Climatology of aerosol optical properties in northern China

The climatology of aerosol optical properties in northern China were examined from the ground-based sun and sky radiance measurements at six sites during 2001–2005. Cases of high aerosol optical thickness (AOT) mostly attributed to the occurrence of dust events in spring and the impacts of pollutant particles in summer. The Alfa parameter decreased with AOT even zero or negative in appearance of dust events. The relationship between Alfa and AOT at sites of downwind regions was characterized by one integrated exponent during summer and autumn and by two integrated exponents during spring and winter. But for the sites near dust source regions, only one integrated exponent was enough in any season. This is a possible way to identify and estimate the effects of different sources on aerosol loading and particle size. The aerosol volume size distributions had three modes, one accumulation mode with radius 0.07–0.15 μm and two coarse modes with radii 1.3–1.7 and 3.8–5.0 μm. As AOT larger than 0.5, a pseudo mode appeared between the accumulation mode and two coarse modes with radius 0.3–0.5 μm at sites of dust source regions. The average effective radii during winter and spring were commonly larger than that of summer and autumn. Real parts of the refractive index at 440–1020 nm varied with AOT in range of 1.40–1.54 at sites of dust source regions and in range of 1.45–1.55 at sites of downwind regions. Imaginary parts of the refractive index appeared low sensitivity to wavelengths from 440 to 1020 nm. The averages of imaginary parts during winter and spring were 0.0015–0.0018 for 440–1020 nm at DH and IM sites, and 0.009–0.014 during winter and spring and 0.011–0.058 during summer and autumn at other sites.