Observations of an Atmospheric Chemical Equator and its Implications for the Tropical Warm Pool Region

1
Observations of an Atmospheric Chemical Equator
and its Implications for the Tropical Warm Pool
Region

• Jacqueline F. Hamilton1, Grant Allen2, Nicola M.
  Watson1, James D. Lee1, Julie E. Saxton1,
  Alastair C. Lewis1, Geraint Vaughan2, Keith N.
  Bower2, Michael J. Flynn2, Jonathon Crosier2,
  Glenn D. Carver3, Neil R.P. Harris3, Robert J.
  Parker4, John J. Remedios4, Nigel A.D. Richards5
• 1Department of Chemistry, University of York,
  Heslington, York, YO10 5DD, UK.
• 2School of Earth, Atmospheric and Environmental
  Science, Sackville St Building, Sackville St,
  University of Manchester, Manchester, M60 1QD,
  UK.
• 3Chemistry Department, University of Cambridge,
  Cambridge, CB2 1TN, UK.
• 4Earth Observation Science, Space Research
  Centre, Department of Physics Astronomy,
  University of Leicester, University Road,
  Leicester, LE1 7RH, UK.
• 5Institute for Atmospheric Science, School of
  Earth and Environment, University of Leeds,
  Woodhouse Lane, Leeds, LS2 9JT, UK.
• J. Geophys. Res., 113, D20313, doi10.1029/2008JD0
  09940, (2008).

2
Overview

• Flight tracks
• Meteorology
• Results
• Chemical characteristics
• Trajectory analysis and biomass burning
• Comparison with Satellite and Model data
• Conclusions
• Acknowledgements

3

• 2 measurement periods
• Pre-monsoon (October December 2005)
• Monsoonal (January March 2006)
• Monsoon period was composed of a number of
  different meteorological conditions
• Active monsoon
• Inactive monsoon
• Break Period with intense Hector storms over
  Tiwi islands

Strong Westerly wind in Darwin, inhibited local
convection Flew north looking for the boundary
between northern and southern hemispheric air.
4
Introduce the generic term Chemical Equator to
describe a defined boundary between tropospheric
air of northern and southern hemispheric origin

• Generally associated with the Inter-Tropical
  Convergence Zone (ITCZ)
• ITCZ is a low pressure region circling the globe
  where the trade winds associated with the Hadley
  circulation in NH and SH meet
• Characterised by rapid vertical uplift and heavy
  rainfall
• Provide a meteorological barrier to cross
  equatorial flow in the troposphere exchange
  times around 6 months

5
Previous Studies of Chemical Equators

• There have been a number of studies of the
  characteristics on either side of the chemical
  equators associated with the ITCZ using aircraft
• Chemical Equators (CE) separates polluted NH from
  the pristine SH
• Differences in chemical signatures on each side
  dependant on location. Carbon Monoxide (CO) can
  be used as a tracer for transport of pollution
• PEM-TROPICS B CO 6-15 ppb higher N of ITCZ
• INDOEX average CO was 49 ppb at 5 º S and 175
  ppb at 5 ºN
• Ship measurements during INDOEX showed factor of
  3-4 increase in CO crossing the ITCZ
• Transition was found to be sharp over the
  course of a day.

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Chemical Equator

• Difficult to sample across the ITCZ in aircraft
  as it is a highly convective region
• ITCZ is a complex system can break down and
  reform
• A boundary between air with NH and SH chemical
  signatures does not have to be associated with
  the ITCZ
• Chemical Equator
• Chemical and aerosol data collected across a
  chemical equator using the Dornier during ACTIVE
  will be presented
• High time resolution measurements of CO, O3 and
  aerosol properties across the boundary
• Lower-time resolution measurements of VOCs and
  CFCs give averaged profiles on either side of the
  boundary

7
Flight Tracks

• Flights part of ACTIVE Dornier Survey Flights
• SD019 30th January 2006
• SD022 3rd February 2006

SD022
SD019
8
Meteorology
SD019 30th Jan
SD022 3rd Feb
MTSAT Infrared images 1403 local
ECMWF Mean Sea level pressure and 10 m winds
1530 local
9
Results Time Series
SD019
SD022
CO O3
Aerosol
AMS
10
CO and Ozone

• CO is an ideal tracer for transport of pollution
  sources
• Photo-chemically produced via oxidation of CH4
  and VOCs
• Direct emission from incomplete combustion
  sources (biomass/fossil)
• Ozone by-product of VOC oxidation in presence
  of NOx.
• Coloured flight path by CO (40-150ppb).
  Transition at chemical equator is sharp (CO 40 to
  165 ppb within 50 km)

11
Air Mass Origin
Back trajectories calculated along the flight
track using NOAAs HYSPLIT model
5 day back trajectory
10 day back trajectory
Coloured by CO 40 ppb blue-160 ppb red
SD019
SD022
12
CO and Ozone

• Using trajectory analysis have separated the data
  according to hemispheric origin (over the
  previous five days)
• Definite correlation between CO and O3 in NH air
  in SD022. Not as clear in SD019.
• Ratio of O3CO in polluted NH air was 0.16.
• Similar to INDOEX polluted air masses from
  India 0.14-0.16
• SE Asia biomass burning plumes 0.12-0.2
• Stehr et al., JGR-Atmos., 107, 19, 2002.
• Kondo et al., JGR-Atmos, 109, 2004

SD019
SD022
13
Gas phase organics

• Collected air samples onto absorbent tubes during
  flights and analysed using gas chromatography
  with time of flight mass spectrometry.
• 5 minute sample time 15 tubes per flight.
• Typical SH background concentrations determined
  using other flights under similar met conditions.
  (AD018 and SD020/21 Survey flights to Alice
  Springs)
• Average VOC concentration determined for samples
  collected when air originated in NH.
• Elevated aromatic concentrations indicate a
  larger anthropogenic pollution source north of
  the chemical equator. Tracers for fossil fuel
  burning and transportation (i.e. evaporations
  from petrol stations)
• Also seen with other petroleum markers e.g.
  alkanes

14
Biomass burning

• The Moderate Resolution Imaging Spectroradiometer
  (MODIS) onboard the Terra and Aqua Satellites can
  be used to detect thermal anomalies including
  fire occurrence
• Data obtained from http//landweb.nascom.nasa.gov/
  cgi-bin/browse/browse.cgi

Extensive fires burning in North Sumatara and SE
Asia (Thailand)
Elevated pollutant levels are a result of BIOMASS
BURNING AND HIGHER BACKGROUND IN N. HEMISPHERE
15
Comparison to Satellite data
The chemical equator can clearly been seen in the
Western Pacific region in the TES data
TES CO profile (ppbv) at approximately 600 mbar
(25 January 5 February 2006)
The change in magnitude is not as great as in the
in-situ measurements - due to averaging over
an 11-day time period to obtain sufficient
satellite coverage - averaging over the
vertical column (approximately 5 km) and the
higher tangent altitude of TES observations
(mid-troposphere).
Weekly mean upper troposphere MLS Cloud Filtered
CO profile (ppbv) at approximately 215 mbar (29
January 4 February 2006)
Evidence for uplift in convection?
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Modelling of Chemical equator

• CO modelled using p-TOMCAT chemical transport
  model, using ECMWF operational analyses.
• Models chemistry, emissions, boundary layer
  mixing and convective parameterisation were
  switched off
• Advecting passive tracers only features which
  develop are due to forcing from analysed winds
• Horizontal resolution (0.75 x 0.75 degrees)
• 31 Vertical levels up to 10hPa
• High-resolution model initialised from lower
  resolution run (that included all the models
  processes) at 1st January 2006.

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Modelling of Chemical equator
Horizontal (830mb)
Vertical (130 E)
SD019 30/01/2006
The famous plot!!!
SD022 03/02/2006
18
The Press!!
Appeared in Nature, New Scientist, National
Geographic, Discovery Channel, MSNBC, Fox

• Some of the weirder titles
• 'Chemical equator' protects Antarctica's clean
  air
• There's A 'Chemical Equator' - And We're On The
  Wrong Side Of It
• Discovered Nature Segregates Dirty, Rich Nations
  From Clean, Poor World

19
Conclusions and Implications

• Evidence of a chemical equator was investigated
  using a comprehensive combination of chemical and
  meteorological tools and techniques, over a broad
  range of spatial and temporal scales,  using the
  expertise of a large team of international
  scientists
• Transition was very sharp indicating inhibited
  inter-hemispheric mixing
• CHEMICAL EQUATOR
• The effect of the CE is amplified by the
  landphoon to the south transporting very clean
  air from the Southern Ocean and extensive biomass
  burning in Sumatra and SE Asia to the north.
• In both flights, the air north of the chemical
  equator is highly polluted (CO, Ozone, aerosols
  and aromatic VOCs).
• Back trajectory analysis indicates that this
  polluted air has travelled to the chemical
  equator through a highly active convective
  region.
• Aircraft measurements indicate that deep
  convection in the TWP is an important mechanism
  (via rapid vertical transport) for injecting
  large quantities of highly polluted air to the
  upper troposphere.
• Comparison with satellite and model data
  indicates air lofted in the TWP may be highly
  polluted.

20
Acknowledgements

• Thanks go to the rest of the ACTIVE team who took
  part, particularly those whose data has been used
• Thanks to the pilots of the Dornier and staff at
  the Airborne Remote Sensing Facility (ARSF)
• Thanks to the collaborative projects SCOUT-O3 and
  TWP-ICE and the Australian Bureau of Meteorology.
  Satellite data and Met analysis are courtesy of
  TWP-ICE and BoM.
• Jonathan Jiang at JPL for MLS plots and the TES
  science team at JPL
• Fire count data was obtained from the World Fire
  Atlas project, the Data User Element of the
  European Space Agency, and plotted by Manasvi
  Panchal.