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

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.