Aerosol number fluxes over the Amazon rain forest during the wet season 1Department of Applied Environmental Science, Stockholm University, Sweden
2Institute of Physics, University of São Paulo, Brazil
Received: 17 June 2009 – Published in Atmos. Chem. Phys. Discuss.: 19 August 2009 Abstract. Number fluxes of particles with diameter larger than 10 nm were measured
with the eddy covariance method over the Amazon rain forest during the wet
season as part of the LBA (The Large Scale Biosphere Atmosphere Experiment
in Amazonia) campaign 2008. The primary goal was to investigate whether
sources or sinks dominate the aerosol number flux in the tropical rain
Revised: 27 November 2009 – Accepted: 30 November 2009 – Published: 15 December 2009
During the measurement campaign, from 12 March to 18 May, 60% of the
particle fluxes pointed downward, which is a similar fraction to what has
been observed over boreal forests. The net deposition flux prevailed even in
the absolute cleanest atmospheric conditions during the campaign and
therefore cannot be explained only by deposition of anthropogenic particles.
The particle transfer velocity vt increased with increasing friction
velocity and the relation is described by the equation
vt = 2.4×10−3×u* where u* is the friction velocity.
Upward particle fluxes often appeared in the morning hours and seem to a
large extent to be an effect of entrainment fluxes into a growing mixed
layer rather than primary aerosol emission. In general, the number source of
primary aerosol particles within the footprint area of the measurements was
small, possibly because the measured particle number fluxes reflect mostly
particles less than approximately 200 nm. This is an indication that the
contribution of primary biogenic aerosol particles to the aerosol population
in the Amazon boundary layer may be low in terms of number concentrations.
However, the possibility of horizontal variations in primary aerosol
emission over the Amazon rain forest cannot be ruled out.
Citation: Ahlm, L., Nilsson, E. D., Krejci, R., Mårtensson, E. M., Vogt, M., and Artaxo, P.: Aerosol number fluxes over the Amazon rain forest during the wet season, Atmos. Chem. Phys., 9, 9381-9400, doi:10.5194/acp-9-9381-2009, 2009.