Scholarly article on topic 'A Case Study of Aerosol Characteristics During a Haze Episode Over Beijing'

A Case Study of Aerosol Characteristics During a Haze Episode Over Beijing Academic research paper on "Earth and related environmental sciences"

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{Haze / "Aerosol scattering coefficent" / "Aerosol optical thickness" / "Angstrom exponent" / Beijing}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Wanzhi Xu, He Chen, Donghui Li, Fengsheng Zhao, Yan Yang

Abstract This work presents the variation of aerosol characteristics obtained before, during and after the haze episode in October, 2010 in Beijing, a highly industrialized city which located in the northwest of North China Plain with more than 21 million inhabitants. Surface solar irradiance was measured by pyranometer which showed great differences between hazy and non-hazy days. Aerosol scattering coefficients were obtained by integrated nephelometer and the influence of relative humidity on aerosol scattering coefficient was investigated by means of the Weather Research and Forecasting Model Wave Watch III (WRF-WW3) imagery. Spectral aerosol optical thickness was measured by Sun photometer and Angstrom exponent were calculated to investigate the variation of aerosol loading and modes under different weather conditions. Haze weather process favoured the formation of the secondary aerosol, which contributed to the increase of fine mode particles.

Academic research paper on topic "A Case Study of Aerosol Characteristics During a Haze Episode Over Beijing"

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Procedia Environmental Sciences 18 (2013) 404 - 411

2013 International Symposium on Environmental Science and Technology (2013 ISEST)

A case study of aerosol characteristics during a haze episode

over Beijing

Wanzhi Xua'b'c,He Chend,Donghui Lib,e, Fengsheng Zhaof'*,Yan Yanga'b'c

a Chinese Academy of Meteorological Science, Beijing 100081, China bGraduate University of Chinese Academy of Science, Beijing 100049, China cNational Satellite Meteorological Center, Beijing 100875,China dBeijing Institute of Technology, School of Optoelectronics, Beijing 100081, China eInstitute of Remote Sensing Applications, Chinese Academy of Sciences, Beijing 100101, China f Beijing Normal University, College of Global Change and Earth System Science, Beijing 100875,China

Abstract

This work presents the variation of aerosol characteristics obtained before, during and after the haze episode in October, 2010 in Beijing, a highly industrialized city which located in the northwest of North China Plain with more than 21 million inhabitants. Surface solar irradiance was measured by pyranometer which showed great differences between hazy and non-hazy days. Aerosol scattering coefficients were obtained by integrated nephelometer and the influence of relative humidity on aerosol scattering coefficient was investigated by means of the Weather Research and Forecasting Model Wave Watch III (WRF-WW3) imagery. Spectral aerosol optical thickness was measured by Sun photometer and Angstrom exponent were calculated to investigate the variation of aerosol loading and modes under different weather conditions. Haze weather process favoured the formation of the secondary aerosol, which contributed to the increase of fine mode particles.

© 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Beijing Institute of Technology. Keywords: Haze; Aerosol scattering coefficent; Aerosol optical thickness; Angstrom exponent; Beijing.

1. Introduction

Aerosol particles can scatter and absorb solar radiation and therefore lead to atmospheric opacity and horizontal visibility degradation. Haze is one of the atmospheric phenomena caused by the aerosol accumulation. Because haze is usually rich in toxic and hazardous substances, it can cause serious respiratory and cardiovascular diseases when entered the human body [1]. Moreover, haze has been found

* Corresponding author. Tel.: +86-010-68406174; fax: +86-010-68409374. E-mail address: fszhao1@gmail.com

1878-0296 © 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Beijing Institute of Technology.

doi: 10.1016/j.proenv.2013.04.054

exert an adverse effects on natural and agricultural ecosystems by reducing the intensity of solar radiation and sunshine hours [2]. Scientist throughout the world have carried out many experiments to understand the formation mechanism of haze[3-7].

Beijing is one of the most populous cities in the world. With the economic development and swift urbanization, the atmospheric pollution due to the aerosol emissions has become a matter of concern. In recent years, haze has been observed frequently in Beijing [8], which notoriously influenced the atmospheric visibility. During the haze process the aerosol properties and loading undergo dramatic changes and these aerosol characteristics are essential to assess the influence of aerosol on environment and human health. However, the researches on the haze pollution in Beijing are relatively limited.

Our study measured the surface solar radiation, aerosol scattering coefficient, spectral aerosol optical thickness, and the Angstrom exponent simultaneously before, during and after an intense haze event in October, 2010 in Beijing. The role of relative humidity on aerosol is analyzed by means of WRF-WW3 imagery. The results presented here concern the dramatic change of aerosol characteristics between hazy and clear weather conditions.

2. Site Description and Instrumentation

2.1. Site description

Beijing is a mega city with a population over 21 million in northeaster China (http://www.ebeijing.gov.cn/defalt.htm). The sampling site is located on the loft of an academic building in the campus of the Beijing Institute of technology (39.98 N, 116.38 E), which is in the northwestern urban area of Beijing. There are two major roads to the east (Zhongguancun South Avenue) and north (the third ring road), which are respectively around 50 m and 600 m from the sampling site. A relatively minor road, Weigongcun Road, lies to the south of the sample site, but also hosts heavy traffic, especially in peak hours. The local traffic emissions exert an immediate effect on the aerosol composition and optical properties.

2.2. Instrumentation and measurements

There were three main instruments deployed in the observation site, and the description for each instrument and the retrieval method of the aerosol parameters are briefly given as below.

2.2.1. CIMEL Sun photometer

The CIMEL Sun photometer automatically tracks the sun and makes direct Sun measurements every 15 minutes at wavelengths of 0.44, 0.67, 0.87, 0.94, and 1.02|nn. These solar extinction measurements are then used to derive aerosol optical depth at each wavelength except for the 0.94|xm channel, which is used to retrieve total precipitable water [9]. The instrument calibration is performed by comparing the direct solar irradiance measurements with those measured by a reference instrument calibrated by AERONET (AErosol RObotic NETwork) team, before and after our field experiments. Uncertainties are about 0.010.02 at wavelengths greater than 0.44^m [10].With aerosol optical depth data in specific wavelengths, the Angstrom law can be applied and Angstrom exponent can be calculated. The Angstrom exponent is inversely related to the average size of the particles. Therefore, Angstrom exponent is a useful quantity to assess the particle size of atmospheric aerosols and describe the aerosol modes (coarse/fine).

2.2.2. Kipp & Zonen CM11 pyranometer

The CM11 pyranometer was used to measure broadband downwelling shortwave fluxes in the 0.305-2.80-^ spectral range. It was calibrated by the Kipp & Zonen Company in September 2010. Measurement error of this high-precision pyranometer is estimated to be less than 8 Wm-2.

2.2.3. Ecotech M9003 integrating nephelometer

The Ecotech M9003 integrating nephelometer in our study was used to measure the light scattering coefficient due to particles. The light source is a very reliable and stable light emitting diode array, emitting Lambertian distribution of light at a fixed wavelength (520nm). The optical and electrical background noise is sufficiently low to allow measurements of aerosol scattering coefficient from less than 0.25Mm-1 to greater than 2000 Mm-1. Five-minute averaged data are stored internally and then retrieved every day using a PC in our measurements.

3. Results and discussions

From October 7 to 9, an intense haze invent occurred over Beijing with the degradation of visibility. Before the outburst of this haze, the sky was relatively clear on October 4 and turned obscure on October 5 and 6. After 3-day haze, on October 10, there was a heavy precipitation. On October 11 and 12, the visibility is rather good in the downtown area. In the next few paragraphs, we will present the change of the solar radiation and aerosol characteristics from October 4 to 12(except October 10 when rare data retrievals were available). The reason caused these variations will also be carefully discussed.

3.1. Surface Solar Irradiance

As shown in Fig. 1(a), surface solar irradiance values on the non-haze days were generally larger than those measured under the haze weather condition. The value at 12:00 on October 5 was 323.42 Wm-2 higher than that on October 9 (Beijing time), indicating very strong aerosol extinction during the haze episode. After the torrential rainfall on October 10, aerosol loading declined sharply. Because of the notable cloud contamination, trends fluctuated wildly on the October 11 and 12, as shown in Fig. 1(b). But it is still can be seen that the solar radiation was rather intense, with peak values near or higher than that on October 5.

3.2. Aerosol Scattering Coefficient

Aerosol scattering coefficient is the fraction of light scattering per unit distance in particles .Standard unit for the scattering coefficient is Mm-1. In order to study the variation of aerosol scattering coefficient before, in and after the haze, hourly averaged results are calculated and shown in Fig. 2. This graph unfolds a clear change of aerosol coefficient in different days. On October 4, the value was rather low, and peak value was around 458Mm-1. On October 5, the value started to increase and from October 6 to 9, the scattering coefficients have been very high with some fluctuations. The mean value during the haze is about 1590.06Mm-1. When the haze weather was over, on October 11, the value of coefficient dropped sharply to around 30Mm-1.

In order to analyze this trend of aerosol scattering coefficient, the distribution of relative humidity on these 8 days from WRF-WW3 imagery were employed in our study. The eight graphs (Fig. 3(a) to 3(h) provided clear change of relative humidity in Beijing. On October 4, the relative humidity was rather low,

around 20% to 30%, while on October 9 it reached to more than 80%. On October 11 and 12, the values returned back to less than 40%.

Time Time

Fig. 1. (a) Surface solar irradiance measured by CM11 pyranometer from October 4 to 9, 2010; (b) Surface solar irradiance

measured by CM11 pyranometer on October 11 and 12.

10/4 10/5 10/6 10/7 10/8 10/9 10/11 10/12 Date(Month/Day)

Fig. 2. Hourly mean aerosol scattering coefficient from October 4 to October 12 (except October 10).

Atmospheric oxidation of SO2 and NOX, two important ingredients from traffic, occur by both heterogeneous and homogeneous paths and oxidation rate is increased with increasing relative humidity through paths involving OH production[11,12]. The enhanced conversion ratio by relative humidity is found in both sulphate and nitrate [11]. Higher humidity allows more water amount to be carried by the hygroscopic component, resulting in increased scattering coefficient [13]. Tang [14] calculated the light scattering coefficients as a function of relative humidity for nitrate and sulphate, using the extensive optical and thermodynamic properties involved. He found the scattering coefficient increased with humidity for individual water-soluble salts as well as mixed ones. So the values of aerosol scattering coefficient were much larger with higher relative humidity during the haze process in Beijing, where experiences heavy sulphate and nitrate emission.

Fig. 3. Distribution of relative humidity from October 4 to October 12 (a to h) by WRF-WW3 imagery (except October 10).

3.3. Aerosol Optical Thickness and Angstrom Exponent

The distribution of aerosol optical thickness and Angstrom exponent are presented in Fig. 4 and Fig. 5 Under the haze weather, the aerosol loading varied considerably compared with the clear days. According to Fig. 4, on October 4 and 5, most of the aerosol optical thicknesses are less than 0.5. October 6, as a transitional day, witnessed the value ranged from 0.6 to 2.2. From October 7 to 9, aerosol accumulated notably and all aerosol optical thickness are larger than 2.4. After the haze, aerosols loading decreased sharply and low value of aerosol optical thickness were observed on October 11 and 12.

Fig. 4. Aerosol optical thickness distribution from October 5 to October 12(a to h), 2010 (except October 10).

There are also significant differences among the particulate size in different days. Especially for those days after the rainfall on October 10, most of the Angstrom exponent values were less than 1.2. While on October 9, 62% of measured value ranged from 1.4 to 1.5, which meant relatively fine mode particles on that day. During the haze episode, the amount of secondary aerosol such as sulphate and nitrate aerosol increased in big leaps as described above. The stability of air masses also favoured the long-tern existence of the secondary aerosols. The diameters of these kinds of inorganic salts are usually not more than 1|xm [15]. These small particles contributed to the relatively big value of Angstrom exponent.

Fig. 5. Angstrom exponent distribution from October 5 to October 12(a to h), 2010 (except October 10).

4. Conclusions

From October 7 to October 9, 2010, an intense haze invent occurred in Beijing. The aerosol properties and loading as well as particulate composition varied considerably during this period. The surface solar irradiance decreased sharply in hazy days because the strong light extinction by aerosols. Compared with non-hazy days, the value of aerosol scattering coefficient are much larger under haze weather. The mean value on October 11 is only 1/50 of that measured on October 9. By employing the WRF-WW3 imagery, the role of relative humidity on aerosol scattering coefficient was analyzed. The scattering properties increased with relative humidity notably because the accumulation of water-soluble salts.

The distributions of aerosol optical thickness and Angstrom exponent in hazy days were rather different from those in clear days. All the values of aerosol optical thickness in October 9 were above 3.6 while on October 11 the values below 0.3 accounted for about 98%. The torrential rainfall on October 10 seeped away most particles. The hazy weather favoured the formation and build-up of the secondary aerosol with small diameters, which lead to the higher value of Angstrom exponent. The values on October 9 focused on the range from 1.4 to 1.5, while most of the Angstrom exponent values are less than 1.2 on October 11.

Acknowledgements

Dr. Zhang Yinchao (Beijing Institute of Technology, Beijing, 100101, China) is kindly thanked for providing the experimental platforms. This work was supported by the National Science Foundation of China (40637035).

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