East Asia is suffering from a severe air pollution problem due to intensive emissions of anthropogenic pollutants and mineral dust aerosols. During transport, both Asian dust and pollutants undergo complex mixing processes, which result in great impacts on regional air quality, human health and climate. To characterize these mixing processes, we conducted long-term observations using an optical particle counter equipped with a polarization detection module (POPC) in the megacity of Beijing. Mass concentrations of PM 2.5 (aerodynamic diameters of less than 2.5 μm) and PM 10 derived from POPC compared well with ground-based measurements. Based on size-resolved polarization measurements, anthropogenic pollutants, mineral dust and mixed-type mineral dust (dust particle with pollutant coatings) were well distinguished. We found that the depolarization ratio (DR, termed as the ratio of the intensity of the s-polarized signal to the intensity of the backward scattering signal [s/(s + p)]) for aerosol particles with optical diameters of < 2.0 μm has a pronounced seasonal variation trend, with a winter–spring high and a summer low pattern. Aerosols that originated from Mongolia and western China normally demonstrate non-spherical shapes, with a DR centred on 0.3, whereas the value for fine-mode particles was usually less than 0.1 due to the predominance of secondary water-soluble matter. In Beijing, the morphology of mineral dust changed obviously, particularly in humid conditions. Higher relative humidity accelerated the liquid phase reaction and promoted water absorption due to the coating of hygroscopic soluble compounds on the dust surface. This study also found that 48 % of substandard air quality days featured high coarse-mode particles in the winter–spring time, which implies the great influence of heterogeneous processes on the surface of mineral dust on smog formation.