Measurement report: aerosol hygroscopic properties extended to 600 nm in the urban environment

Submicron particles larger than 300 nm dominate the aerosol light extinction and mass concentration in the atmosphere. The water uptake ability of this size range greatly influences the particle mass, visibility degradation, and particle chemistry. However, most previous field measurements on aerosol hygroscopicity are limited within 350 nm. In this study, the size-resolved aerosol hygroscopic properties over an extended size range (50-600 nm) at 85 % relative humidity were investigated in Beijing winter from 27 November 2019 to 14 January 2020 using a humidity tandem differential mobility analyzer (HTDMA) instrument. The corresponding aerosol optical properties were also analyzed using the Mie scattering theory. Results show that the averaged probability distribution of growth factor (GF-PDF) is generally constituted by a more hygroscopic (MH) group and a less hygroscopic (LH) group (including hydrophobic). For the particles larger than 300 nm, there exists a large fraction of LH group particles, resulting in an unexpected low hygroscopicity. During the development of pollution, when particles are gradually aged and accumulated, the bulk hygroscopicity above 300 nm is enhanced significantly by the growth and expansion of the MH group. This result is supported by previous chemical composition analysis, and we give more direct and detailed evidence from the growth factor and mixing-state aspects. Our calculations indicate that the optical contribution of particles larger than 300 nm constitutes about two-thirds of the total aerosol extinction. The large hygroscopic variation in aerosols above 300 nm will influence the light degradation comparably with the increase in aerosol loading in low-visibility haze events. Our studies highlight that the hygroscopic properties above 300 nm are complex and vary greatly with different pollution stages. Therefore more field measurements and investigations need to be done in the future.