Influence of relative humidity on real-time measurements of particulate matter concentration via light scattering

Highlights

• The angular-scattering method is suitable for PM mass-concentration measurement under wet conditions.

• In wet aerosol with no condensation, the water vapour promotes the agglomeration of aerosol particles, and the agglomeration increases with the relative humidity.

• Water absorption and agglomeration of particles in wet aerosols are the main factors affecting the light scattering method.

Abstract

As the particulate-matter (PM) emission standards in China become increasingly rigorous (their upper limit has decreased to 10 mg/m³ in the power industry), considerable attention has been paid to PM emissions from coal-fired power plants. However, there is a large amount of water vapour in flue gas from power plants, which poses a great challenge to the measurement of the PM mass concentration via light scattering. Although water vapour does not affect the light scattering, changes in the properties of particles caused by water vapour influence it. To investigate the effects of the aerosol relative humidity on the PM mass-concentration measurement via light scattering, a test rig was constructed for wet aerosol light-scattering measurements. Light-scattering and hygroscopic properties, as well as morphological features of fly ash aerosol and pure mineral aerosol (pure powder silica) under different relative-humidity conditions were compared in a laboratory. The linear least-squares method was used to fit the relationship between the scattered light intensity and the PM mass concentration under four different humidity conditions. With increase in relative humidity, the R-squared values and the slopes of the fitted line were 0.9941, 0.9941, 0.9926, and 0.9854 and 35.14, 35.90, 28.59, and 26.91, respectively. Thus, the intensity of light scattering was directly proportional to the PM mass concentration, and the mass sensitivity, i.e. the slope of the fitted curve, changed with respect to relative humidity. From low to high relative-humidity conditions, the enhancement factors at a scattering angle of 20° were 0.868 ± 0.045, 0.814 ± 0.053, and 0.706 ± 0.029 for powder silica and 0.905 ± 0.010, 0.918 ± 0.015, and 0.984 ± 0.019 for fly ash. Thus, humidity had a greater influence on the mass-concentration measurement of powder silica aerosol via light scattering than on that of fly ash aerosol. Scanning electron microscopy results for aerosol particles under different humidity conditions indicated that the agglomeration characteristics of these particles increased with the relative humidity. Considering previous reports on the response characteristics of the hygroscopic particles with respect to their optical properties to relative humidity in the atmosphere, we found that the hygroscopic growth and agglomeration of aerosol particles together determine the measurement accuracy, which is very useful for onsite measurements of the PM mass concentration via the light-scattering method.

Introduction

Particulate matter (PM) with an aerodynamic diameter of <2.5 μm, i.e. PM_2.5, is one of the most important components of atmospheric pollutants and is harmful to human health because it has hazardous components that can easily enter the human body (Z. Chen et al., 2013; M. Xu, Yu, Yao, Liu, & Qiao, 2011). Fossil-fuel combustion is one of the major emission sources of PM_2.5. In China, PM emission from coal-fired power plants has attracted considerable attention owing to the tremendous coal consumption (Gao, Beig, Song, Zhang, & Mcelroy, 2018; Xu, Liu, & Wang, 2019; Y.; Xu et al., 2018). Monitoring of the PM emission concentration is indispensable in PM emission reduction projects. The light-scattering method—one type of PM concentration measurement technique (Boothroyd, Jones, Nicholson, & Wood, 1987; Gajewski, 1999; Gu, Su, & Cai, 2018; Takahashi, Minoura, & Sakamoto, 2008)—has been investigated owing to its excellent characteristics for real-time measurement of PM concentrations (Kelly et al., 2017; Sousan et al., 2016).

When a particle is exposed to light, it is not absorbed in its entirety by the particle; some of it is deflected, reflected, and refracted. These phenomena can be categorized under “light scattering”. The scattered light intensity of particles depends on their physical characteristics, such as size, shape, and refractive index (Mishchenko, Travis, & Lacis, 2002). Based on the relationship between the particle size and the incident wavelength, the scattered light intensity of particles can be calculated using different methods. As the dimensionless particle-size parameter (the ratio of the particle size to the incident wavelength) increases, Rayleigh scattering, Mie scattering, and Fraunhofer diffraction are used successively (Hulst, 1981; Jones, 1999). Kerker, Farone, Smith, and Matijević (1964) emphasised that the scattered light intensity of the particle clouds can be obtained via linear superposition of single particles at a very low concentration, where the distance between the particles is more than three times the particle diameter. Therefore, the scattered light intensity of aerosol (particle clouds) is proportional to the total amount of particles, and the PM concentration of aerosol can be determined using the scattered light intensity (Bindelle et al., 2007; Nagy, Szymanski, Gál, Golczewski, & Czitrovszky, 2007). The most important parameter in the process of mass-concentration inversion is the ratio of the scattered light intensity to the PM mass concentration, which was defined as the mass sensitivity by Smith, Baron, and Murdock (1987). Dick, McMurry, and Bottiger (1994) measured the scattered light from particles with eight sizes ranging from 0.14 to 0.96 μm, and the results indicated that the intensity of the scattered light varied significantly with respect to the particle size. For aerosol with different average sizes, Roebuck, Vaughan, and Chung (1990) directly measured the mass sensitivity using the Optical Scattering Instantaneous Respirable Dust Indicating System and found that particle size had a considerable impact on the light-scattering measurement results. Regarding the complex refractive index of the particle, a larger real part of the particle refractive index can result in a higher mass sensitivity, according to the results of Görner, Bemer, and Fabriès (1995) and Dick et al. (1994) Recently, our team measured the angular-scattering mass sensitivity of quartz particles with four different particle-size distributions (PSDs) in a laboratory. The results indicated that the effect of the PSD on mass sensitivity depended on the detection angle and that ~25° might be a good choice for different PSDs (D. Chen et al., 2018). Additionally, extensive research has been performed on the effects of relative humidity on aerosol light scattering in the atmosphere. Previous studies indicated that the response characteristics of atmospheric aerosols to the relative humidity are significantly affected by the aerosol composition and hygroscopicity (P. Zieger et al., 2015; P. Zieger, Fierz-Schmidhauser, Weingartner, & Baltensperger, 2013). However, few studies have reported the impact of flue gas relative humidity on PM emission measurement via the optical method in coal-fired power plants. Coal combustion and the use of wet flue gas cleaning equipment (Wet Flue Gas Desulfurization and Wet Electrostatic Precipitator) increase the relative humidity of flue gas (Peng et al., 2019). Under wet conditions, the properties of aerosols may change, and the scattered light signal may be distorted under the same emission concentration. The existing measuring instrument is equipped with a flue gas drying system, but the introduction of the drying system increases the overall complexity of the system. Additionally, the measurement error increases, owing to the system instability and loss of volatile particles. The only purpose of the drying system is to reduce relative humidity and prevent the condensation of water vapour; however the effect of humidity on aerosol measurements persists. Currently, the influence of the relative humidity on PM concentration measurements via the light-scattering method is unclear, along with the mechanism of its impact. Investigating the influence of the relative humidity on the real-time measurement of the PM concentration via light scattering is of great significance for field measurement applications.

Thus, in this study, a real-time test rig for PM mass-concentration measurement was designed, which can control the aerosol relative humidity and change the detection angle automatically. The fly ash mentioned here is produced by coal combustion containing a large amount of silica-alumina minerals and minor sodium, potassium, calcium and iron, which has differences from pure minerals (Li et al., 2016). Thus, fly ash from the 4# electrostatic precipitator of coal-fired power plants and pure powder silica were selected as samples for comparative measurements in this study. Scanning electron microscopy (SEM) was performed on particles collected in situ under different relative-humidity conditions, and calculations were conducted. Then, the effect of the aerosol relative humidity on the PM mass-concentration measurement and the influence mechanism were analysed.