African biomass burning plumes over the Atlantic: aircraft based measurements and implications for H2SO4 and HNO3 mediated smoke particle activation 1Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, 82234 Wessling, Germany
2Max-Planck Institute for Nuclear Physics, (MPIK), Atmospheric Physics Division, P.O. Box 103980, 69029 Heidelberg, Germany
3Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
4Department of Technology, Metropolia University of Applied Sciences, P.O. Box 4000, 00180 Helsinki, Finland
Received: 18 Feb 2010 – Published in Atmos. Chem. Phys. Discuss.: 25 Mar 2010 Abstract. Airborne measurements of trace gases and aerosol particles have been made in
two aged biomass burning (BB) plumes over the East Atlantic (Gulf of Guinea).
The plumes originated from BB in the Southern-Hemisphere African savanna
belt. On the day of our measurements (13 August 2006), the plumes had ages of
about 10 days and were respectively located in the middle troposphere (MT) at
3900–5500 m altitude and in the upper troposphere (UT) at 10 800–11 200 m.
Probably, the MT plume was lifted by dry convection and the UT plume was
lifted by wet convection. In the more polluted MT-plume, numerous measured
trace species had markedly elevated abundances, particularly SO2 (up
to 1400 pmol mol−1), HNO3 (5000–8000 pmol mol−1) and smoke particles with
diameters larger than 270 nm (up to 2000 cm−3). Our MT-plume measurements
indicate that SO2 released by BB had not experienced significant loss
by deposition and cloud processes but rather had experienced OH-induced
conversion to gas-phase sulfuric acid. By contrast, a significant fraction of
the released NOy had experienced loss, most likely as HNO3 by
deposition. In the UT-plume, loss of NOy and SO2 was more
pronounced compared to the MT-plume, probably due to cloud processes.
Building on our measurements and accompanying model simulations, we have
investigated trace gas transformations in the ageing and diluting plumes and
their role in smoke particle processing and activation. Emphasis was placed
upon the formation of sulfuric acid and ammonium nitrate, and their influence
on the activation potential of smoke particles. Our model simulations reveal
that, after 13 August, the lower plume traveled across the Atlantic and
descended to 1300 m and hereafter ascended again. During the travel across
the Atlantic, the soluble mass fraction of smoke particles and their mean
diameter increased sufficiently to allow the processed smoke particles to act
as water vapor condensation nuclei already at very low water vapor
supersaturations of only about 0.04%. Thereby, aged smoke particles had
developed a potential to act as water vapor condensation nuclei in the
formation of maritime clouds.
Revised: 09 Jan 2011 – Accepted: 18 Mar 2011 – Published: 05 Apr 2011
Citation: Fiedler, V., Arnold, F., Ludmann, S., Minikin, A., Hamburger, T., Pirjola, L., Dörnbrack, A., and Schlager, H.: African biomass burning plumes over the Atlantic: aircraft based measurements and implications for H2SO4 and HNO3 mediated smoke particle activation, Atmos. Chem. Phys., 11, 3211-3225, doi:10.5194/acp-11-3211-2011, 2011.