Measurement of Aerosol and VOC Turbulent Fluxes Over a Pristine Forest in Amazonia

The Amazon tropical forest constitutes a complex ecosystem, in which the biosphere and the atmosphere are intrinsically related through the turbulent exchange of water vapor, aerosol particles and trace gases such as carbon dioxide and volatile organic compounds. As a result of the intense convective activity typically observed in the tropics, these atmospheric constituents can be transported over large distances, in a way that the forest acts as a natural global source. Eddy covariance fluxes of fine mode aerosol particles and VOC were measured at a 60m high tower over a pristine forest in the Cuieiras reservation, near Manaus. Aerosol physical and chemical properties were also measured in three levels of the flux tower: 2, 28 and 40 m. Observed black carbon concentrations averaged 520(220) ng/m3, typical of a background environment. Coarse mode aerosol particles were in average 15% more abundant at the soil level, with a strong vertical gradient of phosphorus and potassium concentrations at night. It suggests the existence of a nocturnal source of biogenic particles close to the ground. The submicrometer aerosol number size distributions observed over the canopy were bimodal in 87% of the cases, with the presence of Aitken and accumulation modes. The nucleation mode appeared in 13% of the size distributions. Bursts of Aitken mode particles were recurrently observed at night, with no apparent relationship with aerosol, VOC or heat fluxes. Events of particle nucleation were not clearly observed during the experiment. Isoprene and monoterpenes fluxes (gaseous precursors of secondary organic aerosol) reached respectively 7.4 and 0.82 mg/m2/s around noon, declining to zero at sunset. An average particle number flux of 5(10)exp4 m-2 s-1 was calculated for the experiment. It means that the net fine mode aerosol exchange between the primary forest and the atmosphere is near zero. Nevertheless, peaks of positive fluxes (ascendant transport) were frequently observed during daytime, while peaks of negative fluxes were observed mostly at night. No significant correlations were found between VOC and fine mode aerosol concentrations or fluxes. Nocturnal bursts of aerosol particles with diameters between 0.5 and 5 um were registered in 8 of the 14 sampling days, associated with negative peaks of particle flux (deposition) and positive peaks of CO2 flux (emission). Those events can be attributed to nocturnal biogenic emissions or to activation of particles due to the saturation of water vapor inside the canopy at night.