Toward characterization of the aerosol optical properties over Loess Plateau of Northwestern China

Aerosol optical properties were obtained from a CIMEL sunphotometer of the Aerosol Robotic Network at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL). SACOL is located over the Loess Plateau of the Northwestern China. The observed data are analyzed for the period of August 2006-October 2008. We find that aerosol optical depths (AODs) have a pronounced annual cycle, with a maximum dust aerosol loading during the spring. The 2-year average values of AOD, Ångström exponent (α), and water vapor path (WVP) along with their standard deviation (in parenthesis) are 0.35 (0.21), 0.93 (0.34), and 0.77 cm (0.52 cm), respectively. The probability distributions of these quantities all have one modal value, which are 0.3, 1.1, and 0.5 cm, respectively. There is a notable feature in the relationship between daily averaged AOD and Ångström exponent: a wide range of α corresponding to moderate to low aerosol optical depths (<0.8). There is no significant correlation between daily averaged WVP and AOD. However the daily averaged Ångström exponent and WVP show a significant positive correlation, indicating that the smaller aerosol particles present when the WVP is large. Variations of the retrieved aerosol volume size distributions are mainly associated with the changes in the concentration of the coarse aerosol fraction. The geometric mean radii for the fine and coarse aerosols are 0.18 μm (±0.03 μm), and 2.53 μm (±0.25 μm), respectively. The spectral dependences of the single scattering albedos are different between the dusty and non-dusty conditions. In the presence of dust, the SSAs increase slightly with wavelength. When dust is not a major component, the corresponding values decrease with wavelength.

Research Highlights► The aerosol optical depths have a pronounced annual cycle, with a maximum dust aerosol loading during the spring over the Loess Plateau of Northwest China. ► The relationship between daily averaged WVP and AOD shows no significant correlation. But the daily averaged Ângström exponent and WVP present a significant positive correlation, indicating that smaller aerosol particles are present when the WVP is large. ► Although the semiarid regions between China and the USA have similar climatic conditions, but the multi-year mean AOD at 500 nm over the semiarid area of northwest China (∼0.35) is much higher than that over the USA (□0.10). ► The variations of aerosol volume size distributions at SACOL are largely due to the change in the concentration of the coarse aerosol fraction. There is an apparent bimodal lognormal volume size distribution for the whole period. ► The spectral dependencies of single scattering albedos are different between the dusty and the non-dusty conditions. In the presence of dust, the SSAs increase with wavelength. When dust is not a major component, the corresponding values decrease with wavelength.