1. Willem W. Verstraeten 1, Jessica L. Neu 2, Jason E. Williams 1, Kevin W. Bowman 2, John R. Worden 2, K. Folkert Boersma 1,3 Rapid increases in tropospheric ozone levels over China: The view from AURA 1. KNMI, the Netherlands; 2. JPL, USA; 3. Wageningen University, the Netherlands A short story about ozone made in China

2. O 3 increased at many background locations in the 1990s. It continued mainly over Asia and flattened over Europe during the last decade. Situation in North America is mixed. Relatively long O 3 time series are available, but only for few background measurement locations (possible sampling bias). Satellite observations can bridge the gap thanks to both the good spatial as well as temporal coverage compared to ozone sondes, ground-based instruments, MOZAIC etc. Trends in tropospheric ozone Europe IPCC 2012 Asia and North America China W-US 1.08%/yr 0.56%/yr Satellite tropospheric O 3 Surface O 3

3. What did we do? Compiling short-term time series of tropospheric O 3 satellite observations for eastern China and western United States. Derive short-term O 3 trends over these regions. Attributing the sources of the observed O 3 trends using TM5 constrained with AURA data. Changes in tropospheric O 3 as observed from AURA-TES (Thermal Emission Spectrometer) might be due to Changes in anthropogenic emissions such as NO X : => AURA-OMI (Ozone Monitoring Instrument); AURA-MLS Variation in the stratospheric contribution to tropospheric O 3 => AURA-MLS (Microwave Limb Sounder); Transport of O 3 and its precursors => TM5 (3x2 lon/lat).

4. TES tropospheric O 3 between 2005 and 2010 DIFFERENCE 3-9 km partial O 3 column summer 2005-2006 3-9 km partial O 3 column summer 2009-2010

5. OMI tropospheric NO 2 between 2005 and 2010 China: +5.4%/yr W-US: -2.7%/yr NO 2 column summer 2005-2006 Bb DIFFERENCE NO 2 column summer 2009-2010

6. Attribution of anthropogenic emissions to tropospheric O 3 trends Constraining TM5 anthropogenic emission inventory using OMI NO 2 (mass balance approach, Lamsal et al, 2011). => Updated NO X emissions for 2005-2010 in TM5 O 3 trend explained by changes in anthropogenic NO X emissions Removal of the STE effect! Only effect on NO X update!

7. Stratospheric O 3 in TM5 is constrained by 30-year (assimilated) 14-satellite dataset of total O 3 (van der A. et al., ACP, 2010) Net stratosphere-troposphere exchange is governed by ECMWF meteo + MSR constraints STE can be tracked in TM5 with tagged tracer ‘O 3S ’ Stratosphere-troposphere exchange contributes to tropospheric O 3 trends

8. Computed STE trends may be large compared to observations due to the use of ECMWF data (Škerlak et al. 2014, ACP). MLS observations in the NH only show a small O 3 increase in the lower stratosphere (Neu et al., 2014 NGS). As a lower limit, TM5 STE is constrained by MLS by forcing the model at 200-250 hPa to MLS data. O 3 trend explained by changes in STE

9. The view from AURA: Overview of the sources explaining O 3 trends Verstraeten et al. (2015) Nature Geoscience

10. Asian transport of O 3 and precursors towards western US Difference in TM5 runs April-May 2010 O 3 at ~800 hPa (±1.5 km) O 3 at ~400 hPa (±6 km) Transport from Asia offsets 43% of the local efforts in W-USA! +0.03 DU yr -1 -0.04 DU yr -1 TM5_update – TM5_update_except_China_ref

11. What about the ozone levels in the lower troposphere near the surface? +0.35 ±0.16 ppbv yr -1 +0.29 ±0.14 ppbv yr -1 -0.40 ±0.18 ppbv yr -1 -0.73 ±0.27 ppbv yr -1 China W-USA

12. Effect of ozone pollution from China on the W- USA near the surface from TM5 0.05 ppbv on 45 ppbv = 0.1% yr -1 0.05 -------------- x 100 = 6.8% from China 0.73+0.05

13. Conclusions 2005-2010 tropospheric O 3 (3-9 km partial column) over China increased with 1.08% yr -1. OMI NO 2 constrains on TM5 increases the ability to reproduce the TES observed O 3 trend over China. 56% of the O 3 trend over China is explained by anthropogenic sources, and 44% can be attributed to STE. China offsets 43% of the domestic O 3 reductions in the western US for 3-9 km partial column. In the lower troposphere near the surface the offset is lower but substantial.

14. More info: Verstraeten et al., 2015, Nature Geoscience, 8, 690-695