  The Chinese "coal-to-gas" strategy aims at reducing coal consumption and
related air pollution by promoting the use of clean and low carbon fuels in
northern China. Here we show that on top of meteorological influences, these
measures achieved an average decrease of fine particulate matter (PM2.5)
concentrations of ~14% during winter 2017 in Beijing and surrounding areas (the
"2+26" pilot cities). However, the localized air quality improvement was
accompanied by a contemporaneous ~15% upsurge of PM2.5 concentrations over
large areas in southern China. We find that the pollution transfer that
resulted from a shift in emissions was caused by a natural gas shortage in the
south due to the "coal-to-gas" transition in the north. The overall shortage of
natural gas greatly jeopardized the air quality benefits of the "coal-to-gas"
strategy in winter 2017 and reflects structural challenges and potential
threats in China's clean energy transition.
