Fig. 1. Illustration of the changes in urban and rural RTC emissions and PM_2.5 concentrations due to population migration.

Distributions of RTC emissions and PM_2.5 concentrations before (A) and after (B) migration. Compared with rural areas, urban areas are associated with lower per-capita RTC emissions due to cleaner RTC energy mix used by the urban population. A consequence of the population migration is the decrease in rural population density, which leads to decreases in RTC emissions and PM_2.5 concentration in rural areas. In contrast, migration-induced change in urban concentration is a competition between increased urban emissions and declined background concentrations contributed by decreased rural emissions. In addition, migration to cities causes more people to be exposed to polluted urban air. The overall change in population exposure concentration and premature deaths across China due to migration is the consequence of the concurrent changes in both concentrations and population distributions.