  Human migration during the Chinese Spring Festival (SF) is the largest
collective human activity of its kind in the modern era-involving about
one-tenth of the world population and over six percent of the earth's land
surface area. The festival results in a drop of air pollutant emissions that
causes dramatic changes of atmospheric composition over China's most polluted
regions. Based on satellite and in-situ measurements for the years 2005-2019
over 50 cities in eastern China, we find that the atmospheric NO2 pollution
dropped by ~40% during the SF week, and fine particulate matter (PM2.5)
decreased by ~30% in the following week, reflecting the effectiveness of
precursor emission controls on the mitigation of secondary PM2.5 formation.
However, although human activity and emissions are at the lowest level, air
pollution over eastern China during the SF still far exceeds that over other
worldwide pollution hotspots. Our analyses suggest that measures based solely
on end-of-pipe controls and industry upgrades may not suffice to meet air
quality goals. Further cleaning of the air in China depends fundamentally on
sustainable advances in both heavy industry upgrades and clean energy
transition.
