the period when the MCC stove program was initiated.
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Such information is critical to studies analyzing the impact of such programs on ambient PM.
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We have collected twice-monthly samples from a residential location in Ulaanbaatar, as
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described below, over ten months (June 2012 to March 2013; Fig. 2). Samples were analyzed
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for type and elemental composition by transmission electron microscopy (TEM) and energy
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dispersive X-ray spectroscopy (EDS) as well as for ice nucleation onset. We also report nine
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months of concurrent monthly average PM10, SO2, NOx, and O3 levels as well as meteorological
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data, as measured at a nearby location.
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Materials and Methods
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Description of Sampling Site
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approximately 10 km from the city’s coal-fired power plants and 600 meters away from the
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southern edge of the ger district. The sampling site was on a balcony, approximately 15 m above
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the ground, facing south. A downward facing funnel, filled with a coarse wire mesh was used to
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eliminate water droplets and very coarse material entering the inlet. No PM filter was used to
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prevent the collection of larger particles. Please see the Supplementary Information for more
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details on the sampling collection methods.
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The sampling site (Fig. 2) was located east and slightly north of the city center,
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TEM/EDS Studies
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Transmission Electron Microscopy (TEM) samples were prepared by impacting particles
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on TEM grids (200 mesh copper coated with continuous carbon, Electron Microscopy Science,
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Hatfield, PA). Particles were imaged using a Philips EM420T operated at an accelerating voltage
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of 120kV. Example images for two different months taken at the magnification used for the
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analysis are shown in Fig. S1. Particles were classified based on shape (spherical, fractal,
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irregular). The area of each particle was determined using ImageJ software (National Institutes
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of Health, Bethesda, MD). A size distribution for each shape was calculated by converting the
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area calculated for each particle to an area-equivalent diameter, assuming the particles are
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spheres and consequently, the projected area of the particles on the TEM images are circles. To
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obtain the elemental composition of the particles, the EDS detector on the Philips EM420T was
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used.
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Ice Nucleation Studies
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The experimental setup used for ice nucleation experiments has been described previously (Baustian et al., 2010; Schill et al., 2013). Briefly, a Nicolet Almega XR Raman
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microscope, equipped with an Olympus BX-51 microscope, has been coupled to a Linkam
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THMS600 environmental cell. The temperature inside the cell is controlled by a Linkam TMS94
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temperature controller (accuracy of ±0.1 K). Water vapor inside the environmental cell is
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controlled by mixing dry and humidified flows of N2; the partial pressure of water is measured
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by a Buck Research CR-1A chilled mirror hygrometer in line with the cell (accuracy of ±0.15
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K). As in Schill et al. (2013), the relative humidity over the sample was controlled by keeping
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the water partial pressure constant, and slowly lowering the temperature at a rate of 0.1 K min-1.
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Ice particles were detected visually, confirmed spectrally via the Raman signature for ice, and
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video of the ice sublimation process revealed the IN.
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Air quality and Meteorological Data Air quality and meteorological data collected concurrently during the study period (with
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the exception of March 2013) at a monitoring site run from the National Air Quality Agency in
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Mongolia are also reported. Specifically, the site, which was located approximately 1km away
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from our sampling site (see Fig. 2), collected hourly PM10, SO2, NOx, and ozone (O3)
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concentrations. Hourly meteorological data (temperature, RH, wind speed, and wind direction)
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were also obtained. The site is at the southern edge of the ger district, in a residential and urban
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area. These data, shown as monthly averages to correspond with our measurements below, are
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displayed in Figure 3 and also publicly available in tabular form from ScholarSphere
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(http://scholarsphere.psu.edu).
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Results and Discussion
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Air Quality and Meteorological Data
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Numerous seasonal differences are seen in the air quality and meteorological data shown in Fig.
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3. The winter months (Nov 2012 – February 2013) have colder temperatures, but similar relative
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humidities compared with the summer months. The winter has higher PM10 and SO2, which is
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consistent with increased fuel consumption for residential and commercial heating. Note that all
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the collected data are taken as the number of energy efficient stoves in use rises from the MCC
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Stove Project, indicating that wintertime particulate levels may have been higher in previous
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years. In addition, the winter months have higher NOx concentrations, and such concentrations
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are consistent with a lower boundary layer. Ozone concentrations are lower during the winter,
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which is consistent with reduced photochemical production.
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Characterization of Particle Types Fig. 4 shows representative TEM images of the types of particles we observed. The type
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of particle is determined first by its characteristic shape (spherical, fractal, irregular) and then by
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its elemental composition. Spherical particles are approximately circular in the TEM images,
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fractal particles are composed of two or more discrete, smaller particles, and irregular particles
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are single particles that are non-spherical in shape. Based on composition, spherical particles
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were assigned to be organic and sulfate-organic particles with a small amount of fly ash and
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possibly also tar balls. Fractal particles are composed of soot and fly ash, where the soot can be
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partially collapsed due to additional material. Irregularly shaped particles are assumed to be
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mineral dust because they primarily contain Al, Si, and Ca, with a small fraction by number of
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anthropogenic particles and fully collapsed soot (Figs. 4, S2).
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The effect of particulates on human health depends in part on their size, where particles
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less than 10 µm are inhalable, and particles less than 2.5 µm have the strongest correlation with
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mortality rates (Spengler and Thurston, 1983; Chow and Spengler, 1986a and 1986b; Dockery et
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al., 1993). To determine the fraction of respirable particles by number, size distributions of the
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spherical, fractal, and irregular particles were determined from the two-dimensional projections
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of the particles observed in the TEM images and for the particles studied with optical
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microscopy for the ice nucleation experiments. As shown in Fig. 5, all particles observed with
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TEM are in the inhalable range, with almost all particles less than 2.5 µm. We also obtained data
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on particle size from the ice nucleation experiments, which can be compared to the size range
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observed with TEM. For the samples collected for the ice nucleation experiments, the average
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particle size detected with optical microscopy was 1.9 ± 1.5 µm. Less than 1% of the particles
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observed each month were greater than 10 µm in diameter. The size distribution of the particles
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collected for the ice nucleation experiments was analyzed using optical microscopy, which is
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sensitive to larger particle sizes (> 500nm).
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microscopy were most likely irregularly shaped particles. The particles fluoresced in the Raman
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microscope, prohibiting chemical characterization. Note that mineral dust and soot commonly
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fluoresce, suggesting that the particles observed with optical microscopy have the same
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composition as the irregularly shaped particles that we characterize with TEM and EDS. From
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the TEM and optical microscopy analysis, it can be seen that almost all particles collected fall
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within the respirable aerosol fraction, approximately 10 µm in aerodynamic diameter or smaller.
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Note that our particle numbers observed with TEM cannot be directly converted to PM10 or
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PM2.5. As shown below, however, increases in particle numbers with season observed with
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TEM agree with increases observed in PM10, as shown in Fig. 3.
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As a result, particles observed with optical
Seasonal Particle Concentration and Sulfur Content of Particles To probe the seasonal dependence of Ulaanbaatar particulate, we estimate the particle
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number concentration in each month. The relative particle concentrations are estimated in a
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given month by plotting the number of particles per TEM image per hour of collection time,
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averaging over the two monthly samples (Fig. 6). Note that all the TEM images were taken at
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the same magnification. Relating this calculation to the absolute particle concentrations assumes
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that the collection efficiency is the same in every collection and that the levels of particulate
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matter on the collection days are representative of that month. Even with the variability shown in
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Fig. 6, however, the particulate concentrations are consistently higher during the colder months
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(Nov-March) than in the warmer months (June-Oct). Increasing concentrations of particulates
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during the winter months is consistent with previous literature, as well as the data in Fig. 3 (Air
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Quality Analysis of Ulaanbaatar, 2011; Allen et al., 2013; Nishikawa et al., 2011). We have investigated how the amount of sulfur in the particles varies during the year for
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spherical, fractal, and irregular particles (Figs. 6, S3, S4). Some of the sulfur is native to the
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particles, other presumably accumulates through heterogeneous chemistry. For example, native
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sulfur could result from gypsum (CaSO4 2H2O) inclusions in minerals. The presence of sulfur
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due to heterogeneous chemistry of mineral dust with Asian pollution has been observed in
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Sullivan et al. (2007) and McNaughton et al. (2009). For all particle types, sulfur is seen in the
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EDS spectra in a larger number of particles in November – March compared with June –
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October, which is consistent with the presence of higher ambient SO2 concentrations from
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measurements obtained from the nearby air quality station (Fig. 3). The increase in sulfur
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observed in winter in Ulaanbaatar is consistent with increased domestic stove, HOBs, and coal-
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fired power plant use in winter, given the elemental profiles of these energy sources from Davy
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et al. (2011), and that these three sources are associated with 95% of sulfur dioxide emissions in
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Ulaanbaatar (Guttikunda et al., 2013). Note that the increasing use of energy efficient stoves
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changes the amount of ambient PM2.5, but because the same types of coal are being burned, use
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of these stoves is not likely to change the sulfur content of the particles.
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Ice Nucleation
In addition to classifying the ambient particles by size, shape, and composition, the
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critical ice saturation ratios (Sice) required for the onset of heterogeneous ice nucleation were
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determined (Figs. 7, S5).
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nucleation studies are useful because they provide additional insight into particle composition
While not a standard analysis technique for field samples, ice
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and hygroscopicity. For example, salts and minerals can nucleate ice at low saturation ratios,
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whereas organic compounds tend to nucleate ice at higher saturation ratios (Hoose and Möhler,
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2012; Baustian et al., 2010). Hygroscopic materials may uptake water prior to freezing at some
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temperatures. Sice is calculated as the ratio of the partial pressure of water vapor to the saturation
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vapor pressure over ice at a given temperature. For each month, Sice values are averaged over
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three temperatures, 230, 235, and 240 K. A minimum of nine experiments were performed for
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each month, collected during two sample periods over each month. Water uptake prior to ice
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nucleation was not observed within the limits of our instrument, suggesting that deliquescent
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salts and hygroscopic organics do not contribute a large mass fraction to heterogeneous ice
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nuclei. In other words, the irregular particles do not contain sufficient amounts of salts and
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organics to visibly uptake water. The resolution of the optical microscope did not allow for
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visual identification of the types of particles (spherical, fractal, irregular) that nucleated ice.
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Particle number, diameter, and surface area loadings from month-to-month are comparable by
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visual inspection via optical microscopy, however, suggesting that differences in particle type
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and chemistry are dictating differences in ice nucleation ability (Figs. S6, S7). The onset Sice
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values show clear seasonal variation; particles are generally more efficient ice nuclei (require
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lower supersaturations with respect to ice for the onset of nucleation) from June - September
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than December – March. For June – September, the range of onset Sice values observed is
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approximately 1.20-1.24.
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supermicron aluminosilicate clay mineral dust aerosol particles shows Sice values from
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approximately 1.04 – 1.27 in the temperature range from 230 K – 240 K (Hoose and Möhler,
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2012; Kanji and Abbatt, 2006; Salam et al., 2006; Dymarska et al., 2006; Eastwood et al., 2008;
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Laboratory studies of the onset of nucleation on initially dry
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Chernoff and Bertram, 2010; Wang and Knopf, 2011). Our result is therefore in the upper part
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of the range observed in laboratory studies for a common type of mineral dust aerosol. Ice nucleation activity is lowest when the fraction of particles containing sulfur is highest.
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Deactivation of kaolinite, montmorillonite, illite, quartz, and Arizona test dust due to exposure to
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sulfuric and nitric acid has been observed in heterogeneous ice nucleation experiments in which
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particles are exposed to subsaturated water vapor (Cziczo et al., 2009; Eastwood et al., 2009;
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Chernoff et al., 2010; Niedermeier et al., 2010; Sihvonen et al., 2014; Tobo et al., 2012; Wex et
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al., 2014). The degree of deactivation varies depending on experiment, from little change in Sice
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to an approximately 30% increase in the temperature range of interest (Cziczo et al., 2009;
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Eastwood et al., 2009; Chernoff et al., 2010; Sihvonen et al., 2014; Hoose and Mohler, 2012;
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Sullivan et al., 2010). The onset Sice values observed from December – March range from 1.28 –
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1.32, which is within the range of the laboratory studies for acid-treated dust (Cziczo et al., 2009;
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Eastwood et al., 2009; Chernoff et al., 2010; Sihvonen et al., 2014). Based on these experiments,
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we hypothesize that the observed deactivation in the winter months is due to changes in the
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degree of processing as indicated by the fraction of particles that contain sulfur. This result is
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consistent with field studies that have found reduced sulfate content in cirrus cloud residuals
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(Pratt et al., 2009; Cziczo et al., 2013; Cziczo et al., 2004). As a result of processing, mineral
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dust particles in the region may be more likely to take up water and participate in low to mid-
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altitude clouds. The number of particles that form cloud droplets in a cloud determines in part the
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optical properties, lifetime, and precipitation from the cloud. Particles that do not form cloud
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condensation nuclei could form ice nuclei. As a result, chemical processing of particles could
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affect the types and properties of clouds formed in this region, which could in turn affect
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regional climate in this semi arid region.
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Summary We have characterized twice monthly samples of aerosol particles from Ulaanbaatar, Mongolia over a 10-month period from June 2012 to March 2013. These samples primarily
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consist of spherical particles (organic and sulfate/organic), fractal soot particles, and irregularly
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shaped mineral dust particles. Almost all particles collected were less than 10 µm in diameter,
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and are therefore, inhalable.
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increases and the sulfur content of the particles increases, which is consistent with increased
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residential heating. The heterogeneous ice nucleation activity of the particles decreases, which
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we hypothesize is due to a deactivating reaction with sulfur-containing compounds (Sihvonen et
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al., 2014). This decrease could affect the microphysical properties of clouds that are formed in
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the winter months in this semi-arid region.
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In the winter months, the concentration of particulate matter
Our characterization of Ulaanbaatar PM during the MCC stove program details the
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properties of aerosol particles that are important for their health effects and their direct and
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indirect effects on climate, as well as provides an unprecedented view of particulate matter in
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Ulaanbaatar by season. These characteristics, along with the reported pollutant information from
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the National Air Quality Agency in Mongolia (made publicly available) will help inform future
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air quality studies. As more energy efficient stoves and new fuels are introduced to this region, it
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will be important to repeat these studies both to determine the amount of PM2.5, its composition,
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and the effect of composition on cloud formation properties.
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In summary, the field study we present is a comprehensive, year-long look at the
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physical, chemical and ice-nucleating properties of aerosol in a highly polluted city, one of very
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few such studies conducted in the region or in such a highly polluted environment. We hope the
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scientific results of this study can inform work done in other similarly situated and polluted cities
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in Asia and the Middle East, such as Kabul, Afghanistan, Kathmandu, Nepal and Almaty,
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Kazakhstan.
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3 Acknowledgements
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C. A. H. was supported by fellowships through the NSF International Research Fellowship
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Program (IRFP) and the U.S. Fulbright Program. S. L. acknowledges support from the USAID
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Partnerships for Enhanced Engagement in Research (PEER) Program. D. P. V. and M. A. F.
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were supported by the Pennsylvania State University. We thank the Materials Characterization
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Lab at Penn State for use of the Philips EM420T TEM. G. P. S. and M. A. T. acknowledge
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support through NSF grant AGS 1048536.
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Supporting Information Available: Additional details on methods and results.
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Cziczo, D. J.; Froyd, K. D.; Gallavardin, S. J.; Moehler, O.; Benz, S.; Saathoff, H.; Murphy, D. M. Deactivation of Ice Nuclei Due to Atmospherically Relevant Surface Coatings, Environ. Res. Lett. 2009, 4, 44013.
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Cziczo, D. J.; Froyd, K. D.; Hoose, C.; Jensen, E. J.; Diao, M.; Zondlo, M. A.; Smith, J. B.; Twohy, C. H.; Murphy, D. M. Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation, Science, 2013, 340 , 1320–1324.
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Davy, P. K.; Gunchin, G.; Markwitz, A.; Trompetter, W. J.; Barry, B. J.; Shagjjamba, D.; Lodoysamba, S. Air Particulate Matter Pollution in Ulaanbaatar, Mongolia: Determination of Composition, Source Contributions and Source Locations, Atmos. Pollut. Res. 2011, 2, 126–137.
29 30 31
Dockery, D. W.; Pope, C. A.; Xu, X.; Spengler, J. D.; Ware, J. H.; Fay, M. E.; Ferris, B. G.; Speizer, F. E. An Association Between Air Pollution and Mortality in Six U.S. Cities, N. Engl. J. Med. 1993, 329, 1753–1759.
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Eastwood, M. L.; Cremel, S.; Wheeler, M.; Murray, B. J.; Girard, E.; Bertram, A. K. Effects of Sulfuric Acid and Ammonium Sulfate Coatings on the Ice Nucleation Properties of Kaolinite Particles, Geophys. Res. Lett. 2009, 36, L02811.
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Guttikunda, S. K. Urban Air Pollution Analysis for Ulaanbaatar, Soc. Sci. Res. Network, 2008, doi: 10.2139/ssrn.1288328.
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Guttikunda, S. K.; Lodoysamba, S.; Bulgansaikhan, B.; Dashdondog, B. Particulate Pollution in Ulaanbaatar, Mongolia, Air Qual. Atmos. Heal. 2013, 6, 589–601.
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Hoose, C.; Möhler, O. Heterogeneous Ice Nucleation on Atmospheric Aerosols: A Review of Results from Laboratory Experiments, Atmos. Chem. Phys. 2012, 12, 9817-9854.
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McNaughton, C. S. et al. Observations of Heterogeneous Reactions between Asian Pollution and Mineral Dust over the Eastern North Pacific during INTEX-B, Atmos. Chem. Phys. 2009, 9, 8283-8308.
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Millenium Challenge Corporation, Mongolia - Energy and Environment Project, Stove Subsides Component, MCC Independent Impact Evaluation, ID Number: DDI-MCC-MNG-EEPIE-2014v01, 2014a, https://data.mcc.gov/evaluations/index.php/catalog/133/related_materials (accessed Jan 8, 2016).
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Millenium Challenge Corporation, MCC Summary: Measuring Results of the Mongolia Energy and Environment Project Stove Subsidies Component, 2014b, https://www.mcc.gov/resources/doc/summary-measuring-results-of-mng-eep (accessed Jan 8, 2016).
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Niedermeier, D.; Hartmann, S.; Shaw, R. A.; Covert, D.; Mentel, T. F.; Schneider, J.; Poulain, L.; Reitz, P.; Spindler, C.; Clauss, T.; Kiselev, A.; Hallbauer, E.; Wex, H.; Mildenberger, K.; Stratmann, F. Heterogeneous Freezing of Droplets with Immersed Mineral Dust Particles – Measurements and Parameterization, Atmos. Chem. Phys. 2010, 10, 3601–3614.
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Nishikawa, M.; Matsui, I.; Batdorj, D.; Jugder, D.; Mori, I.; Shimizu, A.; Sugimoto, N.; Takahashi, K. Chemical Composition of Urban Airborne Particulate Matter in Ulaanbaatar, Atmos. Environ. 2011, 45, 5710–5715.
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Pratt, K. A.; DeMott, P. J.; French, J. R.; Wang, Z.; Westphal, D. L.; Heymsfield, A. J.; Twohy, C. H.; Prenni, A. J.; Prather, K. A. In Situ Detection of Biological Particles in Cloud IceCrystals, Nat. Geosci. 2009, 2, 398–401.
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Schill, G. P.; Tolbert, M. A. Heterogeneous Ice Nucleation on Phase-Separated Organic-Sulfate Particles: Effect of Liquid vs. Glassy Coatings, Atmos. Chem. Phys. 2013, 13, 4681–4695.
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Sihvonen, S. K.; Schill, G. P.; Lyktey, N. A.; Veghte, D. P.; Tolbert, M. A.; Freedman, M. A. Chemical and Physical Transformations of Aluminosilicate Clay Minerals Due to Acid Treatment and Consequences for Heterogeneous Ice Nucleation, J. Phys. Chem. A 2014, 118, 8787–8796.
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Spengler, J.D. and Thurston, G.D. Mass and Elemental Composition of Fine and Coarse Particles in Six U.S. Cities, J. Air Pollut. Contr. Assoc., 1983, 33, 1162–1171.
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Sullivan, R. C.; Guazzotti, S. A.; Sodeman, D. A.; Prather, K. A. Direct observations of the atmospheric processing of Asian mineral dust, Atmos. Chem. Phys. 2007, 7, 1213-1236.
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Sullivan, R. C.; Petters, M. D.; DeMott, P. J.; Kreidenweis, S. M.; Wex, H.; Niedermeier, D.; Hartmann, S.; Clauss, T.; Stratmann, F.; Reitz, P.; J. Schneider; Sierau, B. Irreversible Loss of Ice Nucleation Active Sites in Mineral Dust Particles Caused by Sulphuric Acid Condensation, Atmos. Chem. Phys. 2010, 10, 11471-11487.
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Tobo, Y.; DeMott, P. J.; Raddatz, M.; Niedermeier, D.; Hartmann, S.; Kreidenweis, S. M.; Stratmann, F.; Wex, H. Impacts of Chemical Reactivity on Ice Nucleation of Kaolinite Particles: A Case Study of Levoglucosan and Sulfuric Acid, Geophys. Res. Lett. 2012, 39, L19083.
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United States Department of State, MCC Fact Sheet on Mongolia Project to Address Air Pollution, 2013, http://iipdigital.usembassy.gov/st/english/texttrans/2013/04/20130424146351.html#ixzz3FaQ7rC Ab (accessed Oct 8, 2014).
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Veghte, D. P.; Freedman, M. A. The Necessity of Microscopy to Characterize the Optical Properties of Size-Selected, Nonspherical Aerosol Particles, Anal. Chem. 2012, 84, 9101–9108.
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Wang, Y.; Zhang, R.; Saravanan, R. Asian Pollution Climatically Modulates Mid-Latitude Cyclones Following Hierarchical Modelling and Observational Analysis, Nat. Comm. 2014a, 5, 3098.
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Wang, Y.; Wang, M.; Zhang, R.; Ghan, S. J.; Lin, Y.; Hu, J.; Pan, B.; Levy, M.; Jiang, J. H.; Molina, M. J. Assessing the Effects of Anthropogenic Aerosols on Pacific Storm Track Using a Multiscale Global Climate Model, Proceed. Natl. Acad. Sci. USA, 2014b, 111, 6894-6899.
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Wex, H.; DeMott, P. J.; Tobo, Y.; Hartmann, S.; Rösch, M.; Clauss, T.; Tomsche, L.; Niedermeier, D.; Stratmann, F. Kaolinite Particles as Ice Nuclei: Learning from the Use of Different Kaolinite Samples and Different Coatings, Atmos. Chem. Phys. 2014, 14, 5529–5546.
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World Bank, Air Quality Analysis of Ulaanbaatar: Improving Air Quality to Reduce Health Impacts; Washington DC, 2011.
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World Health Organization, Seven Million Premature Deaths Annually Linked to Air Pollution, News Release, 2014.
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World Health Organization, Urban Outdoor Air Pollution Database; Geneva, Switzerland, 2011.
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Zagdjav, D. Air Pollution in Ulaanbaatar City, Second Meeting of the Kitakyushu Initiative Network, Weihai, China, 2003.
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Zhang, R.; Li, G.; Fan, J.; Wu, D. L.; Molina, M. J. Intensification of Pacific Storm Track Linked to Asian Pollution, Proceed. Natl. Acad. Sci. USA, 2007, 104, 5295-5299.
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Tables
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Table 1: Previous ambient PM studies on Ulaanbaatar, Mongolia and this study
PM2.5, analyzed for: • water-soluble dicarboxylic acids and related compounds • OC, EC, WSOC, and inorganic ions
1.25 months, 29 Nov 2007 to 6 Jan 2008
1 location
No
3.5 years, October 2004 to April 2008
1 location
No
1 location
No
1 location
No
1 location
No
38 locations throughout city mobile detection along 2 routes 1 location
No
1 location
No
PM10-2.5, PM2.5, analyzed for elemental composition
PM10, analyzed for: • ionic components (F, Cl, SO4, NO3, NH4) • elements (Al, As, Ca, Cu, Fe, K, Mg, Mn, Na, Pb, Zn) • OC, EC
Two season chemical composition of bulk aerosol
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Multi-year, Seasonal elemental composition, source contribution and location via analysis of bulk aerosol
Temporal Coverage
Epidemiological
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Batmunkh, et al. (2013)
Chemical characterization of a pollution event
Guttikunda et al. (2013)
Development of emissions inventory,
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Wintertime chemical composition of bulk aerosol
PM2.5, SO2, PM10 mass concentrations (Obtained from monitoring program by UB Env. Monitoring Agency) PM10, PM2.5 mass concentrations NO2 and SO2 mass concentrations PM2.5 mobile monitoring
PM2.5 mass concentration, analyzed for: • OC and EC • Water soluble ionic components PM2.5 concentrations
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Geographic coverage
Publicly available in tabular form?
Pollutants measured
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Jung et al. (2010)
Purpose of Study
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6 days in Winter 2008 (12 Jan- 17 Jan) 6 days in Spring 2008(31 May 6 Jun) 2008 1 year, June 1, 2009 to May 31, 2010 5 months, Oct 2008 – Mar 2010 2 weeks, Feb 24 to Mar 11, 2010 Feb 24, 25, and 26, 2010 Jan 9 – Feb 17, 2008
4 years, Dec 2007 – Dec 2011
No
No
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1 location
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Jun 2012- Mar 2013, 10 months
1 location
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Jun 2012- Feb 2013, 9 months
Allen et al., Air Qual. Atmos. Health, 2013, 6, 137-150. Batmunkh et al. J. Air Waste Management Assoc. 2013, 63, 659-670. Davy et al. Atmos. Pollut. Res. 2011, 2, 126–137. Guttikunda et al., Air Qual. Atmos. Health, 2013, 6, 589-601. Jung et al. J. Geophys. Res. 2010, 115, D22203. Nishikawa et al. Atmos. Environ. 2011, 45, 5710–5715.
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PM samples collected and analyzed particle-by particle-for: • size distribution • shape (irregular fractal spherical) • Chemical comp. via EDS and general type (mineral dust, soot, and sulfate-organic) • Sice at ice nucleation PM10, NOx, O3, SO2, concentrations Daily meteorological data (temp, RH, wind direction, windspeed) (Obtained from monitoring program by UB Env. Monitoring Agency)
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dispersion modeling Monthly chemical, size, morphological, and ice nucleating-ability analysis of aerosol on a particle-byparticle basis
Yes (hourly data available )
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Figures
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Fig. 1. The relationship of annual average PM10 concentration in selected cities reported in 2011 relative to the number of peer-reviewed scientific papers that have been published about air pollution in that city prior to and including 2011. Shown are some of the most well-studied cities (in blue), as well as the ten most PM10-polluted cities from the 2011 WHO report of over 1000 places worldwide (in red).3 The vertical, dashed gray line shows the sum of all of papers published on air pollution in those ten most PM-polluted cities. The WHO guideline for PM10 annual average concentration (20 µg m-3) is the horizontal, dashed gray line.
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Fig. 2. Map of Ulaanbaatar Mongolia with relevant sites for this study marked. Ulaanbaatar is located at 1310 m in a valley between two mountainous areas. The main sources of particulate matter pollution are from the ger district, which blankets the Northern half of the city, as well as three coal-fired power plants in the outer areas of the western portion of the city. Note that Power Plant #1 is no longer functioning. The measurement site from this study is shown as the yellow marker and the nearby monitoring site is indicated as a teal marker. To note: Ulaanbaatar experiences low winter wind speeds due to the Siberian High that sits over the region throughout the winter and highest windspeeds in the spring time.
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Fig. 3. Monthly average PM10, O3, SO2, and NOx levels (top panel) and temperature, windspeed, RH, and wind direction (bottom panel) at an air quality monitoring site run by the National Agency of Meteorology and Environmental Monitoring in Mongolia, located approximately 1 km away from our station. The site is located just over the southern border of the ger district in a residential and urban area (see Fig. 2).
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Fig. 4. Representative particles collected in Ulaanbaatar, Mongolia. Particles are classified first according to their shape (spherical, fractal, irregular) and then according to their elemental composition. For the shape classification: fractal particles were considered as any particle composed of two or more discrete, smaller particles, while irregular particles were considered as any single particle that was non-spherical in shape.
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Fig. 5. Size distributions for spherical, fractal, and irregularly shaped particles averaged over all the particles collected in this study as analyzed by TEM. The area of the particles in the TEM images was measured and converted to an area equivalent diameter by assuming that the particles are spheres and therefore are circular in the TEM images. The bins are the same size for spherical and fractal particles and larger for irregular particles. The normalized number of fractal particles in the smallest bin is 0.24. Irregular particles were not observed below 200 nm. The number of each particle type on the TEM grids is given in Fig. 6.
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Fig. 6. The number of particles collected per TEM image per hour collection time for a) all particles, b) spherical particles, c) fractal particles, and d) irregularly shaped particles. The months November – March are shaded blue. Note that all of the panels have different y-axis scales. For June - Sept (labeled “summer”) and Nov – March (labeled “winter”), the percentage of the particles by number that contain sulfur is indicated in red, and the percentage by number without sulfur is indicated in black.
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Fig. 7. The supersaturation with respect to ice, Sice, at the onset of heterogeneous ice nucleation for samples collected in this study. Error bars are the standard deviation of repeated experiments. The months November – March are shaded in blue.
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Highlights Ulaanbaatar, Mongolia has the second highest PM10 concentrations globally Almost all particles are inhalable, composed of minerals, soot, and sulfate-organic Particle concentration and sulfur content of particles increases in winter Particles collected in winter have lower ice nucleation activity
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• • • •
S1352-2310(16)30384-3
DOI:
10.1016/j.atmosenv.2016.05.037
Reference:
AEA 14625
To appear in:
Atmospheric Environment
Received Date: 28 January 2016 Revised Date:
18 May 2016
Accepted Date: 21 May 2016
Please cite this article as: Hasenkopf, C.A., Veghte, D.P., Schill, G.P., Lodoysamba, S., Freedman, M.A., Tolbert, M.A., Ice nucleation, shape, and composition of aerosol particles in one of the most polluted cities in the world: Ulaanbaatar, Mongolia, Atmospheric Environment (2016), doi: 10.1016/ j.atmosenv.2016.05.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Ice Nucleation, Shape, and Composition of Aerosol Particles in One of the Most Polluted Cities in the World: Ulaanbaatar, Mongolia
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Christa A. Hasenkopf a,*, Daniel P. Veghteb, Gregory P. Schillc, Sereeter Lodoysambad, Miriam Arak Freedmanb,*, Margaret A. Tolbert c,*
Addresses: a) OpenAQ, Washington, DC 20009
b) Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
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c) CIRES & Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO 80309
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d) German-Mongolian Institute for Resources and Technology, Nalaikh, 2 Khoroo, Ulaanbaatar, Mongolia and National University of Mongolia, Ikh Surguuliin Gudamj–1, Ulaanbaatar, Mongolia
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* To whom all correspondence should be addressed: Christa Hasenkopf OpenAQ Washington, D.C. 20009 Phone: (507) 246-2097 Fax: Not Available [email protected]
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Miriam Freedman Department of Chemistry The Pennsylvania State University University Park, PA 16802 Phone: (814) 867-4267 Fax: (814) 865-3314 [email protected]
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Margaret Tolbert CIRES & Department of Chemistry and Biochemistry University of Colorado at Boulder Boulder, CO 80309 Phone: (303) 492-3179 Fax: (303) 492-1149 [email protected]
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Abstract Air pollution is attributable to 7 million deaths per year, or one out of every eight deaths globally. In particular, high concentrations of particulate matter (PM), a major air pollutant, have significant impacts on health and regional climate in urban centers. Many of the most polluted places, largely in developing countries, go severely understudied. Additionally, high particulate matter levels can have an impact on the microphysical properties of clouds, impacting precipitation and regional climate. Semi-arid regions can be especially affected by small changes in precipitation. Here we characterize the physical and chemical properties of PM in one of the most PM-polluted cities in the world: Ulaanbaatar, Mongolia, a semi-arid region in central Asia. Twice monthly aerosol samples were collected over 10 months from a central location and analyzed for composition and ice nucleation activity. Almost all particles collected were inhalable, consisting primarily of mineral dust, soot, and sulfate-organic. In winter, all classes of PM increase in concentration, with increased sulfur concentrations, and the particles are less active towards heterogeneous ice nucleation. In addition, concurrent monthly average PM10, SO2, NOx, and O3 levels and meteorological data at a nearby location are reported and made publicly available. These measurements provide an unprecedented seasonal characterization of the size, shape, chemical structure, and ice nucleating activity of PM data from Ulaanbaatar. This 10month field study, exploring a variety of aerosol properties in Ulaanbaatar, Mongolia, is one of very few such studies conducted in the region or in such a highly polluted environment. The results of this study may inform work done in other similarly situated and polluted cities in Asia and elsewhere.
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Keywords: Air pollution; climate; ice nucleation; electron microscopy; particulate matter; sulfur
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Introduction Driven by urbanization, overall increasing populations, and improved economic
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development, developing countries must often meet higher energy demands with fuels that
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produce airborne-pollutants as byproducts. In addition to affecting climate, these pollutants
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compromise the health of billions, especially those in developing countries, which account for
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more than 80% of the world’s population (Central Intelligence Agency, 2013). The World Health
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Organization (WHO) deems air pollution the most lethal environmental hazard on the planet,
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responsible for one of every eight deaths (WHO, 2014).
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Despite the threats air pollution poses, the most polluted places are vastly understudied.
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Fig. 1 compares the annual average concentration of PM10 (particulate matter, PM, 10µm or less
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in diameter) in cities around the world with the number of scientific papers that have been
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published about air pollution in a given city through 2011 (WHO, 2011). As well as showing
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well-researched cities, Fig. 1 shows the ten most PM10-polluted cities from the 2011 WHO
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Outdoor PM10 Survey of over 1000 cities (WHO, 2011).
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According to Fig. 1, Ulaanbaatar, Mongolia is the city with the second highest annual
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average PM10 levels. Additionally, a 2011 World Bank study reports monthly Ulaanbaatar
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average PM2.5 (particles with diameters less than 2.5µm) concentrations in December 2008 and
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January 2009 exceeding 1000 µg m-3 – more than 40 times the PM2.5 daily average WHO
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guidelines (World Bank, 2011). A study by Allen et al. conservatively estimates that up to 25%
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of deaths annually in Ulaanbaatar are caused by PM (Allen et al., 2013). In addition, Asian air
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pollution can intensify storms over the Pacific (Zhang et al., 2007; Wang et al., 2014a & 2014b).
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The majority of Ulaanbaatar’s 1.3 million people live in the ger district, comprised of traditional
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Mongolian homes (gers) and wooden buildings, typically heated by coal-burning stoves that emit
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incompletely combusted fuel as soot from chimneys 3-4m high (Zagdjav, 2003). Coal-fired heat-
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only boilers (HOBs) are also prevalent in the ger district for heating in larger buildings. With
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60% of the city’s residents living in the ger district, it is unsurprising that the PM emissions from
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stoves and HOBs have been found to be main sources of PM-pollution (Guttikunda, 2008; Davy
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et al., 2011). Ulaanbaatar is situated at an altitude of 1,350m in a valley. Like many higher
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altitude valleys, the city experiences temperature inversions and horizontal airflow is blocked to
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the north and south due to surrounding mountains. Additionally, Ulaanbaatar’s sub-arctic,
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semiarid climate influenced by the Siberian High exacerbates the effects of pollution from the
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ger district. Its semi-arid climate also leaves Ulaanbaatar susceptible to potential changes in the
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regional climate, such as precipitation, due to the presence of high PM concentrations.
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To date, there have been few studies spanning all four seasons on ambient PM levels in
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Ulaanbaatar (See Table 1 for full comparison). Except for Davy et al. (2011), previous studies
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have explored the concentration and chemical composition of PM over limited time periods or
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have relied city air quality monitoring for longer-term measurements.
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sources, chemistry, and consequences of PM for air quality and regional climate, however, long-
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term data sets are needed. Recently, donor organizations such as the U.S.’s Millennium
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Challenge Corporation (MCC) and World Bank have, along with the Mongolian Government,
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taken several PM-pollution mitigation measures. These measures included a 41 million USD
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effort by MCC to sell over 100,000 efficient stoves and Mongolian government-subsidized
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alternative fuels (United States Department of State, 2013). The stove subsidy started in 2011,
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and use of these stoves was evaluated during three survey periods spanning October 2012
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through March 2013 (MCC, 2014a & 2014b). The MCC stove program found that while ambient
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PM2.5 was estimated to have been reduced by 30%, no statistically significant reduction in fuel
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consumption resulted, perhaps due to households heating to warmer indoor temperatures than
To understand the
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with less efficient stoves (MCC Summary: Measuring Results of the Mongolia Energy and
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Environment Project Stove Subsides Component, 2014). Further, no data for the chemical
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composition of the PM2.5 were reported. There are no other published peer-reviewed studies
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that characterize and report PM during the period when the MCC stove program was initiated.
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Such information is critical to studies analyzing the impact of such programs on ambient PM.
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We have collected twice-monthly samples from a residential location in Ulaanbaatar, as
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described below, over ten months (June 2012 to March 2013; Fig. 2). Samples were analyzed
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for type and elemental composition by transmission electron microscopy (TEM) and energy
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dispersive X-ray spectroscopy (EDS) as well as for ice nucleation onset. We also report nine
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months of concurrent monthly average PM10, SO2, NOx, and O3 levels as well as meteorological
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data, as measured at a nearby location.
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Materials and Methods
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Description of Sampling Site
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approximately 10 km from the city’s coal-fired power plants and 600 meters away from the
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southern edge of the ger district. The sampling site was on a balcony, approximately 15 m above
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the ground, facing south. A downward facing funnel, filled with a coarse wire mesh was used to
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eliminate water droplets and very coarse material entering the inlet. No PM filter was used to
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prevent the collection of larger particles. Please see the Supplementary Information for more
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details on the sampling collection methods.
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The sampling site (Fig. 2) was located east and slightly north of the city center,
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TEM/EDS Studies
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Transmission Electron Microscopy (TEM) samples were prepared by impacting particles
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on TEM grids (200 mesh copper coated with continuous carbon, Electron Microscopy Science,
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Hatfield, PA). Particles were imaged using a Philips EM420T operated at an accelerating voltage
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of 120kV. Example images for two different months taken at the magnification used for the
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analysis are shown in Fig. S1. Particles were classified based on shape (spherical, fractal,
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irregular). The area of each particle was determined using ImageJ software (National Institutes
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of Health, Bethesda, MD). A size distribution for each shape was calculated by converting the
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area calculated for each particle to an area-equivalent diameter, assuming the particles are
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spheres and consequently, the projected area of the particles on the TEM images are circles. To
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obtain the elemental composition of the particles, the EDS detector on the Philips EM420T was
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used.
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Ice Nucleation Studies
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The experimental setup used for ice nucleation experiments has been described previously (Baustian et al., 2010; Schill et al., 2013). Briefly, a Nicolet Almega XR Raman
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microscope, equipped with an Olympus BX-51 microscope, has been coupled to a Linkam
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THMS600 environmental cell. The temperature inside the cell is controlled by a Linkam TMS94
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temperature controller (accuracy of ±0.1 K). Water vapor inside the environmental cell is
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controlled by mixing dry and humidified flows of N2; the partial pressure of water is measured
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by a Buck Research CR-1A chilled mirror hygrometer in line with the cell (accuracy of ±0.15
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K). As in Schill et al. (2013), the relative humidity over the sample was controlled by keeping
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the water partial pressure constant, and slowly lowering the temperature at a rate of 0.1 K min-1.
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Ice particles were detected visually, confirmed spectrally via the Raman signature for ice, and
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video of the ice sublimation process revealed the IN.
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Air quality and Meteorological Data Air quality and meteorological data collected concurrently during the study period (with
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the exception of March 2013) at a monitoring site run from the National Air Quality Agency in
5
Mongolia are also reported. Specifically, the site, which was located approximately 1km away
6
from our sampling site (see Fig. 2), collected hourly PM10, SO2, NOx, and ozone (O3)
7
concentrations. Hourly meteorological data (temperature, RH, wind speed, and wind direction)
8
were also obtained. The site is at the southern edge of the ger district, in a residential and urban
9
area. These data, shown as monthly averages to correspond with our measurements below, are
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displayed in Figure 3 and also publicly available in tabular form from ScholarSphere
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(http://scholarsphere.psu.edu).
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Results and Discussion
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Air Quality and Meteorological Data
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Numerous seasonal differences are seen in the air quality and meteorological data shown in Fig.
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3. The winter months (Nov 2012 – February 2013) have colder temperatures, but similar relative
17
humidities compared with the summer months. The winter has higher PM10 and SO2, which is
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consistent with increased fuel consumption for residential and commercial heating. Note that all
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the collected data are taken as the number of energy efficient stoves in use rises from the MCC
20
Stove Project, indicating that wintertime particulate levels may have been higher in previous
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years. In addition, the winter months have higher NOx concentrations, and such concentrations
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are consistent with a lower boundary layer. Ozone concentrations are lower during the winter,
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which is consistent with reduced photochemical production.
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Characterization of Particle Types Fig. 4 shows representative TEM images of the types of particles we observed. The type
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of particle is determined first by its characteristic shape (spherical, fractal, irregular) and then by
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its elemental composition. Spherical particles are approximately circular in the TEM images,
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fractal particles are composed of two or more discrete, smaller particles, and irregular particles
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are single particles that are non-spherical in shape. Based on composition, spherical particles
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were assigned to be organic and sulfate-organic particles with a small amount of fly ash and
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possibly also tar balls. Fractal particles are composed of soot and fly ash, where the soot can be
10
partially collapsed due to additional material. Irregularly shaped particles are assumed to be
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mineral dust because they primarily contain Al, Si, and Ca, with a small fraction by number of
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anthropogenic particles and fully collapsed soot (Figs. 4, S2).
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The effect of particulates on human health depends in part on their size, where particles
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less than 10 µm are inhalable, and particles less than 2.5 µm have the strongest correlation with
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mortality rates (Spengler and Thurston, 1983; Chow and Spengler, 1986a and 1986b; Dockery et
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al., 1993). To determine the fraction of respirable particles by number, size distributions of the
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spherical, fractal, and irregular particles were determined from the two-dimensional projections
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of the particles observed in the TEM images and for the particles studied with optical
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microscopy for the ice nucleation experiments. As shown in Fig. 5, all particles observed with
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TEM are in the inhalable range, with almost all particles less than 2.5 µm. We also obtained data
21
on particle size from the ice nucleation experiments, which can be compared to the size range
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observed with TEM. For the samples collected for the ice nucleation experiments, the average
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particle size detected with optical microscopy was 1.9 ± 1.5 µm. Less than 1% of the particles
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observed each month were greater than 10 µm in diameter. The size distribution of the particles
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collected for the ice nucleation experiments was analyzed using optical microscopy, which is
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sensitive to larger particle sizes (> 500nm).
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microscopy were most likely irregularly shaped particles. The particles fluoresced in the Raman
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microscope, prohibiting chemical characterization. Note that mineral dust and soot commonly
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fluoresce, suggesting that the particles observed with optical microscopy have the same
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composition as the irregularly shaped particles that we characterize with TEM and EDS. From
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the TEM and optical microscopy analysis, it can be seen that almost all particles collected fall
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within the respirable aerosol fraction, approximately 10 µm in aerodynamic diameter or smaller.
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Note that our particle numbers observed with TEM cannot be directly converted to PM10 or
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PM2.5. As shown below, however, increases in particle numbers with season observed with
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TEM agree with increases observed in PM10, as shown in Fig. 3.
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As a result, particles observed with optical
Seasonal Particle Concentration and Sulfur Content of Particles To probe the seasonal dependence of Ulaanbaatar particulate, we estimate the particle
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number concentration in each month. The relative particle concentrations are estimated in a
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given month by plotting the number of particles per TEM image per hour of collection time,
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averaging over the two monthly samples (Fig. 6). Note that all the TEM images were taken at
19
the same magnification. Relating this calculation to the absolute particle concentrations assumes
20
that the collection efficiency is the same in every collection and that the levels of particulate
21
matter on the collection days are representative of that month. Even with the variability shown in
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Fig. 6, however, the particulate concentrations are consistently higher during the colder months
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(Nov-March) than in the warmer months (June-Oct). Increasing concentrations of particulates
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during the winter months is consistent with previous literature, as well as the data in Fig. 3 (Air
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Quality Analysis of Ulaanbaatar, 2011; Allen et al., 2013; Nishikawa et al., 2011). We have investigated how the amount of sulfur in the particles varies during the year for
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spherical, fractal, and irregular particles (Figs. 6, S3, S4). Some of the sulfur is native to the
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particles, other presumably accumulates through heterogeneous chemistry. For example, native
6
sulfur could result from gypsum (CaSO4 2H2O) inclusions in minerals. The presence of sulfur
7
due to heterogeneous chemistry of mineral dust with Asian pollution has been observed in
8
Sullivan et al. (2007) and McNaughton et al. (2009). For all particle types, sulfur is seen in the
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EDS spectra in a larger number of particles in November – March compared with June –
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October, which is consistent with the presence of higher ambient SO2 concentrations from
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measurements obtained from the nearby air quality station (Fig. 3). The increase in sulfur
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observed in winter in Ulaanbaatar is consistent with increased domestic stove, HOBs, and coal-
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fired power plant use in winter, given the elemental profiles of these energy sources from Davy
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et al. (2011), and that these three sources are associated with 95% of sulfur dioxide emissions in
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Ulaanbaatar (Guttikunda et al., 2013). Note that the increasing use of energy efficient stoves
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changes the amount of ambient PM2.5, but because the same types of coal are being burned, use
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of these stoves is not likely to change the sulfur content of the particles.
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Ice Nucleation
In addition to classifying the ambient particles by size, shape, and composition, the
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critical ice saturation ratios (Sice) required for the onset of heterogeneous ice nucleation were
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determined (Figs. 7, S5).
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nucleation studies are useful because they provide additional insight into particle composition
While not a standard analysis technique for field samples, ice
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and hygroscopicity. For example, salts and minerals can nucleate ice at low saturation ratios,
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whereas organic compounds tend to nucleate ice at higher saturation ratios (Hoose and Möhler,
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2012; Baustian et al., 2010). Hygroscopic materials may uptake water prior to freezing at some
4
temperatures. Sice is calculated as the ratio of the partial pressure of water vapor to the saturation
5
vapor pressure over ice at a given temperature. For each month, Sice values are averaged over
6
three temperatures, 230, 235, and 240 K. A minimum of nine experiments were performed for
7
each month, collected during two sample periods over each month. Water uptake prior to ice
8
nucleation was not observed within the limits of our instrument, suggesting that deliquescent
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salts and hygroscopic organics do not contribute a large mass fraction to heterogeneous ice
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nuclei. In other words, the irregular particles do not contain sufficient amounts of salts and
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organics to visibly uptake water. The resolution of the optical microscope did not allow for
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visual identification of the types of particles (spherical, fractal, irregular) that nucleated ice.
13
Particle number, diameter, and surface area loadings from month-to-month are comparable by
14
visual inspection via optical microscopy, however, suggesting that differences in particle type
15
and chemistry are dictating differences in ice nucleation ability (Figs. S6, S7). The onset Sice
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values show clear seasonal variation; particles are generally more efficient ice nuclei (require
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lower supersaturations with respect to ice for the onset of nucleation) from June - September
18
than December – March. For June – September, the range of onset Sice values observed is
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approximately 1.20-1.24.
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supermicron aluminosilicate clay mineral dust aerosol particles shows Sice values from
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approximately 1.04 – 1.27 in the temperature range from 230 K – 240 K (Hoose and Möhler,
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2012; Kanji and Abbatt, 2006; Salam et al., 2006; Dymarska et al., 2006; Eastwood et al., 2008;
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Laboratory studies of the onset of nucleation on initially dry
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Chernoff and Bertram, 2010; Wang and Knopf, 2011). Our result is therefore in the upper part
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of the range observed in laboratory studies for a common type of mineral dust aerosol. Ice nucleation activity is lowest when the fraction of particles containing sulfur is highest.
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Deactivation of kaolinite, montmorillonite, illite, quartz, and Arizona test dust due to exposure to
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sulfuric and nitric acid has been observed in heterogeneous ice nucleation experiments in which
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particles are exposed to subsaturated water vapor (Cziczo et al., 2009; Eastwood et al., 2009;
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Chernoff et al., 2010; Niedermeier et al., 2010; Sihvonen et al., 2014; Tobo et al., 2012; Wex et
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al., 2014). The degree of deactivation varies depending on experiment, from little change in Sice
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to an approximately 30% increase in the temperature range of interest (Cziczo et al., 2009;
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Eastwood et al., 2009; Chernoff et al., 2010; Sihvonen et al., 2014; Hoose and Mohler, 2012;
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Sullivan et al., 2010). The onset Sice values observed from December – March range from 1.28 –
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1.32, which is within the range of the laboratory studies for acid-treated dust (Cziczo et al., 2009;
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Eastwood et al., 2009; Chernoff et al., 2010; Sihvonen et al., 2014). Based on these experiments,
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we hypothesize that the observed deactivation in the winter months is due to changes in the
15
degree of processing as indicated by the fraction of particles that contain sulfur. This result is
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consistent with field studies that have found reduced sulfate content in cirrus cloud residuals
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(Pratt et al., 2009; Cziczo et al., 2013; Cziczo et al., 2004). As a result of processing, mineral
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dust particles in the region may be more likely to take up water and participate in low to mid-
19
altitude clouds. The number of particles that form cloud droplets in a cloud determines in part the
20
optical properties, lifetime, and precipitation from the cloud. Particles that do not form cloud
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condensation nuclei could form ice nuclei. As a result, chemical processing of particles could
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affect the types and properties of clouds formed in this region, which could in turn affect
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regional climate in this semi arid region.
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1 2 3 4
Summary We have characterized twice monthly samples of aerosol particles from Ulaanbaatar, Mongolia over a 10-month period from June 2012 to March 2013. These samples primarily
6
consist of spherical particles (organic and sulfate/organic), fractal soot particles, and irregularly
7
shaped mineral dust particles. Almost all particles collected were less than 10 µm in diameter,
8
and are therefore, inhalable.
9
increases and the sulfur content of the particles increases, which is consistent with increased
10
residential heating. The heterogeneous ice nucleation activity of the particles decreases, which
11
we hypothesize is due to a deactivating reaction with sulfur-containing compounds (Sihvonen et
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al., 2014). This decrease could affect the microphysical properties of clouds that are formed in
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the winter months in this semi-arid region.
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In the winter months, the concentration of particulate matter
Our characterization of Ulaanbaatar PM during the MCC stove program details the
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properties of aerosol particles that are important for their health effects and their direct and
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indirect effects on climate, as well as provides an unprecedented view of particulate matter in
17
Ulaanbaatar by season. These characteristics, along with the reported pollutant information from
18
the National Air Quality Agency in Mongolia (made publicly available) will help inform future
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air quality studies. As more energy efficient stoves and new fuels are introduced to this region, it
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will be important to repeat these studies both to determine the amount of PM2.5, its composition,
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and the effect of composition on cloud formation properties.
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In summary, the field study we present is a comprehensive, year-long look at the
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physical, chemical and ice-nucleating properties of aerosol in a highly polluted city, one of very
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few such studies conducted in the region or in such a highly polluted environment. We hope the
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scientific results of this study can inform work done in other similarly situated and polluted cities
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in Asia and the Middle East, such as Kabul, Afghanistan, Kathmandu, Nepal and Almaty,
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Kazakhstan.
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3 Acknowledgements
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C. A. H. was supported by fellowships through the NSF International Research Fellowship
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Program (IRFP) and the U.S. Fulbright Program. S. L. acknowledges support from the USAID
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Partnerships for Enhanced Engagement in Research (PEER) Program. D. P. V. and M. A. F.
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were supported by the Pennsylvania State University. We thank the Materials Characterization
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Lab at Penn State for use of the Philips EM420T TEM. G. P. S. and M. A. T. acknowledge
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support through NSF grant AGS 1048536.
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Supporting Information Available: Additional details on methods and results.
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Cziczo, D. J.; Froyd, K. D.; Hoose, C.; Jensen, E. J.; Diao, M.; Zondlo, M. A.; Smith, J. B.; Twohy, C. H.; Murphy, D. M. Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation, Science, 2013, 340 , 1320–1324.
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Sullivan, R. C.; Petters, M. D.; DeMott, P. J.; Kreidenweis, S. M.; Wex, H.; Niedermeier, D.; Hartmann, S.; Clauss, T.; Stratmann, F.; Reitz, P.; J. Schneider; Sierau, B. Irreversible Loss of Ice Nucleation Active Sites in Mineral Dust Particles Caused by Sulphuric Acid Condensation, Atmos. Chem. Phys. 2010, 10, 11471-11487.
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Wang, Y.; Wang, M.; Zhang, R.; Ghan, S. J.; Lin, Y.; Hu, J.; Pan, B.; Levy, M.; Jiang, J. H.; Molina, M. J. Assessing the Effects of Anthropogenic Aerosols on Pacific Storm Track Using a Multiscale Global Climate Model, Proceed. Natl. Acad. Sci. USA, 2014b, 111, 6894-6899.
22 23 24
Wex, H.; DeMott, P. J.; Tobo, Y.; Hartmann, S.; Rösch, M.; Clauss, T.; Tomsche, L.; Niedermeier, D.; Stratmann, F. Kaolinite Particles as Ice Nuclei: Learning from the Use of Different Kaolinite Samples and Different Coatings, Atmos. Chem. Phys. 2014, 14, 5529–5546.
25 26
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27 28
World Health Organization, Seven Million Premature Deaths Annually Linked to Air Pollution, News Release, 2014.
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World Health Organization, Urban Outdoor Air Pollution Database; Geneva, Switzerland, 2011.
30 31
Zagdjav, D. Air Pollution in Ulaanbaatar City, Second Meeting of the Kitakyushu Initiative Network, Weihai, China, 2003.
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Zhang, R.; Li, G.; Fan, J.; Wu, D. L.; Molina, M. J. Intensification of Pacific Storm Track Linked to Asian Pollution, Proceed. Natl. Acad. Sci. USA, 2007, 104, 5295-5299.
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Tables
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Table 1: Previous ambient PM studies on Ulaanbaatar, Mongolia and this study
PM2.5, analyzed for: • water-soluble dicarboxylic acids and related compounds • OC, EC, WSOC, and inorganic ions
1.25 months, 29 Nov 2007 to 6 Jan 2008
1 location
No
3.5 years, October 2004 to April 2008
1 location
No
1 location
No
1 location
No
1 location
No
38 locations throughout city mobile detection along 2 routes 1 location
No
1 location
No
PM10-2.5, PM2.5, analyzed for elemental composition
PM10, analyzed for: • ionic components (F, Cl, SO4, NO3, NH4) • elements (Al, As, Ca, Cu, Fe, K, Mg, Mn, Na, Pb, Zn) • OC, EC
Two season chemical composition of bulk aerosol
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Multi-year, Seasonal elemental composition, source contribution and location via analysis of bulk aerosol
Temporal Coverage
Epidemiological
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Batmunkh, et al. (2013)
Chemical characterization of a pollution event
Guttikunda et al. (2013)
Development of emissions inventory,
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Davy et al. (2011)
Wintertime chemical composition of bulk aerosol
PM2.5, SO2, PM10 mass concentrations (Obtained from monitoring program by UB Env. Monitoring Agency) PM10, PM2.5 mass concentrations NO2 and SO2 mass concentrations PM2.5 mobile monitoring
PM2.5 mass concentration, analyzed for: • OC and EC • Water soluble ionic components PM2.5 concentrations
18
Geographic coverage
Publicly available in tabular form?
Pollutants measured
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Jung et al. (2010)
Purpose of Study
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6 days in Winter 2008 (12 Jan- 17 Jan) 6 days in Spring 2008(31 May 6 Jun) 2008 1 year, June 1, 2009 to May 31, 2010 5 months, Oct 2008 – Mar 2010 2 weeks, Feb 24 to Mar 11, 2010 Feb 24, 25, and 26, 2010 Jan 9 – Feb 17, 2008
4 years, Dec 2007 – Dec 2011
No
No
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1 location
No
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Jun 2012- Mar 2013, 10 months
1 location
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Jun 2012- Feb 2013, 9 months
Allen et al., Air Qual. Atmos. Health, 2013, 6, 137-150. Batmunkh et al. J. Air Waste Management Assoc. 2013, 63, 659-670. Davy et al. Atmos. Pollut. Res. 2011, 2, 126–137. Guttikunda et al., Air Qual. Atmos. Health, 2013, 6, 589-601. Jung et al. J. Geophys. Res. 2010, 115, D22203. Nishikawa et al. Atmos. Environ. 2011, 45, 5710–5715.
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PM samples collected and analyzed particle-by particle-for: • size distribution • shape (irregular fractal spherical) • Chemical comp. via EDS and general type (mineral dust, soot, and sulfate-organic) • Sice at ice nucleation PM10, NOx, O3, SO2, concentrations Daily meteorological data (temp, RH, wind direction, windspeed) (Obtained from monitoring program by UB Env. Monitoring Agency)
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dispersion modeling Monthly chemical, size, morphological, and ice nucleating-ability analysis of aerosol on a particle-byparticle basis
Yes (hourly data available )
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Fig. 1. The relationship of annual average PM10 concentration in selected cities reported in 2011 relative to the number of peer-reviewed scientific papers that have been published about air pollution in that city prior to and including 2011. Shown are some of the most well-studied cities (in blue), as well as the ten most PM10-polluted cities from the 2011 WHO report of over 1000 places worldwide (in red).3 The vertical, dashed gray line shows the sum of all of papers published on air pollution in those ten most PM-polluted cities. The WHO guideline for PM10 annual average concentration (20 µg m-3) is the horizontal, dashed gray line.
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Fig. 2. Map of Ulaanbaatar Mongolia with relevant sites for this study marked. Ulaanbaatar is located at 1310 m in a valley between two mountainous areas. The main sources of particulate matter pollution are from the ger district, which blankets the Northern half of the city, as well as three coal-fired power plants in the outer areas of the western portion of the city. Note that Power Plant #1 is no longer functioning. The measurement site from this study is shown as the yellow marker and the nearby monitoring site is indicated as a teal marker. To note: Ulaanbaatar experiences low winter wind speeds due to the Siberian High that sits over the region throughout the winter and highest windspeeds in the spring time.
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Fig. 3. Monthly average PM10, O3, SO2, and NOx levels (top panel) and temperature, windspeed, RH, and wind direction (bottom panel) at an air quality monitoring site run by the National Agency of Meteorology and Environmental Monitoring in Mongolia, located approximately 1 km away from our station. The site is located just over the southern border of the ger district in a residential and urban area (see Fig. 2).
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Fig. 4. Representative particles collected in Ulaanbaatar, Mongolia. Particles are classified first according to their shape (spherical, fractal, irregular) and then according to their elemental composition. For the shape classification: fractal particles were considered as any particle composed of two or more discrete, smaller particles, while irregular particles were considered as any single particle that was non-spherical in shape.
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Fig. 5. Size distributions for spherical, fractal, and irregularly shaped particles averaged over all the particles collected in this study as analyzed by TEM. The area of the particles in the TEM images was measured and converted to an area equivalent diameter by assuming that the particles are spheres and therefore are circular in the TEM images. The bins are the same size for spherical and fractal particles and larger for irregular particles. The normalized number of fractal particles in the smallest bin is 0.24. Irregular particles were not observed below 200 nm. The number of each particle type on the TEM grids is given in Fig. 6.
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Fig. 6. The number of particles collected per TEM image per hour collection time for a) all particles, b) spherical particles, c) fractal particles, and d) irregularly shaped particles. The months November – March are shaded blue. Note that all of the panels have different y-axis scales. For June - Sept (labeled “summer”) and Nov – March (labeled “winter”), the percentage of the particles by number that contain sulfur is indicated in red, and the percentage by number without sulfur is indicated in black.
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Fig. 7. The supersaturation with respect to ice, Sice, at the onset of heterogeneous ice nucleation for samples collected in this study. Error bars are the standard deviation of repeated experiments. The months November – March are shaded in blue.
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Highlights Ulaanbaatar, Mongolia has the second highest PM10 concentrations globally Almost all particles are inhalable, composed of minerals, soot, and sulfate-organic Particle concentration and sulfur content of particles increases in winter Particles collected in winter have lower ice nucleation activity
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