On the severe East Asian dust outbreak in March 2021: from atmospheric dynamics to air quality impact

The Gobi and the Taklamakan Desert are the two main dust source regions in East Asia. Extra-tropical cyclones are known as atmospheric driver for severe dust outbreaks in East Asia. Although previous studies show that dust storm frequency in Northern China have decreased associated with a decrease in near-surface wind speed, a severe dust storm occurred in mid-March 2021. This exceptional dust storm was driven by a Mongolian extra-tropical cyclone and had adverse socio-economic and health impacts. The aim of our study is to investigate the atmospheric dynamics, dust-aerosol contributions from the Gobi Desert and the Taklamakan Desert, as well as dust emission mechanisms involved in the event. We use ground-based observations from Chinese observational networks, satellite images from MODIS, data from ERA5 re-analysis, CAMS forecasts, and MERRA-2 re-analysis.

The passage of the Mongolian cyclone first induced high dust-emitting winds along its cold front. The maximum wind speeds at 10m a.g.l. over the Gobi Desert exceeded the 99th percentile of the 30-year climatology (1992-2021) for March by around 6 ms⁻¹ . The dust aerosols were emitted by these exceptionally strong near-surface winds and transported southeastwards along with the passage of the frontal system of the Mongolian cyclone from the afternoon of 14th March to the morning of 15th March 2021. Hence, high atmospheric PM₁₀ concentrations were first recorded in Northern China on 15th March. As a consequence of the associated poor air quality caused by the high PM₁₀ concentrations, 19 out of 218 stations recorded the lowest atmospheric visibility for March since the past 30 years.

The passage of the Mongolian cyclone then led to a cold air intrusion into the Taklamakan Desert from the afternoon of the 15th onward, which was a few hours after the dust emissions in the Gobi Desert. The cold air intrusion with the associated near-surface temperature inversion was favourable for the formation of Nocturnal Low-Level Jets (NLLJs), which are known as an important mechanism for dust emissions in the dust source regions (e.g., East Asia and North Africa). By comparing the NLLJs from radiosonde observations and an automated detection algorithm applied to ERA5, stronger NLLJs were seen in the Taklamakan Desert in the mornings of 16, 17, and 18 March. The NLLJs breakdown during the morning hours led to sufficiently strong dust-emitting winds in this desert. Consequently, dust emissions are simulated in the mornings of 16, 17, and 18 March 2021 by both CAMS forecasts and MERRA-2 re-analysis. The impacts of the dust aerosols from the Taklamakan Desert were, however, limited to the west of China, supported by spatio-temporal distributions of station observations of the atmospheric PM₁₀ concentrations and visibility.