Modeling particle number concentrations and sources in the Beijing and Shanghai metropolitan areas with the UCD/CIT model - Yanhong Zhu; Jianjiong Mao; Kangjia Gong; Xiaodong Xie; Qiong Liao; Zelong Wang; Huaiqiu Xiao; Hongguang Liao; Jianlin Hu (Journal of Environmental Sciences, 2026)