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Surface components of PM2.5 during clear and hazy days in Shanghai by ToF-SIMS
Atmospheric Environment 148 (2017) 175e181
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Surface components of PM2.5 during clear and hazy days in Shanghai by ToF-SIMS Di Huang a, Guangli Xiu a, *, Meng Li a, Xin Hua b, Yitao Long a, b a State Environmental Protection Key Lab of Environmental Risk Assessment and Control on Chemical Processes, School of Resources & Environmental Engineering, East China University of Science and Technology, Shanghai 200237, PR China b School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, PR China
h i g h l i g h t s Comparison between the surface compositions of PM2.5 collected on different weather is demonstrated. ToF-SIMS results show hazardous metals like lead can only be detected on hazy day. Comparison of two different hazy days is demonstrated by ToF-SIMS.
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Article history: Received 20 April 2016 Received in revised form 14 October 2016 Accepted 20 October 2016 Available online 20 October 2016
The compositions of atmospheric particles change greatly on hazy days and could threaten human health. In this study, fine mode particles (PM2.5) were collected and divided according to hazy and nonhazy days in Shanghai from December 8th, 2015 to January 12th, 2016. Versatile ToF-SIMS was performed on the samples to reveal chemical information from the surface of PM2.5. Normalized intensities of Na, Mn, K, V, Al, Fe, Ca, Ti, Cl, NOx and ammonia were higher on clear days while peak intensities of detected bromine and sulfur-contained species were much higher on hazy days. Some hazardous species (Pb, Cr, Ni, As, CHS, SO2) and high-mass aromatic hydrocarbon fractions could only be detected by ToFSIMS from PM2.5 collected on hazy days. Comparison of metallic elements and phthalates implied that haze pollution in Shanghai was mainly the mixing of coal combustion with vehicle emission. In addition, comparison of different haze pollution was also exhibited. Some nitrogen-containing organic compounds þ were detected only from PM2.5 of December 15th, and ToF-SIMS ion signals of N2Hþ 5 , NH4 , CN , NO2 and NO3 from PM2.5 of December 15th were much more intense, compared with PM2.5 collected on December 25th. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Haze pollution PM2.5 ToF-SIMS Surface analysis
1. Introduction In recent years, recurrent haze pollution has raised an increasing environmental concern in China due to the role it plays in environmental and health impacts (Cao et al., 2004; Chen et al., 2013; Che et al., 2009; Tao et al., 2014). During haze episodes, the increased atmospheric loading of PM2.5 (Xu et al., 2015) and enhanced secondary organic aerosol production (Kaul et al., 2011) with larger surface area can result in the atmospheric visibility reduction (Kim et al., 2006), as well as human cardiorespiratory disease (Schwartz, 1993). Meanwhile, the elemental compositions and concentration of numerous species during haze periods are
* Corresponding author. E-mail address: [email protected] (G. Xiu). http://dx.doi.org/10.1016/j.atmosenv.2016.10.036 1352-2310/© 2016 Elsevier Ltd. All rights reserved.
different from those on clear days (Yang et al., 2010). As dominant reason of haze pollution and representative inhalable aerosol, PM2.5 usually acts as significant carrier for various carcinogenic substances such as water-soluble ions (sulfates, nitrates, ammonia) (Reiss et al., 2007; Gao et al., 2011), carbonaceous components (organic carbon and elemental carbon) (Zhao et al., 2015) and heavy metals (Zereini et al., 2005; Birmili et al., 2006) seriously threatening human health and leading to higher mortality rates. Thus, it is required for haze pollution control to figure out characteristics and sources of particulate matter. Despite previous achievements in aerosol study, conventional techniques including fourier transform infrared spectroscopy (FTIR), ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS) are limited to element composition (Ham et al., 2000). None of them is capable of simultaneously
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involving surface chemistry and light elements (usually Z < 11) common in particles. With respect to morphology information of aerosols, scanning electron microscope (SEM) reflects elemental distribution under several micrometers instead of surface information. X-ray photoelectron spectroscopy (XPS) can provide surface information but is lack of spatial resolution, making it difficult to locate. Surface chemical components are of great significance to determine the origin of particles or formation mechanism. As a powerful technique to characterize surface components of solid particles, time-of-flight secondary ion mass spectrometer (ToFSIMS) has been tried to either use alone or combine with other measurements (Choung et al., 2016; Tomiyasu et al., 2004; Lazzeri et al., 2003). Because of its high mass resolution (m/△m z 104), high spatial resolution (~100 nm), high surface sensitivity (up to one or two monolayers) and extremely high detection sensitivity (ppm or ppb) (Zhu et al., 2001), ToF-SIMS is complementary to established methods. Plenty of studies about atmospheric particles have been carried out by ToF-SIMS to get surface chemical information, including species information (Peterson and Tyler, 2002, 2003), molecular imaging (Ham et al., 2006), chemical derivatives (Kirchner et al., 2003), etc. Nevertheless, studies about particulate matter on different haze pollution with the help of ToF-SIMS remain rare. Shanghai is a metropolis with 24 million population, 3 million vehicle volume and nearly 60 million ton of standard coal per year, the source of PM2.5 in Shanghai has been reported to include coal burning, vehicle exhaust emission, biomass burning and suspended mineral dust. The complex source must bring about complex components of particle. Previous study has found that components changed greatly on hazy days (Qiao et al., 2015). In this study, we want to compare the different surface components of PM2.5 samples on clear days and hazy days in winter time. 2. Experiment
campus (31090 9.9500 N, 121250 53.3500 ), around 17 m above ground, which is located in the southwestern part of Shanghai (Xu-hui District), the downwind of the whole Shanghai urban. The sampling site is located in the urban resident district without high-rise buildings and anthropogenic industrial sources around. As shown in Fig. 1, the sampling location is approximately 0.8 km away from Hu-min road and 1.5 km from the central ring road, which are the major nearby traffic arteries. The PM2.5 samples were collected from December 8th, 2015 to January 12th, 2016, including 20 clear days (C samples) and 16 hazy days (H samples) according to haze information published by Shanghai Environmental Monitoring Center (http:// www.semc.gov.cn/aqi/home/Index.aspx). Variations of the weather condition happened during this period. PM2.5 was deposited on 90 mm quartz filters (Whatman) using a mediumvolume sampler (TH-150C, China) with a flow rate of 100 L min1. Sampling was always performed from around 8:00 a.m. to 8:00 a.m. of the next day. Before sampled, all the filters were pre-fired at 600 C for 4 h and conditioned at a temperature of 25 ± 1 C and relative humidity of (40 ± 5)% for about 24 h. After sampled, all filters were folded and packed individually by aluminium foils and then sealed in clean plastic bags and stored at 18 C until the analysis could be performed. 2.2. The meteorological data and PM2.5 concentration The meteorological data of Hongqiao Airport including wind speed, temperature, relative humidity (RH), pressure and visibility were obtained from Weather Underground (http:// www.wunderground.com/). Hongqiao airport is nearly 8 km far away from the sampling site. The visibility for December 8th, 15th and 25th were 10.0 km, 3.1 km and 2.7 km, respectively. PM2.5 concentration published by Shanghai Environmental Monitoring Center for December 8th, 15th and 25th were 34 mg/m3, 218 mg/m3 and 179 mg/m3, respectively.
2.1. Sampling location and PM2.5 collection 2.3. ToF-SIMS measurements Samples were collected on the roof of a four-story building on the East China University of Science and Technology (ECUST)
The chemical analysis was performed with a ToF-SIMS
Fig. 1. Location of sampling site in Shanghai.
D. Huang et al. / Atmospheric Environment 148 (2017) 175e181
Fig. 2. ToF-SIMS spectra of blank quartz filter.
instrument at ECUST. Blank quartz filters were used to control experiments to examine the original composition on blank filters. Both C samples and H samples were analyzed by static ToF-SIMS measurements, and results of the samples collected on December 8th (clear day), December 15th (hazy day 1, H1 sample) and December 25th (hazy day 2, H2 sample) were shown in this passage. And H2 sample was chosen to represent hazy days for the comparison with C sample. The filters were cut into small pieces (10 mm 8 mm) to match the ToF-SIMS sample holder. The apparatus was ToF-SIMS V (ION-ToF GmbH, Germany). Secondary ion mass spectra were collected under both positive and negative operation modes. The surfaces of the samples were respectively bombarded with a pulsed beam of bismuth primary ions, and the depth of analysis reached several molecular layers. The typical operating energy of the bismuth ion beam was 30 keV with pulsed current of 1 pA and DC current of 30 nA. The operating pressure in the main chamber was 108 mbar. Acquisition time was about 300 s for each sample area of 100 mm 100 mm. Charge neutralization was achieved utilizing a low-energy electron flood-gun supplied with the instrument to compensate sample charging. The SIMS spectra were calibrated to Hþ, Cþ, CHþ, CHþ 2 for positive polarity, and C, CH, CH 2 , OH for negative polarity. 2.4. Blank test ToF-SIMS spectra of blank quartz filter are shown in Fig. 2. The ± secondary ions such as H±, SiO± 2 and CxHy fragment ions were strongly detected in both positive- and negative-ion ToF-SIMS spectra. In the negative-ion mass spectra, O and OH ions were also observed at high intensity on the blank quartz filter. And the contamination on the blank quartz filter is negligible. 3. Results and discussion
Fig. 3. The positive-ion ToF-SIMS spectra of PM2.5 on different weather.
positive ions are presented only between 0 and 100 amu in this paper because accessional information about cations at high mass ranges is less useful. Regular ions were detected in both H sample and C sample. The most intense peak was at m/z ¼ 22.9 (Naþ), descendingly followed þ by Mnþ (m/z ¼ 55.0), Kþ (m/z ¼ 39.0), NHþ 4 (m/z ¼ 18.0), V (m/ þ þ þ z ¼ 50.9), Al (m/z ¼ 26.9), Fe (m/z ¼ 56.0), Ca (m/z ¼ 40.0) and Tiþ (m/z ¼ 47.9). Because such metals are indicators of different sources such as suspended dust (Caþ), biomass burning (Kþ), vehicle exhaust (Mnþ and Vþ), metal smelting (Feþ), their contribution to ambient air quality is complex. In addition to Pbþ, positive ions such as Crþ (m/z ¼ 52.0), Niþ (m/z ¼ 58.6), Asþ (m/z ¼ 74.9) were also detected at a trace level from H sample rather than C sample. This in conjunction with the detection of Pbþ likely resulted in the conclusion that coal burning (based on Asþ and Pbþ) and vehicle emission (based on Crþ and Niþ) might enhanced that haze pollution episode. Although Si(CH3)þ 3 (m/z ¼ 73.0) was detected both from these two samples, its intensity in C sample (counts ¼ 75) was negligible in comparison with H sample (counts ¼ 1472). And B. Tomiyasu (Tomiyasu et al., 2006) proved þ that Si(CH3)þ 3 together with NH4 could be strongly detected by ToFSIMS from diesel exhaust particles. A comparison of the normalized intensities of regular secondary cations between C sample and H sample is shown in Table 1. All positive-ion peaks from each sample were normalized to total intensity, respectively. It is clear that the ratio IC/IH for those regular cations is greater than 1, which indicates dilution of these species during 25th haze episode by pollutants.
Table 1 Comparison of secondary ion intensity between C sample and H sample.
3.1. Metallic components and specific cations Fig. 3 shows positive ToF-SIMS spectra of PM2.5 collected on both clear and hazy days. Pbþ (m/z ¼ 207.2), which particularly affects children's neurogenic system (Canfield et al., 2003), was detected from H sample but not from C sample. Because the use of leaded gasoline has been prohibited since 1997, coal burning and associated industrial emission have become the predominant sources of lead in Shanghai ambient air (Li et al., 2009; Chen et al., 2005). Although the mass range of a ToF-SIMS analysis is unlimited, other
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*
Secondary ion
IC/IH
Secondary ion
Naþ Mnþ Kþ NHþ 4 Vþ þ Al Feþ Caþ Tiþ
2.14 1.52 1.29 1.63 1.51 5.82 1.33 1.36 1.33
Cl Br NO 2 NO 3 SO 3 SO 4 HSO 4
IC/IH Cl 37 Cl 79 Br 81 Br 35
3.21 1.05 0.71 0.69 1.09 1.23 0.61 0.49 0.24
IC is the normalized intensity of secondary ions from C sample. IH is the normalized intensity of secondary ions from H sample.
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3.2. Organic matter As typical precursor to secondary organic aerosol (SOA) (Ng et al., 2007; Johnson et al., 2005), aromatic hydrocarbon has fragmentation peaks at m/z ¼ 39, 50, 51, 63, 65, 76, 77, 78, 91, 105, 119, 127, 128, 141, 142. These peaks were picked out and shown in Fig. 4. According to foregoing discussion, the peak at m/z ¼ 39 with relatively high intensity may also be attributed to Kþ. No more clear peak could be found beyond m/z ¼ 105 from C sample, as observed from ToF-SIMS measurements. However, spectra of H sample displays distinct peaks at these positions, indicating that on hazy days, aromatic hydrocarbon may tend to attach the surface of PM2.5 and be measured by ToF-SIMS more easily than on clear days. Therefore, more SOA was created during haze pollution episodes. Fig. 5 shows ToF-SIMS spectra of PM2.5 collected on different weather at m/z ¼ 149 (C8H5Oþ 3 ). It is observed that the intensity of C sample at m/z ¼ 149 is very low without a recognizable peak. However, the mass-peak of 149 is relatively high with a sharp peak for H sample. The mass-peak of 149 is the major fragmentation peak of phthalates and has been regarded as representative components of diesel engine exhaust (Tomiyasu et al., 2003). In this case, diesel vehicle exhaust emissions made more contribution on PM2.5 on hazy days.
Fig. 4. Aromatic hydrocarbon of PM2.5 on different weather.
Fig. 6. The negative-ion ToF-SIMS spectra of PM2.5 on different weather.
3.3. Negative ions Fig. 6 is negative ToF-SIMS spectra acquired from the surface of C sample and H sample. As regular negative ions, Cl (m/z ¼ 35.0, 37.0), NO 2 (m/z ¼ 46.0), NO3 (m/z ¼ 62.0), Br (m/z ¼ 79.0, 81.0), SO (m/z ¼ 80.0), SO (m/z ¼ 96.0) and HSO 3 4 4 (m/z ¼ 97.0) were detected by ToF-SIMS from both C sample and H sample. But the peaks of m/z ¼ 45.0 (CHS) and m/z ¼ 64.0 (SO 2 ), which tend to transform to SO 4 triggering severe haze pollution (Wang et al., 2015), only appear in the spectra of H sample. Furthermore, a comparison of the normalized intensities of regular secondary anions between C sample and H sample is shown in Table 1. All negative-ion peaks from each sample were normalized to total intensity, respectively. It is observed that chlorine from C sample are more intense than those from H sample. Nitrogen oxide ions are also slightly higher in C sample. Contrarily, the normalized intensities of bromine and sulfur oxide ions from H sample are higher than those from C sample. Therefore, PM2.5 sample collected on 25th hazy day was attached by not only regular anions (i.e. Cl and NO x ) but more bromine and sulfur-rich ions, potential contributors to aerosol aging (Yuan et al., 2015). As a matter of fact, there is no power plant or other sulfur-using enterprise in the neighborhood of the sampling site, this sudden increase in sulfur-containing contents on hazy days may be attributed to local petroleum burning inside of vehicle engines in Shanghai or long-range transport of sulfur from the Northwest. 3.4. Lateral distribution of chemical components Fig. 7 illustrates some ToF-SIMS images from C sample and H sample. The brighter the image is, the more intense the signal is. These images visually display the lateral distribution of specific species on the surface of collected PM2.5. Besides, images of different species show visible difference. This might be because various contaminants attached to individual particles (or cluster) and then deposited on different locations to be detected, indicating complex sources and formation mechanism of the particles in the atmosphere. It is also intriguing to note that similar species display similar distribution images, such as Naþ and Kþ, Cl and Br, SO 4 and HSO 4. 3.5. Different haze pollution
Fig. 5. ToF-SIMS spectra of different PM2.5 samples at m/z ¼ 149.
The positive- and negative-ion ToF-SIMS spectra of hazy day 1 (December 15th) and 2 (December 25th) are shown in Figs. 8 and 9,
D. Huang et al. / Atmospheric Environment 148 (2017) 175e181
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Fig. 7. Secondary ion images from C sample (the upper row) and H sample (the nether row). Field of view: 100 mm 100 mm, 256 256 pixel. (a) positive ions; (b) negative ions.
Fig. 8. The positive-ion ToF-SIMS spectra of different haze pollution.
respectively. As shown in the positive-ion mass spectra, PM2.5 collected on December 15th produced secondary ion for N2Hþ 5 (m/ z ¼ 33.0) while PM2.5 collected on December 25th showed distinct peak for Tiþ (m/z ¼ 47.9), partly indicating the different particulate surface composition between different haze pollution. In the negative-ion mass spectra, only PM2.5 collected on December 15th contained intense ions of C3H9N (m/z ¼ 59.0), C4H11N (m/ z ¼ 73.0), C5H11NO 2 (m/z ¼ 99.0) and C5H13NO2 (m/z ¼ 119.0). However, there is no anthropogenic incineration activity around the sampling site, such nitrogen-containing organic compounds may be attributed to external coal combustion and biomass burning.
Fig. 9. The negative-ion ToF-SIMS spectra of different haze pollution.
Table 2 shows the normalized intensities of NHþ 4 and NOx re action products (CN, NO 2 , NO3 at m/z ¼ 26.0, 46.0, 62.0, respectively) from H1 sample and H2 sample. The ratio IH1/IH2 is consistently greater than 1. It means all the normalized intensities of NHþ 4 , CN , NO2 and NO3 from H1 sample are higher than those from H2 sample. This confirms an overwhelming predominance of 15th haze pollution in terms of nitrogen-containing species. 4. Conclusions PM2.5 samples were collected from December 8th, 2015 to
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D. Huang et al. / Atmospheric Environment 148 (2017) 175e181 Table 2 The normalized intensities of nitrogen-containing species on hazy days. Secondary ion
IH1/IH2
NHþ 4 CN NO2 NO 3
2.20 1.02 2.73 4.32
* IH1 is the normalized intensity of secondary ions from H1 sample. IH2 is the normalized intensity of secondary ions from H2 sample.
January 12th, 2016 during clear and hazy days, and chemical cations as well as anions on the surface of those particles were measured by ToF-SIMS, respectively. The following main conclusions were drawn: (1) Positive-ion signals including Naþ, Mnþ, Kþ, þ þ þ þ Alþ, NHþ 4 , Ca , Fe , Ti , V and negative-ion signals including Cl , NO , NO , Br , SO , SO , HSO were detected from PM samples 2 3 3 4 4 2.5 collected on both clear and hazy days. On hazy days, components containing Na, Mn, K, Al, Ca, Fe, Ti, V, nitrites, nitrates, chlorine and ammonia might be diluted, but bromine and sulfate-containing components increased. According to detailed comparison, hazardous ions such as Pbþ, Crþ, Niþ, Asþ, CHS, SO 2 could only be detected from PM2.5 sample collected on hazy days, indicating that coal burning and vehicle exhaust became dominant contributor to PM2.5 during haze episodes; (2) Aromatic hydrocarbon fractions were completely detected from PM2.5 sample collected on hazy days, but failed to be detected from PM2.5 sample collected on clear days at high-mass peak positions, indicating an increase in SOA production during haze episodes; (3) As components of diesel engine exhaust, the major fragmentation peak of phthalates was also highly detected only from PM2.5 sample collected on hazy days, confirming pollution contribution from vehicle emission on hazy days. These differences between different weather conditions might result from the heavy pollution in Shanghai caused by vehicle exhaust and coal combustion during haze episodes. Further research on source apportionment needs to be accomplished in later discussion; (4) ToF-SIMS images illustrated that secondaryion signals were more intense on hazy days and similar species displayed similar lateral distribution; (5) Compared with December 25th, PM2.5 of December 15th was attached by more nitrogencontaining species (both organic and inorganic matter), elucidating different surface chemical components and sources of PM2.5 on different hazy days; (6) ToF-SIMS was found to complement the traditional characterization of particles by providing the surface composition. Acknowledgements This publication was supported financially by NSFC (21277044), Ph.D. Programs Foundation of Ministry of Education of China (No.20120074140001). This work was also supported by AustraliaChina Centre for Air Quality Science and Management (ACC-AQSM). References Birmili, W., Allen, A.G., Bary, F., Harrison, R.M., 2006. Trace metal concentrations and water solubility in size-fractioned atmospheric particles and influence of road traffic. Environ. Sci. Technol. 40, 1144e1153. Canfield, R.L., Henderson Jr., C.R., Cory-Slechta, D.A., Cox, C., Jusko, T.A., Lanphear, B.P., 2003. Intellectual impairment in Children with blood lead concentrations below 10 mg per deciliter. New. Engl. J. Med. 348, 1517e1526. Cao, J.J., Lee, S.C., Ho, K.F., Zou, S.C., Fung, K., Li, Y., Watson, J.G., Chow, J.C., 2004. Spatial and seasonal variations of atmospheric organic carbon and elemental carbon in Pearl River Delta Region, China. Atmos. Environ. 38, 4447e4456. Che, H.Z., Zhang, X.Y., Li, Y., Zhou, Z.J., Qu, J.J., Hao, X.J., 2009. Haze trends over the
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Surface components of PM2.5 during clear and hazy days in Shanghai by ToF-SIMS Di Huang a, Guangli Xiu a, *, Meng Li a, Xin Hua b, Yitao Long a, b a State Environmental Protection Key Lab of Environmental Risk Assessment and Control on Chemical Processes, School of Resources & Environmental Engineering, East China University of Science and Technology, Shanghai 200237, PR China b School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, PR China
h i g h l i g h t s Comparison between the surface compositions of PM2.5 collected on different weather is demonstrated. ToF-SIMS results show hazardous metals like lead can only be detected on hazy day. Comparison of two different hazy days is demonstrated by ToF-SIMS.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2016 Received in revised form 14 October 2016 Accepted 20 October 2016 Available online 20 October 2016
The compositions of atmospheric particles change greatly on hazy days and could threaten human health. In this study, fine mode particles (PM2.5) were collected and divided according to hazy and nonhazy days in Shanghai from December 8th, 2015 to January 12th, 2016. Versatile ToF-SIMS was performed on the samples to reveal chemical information from the surface of PM2.5. Normalized intensities of Na, Mn, K, V, Al, Fe, Ca, Ti, Cl, NOx and ammonia were higher on clear days while peak intensities of detected bromine and sulfur-contained species were much higher on hazy days. Some hazardous species (Pb, Cr, Ni, As, CHS, SO2) and high-mass aromatic hydrocarbon fractions could only be detected by ToFSIMS from PM2.5 collected on hazy days. Comparison of metallic elements and phthalates implied that haze pollution in Shanghai was mainly the mixing of coal combustion with vehicle emission. In addition, comparison of different haze pollution was also exhibited. Some nitrogen-containing organic compounds þ were detected only from PM2.5 of December 15th, and ToF-SIMS ion signals of N2Hþ 5 , NH4 , CN , NO2 and NO3 from PM2.5 of December 15th were much more intense, compared with PM2.5 collected on December 25th. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Haze pollution PM2.5 ToF-SIMS Surface analysis
1. Introduction In recent years, recurrent haze pollution has raised an increasing environmental concern in China due to the role it plays in environmental and health impacts (Cao et al., 2004; Chen et al., 2013; Che et al., 2009; Tao et al., 2014). During haze episodes, the increased atmospheric loading of PM2.5 (Xu et al., 2015) and enhanced secondary organic aerosol production (Kaul et al., 2011) with larger surface area can result in the atmospheric visibility reduction (Kim et al., 2006), as well as human cardiorespiratory disease (Schwartz, 1993). Meanwhile, the elemental compositions and concentration of numerous species during haze periods are
* Corresponding author. E-mail address: [email protected] (G. Xiu). http://dx.doi.org/10.1016/j.atmosenv.2016.10.036 1352-2310/© 2016 Elsevier Ltd. All rights reserved.
different from those on clear days (Yang et al., 2010). As dominant reason of haze pollution and representative inhalable aerosol, PM2.5 usually acts as significant carrier for various carcinogenic substances such as water-soluble ions (sulfates, nitrates, ammonia) (Reiss et al., 2007; Gao et al., 2011), carbonaceous components (organic carbon and elemental carbon) (Zhao et al., 2015) and heavy metals (Zereini et al., 2005; Birmili et al., 2006) seriously threatening human health and leading to higher mortality rates. Thus, it is required for haze pollution control to figure out characteristics and sources of particulate matter. Despite previous achievements in aerosol study, conventional techniques including fourier transform infrared spectroscopy (FTIR), ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS) are limited to element composition (Ham et al., 2000). None of them is capable of simultaneously
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involving surface chemistry and light elements (usually Z < 11) common in particles. With respect to morphology information of aerosols, scanning electron microscope (SEM) reflects elemental distribution under several micrometers instead of surface information. X-ray photoelectron spectroscopy (XPS) can provide surface information but is lack of spatial resolution, making it difficult to locate. Surface chemical components are of great significance to determine the origin of particles or formation mechanism. As a powerful technique to characterize surface components of solid particles, time-of-flight secondary ion mass spectrometer (ToFSIMS) has been tried to either use alone or combine with other measurements (Choung et al., 2016; Tomiyasu et al., 2004; Lazzeri et al., 2003). Because of its high mass resolution (m/△m z 104), high spatial resolution (~100 nm), high surface sensitivity (up to one or two monolayers) and extremely high detection sensitivity (ppm or ppb) (Zhu et al., 2001), ToF-SIMS is complementary to established methods. Plenty of studies about atmospheric particles have been carried out by ToF-SIMS to get surface chemical information, including species information (Peterson and Tyler, 2002, 2003), molecular imaging (Ham et al., 2006), chemical derivatives (Kirchner et al., 2003), etc. Nevertheless, studies about particulate matter on different haze pollution with the help of ToF-SIMS remain rare. Shanghai is a metropolis with 24 million population, 3 million vehicle volume and nearly 60 million ton of standard coal per year, the source of PM2.5 in Shanghai has been reported to include coal burning, vehicle exhaust emission, biomass burning and suspended mineral dust. The complex source must bring about complex components of particle. Previous study has found that components changed greatly on hazy days (Qiao et al., 2015). In this study, we want to compare the different surface components of PM2.5 samples on clear days and hazy days in winter time. 2. Experiment
campus (31090 9.9500 N, 121250 53.3500 ), around 17 m above ground, which is located in the southwestern part of Shanghai (Xu-hui District), the downwind of the whole Shanghai urban. The sampling site is located in the urban resident district without high-rise buildings and anthropogenic industrial sources around. As shown in Fig. 1, the sampling location is approximately 0.8 km away from Hu-min road and 1.5 km from the central ring road, which are the major nearby traffic arteries. The PM2.5 samples were collected from December 8th, 2015 to January 12th, 2016, including 20 clear days (C samples) and 16 hazy days (H samples) according to haze information published by Shanghai Environmental Monitoring Center (http:// www.semc.gov.cn/aqi/home/Index.aspx). Variations of the weather condition happened during this period. PM2.5 was deposited on 90 mm quartz filters (Whatman) using a mediumvolume sampler (TH-150C, China) with a flow rate of 100 L min1. Sampling was always performed from around 8:00 a.m. to 8:00 a.m. of the next day. Before sampled, all the filters were pre-fired at 600 C for 4 h and conditioned at a temperature of 25 ± 1 C and relative humidity of (40 ± 5)% for about 24 h. After sampled, all filters were folded and packed individually by aluminium foils and then sealed in clean plastic bags and stored at 18 C until the analysis could be performed. 2.2. The meteorological data and PM2.5 concentration The meteorological data of Hongqiao Airport including wind speed, temperature, relative humidity (RH), pressure and visibility were obtained from Weather Underground (http:// www.wunderground.com/). Hongqiao airport is nearly 8 km far away from the sampling site. The visibility for December 8th, 15th and 25th were 10.0 km, 3.1 km and 2.7 km, respectively. PM2.5 concentration published by Shanghai Environmental Monitoring Center for December 8th, 15th and 25th were 34 mg/m3, 218 mg/m3 and 179 mg/m3, respectively.
2.1. Sampling location and PM2.5 collection 2.3. ToF-SIMS measurements Samples were collected on the roof of a four-story building on the East China University of Science and Technology (ECUST)
The chemical analysis was performed with a ToF-SIMS
Fig. 1. Location of sampling site in Shanghai.
D. Huang et al. / Atmospheric Environment 148 (2017) 175e181
Fig. 2. ToF-SIMS spectra of blank quartz filter.
instrument at ECUST. Blank quartz filters were used to control experiments to examine the original composition on blank filters. Both C samples and H samples were analyzed by static ToF-SIMS measurements, and results of the samples collected on December 8th (clear day), December 15th (hazy day 1, H1 sample) and December 25th (hazy day 2, H2 sample) were shown in this passage. And H2 sample was chosen to represent hazy days for the comparison with C sample. The filters were cut into small pieces (10 mm 8 mm) to match the ToF-SIMS sample holder. The apparatus was ToF-SIMS V (ION-ToF GmbH, Germany). Secondary ion mass spectra were collected under both positive and negative operation modes. The surfaces of the samples were respectively bombarded with a pulsed beam of bismuth primary ions, and the depth of analysis reached several molecular layers. The typical operating energy of the bismuth ion beam was 30 keV with pulsed current of 1 pA and DC current of 30 nA. The operating pressure in the main chamber was 108 mbar. Acquisition time was about 300 s for each sample area of 100 mm 100 mm. Charge neutralization was achieved utilizing a low-energy electron flood-gun supplied with the instrument to compensate sample charging. The SIMS spectra were calibrated to Hþ, Cþ, CHþ, CHþ 2 for positive polarity, and C, CH, CH 2 , OH for negative polarity. 2.4. Blank test ToF-SIMS spectra of blank quartz filter are shown in Fig. 2. The ± secondary ions such as H±, SiO± 2 and CxHy fragment ions were strongly detected in both positive- and negative-ion ToF-SIMS spectra. In the negative-ion mass spectra, O and OH ions were also observed at high intensity on the blank quartz filter. And the contamination on the blank quartz filter is negligible. 3. Results and discussion
Fig. 3. The positive-ion ToF-SIMS spectra of PM2.5 on different weather.
positive ions are presented only between 0 and 100 amu in this paper because accessional information about cations at high mass ranges is less useful. Regular ions were detected in both H sample and C sample. The most intense peak was at m/z ¼ 22.9 (Naþ), descendingly followed þ by Mnþ (m/z ¼ 55.0), Kþ (m/z ¼ 39.0), NHþ 4 (m/z ¼ 18.0), V (m/ þ þ þ z ¼ 50.9), Al (m/z ¼ 26.9), Fe (m/z ¼ 56.0), Ca (m/z ¼ 40.0) and Tiþ (m/z ¼ 47.9). Because such metals are indicators of different sources such as suspended dust (Caþ), biomass burning (Kþ), vehicle exhaust (Mnþ and Vþ), metal smelting (Feþ), their contribution to ambient air quality is complex. In addition to Pbþ, positive ions such as Crþ (m/z ¼ 52.0), Niþ (m/z ¼ 58.6), Asþ (m/z ¼ 74.9) were also detected at a trace level from H sample rather than C sample. This in conjunction with the detection of Pbþ likely resulted in the conclusion that coal burning (based on Asþ and Pbþ) and vehicle emission (based on Crþ and Niþ) might enhanced that haze pollution episode. Although Si(CH3)þ 3 (m/z ¼ 73.0) was detected both from these two samples, its intensity in C sample (counts ¼ 75) was negligible in comparison with H sample (counts ¼ 1472). And B. Tomiyasu (Tomiyasu et al., 2006) proved þ that Si(CH3)þ 3 together with NH4 could be strongly detected by ToFSIMS from diesel exhaust particles. A comparison of the normalized intensities of regular secondary cations between C sample and H sample is shown in Table 1. All positive-ion peaks from each sample were normalized to total intensity, respectively. It is clear that the ratio IC/IH for those regular cations is greater than 1, which indicates dilution of these species during 25th haze episode by pollutants.
Table 1 Comparison of secondary ion intensity between C sample and H sample.
3.1. Metallic components and specific cations Fig. 3 shows positive ToF-SIMS spectra of PM2.5 collected on both clear and hazy days. Pbþ (m/z ¼ 207.2), which particularly affects children's neurogenic system (Canfield et al., 2003), was detected from H sample but not from C sample. Because the use of leaded gasoline has been prohibited since 1997, coal burning and associated industrial emission have become the predominant sources of lead in Shanghai ambient air (Li et al., 2009; Chen et al., 2005). Although the mass range of a ToF-SIMS analysis is unlimited, other
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*
Secondary ion
IC/IH
Secondary ion
Naþ Mnþ Kþ NHþ 4 Vþ þ Al Feþ Caþ Tiþ
2.14 1.52 1.29 1.63 1.51 5.82 1.33 1.36 1.33
Cl Br NO 2 NO 3 SO 3 SO 4 HSO 4
IC/IH Cl 37 Cl 79 Br 81 Br 35
3.21 1.05 0.71 0.69 1.09 1.23 0.61 0.49 0.24
IC is the normalized intensity of secondary ions from C sample. IH is the normalized intensity of secondary ions from H sample.
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3.2. Organic matter As typical precursor to secondary organic aerosol (SOA) (Ng et al., 2007; Johnson et al., 2005), aromatic hydrocarbon has fragmentation peaks at m/z ¼ 39, 50, 51, 63, 65, 76, 77, 78, 91, 105, 119, 127, 128, 141, 142. These peaks were picked out and shown in Fig. 4. According to foregoing discussion, the peak at m/z ¼ 39 with relatively high intensity may also be attributed to Kþ. No more clear peak could be found beyond m/z ¼ 105 from C sample, as observed from ToF-SIMS measurements. However, spectra of H sample displays distinct peaks at these positions, indicating that on hazy days, aromatic hydrocarbon may tend to attach the surface of PM2.5 and be measured by ToF-SIMS more easily than on clear days. Therefore, more SOA was created during haze pollution episodes. Fig. 5 shows ToF-SIMS spectra of PM2.5 collected on different weather at m/z ¼ 149 (C8H5Oþ 3 ). It is observed that the intensity of C sample at m/z ¼ 149 is very low without a recognizable peak. However, the mass-peak of 149 is relatively high with a sharp peak for H sample. The mass-peak of 149 is the major fragmentation peak of phthalates and has been regarded as representative components of diesel engine exhaust (Tomiyasu et al., 2003). In this case, diesel vehicle exhaust emissions made more contribution on PM2.5 on hazy days.
Fig. 4. Aromatic hydrocarbon of PM2.5 on different weather.
Fig. 6. The negative-ion ToF-SIMS spectra of PM2.5 on different weather.
3.3. Negative ions Fig. 6 is negative ToF-SIMS spectra acquired from the surface of C sample and H sample. As regular negative ions, Cl (m/z ¼ 35.0, 37.0), NO 2 (m/z ¼ 46.0), NO3 (m/z ¼ 62.0), Br (m/z ¼ 79.0, 81.0), SO (m/z ¼ 80.0), SO (m/z ¼ 96.0) and HSO 3 4 4 (m/z ¼ 97.0) were detected by ToF-SIMS from both C sample and H sample. But the peaks of m/z ¼ 45.0 (CHS) and m/z ¼ 64.0 (SO 2 ), which tend to transform to SO 4 triggering severe haze pollution (Wang et al., 2015), only appear in the spectra of H sample. Furthermore, a comparison of the normalized intensities of regular secondary anions between C sample and H sample is shown in Table 1. All negative-ion peaks from each sample were normalized to total intensity, respectively. It is observed that chlorine from C sample are more intense than those from H sample. Nitrogen oxide ions are also slightly higher in C sample. Contrarily, the normalized intensities of bromine and sulfur oxide ions from H sample are higher than those from C sample. Therefore, PM2.5 sample collected on 25th hazy day was attached by not only regular anions (i.e. Cl and NO x ) but more bromine and sulfur-rich ions, potential contributors to aerosol aging (Yuan et al., 2015). As a matter of fact, there is no power plant or other sulfur-using enterprise in the neighborhood of the sampling site, this sudden increase in sulfur-containing contents on hazy days may be attributed to local petroleum burning inside of vehicle engines in Shanghai or long-range transport of sulfur from the Northwest. 3.4. Lateral distribution of chemical components Fig. 7 illustrates some ToF-SIMS images from C sample and H sample. The brighter the image is, the more intense the signal is. These images visually display the lateral distribution of specific species on the surface of collected PM2.5. Besides, images of different species show visible difference. This might be because various contaminants attached to individual particles (or cluster) and then deposited on different locations to be detected, indicating complex sources and formation mechanism of the particles in the atmosphere. It is also intriguing to note that similar species display similar distribution images, such as Naþ and Kþ, Cl and Br, SO 4 and HSO 4. 3.5. Different haze pollution
Fig. 5. ToF-SIMS spectra of different PM2.5 samples at m/z ¼ 149.
The positive- and negative-ion ToF-SIMS spectra of hazy day 1 (December 15th) and 2 (December 25th) are shown in Figs. 8 and 9,
D. Huang et al. / Atmospheric Environment 148 (2017) 175e181
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Fig. 7. Secondary ion images from C sample (the upper row) and H sample (the nether row). Field of view: 100 mm 100 mm, 256 256 pixel. (a) positive ions; (b) negative ions.
Fig. 8. The positive-ion ToF-SIMS spectra of different haze pollution.
respectively. As shown in the positive-ion mass spectra, PM2.5 collected on December 15th produced secondary ion for N2Hþ 5 (m/ z ¼ 33.0) while PM2.5 collected on December 25th showed distinct peak for Tiþ (m/z ¼ 47.9), partly indicating the different particulate surface composition between different haze pollution. In the negative-ion mass spectra, only PM2.5 collected on December 15th contained intense ions of C3H9N (m/z ¼ 59.0), C4H11N (m/ z ¼ 73.0), C5H11NO 2 (m/z ¼ 99.0) and C5H13NO2 (m/z ¼ 119.0). However, there is no anthropogenic incineration activity around the sampling site, such nitrogen-containing organic compounds may be attributed to external coal combustion and biomass burning.
Fig. 9. The negative-ion ToF-SIMS spectra of different haze pollution.
Table 2 shows the normalized intensities of NHþ 4 and NOx re action products (CN, NO 2 , NO3 at m/z ¼ 26.0, 46.0, 62.0, respectively) from H1 sample and H2 sample. The ratio IH1/IH2 is consistently greater than 1. It means all the normalized intensities of NHþ 4 , CN , NO2 and NO3 from H1 sample are higher than those from H2 sample. This confirms an overwhelming predominance of 15th haze pollution in terms of nitrogen-containing species. 4. Conclusions PM2.5 samples were collected from December 8th, 2015 to
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D. Huang et al. / Atmospheric Environment 148 (2017) 175e181 Table 2 The normalized intensities of nitrogen-containing species on hazy days. Secondary ion
IH1/IH2
NHþ 4 CN NO2 NO 3
2.20 1.02 2.73 4.32
* IH1 is the normalized intensity of secondary ions from H1 sample. IH2 is the normalized intensity of secondary ions from H2 sample.
January 12th, 2016 during clear and hazy days, and chemical cations as well as anions on the surface of those particles were measured by ToF-SIMS, respectively. The following main conclusions were drawn: (1) Positive-ion signals including Naþ, Mnþ, Kþ, þ þ þ þ Alþ, NHþ 4 , Ca , Fe , Ti , V and negative-ion signals including Cl , NO , NO , Br , SO , SO , HSO were detected from PM samples 2 3 3 4 4 2.5 collected on both clear and hazy days. On hazy days, components containing Na, Mn, K, Al, Ca, Fe, Ti, V, nitrites, nitrates, chlorine and ammonia might be diluted, but bromine and sulfate-containing components increased. According to detailed comparison, hazardous ions such as Pbþ, Crþ, Niþ, Asþ, CHS, SO 2 could only be detected from PM2.5 sample collected on hazy days, indicating that coal burning and vehicle exhaust became dominant contributor to PM2.5 during haze episodes; (2) Aromatic hydrocarbon fractions were completely detected from PM2.5 sample collected on hazy days, but failed to be detected from PM2.5 sample collected on clear days at high-mass peak positions, indicating an increase in SOA production during haze episodes; (3) As components of diesel engine exhaust, the major fragmentation peak of phthalates was also highly detected only from PM2.5 sample collected on hazy days, confirming pollution contribution from vehicle emission on hazy days. These differences between different weather conditions might result from the heavy pollution in Shanghai caused by vehicle exhaust and coal combustion during haze episodes. Further research on source apportionment needs to be accomplished in later discussion; (4) ToF-SIMS images illustrated that secondaryion signals were more intense on hazy days and similar species displayed similar lateral distribution; (5) Compared with December 25th, PM2.5 of December 15th was attached by more nitrogencontaining species (both organic and inorganic matter), elucidating different surface chemical components and sources of PM2.5 on different hazy days; (6) ToF-SIMS was found to complement the traditional characterization of particles by providing the surface composition. Acknowledgements This publication was supported financially by NSFC (21277044), Ph.D. Programs Foundation of Ministry of Education of China (No.20120074140001). This work was also supported by AustraliaChina Centre for Air Quality Science and Management (ACC-AQSM). References Birmili, W., Allen, A.G., Bary, F., Harrison, R.M., 2006. Trace metal concentrations and water solubility in size-fractioned atmospheric particles and influence of road traffic. Environ. Sci. Technol. 40, 1144e1153. Canfield, R.L., Henderson Jr., C.R., Cory-Slechta, D.A., Cox, C., Jusko, T.A., Lanphear, B.P., 2003. Intellectual impairment in Children with blood lead concentrations below 10 mg per deciliter. New. Engl. J. Med. 348, 1517e1526. Cao, J.J., Lee, S.C., Ho, K.F., Zou, S.C., Fung, K., Li, Y., Watson, J.G., Chow, J.C., 2004. Spatial and seasonal variations of atmospheric organic carbon and elemental carbon in Pearl River Delta Region, China. Atmos. Environ. 38, 4447e4456. Che, H.Z., Zhang, X.Y., Li, Y., Zhou, Z.J., Qu, J.J., Hao, X.J., 2009. Haze trends over the
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