“…Of the above‐mentioned assimilation approaches, the 3D‐Var method, which minimizes a cost function depending on the error covariance matrix, is the most computationally efficient (Feng et al., 2018; Z. Jiang et al., 2013; Z. Li et al., 2013). Frequent and long‐lasting air pollution has long been recognized as one of the top environmental challenges by both governments and academia due to its widespread, severe impact on climate change and human health (An et al., 2019; Chang et al., 2020; Che et al., 2020; G. Wang et al., 2016). The spatiotemporal distribution of air pollutants is a rich source of information for policymaking (Cai et al., 2017; Zhang, Fung, Lau, et al., 2020; Zhang, Fung, Zhang, et al., 2020), health benefit calculation (Chang et al., 2020; R. Wu et al., 2019; Xue, Zhu, et al., 2019), and chemical mechanism‐driven simulation (Glasius & Goldstein, 2016; Jathar et al., 2017).…”