Comparison of size-resolved hygroscopic growth factors of urban aerosol by different methods in Tianjin during a haze episode.

Size-resolved hygroscopic growth factors of urban aerosol during a haze episode were measured using a Humidified Tandem Differential Mobility Analyzer (HTDMA) (g _m (RH)). These factors were also derived from size-resolved particulate chemical composition combined with the κ-Köhler theory (g _κ (RH)) and the thermodynamic model ISORROPIA-II running in forward mode (g _iso-f (RH)) and reverse mode (g _iso-r (RH)), respectively. In terms of agreement among these hygroscopic growth factors, g _κ (RH) matched g _m (RH) best, followed by g _iso-r (RH). In contrast, g _iso-f (RH) demonstrated a poorer agreement with g _m (RH). The good consistency among g _m (RH), g _κ (RH), and g _iso-r (RH) was because they only focus on the physical hygroscopic process, whereas g _iso-f (RH) contains not only the direct influence of relative humidity (RH) on particle size but also the influence of gaseous precursor on the particle chemical composition, which indirectly affects the hygroscopicity of the particles. In this sense, size-resolved g _κ (RH) and g _iso-r (RH) in a wide size range are more adequate to investigate the impact of RH on light scattering and aerosol radiative forcing. At RH = 80%, g _κ (RH) for accumulation mode particles was 1.30-1.45 on polluted days and higher than that on clean days (1.2-1.3). Whereas on both polluted and clean days, g _κ (RH) of ultrafine and coarse mode particles were generally lower than 1.25. The strong hygroscopicity of accumulation mode particles observed on polluted days can deteriorate visibility due to their high extinction efficiency.
(Copyright © 2019. Published by Elsevier B.V.)