A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest L. Ahlm1, E. D. Nilsson1, R. Krejci1, E. M. Mårtensson1, M. Vogt1, and P. Artaxo2 1Department of Applied Environmental Science, Stockholm University, Sweden 2Institute of Physics, University of São Paulo, Brazil
Abstract. Vertical number fluxes of aerosol particles and vertical fluxes of CO2
were measured with the eddy covariance method at the top of a 53 m high
tower in the Amazon rain forest as part of the LBA (The Large Scale
Biosphere Atmosphere Experiment in Amazonia) experiment. The observed
aerosol number fluxes included particles with sizes down to 10 nm
in diameter. The measurements were carried out during the wet and dry season
in 2008. In this study focus is on the dry season aerosol fluxes, with
significant influence from biomass burning, and these are compared with
aerosol fluxes measured during the wet season.
Net particle deposition fluxes dominated in daytime in both seasons and the
deposition flux was considerably larger in the dry season due to the much
higher dry season particle concentration. The particle transfer
velocity increased linearly with increasing friction velocity in both
seasons. The difference in transfer velocity between the two seasons was
small, indicating that the seasonal change in aerosol number size
distribution is not enough for causing any significant change in deposition
velocity. In general, particle transfer velocities in this study are low
compared to studies over boreal forests. The reasons are probably the high
percentage of accumulation mode particles and the low percentage of
nucleation mode particles in the Amazon boundary layer, both in the dry and
wet season, and low wind speeds in the tropics compared to the midlatitudes.
In the dry season, nocturnal particle fluxes behaved very similar to the
nocturnal CO2 fluxes. Throughout the night, the measured particle flux
at the top of the tower was close to zero, but early in the morning there
was an upward particle flux peak that is not likely a result of entrainment
or local pollution. It is possible that these morning upward particle fluxes
are associated with emission of primary biogenic particles from the rain
forest. Emitted particles may be stored within the canopy during stable
conditions at nighttime, similarly to CO2, and being released from the
canopy when conditions become more turbulent in the morning.
Citation: Ahlm, L., Nilsson, E. D., Krejci, R., Mårtensson, E. M., Vogt, M., and Artaxo, P.: A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest, Atmos. Chem. Phys., 10, 3063-3079, doi:10.5194/acp-10-3063-2010, 2010.