The hygroscopic properties of mixed aerosol particles are crucial for the application of remote sensing products of aerosol optical parameters in the study of air quality and climate at multiple scales. In this study, the authors investigated aerosol optical properties as a function of relative humidity (RH) for two representative metropolises: Beijing and Hong Kong. In addition to the RH data, mass concentrations of PM10 (particulate matter up to 10 μm in diameter) and aerosol scattering extinction coefficient (σext) data were used. The relationship between the mass scattering extinction efficiency (MEE, defined as σext /PM10) and RH can be expressed by regression functions as f = 1.52x + 0.29 (r2 = 0.77), f = 1.42x + 1.53 (r2 = 0.58), f = 1.19x + 0.65 (r2 = 0.59), and f = 1.58x + 1.30 (r2 = 0.61) for spring, summer, autumn, and winter, respectively, in Beijing. Here, f represents MEE, x represents 1/(1?RH), and the coefficients of determination are presented in parentheses. Conversely, in Hong Kong, the corresponding functions are f = 1.98x ? 1.4 (r2 = 0.55), f = 1.32x ? 0.36 (r2 = 0.26), f = 1.87x ? 0.65 (r2 = 0.64), and f = 2.39x ? 1.47 (r2 = 0.72), respectively. The MEE values for Hong Kong at high RHs (RH > 70%) are higher than those for Beijing, except in summer; this suggests that aerosols in Hong Kong are more hygroscopic than those in Beijing for the other three seasons, but the aerosol hygroscopicity is similarly high in summer over both cities. This study describes the effects of moisture on aerosol scattering extinction coefficients and provides a potential method of studying atmospheric visibility and ground-level air quality using some of the optical remote sensing products of satellites.