2015-09-23: 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 last decade, with highly uncertain impacts on regional and global tropospheric ozone (O3). Ozonesonde measurements from two Asian sites show that tropospheric O3 has increased by 1 to 3% per year since 2000, which is thought to contribute to positive O3 trends observed at North America’s West Coast.
However, model estimates of the Asian contribution to North American O3 have not been well-constrained by observations. Here we interpret Aura satellite measurements of tropospheric O3 and its precursor NO2 along with its largest natural source, stratospheric O3, using the TM5 global chemistry-transport model. We show that tropospheric O3 over China has increased by ~7% from 2005-2010 in response to both a ~21% rise in Chinese emissions and increased downward transport of stratospheric O3 (Figure 1). Furthermore, we find that Chinese export of O3 and its precursors have offset ~43% of the 0.42 DU reduction in free-tropospheric O3 over the western United States that should have occurred during 2005-2010 via emissions reductions associated with federal, state, and local air quality policies (Figure 2). We conclude that global efforts may be required to address regional air quality and climate change.

Read the full article here.
Figure 1. 2005–2010 observed and simulated changes in monthly tropospheric partial O3 columns over eastern China and the western US. a–d, Eastern China. e–h, Western US. a,e, TES 3–9 km O3 time series (dashed black curve) and TM5 simulations (dashed red) with OMI-inferred anthropogenic NOX emissions. b–d,f–h, Short-term trends in deseasonalized O3 columns. b,f, TES deseasonalized time series (solid black line). c,g, 2005–2010 changes in the contribution of stratospheric O3 to the tropospheric column for TM5 (dark blue) and TM5MLS (bright blue). d,h, Effect of changing NOX emissions (OMI-inferred 2005–2010 NOX emissions run minus the constant year 2005 emissions run).
Figure 2. The effect of Chinese emissions and long-range transport on tropospheric O3 over the western US. a, Monthly deseasonalized difference between the global TM5 run with observationally constrained 2005–2010 anthropogenic NOX emissions and the TM5 run with constant 2005 NOX emissions for China and observationally constrained emissions everywhere else. b, Difference in O3 in the free troposphere (~460 hPa) between the two TM5 runs for April–May 2010 sampled at the Aura overpass time. c, As for b but showing the lower troposphere (~800 hPa).