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China implemented a one-month lockdown after the 2020 Spring Festival to prevent the spread of COVID-19. The closure measures provide a rare opportunity to understand the resulting changes in air pollution levels and to test the effectiveness of previous environmental protection measures. We used the time series decomposition method to quantify the air pollution in Lanzhou during the closure period. The results showed that during the epidemic lockdown period, although the concentration of SO2 in Lanzhou decreased substantially, there was a significant increase in the concentration of O3 (by 19.14%), followed by a gradual return to the normal level. Most of the changes during the COVID-19 lockdown were within the range of fluctuations over the past five years. The trend of decreasing SO2 and CO in 2020 was less than that during 2015–2019, and the continuous decline of the PM10 concentration exceeded expectations. NO2, PM2.5 and O3 maintained the trend of the previous five years. Our results show that temporary social closure measures have a limited effect on improving air quality in Lanzhou, and they emphasize the importance of reducing the O3 concentration in the future.

During the winter half-year (previous October–April), the mid-latitude westerlies flows around the Tibetan Plateau (TP) and generate a dynamic low-pressure trough on its south side and a high-pressure ridge on its north side. In this study, we define the vorticity perturbation as the difference between local vorticity and the meridionally-averaged vorticity. Then, the difference of averaged vorticity perturbation at 600 hPa in the two key areas where the trough and ridge are located is used to represent the intensity of the flows around the Tibetan Plateau (FAT). The evolution characteristics of FAT in the winter half-year, as well as in autumn, winter and spring, are analyzed. Moreover, under global warming, in winter the relationship of FAT to the precipitation and temperature in China are discussed. The results show that FAT steadily exists on both sides of TP during the winter half-year. With the north-south migration of the mid-latitude westerlies, the FAT gradually strengthens in autumn, with the strongest intensity and the widest range in winter, and begins to weaken in spring. The intensity of FAT (IFAT) has a decreasing trend and a quasi-4a period variation on the interannual scale in all the time, both of which are closely related to the mid-latitude westerlies upstream of TP. Represented by the winter when IFAT is strongest, the IFAT is significantly correlated to the change of precipitation and temperature in most parts of China under global warming, especially in the convergence area over central-eastern China. There is also a significant correlation between IFAT and the precipitation and temperature anomalies in Northwest China, TP, and Northeast China. This relationship between the FAT anomaly and the climate anomaly in China in winter can be well explained by analyzing the anomalies of large-scale circulation, outgoing longwave radiation and water vapor flux divergence. The FAT anomaly maybe one of the reasons for the climate extreme events in China in winter.