A comparative analysis of aerosol properties in dust and haze-fog days in a Chinese urban region

A comparative study on the aerosol optical properties of two severe pollution phenomena occurred in Beijing, i.e. dust and haze-fog, was analyzed using solar and sky radiance measurements from 2001 to 2007. The aerosol optical depth (AOD) showed a distinct variation under different weather conditions, with an average 1.70 in dust days and 1.26 in haze-fog days. The values of Ångström exponent (α) in dust days were significantly lower than those of haze-fog days, with an average of 0.48 in dust days and 1.11 in haze-fog days. The fine modes of volume size distributions showed the maxima peak at radius 0.09–0.25 μm in dust days and radius 0.11–0.25 μm in haze-fog days. The coarse modes showed the maxima peak at radius 2.2–2.9 μm in dust days, and radius 2.2–3.8 μm in haze-fog days. The size distributions showed a distinct difference in dominant mode for the different weather conditions. For haze-fog days, the fine mode was dominant in the aerosol size distribution. However, the coarse mode was dominant in the aerosol size distribution of dust days with the average volume concentration ratio of coarse to fine modes being 8.3. The averages of single scattering albedo (SSA) were found to be about 0.92 for dust days and 0.89 for haze-fog days at 440, 675, 870 and 1020 nm. In comparison with dust days and haze-fog days, the growth in SSA was due to the addition of amount of dust particles. In view of climate, the asymmetry factor at wavelengths 440–1020 nm were about 0.70 for dust days and 0.65 for haze-fog days in Beijing. The scattering phase functions of dust days at forward and backward directions were commonly larger than those of haze-fog days, with values of 381.18 at 0° and 0.23 at 180° for dust days, and lower values of 86.48 at 0° and 0.20 at 180° for haze-fog days.