Four-dimensional variational assimilation for SO2 emission and its application around the COVID-19 lockdown in the spring 2020 over China. - Hu, Yiwen; Zang, Zengliang; Ma, Xiaoyan; Li, Yi; Liang, Yanfei; You, Wei; Pan, Xiaobin; Li, Zhijin (Atmospheric Chemistry & Physics, 2022)