Why did air quality experience little improvement during the COVID-19 lockdown in megacities, northeast China?

To inhibit the COVID-19 (Coronavirus disease 2019) outbreak, unprecedented nationwide lockdowns were implemented in China in early 2020, resulting in a marked reduction of anthropogenic emissions. However, reasons for the insignificant improvement in air quality in megacities of northeast China, including Shenyang, Changchun, Jilin, Harbin, and Daqing, were scarcely reported. We assessed the influences of meteorological conditions and changes in emissions on air quality in the five megacities during the COVID-19 lockdown (February 2020) using the WRF-CMAQ model. Modeling results indicated that meteorology contributed a 14.7% increment in Air Quality Index (AQI) averaged over the five megacities, thus, the local unfavorable meteorology was one of the causes to yield little improved air quality. In terms of emission changes, the increase in residential emissions (+15%) accompanied by declining industry emissions (-15%) and transportation (-90%) emissions resulted in a slight AQI decrease of 3.1%, demonstrating the decrease in emissions associated with the lockdown were largely offset by the increment in residential emissions. Also, residential emissions contributed 42.3% to PM2.5 concentration on average based on the Integrated Source Apportionment tool. These results demonstrated the key role residential emissions played in determining air quality. The findings of this study provide a scenario that helps make appropriate emission mitigation measures for improving air quality in this part of China.

[Difference of rhizosphere microbe quantity and functional diversity among three flue-cured tobacco cultivars with different resistance].

Field experiments were conducted in Shilin and Xundian respectively to study the diffe- rence of rhizosphere microbe quantity and functional diversity with plate culture method and Biolog technique among Hongda (high susceptibility, S), Yun87 (middle resistance, MR) and K326 (high resistance, R) , three flue-cured tobacco cultivars with different resistance to bacterial wilt and black shank. The results indicated that the amounts of bacteria, actinomycetes and the total number of microbes in tobacco plants' rhizosphere were positively correlated with the cultivar' s re- sistance, while it was opposite for the fungi. The consistent tendency was obtained not only at 35 d, 55 d and 75 d after transplanting, but also at two experimental sites. Cultivar and experimental con- ditions greatly affected the utilization of six types of carbon source by rhizospheric microbes, as well as the AWCD value. In Xundian site, rhizospheric microbes' utilization of carbohydrates, amino acids, carboxylic acids, polymers,. amines and the AWCD value were all higher at 55 d and 75 d after transplanting for the resistant cultivar than the susceptible one, but it was opposite at 75 d after transplanting for the phenolic acids. In Shilin, significant differences existed among the three culti- vars at 35 d, 55 d and 75 d after transplanting for the indices mentioned above, although they were not consistent with cultivars' resistance. Principal component (PC) analysis even showed that utili- zation of carbon sources by rhizosphere microorganisms differed significantly among the three culti- vars at the two sites, and it was better reflected by PC1 and PC2 at 55 d and 75 d after transplan- ting, respectively. In conclusion, rhizosphere microbial community structure and functional diversity were greatly affected not only by the cultivars' difference in resistance, but also by experimental conditions.