Observing the impact of Asia dust aerosols on aridity

Dust aerosols play an important role in the global climate by scattering and absorbing solar and terrestrial radiation, thereby influencing the atmosphere's radiation balance. Asian dust often originates from the Taklimakan and Gobi deserts in late winter and spring. This dust is transported long distances by the prevailing westerlies, passing over northeastern Asia and the Pacific Ocean and reaching North America. During transport, dust aerosols not only reduce local air quality and affect humans, but also act as ice nuclei (IN) and cloud condensation nuclei (CCN), thus changing the radiative properties of clouds, ice water paths, cloud lifetimes, and precipitation.¹

To improve understanding and capture direct evidence of the impact of dust aerosols on semi-arid climate over Loess Plateau, the Semi-Arid Climate & Environment Observatory of Lanzhou University and Lidar network was established in 2005.² Extensive studies concerning Asian dust and anthropogenic aerosols were conducted during several field experiments. Our group has studied the dust aerosols' semi-direct effect on cloud properties over East Asia by using ground and satellite measurements. We found that the water path of dust-contaminated clouds is considerably smaller than that of dust-free clouds. The mean ice water path (IWP) and liquid water path (LWP) of dusty clouds are less than their dust-free counterparts by 23.7% and 49.8%, respectively. The long-term statistical relationship derived from International Satellite Cloud Climatology Project (ISCCP) data confirms that there is a significant negative correlation between the dust storm index, defined as the number of days when dust storms occur each month, and the ISCCP cloud water path. Those studies show some evidence of the semi-direct effect of Asian dust aerosols on cloud properties.¹ Both local anthropogenic dust aerosols and natural dust aerosols transported into a region can significantly reduce the water cloud particle size, optical depth, and LWP.³

Figure 1. The feedback mechanism of precipitation and dust storms.

Cloud droplets form on CCN and reduce the water vapor saturation barrier necessary for droplets to form. Higher aerosol concentrations result in more cloud-droplet embryos competing for available water vapor. Changes in the number and size distribution of cloud droplets create thermodynamic and microphysical feedbacks that have the potential to change cloud evolution and properties. A significant feature of dust-cloud-precipitation interactions over arid and semi-arid areas is that they create a positive feedback loop that begins with a rainfall decrease and a resulting soil moisture deficit (see Figure 1). This leads to an increased occurrence of dust storms. Consequently, dust aerosols in the atmosphere warm clouds, increase the evaporation of cloud droplets, and further reduce the cloud water path (the semi-direct effect). This decreases the low cloud cover and water vapor amount, leading to less rainfall. The occurrence of dust storms would then increase, which could lead to even less rainfall.

The consequences of human activities, such as over cultivation and overgrazing, include drastic reductions in vegetative cover and soil destabilization. Loss of vegetative cover makes the predominantly sandy soils vulnerable to wind and water erosion. Another aspect of human influence on dust activity is land desertification due to water diversion to farmland. The consequences of diminished vegetation cover over drylands may exacerbate the effects of drought, thus leading to a further decline in vegetation cover. This aggravates a general climatic trend toward increasing aridity and may initiate local climatic change.³

If dust aerosols are transported to wet regions (e.g., East Asia and Pacific regions) and suspended in the atmosphere, they may serve as a source of IN which can enhance ice formation by droplet nucleation and intensify precipitation. However, dust aerosol effects and associated feedbacks that modulate cloud properties, rain formation, and cloud lifetimes are still the subject of much debate. As we further our studies, more field experiments are needed. Further research should focus on measurements of physical processes of dust aerosol-cloud interactions.