Pollution Characteristics of PM2.5 during a Typical Haze Episode in Xiamen, China

Fuwang Zhang, Jinsheng Chen, Tianxue Qiu, Liqian Yin, Xiaoqiu Chen, Jianshuan Yu

Environmental Monitoring Center of Fujian, Fuzhou, China.
Environmental Monitoring Center of Xiamen, Xiamen, China.
Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, China.

DOI: 10.4236/acs.2013.34044                

Abstract

In this study, mass concentrations and chemical compositions of fine particles, mass concentrations of coarse particles, light extinction, and meteorological parameters in the atmosphere ofXiamenwere presented and analyzed to study the chemical and optical characteristics of a typical haze episode from Dec 25, 2010 to Jan 1, 2011. The major chemical compositions of PM_2.5, such as water soluble inorganic ions (WSIIs), carbonaceous fractions (mainly composed of organic carbon (OC) and elemental carbon (EC)), and elements were determined. The results showed that with the typical haze episode process, the concentrations of PM_2.5 mass, WSIIs, OC, EC, and TE first increased and then decreased. The average concentrations of PM_2.5 mass in the stages of Before Haze, During Haze, and After Haze were (88.80 ± 19.97), (135.41 ± 36.20), and (96.35 ± 36.26) μg/m³, respectively. The corresponding average concentrations of secondary organic carbon (SOC) were 6.72, 8.18, and 10.39 μg/m³, accounting for 46.5%, 27.0%, and 61.0% of OC, respectively. S_4^2- , NO_3^-, and NH_4⁺ were three major WSIIs species, accounting for 31.4%, 26.0%, and 12.1% of total WSIIs. The major elements in PM_2.5 were S, K, Fe, Zn, Pb, Ti, and Mn, covering 97.9% of the total elements, while the percentage of the other ten elements was only 2.1%. The average value of light extinction coefficients (b_ext) was 371.0 ±147.1 Mm^-1 during the typical haze episode. The average percentage contributions to b_ext were 39.3% for organic mass, 19.9% for elemental carbon, 16.0% for ammonium sulfate, 13.0% for coarse mass, and 11.8% for ammonium nitrate.

Keywords

Fine Particles; Haze; Water Soluble Inorganic Ions; Organic Carbon; Elemental Carbon; Elements; Light Extinction Coefficients; Xiamen

1. Introduction

Haze is defined by China Meteorological Administration as the phenomenon which leads to atmospheric visibility less than 10 km due to the suspended particles, smoke, and vapor in the atmosphere [1]. Closely related to chemical compositions and meteorological conditions, haze has adverse effects on air quality, human health, visibility, cloud formation and even global climate [2-4]. As a worldwide phenomenon, haze has been given much attention in many countries. Previous studies have been carried out for the investigation of characteristics and source apportionment of atmospheric particulate matter during haze episodes [5-9].

Fine particles (aerodynamic diameter less than 2.5 μm, PM_2.5) play an important role on the formation of haze [10]. The major chemical compositions in PM_2.5, such as water soluble inorganic ions (WSIIs), carbonaceous fractions, mainly composed of organic carbon (OC) and elemental carbon (EC), and elements also have been found to be widely associated with health problems [11-15].

The atmospheric condition is relatively more stable during haze episodes, which results in worse accumulation of atmospheric particulate and light extinction. Visibility degradation is primarily attributed to light scattering and light absorption caused by particulate pollutants and gas pollutants in the atmosphere [16,17]. The impacts of aerosol on solar radiation include light scattering and light absorption, in general called light extinction. It is believed that the degradation of visibility is mainly attributed to the scattering and absorption of visible light by PM_2.5 [18]. Therefore, it is very crucial to investigate chemical characteristics and light extinction coefficients of particulate matter, especially the fine particles during haze episodes.

Xiamen is a subtropical climate city located in the coastal line of southeastern China, with an area of 1565.1 km² and a population of approximately 3.5 million. With rapid industrialization and urbanization, like many other cities in China [8,19-21], Xiamen also suffers from air pollution problems, and the haze problems in Xiamen were increasingly severe during the last few years. The rapid increase of traffic vehicles and coal consumptions in Xiamen have been regarded as the main reasons for the deterioration of air quality in urban area [22]. However, there were very few reports available devoted to determining the effects of chemical composition to visibility (light extinction) during haze episodes in Xiamen, especially during a typical haze episode.

In this study, particle mass and chemical compositions of PM_2.5, light extinction, and meteorology in the atmosphere of Xiamen from Dec 25, 2010 to Jan 1, 2011 were analyzed to study the pollution characteristics of a typical haze episode. The objectives of this paper are 1) to present the characteristics and source apportionments of particle mass, water soluble inorganic ions, carbonaceous fractions, and elements in PM_2.5; 2) to estimate the contributions of light extinction coefficients in atmospheric aerosol by IMPROVE algorithm; 3) to investigate the relationships between visibility degradations and chemical compositions in the atmosphere during a typical haze episode.

2. Methodology

2.1. Site and Sampling

Aerosol samples were collected at a laboratory building rooftop of Institute of Urban Environment, Chinese Academy of Sciences, about 30 m above the ground. The sampling site is located in Jimei District of Xiamen (JM) with rapid urbanization during the last few years, which is surrounded by highway, schools, residential buildings and railway station within two kilometers, as illustrated in Figure 1.

Daytime samples (from 7:00 to 19:00, local time) and nighttime samples (from 19:00 to next day 7:00, local time) were collected from Dec 25, 2010 to Jan 1, 2011 during a typical haze episode, based on the historical meteorological data and weather forecast of Xiamen. A middle volume sampler (TH-150C Ш, Tianhong, China) was employed for collecting samples at a flow rate of 100 L/min. Then 16 sets of samples were obtained in this study. Each set of sample contained fine particle and coarse particle samples. The particulate was retained on quartz fiber filters (QFFs Ø75/Ø90 mm, Whatman, UK). QFFs were previously annealed for 5 h at 450˚C in a furnace to remove residual pollutants for the analysis of particulate mass concentration, WSIIs, OC, EC and TE. Then all the filters were kept in baked aluminum foil-

Figure 1. Location of the sampling site.

within sealed polyethylene plastic bags. During sampling period the meteorological parameters, including visibility, ambient temperature, relative humidity, and wind speed, were also supplied by the Institute of Meteorological Science of Fujian Province, China.

2.2. PM_2.5 Mass Analysis

The particulate mass concentrations of fine particle and coarse particle were determined gravimetrically. Each QFF was weighted before and after exposure with an analytical microbalance (T-114, Denver Instrument, USA) after being stabilized under constant temperature (25˚C) and relative humidity (52%) in chamber (HWS-080, Jinghong, China) for 24 hr. Typical uncertainty for gravimetric measurements is ± 20 μg, which represents less than ± 5% of total aerosol mass of field samples.

2.3. Chemical Compositions Analysis

2.3.1. WSIIs Analysis

Water soluble inorganic ions in PM_2.5 (F⁻, Cl⁻, , , Na⁺, K⁺, , Mg²⁺, and Ca²⁺) were analyzed by Ion chromatography system (ICS3000, Dionex, USA). The details were given elsewhere [23]. Field blank values were subtracted from sample concentrations. The detection limits were about 0.05 μg/mL for anions and cations, based on three times the standard deviation of measurement blank.

2.3.2. OC and EC Analysis

The PM_2.5 samples were measured for OC and EC with carbon analyzer (Model-4, Sunset Lab, USA) by the thermal optical transmission method following the National Institute for Occupational Safety and Health protocol. The detailed analysis procedures were described in literature [23,24]. Field blanks were collected and determined to examine the operational contamination of the field during measured periods. Generally, the concentrations of OC and EC of field blanks were less than 1% of the sample batches, and were not subtracted from the samples. The detection limit for EC and OC was 0.1 μg C/cm².

2.3.3. Element Analysis

The samples were cut into strips (width × length = 6 × 20 mm²) and sticked parallel on a frame. The elements in PM_2.5 were analyzed by proton induced X-ray emission (PIXE) in the Institute Low Energy Nuclear Physics of Beijing Normal University. The PIXE analysis was carried out at the 1.7 MV tandem accelerator with a 2.5 MeV proton beam, and the X-rays were collected by a Si/Li detector [25]. 17 elements were determined in this study including S, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Zr, and Pb. The detection limits ranged from 3.0 × 10⁻⁴ to 5.4 × 10⁻² μg/cm² for 17 elements, based on three times the standard deviation of measurement blank. All the values of sample were corrected by blank filters.

2.3.4. Extinction Analysis

Equations developed by Interagency Monitoring of Protected Visual Environments (IMPROVE) program are used to estimate total light extinction coefficients (b_ext) via multiplying the concentrations of each major chemical composition of atmospheric aerosol by compositionspecific light extinction efficiency, and summed over all species. Equation (1) is the IMPROVE algorithm used to reconstruct b_ext [26,27].

(1)

where f(RH) is relative humidity (RH) dependent adjustment factor which illustrates the relationships between RH and scattering efficiencies for sulfates and nitrates; and Equation (1) includes a constant 10 Mm⁻¹ for the Rayleigh scattering of clear air. The light extinction coefficients of atmospheric aerosol are in unit of Mm⁻¹; chemical composition concentrations shown in brackets are in unit of μg/m³; dry efficiency terms are in unit of m²/g; and the hygroscopic growth terms, f(RH), are unitless. Table 1 summarizes different f(RH) values in selected relative humidity ranges [28].

3. Results and Discussion

3.1. Concentrations and Variations of PM_2.5 with Meteorological Conditions

The correlations between PM_2.5 mass and meteorological parameters during the typical haze episode are illustrated in Figure 2. Haze is closely related to atmosphere qual-

Figure 2. Concentration levels and temporal variations of PM_2.5 during the typical haze episode with meteorological parameters (D: day time, N: night time).

ity, for which visibility is one of the most effective indicators. However, fog could also reduce visibility in addition to haze. When relative humidity is greater than 90%, fog is the major factor that reduces visibility, whereas when relative humidity is less than 90%, haze plays a major role. In order to distinguish haze from fog, relative humidity was also analyzed in this study (Figure 2). All the values of relative humidity were less than 90% during the sampling. According to the definition of haze defined by China Meteorological Administration, the 8-day haze episode could be divided into three stages: Before Haze (Dec 25-26, 2010), During Haze (Dec 27-30, 2010), and After Haze (Dec 31, 2010-Jan 1, 2011).

The average PM_2.5 concentrations in the stages of Before Haze, During Haze, and After Haze were (88.80 ± 19.97), (135.41 ± 36.20), and (96.35 ± 36.26) µg/m³, respectively, which far exceeded the 24-hour limit of Ambient Air Quality Standard (AAQS) for PM_2.5 (75 µg/m³) [29]. In particular, the average concentration of PM_2.5 in the stage of During Haze reached as much as 1.8 times of the AAQS standard. The results indicated that there were serious fine particle pollutions in Xiamen during haze episodes.

The 12-hour concentrations of PM_2.5 were in the range of 47.37 - 188.30 µg/m³, with mean and standard deviation of 113.99 ± 38.09 µg/m³, which showed significant temporal variation during the typical haze episode (Figure 2). With the typical haze process, the PM_2.5 concentration first increased and then decreased, except for a little reduction during Dec 29-30, 2010. As there was a negative correlation between PM_2.5 mass and relative humidity in this study (r = −0.3), lower concentrations of PM_2.5 were mainly caused by higher relative humidity during Dec 29-30, 2010. As shown in Figure 2, the variation of wind speed was opposite to PM_2.5 mass but consistent with visibility. Low wind speed is not conductive to the diffusion of PM_2.5, which plays an important role on the formation of haze [10]. Moreover, there was no apparent correlation between PM_2.5 and temperature.

In this study, the vertical variations of meteorological soundings in the atmosphere of Xiamen were also observed by the soundings system of University of Wyoming during the haze episode. The vertical variations during Dec 26-28, 2010 are illustrated in Figure 3. As shown in Figure 3, there were temperature inversions in the surface atmosphere from Dec 26 night to Dec 28 day, especially at Dec 27 night. Although the temperature inversions disappeared on Dec 28 day, whereas the haze phenomenon still existed due to the hysteresis effects of atmospheric process. Overall, vertical temperature inversion phenomenon led to stable atmospheric conditions in haze days, which promoted the accumulation of pollutants.

Dew point is generally associated with relative humidity. The higher the relative humidity, the closer the dew point is to the ambient temperature. Two main results could be deduced from Figure 3: one was that the vertical variation of relative humidity was obvious during the haze episode; the other was that the relative humidity of surface atmosphere on day was lower than that at corresponding night, which was consistent with Figure 2.

3.2. Chemical Compositions and Source Apportionments of PM_2.5

Table 2 summarizes the concentrations of chemical-

Conflicts of Interest

The authors declare no conflicts of interest.

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