Contrast in column-integrated aerosol optical properties during heating and non-heating seasons at Urumqi - Its causes and implications

Aerosol optical properties were retrieved from two years' worth of Sunphotometer measurements at Urumqi, an urban station in western China. Distinct seasonal variations of aerosol optical properties were revealed. During the heating season, mean aerosol optical depth at 550 nm (τa), Angstrom exponent calculated from aerosol optical depth at wavelength of 440 and 870 nm (α) as well as PM2.5 concentration were 0.58 ± 0.33, 1.11 ± 0.34 and 79.5 ± 69.6 μg m− 3, respectively, which contrasted their counterparts during the non-heating season of 0.32 ± 0.22, 0.79 ± 0.26, and 35.0 ± 20.1 μg m− 3. Seasonal variations of τa and PM2.5 at Urumqi contrasted with corresponding values in eastern China. Enhancement of τa was associated with fine mode radius (rf) exceeding 0.15 μm. Relative humidity frequently exceeded 80% during the heating season, which probably resulted in rf > 0.15 as a result of the hygroscopic growth under the humid environment. rf was larger than value assigned to the fine-mode dominated aerosol models used in the dark-target algorithm of the Moderate Resolution Spectroradiometer (MODIS). Annual mean single scattering albedo at 550 nm (ω) was 0.87 that was close to the value assigned to the absorption aerosol model in the MODIS algorithm. ω increased as τa increased, probably as a result of the growth of aerosol size. ω of dust aerosols at Urumqi was slightly lower than that in dust source and downwind regions. Substantial differences in aerosol optical and physical properties and their seasonal variation between western and eastern China require maintaining long-term ground based remote sensing aerosols, which would be expected to play an important role in studying aerosol's effects on weather, climate and atmospheric environment.