Dr. Jun Wang

Diurnal Variability of Dust Aerosol Optical Thickness and Angstrom Exponent over Dust Source Regions in China

Collaborated with Drs. X. Xia and P. Wang in the Institute of Atmospheric Physics, Chinese Academy of Sciences, We recently wrote a paper on the dirunal variability of dust optical properties over the dust source regions in China. The paper has been accepted for publication in Geophys. Res. Lett. Here is the reference of the paper:

Wang, J., X. Xia, P.Wang, and S. A. Christopher, Diurnal Variability of Dust Aerosol Optical Thickness and Angstrom Exponent over Dust Source Regions in China, Geophys. Res. Lett., 31, 10.1029/2003GL019580, 2004. (phf file)

The following are some brief descriptions and highlights.

1. Major finding

Using 22 months of Aerosol Optical Thickness (AOT) data collected from a Sunphotometer located near the Takalamakan and Gobi dust source regions (Dunhung, 40.09°N, 94.41°E) in 1999 and 2000, we examine the diurnal and seasonal change of dust AOT and Angstrom exponent.   Most dust events are during the Spring through early Summer months and our analysis shows a season-invariant diurnal change of more than ±10% for AOT and ±30% for Angstrom exponent, with larger AOT and smaller Angstrom exponent values late in the afternoon.  Our results are different and complimentary to recent studies [e.g., Smirnov et al., 2002; Kaufman et al., 2000] that have reported a much smaller (±5%) diurnal variability of dust AOT over various AERONET sites where dust is a major contributor to AOT, but lacked the long-term observation data in the Chinese dust source regions.  Large diurnal changes of AOT and Angstrom exponent may be significant enough to be carefully considered in radiative forcing, air quality and numerical modeling studies.

2. Why do we concern about the dust optical properties, especially in East Asian dust source region

The effect of aerosols on climate is one of the largest uncertainties in current global climate models [Hansen et al., 1997].  Current understanding of the radiative forcing of dust aerosols is limited, especially over dust source regions where ground observations are sparse and polar orbiting, multi-spectral satellite retrievals at visible to near-infrared wavelengths are often difficult due to the high surface albedo  [Kaufman et al., 2002].  While several studies and field experiments have been conducted to study Saharan dust aerosols [Tanré et al., 2003] , the widely prevalent dust events (“yellow sand”) from the Taklamakan and Gobi deserts in Northwest China, have only gained attention recently [e.g. Husar et al., 2001].   In addition, previous experiments such as ACE-Asia [Hubert et al., 2003] examined aerosol properties  thousands of kilometers downwind from the Taklamakan and Gobi deserts.  To our knowledge, no long-term or mid-term systematic observations of dust radiative properties in the dust source region over China have been presented.   

Using AOT data from the Aerosol Robotic Network (AERONET), recent studies have indicated that the diurnal variability of dust AOT is small (<5%) over various observation sites such as Cape Verde [Kaufman et al., 2000; Smirnov et al., 2002] that is about 500km downwind from the Saharan dust source region .  On the other hand, several cases studies from both ground observations [e.g., Levin et al., 1980] and geostationary satellite retrievals [e.g., Wang et al., 2003] indicate that the diurnal variation of dust aerosols is relatively larger (>15%) and are important for dust radiative forcing calculations [Christopher et al., 2003].  Our current study is different from previous research since we use Sunphotometer inferred aerosol optical thickness (SP AOT) data collected near the Chinese dust source regions over a longer time period to analyze the diurnal and seasonal variations of dust AOT and Angstrom exponent.

3. What makes our observation site and this study unique?

Nearly two-year (1999 and 2000, except October and December 2000) worthy of AOT inferred from a Sunphotometer (Model Pom-01, Prede Inc.) located at Dunhuang airport (40.09°N, 94.41°E), was used to study the wavelength-dependent optical and temporal characteristics of dust aerosols.  The observation site is located at the eastern edge of the Takalamakan desert (35°N~40°N, 80°E~90°E), southern edge of the Gobi desert (about 40°N~45°N, 90°E~110°E), and the western edge of Hexi corridor in Gansu province  (Figure 1).  Due to the existence of Gobi desert and Takalamakan along 40°N, most Chinese dust storms are located in the horizontal zone (35°N~45°N, 80°E~110°E) and this zone becomes the major dust sources in the east Asian region [Sun et al., 2001].  The zone is bounded by different high mountains with elevation usually higher than 3000m (e.g., Himalayan Plateau to the south, the Hindu Kush mountains to the west, and the Tian Shan mountains to the north).  Most time, the lofted dust aerosols in this zone are transported eastward, affecting eastern part of China and its vicinity countries (such as Japan and Korea).   However, in some time,  the cold high pressure from Siberia could entrain the Takalamakan dust (and sometime also small portion of Gobi desert) westward to an elevation larger than 5000m, and then further northward to the 50°N where ultimately dust would be transported by the westerly just stream to the North Pacific Ocean [Sun et al., 2001].  In either case, Dunhuang is affected by dust storms.  The elevation (above the sea level) of Dunhuang city is about 1400 meters, the annual relative humidity is about 40%, and annual precipitation is less than 16mm.  Such geographical and meteorological characteristics of the Dunhuang observation site are different when compared with Cape Verde (16.72°N, 22.93°W) that is commonly used to study Saharan dust properties.  

4. Please find the analysis results in our paper.