1 Intercontinental Transport of Anthropogenic and Biomass Burning Pollution Qinbin Li Department of Earth and Planetary Sciences Harvard University March 2003

2 Statement of problem Intercontinental transport of anthropogenic and biomass burning pollution, particularly ozone, could have important impact on global atmospheric chemistry and regional air quality that needs to be better understood and quantified. 1.Middle East ozone maximum. [Li et al., GRL, 2001] 2.Springtime ozone maximum at Bermuda. [Li et al., JGR, 2002b] 3.Export efficiency of NOy out of continental boundary layer. [ Li et al., JGR, 2003b ] 4.Transatlantic transport of pollution. [ Li et al., 2002a ] 5.Atmospheric budgets of biomass burning tracers HCN and CH 3 CN. [ Li et al., GRL, 2000; Li et al., JGR, 2003a ]

3 Approach GEOS-CHEM global 3-D model simulation: Tagged tracers (CO, ozone) Sensitivity simulation Tracer correlations NARE TRACE-P ASIANORTH AMERICA EUROPENORTH PACIFIC NORTH ATLANTIC Midlatitude westerly MIDDLE EAST Tropical easterly MOZAIC Mace Head Sable Island Bermuda Sonde

4 GEOS-CHEM and limited observations indicate an ozone maximum over the Middle East What is the origin of this ozone maximum? circles: ozonesonde/MOZAIC contours: GEOS-CHEM (July 1997)

5 Origin of the Middle East ozone maximum GEOS-CHEM, July 1997 Combination of three factors: anticyclonic circulation in the middle/upper troposphere with large-scale subsidence over the Middle East. lightning outflow from the India monsoon and pollution from China transported in an easterly tropical jet. northern midlatitude pollution transported in the westerly midlatitude jet. arrows: ozone transport flux contours: ozone production rate

6 Recent confirmation of the Middle East ozone maximum: SAGE II tropospheric ozone observation Kar et al. [2002] Climatological ozone mixing ratio at 7 km from SAGE II ( , ) JulyOctober

7 Interpretation of the springtime ozone maximum at Bermuda GEOS-CHEM reproduces the observed seasonal cycle of surface ozone at Bermuda. ? Stratospheric [Oltmans & Levy II, 1992,1994; Moody et al., 1995] ? Anthropogenic [Dickerson et al., 1995] ?

8 Li et al. [2002] Most of surface ozone at Bermuda in spring originates from North American (outflow behind cold fronts); stratosphere contributes less than 5 ppb. Observations are from S. Oltmans Transport of North American pollution to Bermuda in spring r = 0.82, bias = -1.8 ppb Source attribution in the model

9 Previous argument for stratospheric origin of ozone at Bermuda from back-trajectory analyses Oltmans and Levy II [1994]: “On days with high ozone … the trajectories all come from north of 50°N and altitudes near 600 mb.” Moody et al. [1995]: “High-ozone events are associated with high- speed subsident flow of North American continental origin.” Moody et al. [1995]

10 GEOS-CHEM reproduces the association of high ozone at Bermuda with subsiding trajectories from NW Continental ozone pollution mixes with subsiding air behind cold fronts N. America Cold front Ozone pollution Subsiding air Bermuda March 18, 1996 event: 290 K back-trajectory

11 North Atlantic Regional Experiment (NARE’97): NOy export efficiency (f) out of continental boundary layer Lagrangian mixing model: f = (∆NOy/∆CO) R α f : NOy export efficiency ∆NOy, ∆CO: enhancement over background R: anthropogenic emission ratio CO/NOy α: natural CO enhancement in CBL Lagrangian: along NARE’97 flight tracks f = 9%, NOx/NOy = 8% [Parrish et al., 2003] Eulerian: NOy export flux out of CBL f = 30%, NOx/NOy = 34% [Liang et al., 1998]

12 NOy export efficiency (f) out of the North American boundary layer: NOy-CO correlations along the NARE’97 flight tracks Parrish et al. [2003]this work NOy export efficiency (f) 9±5% (→ 17±13%)11.5±3% (→ 17±7%) as NOx 8%6±4% as PAN 34%36±13% as HNO 3 57%52±14% Curves: relationships expected from the mixing model for different values of the export efficiency of NOy (f).

13 Liang et al. [1998] f = 30%,NOx/NOy = 34% Eulerian Parrish et al. [2003] f = 9±5%, NOx/NOy = 8% Lagrangian, (1) f = 14.5±11% Lagrangian, (1) but CO = 95 ppb f =17±13% Lagrangian, (2) but NOy = 0.1 ppb This work f = 11.5±3% Lagrangian, (1) f = 17±7% Lagrangian, (2) f = 17±6%, NOx/NOy = 6±4% Lagrangian, (2) but model ∆CO, ∆NOy f = 20%, NOx/NOy = 39% Eulerian NOy export efficiency (f): Reconciling Eulerian and Lagrangian analyses (1) background CO =75 ppb, NOy = 0.1 ppb; R = 5.67; α = 1.18 [Parrish et al., 2003] (2) background CO =95 ppb, NOy = 0.3 ppb; R = 6.50; α = 1 [GEOS-CHEM]

14 Ozone production due to exported North American anthropogenic NOy Half the ozone production takes place in near-field driven by exported NOx; the other half is due to exported PAN over NH. The eventual ozone production due to exported NOy is comparable to direct export of ozone pollution. GOES-CHEM, September 1997

15 April July Transatlantic transport of North American pollution: Simulated concentrations and fluxes of North American pollution ozone GEOS-CHEM 1997 L L H H

16 Surface ozone at Mace Head, Ireland: North American pollution signal is there but faint GEOS-CHEMN. America pollution events in the model Time series, Mar-Aug 1997 Model vs. observed stats, Li et al. [2002] Observation

17 Effect of North American sources on violations of European air quality standard (55 ppbv, 8-h average) GEOS-CHEM, JJA 1997 # of violation days (out of 92) # of violation days that would not have been in absence of North American emissions

18 North Atlantic Oscillation (NAO) Index North American ozone pollution enhancement at Mace Head, Ireland (GEOS-CHEM) r = 0.57 NAOI: normalized surface pressure anomaly between Iceland and Azores Transport of North American pollution to Europe: Correlation with the NAO Index Greenhouse warming  NAO index shift  change in transatlantic transport of pollution

19 GEOS-CHEM, JJA 1997 North America Europe Asia Intercontinental transport of pollution: Surface ozone enhancements caused by anthropogenic emissions from different continents

20 Zhao et al. [2000] Atmospheric HCN: Tracer for long-range transport of biomass burning pollution? Conventional view: source: biomass burning [Lobert, 1990] sink: reaction with OH lifetime: 2-5 years well mixed: pptv [Cicerone and Zellner, 1983] Recent observations indicate a much shorter lifetime (less than a year) – missing sink? Rinsland et al. [1998]

21 Proposed atmospheric budget for HCN (atmospheric lifetime = months) HCN(aq)/CN - HCN Biomass burning Tg N yr -1 Ocean uptake Tg N yr -1 (saturation < 0.85) Tropopause Henry’s law constant (298 K) = 8-12 M atm -1 pKa(HCN(aq)/CN - ) = 9.2 HCN + OH 0.2 Tg N yr km HCN + OH 0.1 Tg N yr -1 HCN + O( 1 D) < 0.01 Tg N yr -1 HCN + hν < 0.01 Tg N yr -1 k > 0.2 yr -1

22 TRACE-P observations of background (CO<120 ppb, C 2 Cl 4 <10 ppt) HCN and CH 3 CN: A dominant ocean uptake sink GEOS-CHEM, Feb-Apr 2002 Deposition velocity: 0.13 cm s -1 Saturation ratio: 0.79 for HCN, 0.88 for CH 3 CN Model reproduces the vertical gradients between MBL and FT.

23 TRACE-P observations of HCN-CH 3 CN-CO HCN CO CH 3 CN GEOS-CHEM Feb-Apr 2002 Elevated HCN in Chinese urban plumes. Relatively small enhancements of CH 3 CN in Chinese urban plumes.

24 HCNCH 3 CN Atmospheric burden (Tg N) Atmospheric lifetime (months) Tropospheric burden (Tg N) Tropospheric lifetime (months) Sources (Tg N yr -1 ) Biomass burning Residential coal burning Sinks (Tg N yr -1 ) Ocean uptake Reaction with OH CH 3 CN is a better tracer for biomass burning. Atmospheric budgets of HCN and CH 3 CN GEOS-CHEM 2002

25 Summary of results 1.Middle East ozone maximum is attributed to lightning outflow from India and pollution from China transported in a tropical easterly jet [Li et al., GRL, 2001]. 2.The springtime ozone maximum at Bermuda is attributed to boundary layer outflow of North American pollution behind cold fronts, not to stratospheric sources [Li et al., JGR, 2002b]. 3.Export efficiency of NOy out of continental boundary layer from Eulerian and Lagrangian approaches are consistent (~20%). Ozone production due to exported NOy is comparable to direct export of ozone pollution [Li et al., JGR, 2003b]. 4.Transatlantic transport of pollution: North American anthropogenic emissions enhance surface ozone in Europe by 2-5 ppb on average in summer which is important for European air quality standard. The NAO index is a predictor for transatlantic transport of North American pollution [Li et al., JGR, 2002a]. 5.Atmospheric budgets of HCN and CH 3 CN: ocean uptake is a dominant sink for both HCN and CH 3 CN; CH 3 CN is a better biomass burning tracer [Li et al., GRL, 2000; Li et al., JGR, 2003a].