Impacts of pollution and dust aerosols on the atmospheric optical properties over a polluted rural area near Beijing city

It is well-known that the aerosols in the Huabei region of China are mainly composed of both natural desert dust and pollution components during springtime. However, to our knowledge, the impacts of these mixed aerosols on the optical properties of the atmosphere over the region have not been well quantified. In this study we analyze the data on the size distributions of aerosol masses and chemical composition, as well as aerosol scattering coefficients as a function of relative humidity measured at a rural site of Huabei near Beijing city during April-May 2006. Both mass and optical closure studies are performed using measured total masses, mineral elements, ions, EC, and OC concentrations and scattering coefficients. The aerosol water content is predicted with the aerosol thermodynamic model ISORROPIA II, constrained by the measured ion concentrations. The aerosol diameter hygroscopic growth factor g(RH = 90%) averaged for the period is estimated to be 1.33 for the particles in diameters of 0.65-1.1. μm. The aerosol optical hygroscopic growth factor f(RH = 80%) over the period is 1.47 ± 0.21 by measurement and 1.37 ± 0.18 by model prediction, respectively. Model analyses based on measurement data show that the pollution aerosols make a major contribution to the atmospheric extinction coefficient not only by increasing the amount of dry aerosols but also by increasing the hygroscopicity of the aerosols, the latter accounting for up to 30% of the total extinction coefficient.