A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest

Abstract

Abstract. Vertical number fluxes of aerosol particles and vertical fluxes of CO 2 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 CO 2 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 partiCorrespondence to: L. Ahlm (lars.ahlm@itm.su.se) cle 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 CO 2 , and being released from the canopy when conditions become more turbulent in the morning.

Emission and dry deposition of accumulation mode particles in the Amazon Basin

“…In daytime, v t for all the five size bins show maximum values between 3.5 and 5.5 mm s −1 . These peak values are considerably higher than the median peak in v t at ∼1-2 mm s −1 observed for the total particle size range (D p >10 nm) in Ahlm et al (2010). Most studies of aerosol dry deposition show minimum deposition velocities at diameters around 0.1-0.3 µm (Zhang and Vet, 2006), a diameter range where there is no efficient dry deposition mechanism.…”

“…This minimum, in which particles tend to be accumulated, is one of the key processes behind the formation of the accumulation mode. The fact that the transfer velocities obtained in this study (for particles with D p >0.25 µm) are considerably larger than the transfer velocities for the total particle size range (D p >0.01 µm) in Ahlm et al (2010) indicates that for conditions observed over the rain forest, the minimum deposition velocity (as a function of particle size) is located at lower diameters than 0.25 µm. The higher transfer velocities in this study can thereby be interpreted as higher efficiency of interception and impaction of particles with D p >0.25 µm compared with the corresponding average efficiencies of these processes in the total particle size range, which is dominated by particles smaller than 0.25 µm in diameter (Zhou et al, 2002;Rissler et al, 2006).…”

“…At 15:00 LT, the mixed layer is well developed (Fisch et al, 2004). The net upward fluxes are instead most likely a result of primary biogenic aerosol emission, and the fact that they peak during afternoon also suggests that the source here cannot likely be related to the potential emission observed as upward particle fluxes during early morning in Ahlm et al (2010). However, that study focused on the total aerosol number population (D p >10 nm) whereas the fluxes discussed here only include particles in the diameter range 0.5-2.5 µm.…”

“…The median aerosol number concentration within the OPC size range in this study was 33 cm −3 in the wet season, and 122 cm −3 in the dry season. This can be compared with the total aerosol number concentration measured with a condensational particle counter CPC (D p >10nm) that was 682 cm −3 and 1513 cm −3 in the wet season and dry season, respectively (Ahlm et al, 2010). Hence, the number of particles measured with the Grimm 1.109 in this study represents roughly 5% and 8% of the total number of particles in the wet and dry season, respectively.…”

“…That was an indication that the source of primary biogenic aerosol emission, in terms of number concentrations, may be low when considering the total size range of particles. Also in the dry season, deposition fluxes dominated most of the time (Ahlm et al, 2010). However, upward particle fluxes often appeared in the morning.…”