Cross-hemispheric transport of central African biomass burning pollutants: implications for downwind ozone production 1UPMC Univ. Paris 06, Univ. Versailles St-Quentin, CNRS/INSU, UMR 8190 LATMOS-IPSL, Paris, France
2Institute for Atmospheric Science and Climate, ISAC-CNR, Italy
3Istituto di Scienze dell'Atmosfera e del Clima, Roma, Italy
4Max Planck Institute for Chemistry, Particle Chemistry Department, Germany
5Institute for Atmospheric Physics, Johannes Gutenberg Univ., Mainz, Germany
6Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Institut für Physik der Atmosphäre, 82230 Wessling, Germany
7Institute for Atmospheric and Environmental Sciences, J. W. Goethe-Univ., Frankfurt, Germany
8Institute for Climate and Atmospheric Science School of Earth and Environment, University of Leeds, UK
9Department of Chemistry, Univ. of Reading, Reading, UK
10National Centre for Atmospheric Science Department of Chemistry, Univ. of York, York, UK
11CAO, Dolgoprudny, Russia
12Laboratoire d'Aérologie, Univ. de Toulouse, France
*now at: CEREA, ENPC/EDF, 20 rue Alfred Nobel 77455 – Champs sur Marne, France
**now at: Department of Physics, Univ. of Wuppertal, Germany
Received: 04 May 2009 – Published in Atmos. Chem. Phys. Discuss.: 20 August 2009 Abstract. Pollutant plumes with enhanced concentrations of trace gases and aerosols were
observed over the southern coast of West Africa during August 2006 as
part of the AMMA wet season field campaign. Plumes were observed
both in the mid and upper troposphere. In this study we examined
the origin of these pollutant plumes, and their potential
to photochemically produce ozone (O3) downwind over the Atlantic Ocean. Their
possible contribution to the Atlantic O3 maximum is also discussed.
Runs using the BOLAM mesoscale model including biomass burning carbon
monoxide (CO) tracers were used to confirm an origin
from central African biomass burning fires. The plumes measured in the
mid troposphere (MT) had significantly higher pollutant concentrations over
West Africa compared to the upper tropospheric (UT) plume. The mesoscale model
reproduces these differences and the two different pathways for
the plumes at different altitudes: transport to the north-east of the
fire region, moist convective uplift and transport to West Africa for
the upper tropospheric plume versus north-west transport over the Gulf of Guinea for
the mid-tropospheric plume.
Lower concentrations in the upper troposphere are mainly due to enhanced
mixing during upward transport. Model simulations suggest that MT and UT
plumes are 16 and 14 days old respectively when measured over West Africa.
The ratio of tracer concentrations at 600 hPa and 250 hPa was estimated for
14–15 August in the region of the observed plumes and compares well with the
same ratio derived from observed carbon dioxide (CO2) enhancements in both
plumes. It is estimated that, for the period
1–15 August, the ratio of Biomass Burning (BB) tracer concentration transported in the UT to the
ones transported in the MT is 0.6 over West Africa and the equatorial South Atlantic.
Revised: 04 March 2010 – Accepted: 10 March 2010 – Published: 30 March 2010
Runs using a photochemical trajectory model, CiTTyCAT, initialized with the
observations, were used to estimate in-situ net photochemical O3 production
rates in these plumes during transport downwind of West Africa.
The mid-troposphere plume spreads over altitude between 1.5 and 6 km
over the Atlantic Ocean. Even though the plume was old, it was still
very photochemically active (mean net O3 production rates over 10 days of
2.6 ppbv/day and up to 7 ppbv/day during the first days) above 3 km
especially during the first few days of transport westward. It is also shown
that the impact of high aerosol loads in the MT plume on photolysis rates serves
to delay the peak in modelled O3 concentrations. These results suggest that
a significant fraction of enhanced O3 in mid-troposphere over the Atlantic
comes from BB sources during the summer monsoon period. According to simulated
occurrence of such transport, BB may be the main source for O3 enhancement
in the equatorial south Atlantic MT, at least in August 2006.
The upper tropospheric plume was also still photochemically active, although
mean net O3 production rates were slower (1.3 ppbv/day). The results suggest
that, whilst the transport of BB pollutants to the UT is variable (as shown by the
mesoscale model simulations), pollution from biomass burning can make an
important contribution to additional photochemical production of O3 in
addition to other important sources such as nitrogen oxides (NOx) from lightning.
Citation: Real, E., Orlandi, E., Law, K. S., Fierli, F., Josset, D., Cairo, F., Schlager, H., Borrmann, S., Kunkel, D., Volk, C. M., McQuaid, J. B., Stewart, D. J., Lee, J., Lewis, A. C., Hopkins, J. R., Ravegnani, F., Ulanovski, A., and Liousse, C.: Cross-hemispheric transport of central African biomass burning pollutants: implications for downwind ozone production, Atmos. Chem. Phys., 10, 3027-3046, doi:10.5194/acp-10-3027-2010, 2010.