Rapid increases in tropospheric ozone production and export from China

Rapid population growth and industrialization have driven substantial increases in Asian ozone precursor emissions over the past decade(1), with highly uncertain impacts on regional and global tropospheric ozone levels. According to ozonesonde measurements(2,3), tropospheric ozone concentrations at two Asian sites have increased by 1 to 3% per year since 2000, an increase thought to contribute to positive trends in the ozone levels observed at North America's West Coast(4,5). However, model estimates of the Asian contribution to North American ozone levels are not well-constrained by observations(6,7). Here we interpret Aura satellite measurements of tropospheric concentrations of ozone and its precursor NO2, along with its largest natural source, stratospheric ozone, using the TM5 global chemistry-transport model. We show that tropospheric ozone concentrations over China have increased by about 7% between 2005 and 2010 in response to two factors: a rise in Chinese emissions by about 21% and increased downward transport of stratospheric ozone. Furthermore, we find that transport from China of ozone and its precursors has offset about 43% of the 0.42 DU reduction in free-tropospheric ozone over the western United States that was expected between 2005 and 2010 as a result of emissions reductions associated with federal, state and local air quality policies. We conclude that global efforts may be required to address regional air quality and climate change.