High time-resolution measurement of light scattering hygroscopic growth factor in Beijing: A novel method for high relative humidity conditions

The light scattering hygroscopic growth factor (f(RH)), which quantifies the hygroscopicity of polydisperse ambient aerosols at continuous relative humidity (RH) levels, is usually measured by a humidified nephelometer system. The hygroscopicity of aerosol will change evidently under high RH conditions (>90%). However, in previous studies, only the aerosol hygroscopicity below 90% RH was generally obtained and discussed due to the difficulty in creating a stable high RH and measuring it accurately. In view of these conditions, an improved high-resolution humidified nephelometer system was established to observe the f(RH) of PM2.5 for a wide RH range between 30 and 96% in an urban area of Beijing during three seasons (winter, summer, and autumn) in 2017. Two sensors were used to calculate the humidified RHs, which made the uncertainty in the f(RH) at a high RH much lower than that in previous studies (lower than 10% for the maximum value). It was found that the f(80%) at 525 nm of PM2.5 was evidently higher under polluted conditions and highly correlated with the fractions of all of the water-soluble ions. A two-parameter fitting equation was selected to fit the observed f(RH) data. The f(RH) data under polluted conditions were more uniform with higher fitting R2 values during the summer and autumn. The hygroscopicity of aerosols has probably increased compared with that in the previous study conducted in the NCP. The fitted curves of the seasonal f(RH) data showed a significant dependence on the wavelength and increased with increasing wavelength. The hygroscopicity of PM2.5 for RH > 90% was definitely lower than that for 80% < RH ≤ 90%. The hygroscopic growth of aerosols under high RH conditions can probably be overestimated by only using f(RH) data below a RH of 90%.