Reply to Li et al.: Insufficient evidence for the contribution of regional transport to severe haze formation in Beijing

Contrary to the finding by Guo et al. (1), Li et al. argue that regional transport of fine particulate matter (PM2.5) represents the major cause for severe haze formation in Beijing (2). We noticed that the letter by Li et al. contains numerous nonscientific and unsubstantiated arguments: for example, “Guo et al.’s conclusion…goes against a broadly held view” and “Our [Li et al.’s] conclusion…is consistent with…control measures.” The letter by Li et al. hinges on three sentences describing their methodology and results, but provides little detail to assess the validity of their conclusion.