Fig. 5. Migration-induced spatial changes in PM2.5 emissions and concentrations in China.
Spatial changes in (A) annual primary PM2.5 emissions and (B) annual average PM2.5 concentrations (primary + secondary) in 2010 in mainland China caused by the population migration in the last 30 years. (A) The bubbles represent the changes in emissions within the urban areas of individual cities. The areas of the bubbles are proportional to emission increase. The color of the bubbles shows the change in urban emission density. Emission change in rural area is shown in the background. Because of population migration since 1980, emissions from RTC were spatially relocated. Rural emissions decreased pervasively while urban emissions increased. The national total emissions were also changed by migration because migrants got easier access to clean fuels and spontaneously shifted their energy mix after migrating into urban areas. (B) Migration-induced increase in urban emissions cannot compensate the extensive decrease in rural areas, leading to a reduction of PM2.5 concentrations across the east part of the country. There are some exceptions such as those four megacities. Serving as the destinations of massive migration, these cities showed an increase in PM2.5 concentrations due to increased local emissions by migration.