Hygroscopic property of water-soluble organic-enriched aerosols in Ulaanbaatar, Mongolia during the cold winter of 2007

The hygroscopic properties of the water-soluble matter extracts of atmospheric aerosols collected at an urban site (47.92° N, 106.90° E, ∼1300 m above sea level) in Ulaanbaatar, Mongolia during the cold winter of 2007 were investigated using a hygroscopicity tandem differential mobility analyzer (H-TDMA). Dynamic shape correction factor (χ), the ratio of the actual drag force on a non-spherical aerosol particle to that on a sphere of equivalent volume, of the laboratory generated water-soluble matter (WSM) was found to be 1.09–1.38 (avg. 1.23 ± 0.10), implying that particles generated from the WSM are highly non-spherical. The reduction in the mobility diameter can cause ∼11% underestimation in a hygroscopic growth factor at 85% RH. The hygroscopic growth factors at 85% RH (g(85%)), defined as the ratio of the particle diameter at 85% RH to that at RH < 10% (initial dry diameter), of the WSM (initial dry particle diameter = 100 nm) were 1.32–1.50 (avg. 1.40 ± 0.06). The g(RH) of the water-soluble organic matter (WSOM) was retrieved from the measured g(RH) of the WSM and using the ZSR (Zdanovskii–Stokes–Robinson) approach and the thermodynamic aerosol inorganic model (AIM). We found that the g(85%) of the WSOM were in the range of 1.11–1.35 (avg. 1.22 ± 0.08), which are comparable to those of the biomass burning aerosols.

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► Mobility diameter shows substantial reduction at 20–40% RH.
► Laboratory generated particles are highly non-spherical shape.
► Hygroscopic growth factor of water-soluble organic aerosols is 1.22 ± 0.08 at 85% RH.
► These growth factors are comparable to biomass burning aerosol and aged organics.