Intercontinental Transport of Anthropogenic and Biomass Burning

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					    Intercontinental Transport of
Anthropogenic and Biomass Burning

                     Qinbin Li
     Department of Earth and Planetary Sciences
                Harvard University

                    March 2003

                  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
    CH3CN. [Li et al., GRL, 2000; Li et al., JGR, 2003a]

                GEOS-CHEM global 3-D model simulation:
                • Tagged tracers (CO, ozone)
                • Sensitivity simulation
                • Tracer correlations
    Tropical                      Midlatitude



                                            Sable             Mace
                                            Island Bermuda    Head

MIDDLE   ASIA         NORTH             NORTH       NORTH       EUROPE
 EAST                 PACIFIC          AMERICA     ATLANTIC

GEOS-CHEM and limited observations indicate an
    ozone maximum over the Middle East

circles: ozonesonde/MOZAIC   contours: GEOS-CHEM (July 1997)

          What is the origin of this ozone maximum?

       Origin of the Middle East ozone maximum
arrows: ozone transport flux                          GEOS-CHEM, July 1997
contours: ozone production rate

 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.
  Recent confirmation of the Middle East ozone
maximum: SAGE II tropospheric ozone observation

              July                           October

    Climatological ozone mixing ratio at 7 km from SAGE II
                      (1985-90, 1994-99)
                                             Kar et al. [2002]

 Interpretation of the springtime ozone
         maximum at Bermuda

     ? Stratospheric [Oltmans & Levy II, 1992,1994; Moody et al., 1995]
     ? Anthropogenic [Dickerson et al., 1995]

GEOS-CHEM reproduces the observed seasonal cycle
of surface ozone at Bermuda.

Transport of North American pollution to Bermuda
                    in spring

                                                               are from
                                                               S. Oltmans

          r = 0.82, bias = -1.8 ppb

                            Source attribution
                              in the model

Most of surface ozone at Bermuda in spring originates from North American
  (outflow behind cold fronts); stratosphere contributes less than 5 ppb.

                                                             Li et al. [2002]
   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]

GEOS-CHEM reproduces the association of high ozone
  at Bermuda with subsiding trajectories from NW

                                                  March 18, 1996 event:
                                                  290 K back-trajectory

                                                 air           Cold front

                                                  Ozone pollution

                                                 N. America       Bermuda

  Continental ozone pollution mixes with subsiding air behind cold fronts

 North Atlantic Regional Experiment (NARE’97): NOy
 export efficiency (f) out of continental boundary layer

 Eulerian: NOy export             Lagrangian: along
   flux out of CBL               NARE’97 flight tracks

f = 30%, NOx/NOy = 34%           f = 9%, NOx/NOy = 8%
    [Liang et al., 1998]           [Parrish et al., 2003]

                                 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
NOy export efficiency (f) out of the North American
      boundary layer: NOy-CO correlations
        along the NARE’97 flight tracks

                                                       Curves: relationships
                                                       expected from the
                                                       mixing model for
                                                       different values of
                                                       the export efficiency
                                                       of NOy (f).

                              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 HNO3                      57%                  52±14%

              NOy export efficiency (f):
    Reconciling Eulerian and Lagrangian analyses

 Liang et al. [1998]              This work                      Parrish et al. [2003]

f = 30%,NOx/NOy = 34%             f = 11.5±3%                f = 9±5%, NOx/NOy = 8%
        Eulerian                 Lagrangian, (1)                      Lagrangian, (1)

                                   f = 17±7%                           f = 14.5±11%
                                 Lagrangian, (2)              Lagrangian, (1) but CO = 95 ppb

                        f = 17±6%, NOx/NOy = 6±4%                       f =17±13%
                       Lagrangian, (2) but model ∆CO, ∆NOy    Lagrangian, (2) but NOy = 0.1 ppb

                          f = 20%, NOx/NOy = 39%

  (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]

       Ozone production due to exported
       North American anthropogenic NOy
                                           GOES-CHEM, September 1997

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.
 Transatlantic transport of North American pollution:
Simulated concentrations and fluxes of North American pollution ozone

                                                   GEOS-CHEM 1997





         Surface ozone at Mace Head, Ireland:
    North American pollution signal is there but faint
          Time series, Mar-Aug 1997                    Model vs. observed
                                                        stats, 1993-1997
    Observation    GEOS-CHEM    N. America pollution
                                events in the model

Li et al. [2002]
 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

  Transport of North American pollution to Europe:
          Correlation with the NAO Index
  NAOI: normalized surface pressure anomaly between Iceland and Azores
               North American ozone pollution enhancement
               at Mace Head, Ireland (GEOS-CHEM)
               North Atlantic Oscillation (NAO) Index

                                     r = 0.57

Greenhouse warming a NAO index shift a          change in transatlantic
                                                transport of pollution
Intercontinental transport of pollution: Surface ozone
 enhancements caused by anthropogenic emissions
              from different continents
                                    GEOS-CHEM, JJA 1997

                                    North America



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: 150-170 pptv
 [Cicerone and Zellner, 1983]

                                                        Rinsland et al. [1998]

                                         Recent observations indicate a
                                         much shorter lifetime (less than
                                         a year) – missing sink?

                  Zhao et al. [2000]
         Proposed atmospheric budget for HCN
               (atmospheric lifetime = 2 - 4 months)
 26 km
                 HCN + hν          < 0.01 Tg N yr-1
                 HCN + O(1D)       < 0.01 Tg N yr-1
                 HCN + OH           0.1 Tg N yr-1
                 HCN + OH           0.2 Tg N yr-1


                                                    Ocean uptake
                                                    1.1-2.6 Tg N yr-1
                                                     (saturation < 0.85)
      Biomass burning                                     k > 0.2 yr-1
      1.4-2.9 Tg N yr-1                       HCN(aq)/CN-
                               Henry’s law constant (298 K) = 8-12 M atm-1
                               pKa(HCN(aq)/CN-) = 9.2
    TRACE-P observations of background
 (CO<120 ppb, C2Cl4<10 ppt) HCN and CH3CN:
       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 CH3CN
Model reproduces the vertical gradients between MBL and FT.
       TRACE-P observations of HCN-CH3CN-CO
Feb-Apr 2002                       HCN
                                    Elevated HCN
                                    in Chinese
                                    urban plumes.

                                    Relatively small
                                    of CH3CN in
                                    Chinese urban


       Atmospheric budgets of HCN and CH3CN
                                                  GEOS-CHEM 2002

                                  HCN     CH3CN
Atmospheric burden (Tg N)         0.426    0.28
Atmospheric lifetime (months)      6.2     6.7
Tropospheric burden (Tg N)        0.38     0.25
Tropospheric lifetime (months)     5.3     5.8
Sources (Tg N yr-1)
       Biomass burning            0.63     0.47
       Residential coal burning   0.20     0.03
                                                   CH3CN is a better
Sinks (Tg N yr-1)
                                                   tracer for biomass
       Ocean uptake               0.73     0.36    burning.
       Reaction with OH           0.10     0.14

                         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 CH3CN: ocean uptake is a dominant
  sink for both HCN and CH3CN; CH3CN is a better biomass burning tracer [Li et al.,
  GRL, 2000; Li et al., JGR, 2003a].