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, doi:10.1029/2008JD009940, (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.

6. 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 14:03 local ECMWF Mean Sea level pressure and 10 m winds 15:30 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 NOAA’s 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 O3:CO 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?

16. 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 model’s processes) at 1st January 2006.

17. 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.