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

Abstract

Size-resolved hygroscopic growth factors of urban aerosol during a haze episode were measured using a Humidified Tandem Differential Mobility Analyzer (HTDMA) (gm(RH)). These factors were also derived from size-resolved particulate chemical composition combined with the κ-Köhler theory (gκ(RH)) and the thermodynamic model ISORROPIA-II running in forward mode (giso-f(RH)) and reverse mode (giso-r(RH)), respectively. In terms of agreement among these hygroscopic growth factors, gκ(RH) matched gm(RH) best, followed by giso-r(RH). In contrast, giso-f(RH) demonstrated a poorer agreement with gm(RH). The good consistency among gm(RH), gκ(RH), and giso-r(RH) was because they only focus on the physical hygroscopic process, whereas giso-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 gκ(RH) and giso-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%, gκ(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, gκ(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.