Calculating ambient aerosol surface area concentrations using aerosol light scattering enhancement measurements

Abstract

Aerosol surface area concentration (SA) is crucial for studying atmospheric chemical reactions happened in aerosol water or on aerosol surface. However, there is no commercial instrument that can provide direct measurements of ambient SA. In this paper, we propose a method to calculate ambient SA based only on measurements of a three-wavelength humidified nephelometer system, which measures aerosol optical properties at three wavelengths under dry state and different relative humidity (RH) conditions. Two critical steps are required in this method: (1) Calculating surface area concentration of ambient aerosols in dry state, SA(dry), using a trained random forest machine learning model based only on optical properties measured by the “dry” nephelometer. The proposed machine learning method is evaluated with particle number size distributions (PNSD) datasets from eight field campaigns conducted on the North China Plain during different seasons. The square of correlation coefficients between predicted and calculated SA(dry) for PNSD is about 0.99, the average ratio between predicted and calculated SA(dry) is 1.01 and 70% of data points has a relative difference less than 10%. (2) Calculating the surface area growth factor fS(RH) of ambient aerosol particles due to water uptake using the proposed fS(RH) parameterization scheme fS(RH)=(1+κSRH100−RH)23, and the hygroscopicity parameter κS can be calculated using measured aerosol light scattering enhancement factor and Ångstro¨m exponent. The ambient SA values during two field campaigns which are conducted on the North China Plain (Wangdu campaign and Gucheng campaign, in summer and winter respectively) are calculated using the proposed method. The ambient SA ranges from 42 to 1871 μm2/cm3 with an average of 319 μm2/cm3 during Wangdu campaign, and ranges from 19 to 4156 μm2/cm3 with an average of 788 μm2/cm3 during Gucheng campaign. Drastic daily variations of ambient SA are observed during these two campaigns. The results demonstrate that aerosol hygroscopic growth impacts significantly on variations in ambient SA especially under high RH conditions. The fS(RH) ranges from near 1 to 4 with an average of 1.4 during Wangdu campaign, and ranges from near 1 to 2.7 with an average of 1.3 during Gucheng campaign. The results indicate that the larger RH, the more sensitive fS(RH) becomes to variations in κS which highlights that real-time measurements of aerosol hygroscopicity are required for accurate calculations of ambient SA. The advantage of the proposed method is that the ambient SA can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating real-time measurements of ambient SA and promoting studies in aerosol heterogeneous reactions.