1. Satellite-detection and attribution of rapid increases in tropospheric ozone made in China and its influence on the western United States Folkert Boersma Now also at A short story about ozone made in China Submitted Willem W. Verstraeten 1,2,3*, K. Folkert Boersma 1,2,3, Jason E. Williams 1, Jessica L. Neu 4, Kevin W. Bowman 4 and John R. Worden 4

2. Impact of (SRES-A2) emissions (in ppb) 2030 – 2000 Loss in life expectancy in months Source: EU programme CAFE, CAFE Scenario Analysis Report Nr. 2. (Amann et al., 2004) Szopa et al., 2004 Why do we care about ozone in the troposphere? I live here 1. Ozone is an air pollutant – also in Europe

3. Shindell et al., 2009 Why do we care about ozone in the troposphere? 2. Ozone is the 3 rd most powerful cause of positive radiative forcing O 3 RF: 0.40 W/m 2 CO 2 RF: 1.69 W/m 2 …but uncertainty in ozone forcing (±0.20 W/m 2 ) higher than for CO 2 and CH 4

4. Why do we care about ozone in the troposphere? 3. Ozone drives the oxidative capacity of the atmosphere O 3 + hv  O 2 + O( 1 D) O( 1 D) + H 2 O  2 OH OH is the “detergent” of the atmosphere, determining lifetime of CH 4, CO, and many other pollutants Need to understand ozone to improve our understanding of OH and possible trends therein Model-result: OH very difficult to measure!

5. How is ozone formed in the troposphere? Lifetime tropospheric O 3 : a few weeks STE

6. IPCC AR5-reported trends in tropospheric ozone IPCC conclusions Ozone increases over Europe and eastern USA have levelled off Ozone has likely increased over eastern Asia since the 1990s

7. Ozone increases over western United States Compile and analyse large dataset (1984-2008) of ozone measurements over the western United States Ozonesondes, lidar, aircraft measurements Cooper et al., 2008 EU air quality standard

8. Unclear where that ozone came from High ozone tends to originate from deep within Asia LOW OZONE CONDITIONS HIGH OZONE CONDITIONS Ozone monitoring in the free troposphere since the 1970s is very limited. What can satellite instruments tell us?

9. Observes NO 2 in VIS Surface   Scattering by Earth surface and by atmosphere Satellite instruments: TES and OMI Observes ozone in thermal IR Surface ToTo I (T 0 ) blackbody radiation absorbing gas T1T1  I (T 1 ) Broadly sensitive to free tropospheric ozone Sensitive to NO 2 down to the surface

10. Vertical sensitivity 464 hPa averaging kernel: Broad sensitivity to free tropospheric ozone OMI: sensitivity down to surface

11. Validation and use of TES ozone retrievals Global map of summertime tropospheric ozone from TES (@464 hPa) Strong precursor emissions! + photochemical activity JJA 2005-2010 464 hPa

12. Validation and use of TES ozone retrievals Global validation with WOUDC ozone sondes for period of 6 years Insignificant trend in TES-sonde bias at midlatitudes TES can be safely used for temporal analysis Verstraeten et al., AMT, 2013

13. OMI tropospheric NO 2 retrievals Strong precursor emissions!

14. Increase in Summertime tropospheric O 3 and NO 2 observed by TES and OMI over China 2005-2006: May-August2009-2010: May-August2010-2005 Difference TES: Strong increase in FT ozone over and downwind of China OMI: Strong increase in BL NO2 pollution over China TES 464 hPa Increase of 1.1 ppbv/yr ozone Increase of 1.1 ppbv ozone/yr at 464 hPa

15. TES 3-9 km partial ozone column 2009-2010 Also increasing but not at a statistically significant pace Increase in tropospheric O 3 observed by TES over western United States

16. Interpretation of the increases with TM5 CTM Research Question 1: Can emissions explain the increase in FT O 3 over/downwind of China? 3-D global chemistry transport model 34 vertical layers (surface-0.1hPa), 3˚×2˚ ECMWF meteorological fields (ERA-interim) Emission inventories: Anthropogenic inventories: RETRO project + REAS inventory for Asian Lightning NOx emissions: ~5 Tg N/yr Biomass burning emissions: GFEDv2 Tropospheric chemistry: 42 species and > 60 reactions (CBM-IV) Reference: Huijnen et al., 2010. 2 TM5 simulations: 1.Invariable NO x emissions 2.OMI-constrained NO x emissions

17. OMI-constraints on NO x emissions (2005-2010) Update a priori emissions to follow OMI-observed trend Update for all grid cells (no smearing) Take into account non-linear sensitivities to chemical regime NO 2 New top-down emissions A priori emissions Change in year j relative to 2006 in NO 2 observed with OMI Sensitivity of NO 2 column to changing emissions

18. Taking into account chemical regime (β) Evaluate TM5 response to perturbing NO x emissions (Lamsal et al., 2009): β=1: linear local response between NO x and NO 2 column β>1: Rapid negative feedback on NO 2 via increased OH β<1: Accumulation of NO 2 in response to emissions JANUARY JULY Increasing emissions by +15% In winter: β<1 (long lifetime NO 2 ) In summer: β>1 (enhanced loss)

19. OMI-constrained NO x emissions (2010-2005) Clean-up of USA, Europe and Japan Strong increases in Asia

20. OMI-constrained NO x emissions over China (2010-2005) TM5 with OMI emissions captures OMI time series

21. Reproducing the TES-observed O 3 increase over China TM5 with updated OMI emissions better captures TES time series But substantial increase also in TM5 simulation with constant NOx emissions

22. Stratosphere-troposphere exchange (STE) over China Research Question 2: What is the role of stratosphere-troposphere exchange? Use TM5 Stratospheric ozone in TM5 is constrained by 30-year (assimilated) 14-satellite dataset of total ozone (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 ‘O3S’ O 3 increase due to STEOverall O 3 increase 2010-2005 difference China 2010-2005 difference China

23. Stratosphere-troposphere exchange (STE) Deseasonalized changes in tropospheric ozone Answers to Research Questions: STE and increasing NO x both explain the observed increase over China STE stronger effect From subtracting TM5 simulations 1 and 2 (same STE)

24. What is happening with O 3 over the western United States? TES 3-9 km partial ozone column 2009-2010 Also increasing but not at a statistically significant pace

25. Reduced NO x emissions … Substantial reductions in NO x emissions over the western USA (- 12% between 2005-2010) Overall O 3 increase O 3 increase due to STE …but also strong O 3 increase due to STE over the western USA STE associated with enhanced stratospheric circulation in response to vigorous El Nino & QBO in 2010 relative to 2005

26. 2005-2010 ozone changes over the western US Deseasonalized changes in tropospheric ozone: increase! TES indicates increase over western US In spite of local decreases in NO x STE masks out effects reductions in anthropogenic NO x emissions How much of the western US ozone increase originated from China?

27. Compare two 6-year TM5 simulations 2. TM5 with OMI-constrained NO x emissions throughout the world 3. As (2) but with constant REAS-2006 NO x emissions over China Difference between (2) and (3) indicative of the effect of increasing Chinese NO x emissions on western United States Change in Anthropogenic NO X emissions between 2010 and 2006

28. Δ O 3 due to increasing Chinese NO x emissions ΔO 3 between 2010 and 2005 at ~900 hPa (±1 km) Western US O 3 increases by +0.1 ppbv at ±1 km because of +15% Chinese NOx emission increases ΔO 3 between 2010 and 2005 at ~460 hPa (±6 km) Western US O 3 increases by +0.2 ppbv at ±6 km because of large-scale transport & sustained O 3 production driven by decomposition of nitrogen reservoirs in descending air masses

29. Δ O 3 due to increasing Chinese NO x emissions Deseasonalized timeseries 3-9 km correcting for STE and local effects FT (3-9 km) O 3 increases by 0.01 DU/yr (0.1%/yr) This import neutralizes one-third of the O 3 decreases related to reduction in surface emissions due to stricter air pollution policies.

30. Discussion Combined use of TES and OMI provides a new perspective on changing tropospheric composition in 2005-2010 TES observes a 10% increase in 6 years in FT O 3 over China Supported by TM5 when OMI-constrained NO x (+2%) and STE changes (+8%) are accounted for TES indicates a 4% increase in FT O 3 over western US Policy-driven NO x reductions alone would have reduced O 3 (-1%) over the western US in the absence of other effects STE (+5%) and import of ozone made in China (+0.5%) offset local policy measures

32. Support for increased STE over China throughout 2005-2010 Wang et al, 2012, ACP OZONE SONDES OVER BEIJING --- Sonde: influenced by photochemical production and STE --- Model (CLaMS): only STE