Biogenic aerosols from Amazonia: Composition, size distributions and optical properties

Amazonia is an excellent laboratory to study atmospheric processes that are characteristic of natural conditions, as they existed prior to the impact of industrialization on the regional and global atmosphere. Biogenic aerosols dominate the particle population in Amazonia, showing a strong link between forest biology and atmospheric composition. In the fine fraction, aerosols are mostly secondary organic particles formed from biogenic emissions of trace gases, with a contribution of primary particles. In the coarse mode, primary biogenic particles dominates the picture. Aerosols have been continuously measured at the TT34 LBA tower at the ZF2 ecological station about 55 Km North of Manaus since January 2008. Fine mode aerosol mass concentration is very low in Amazonia, with PM2.5 of about 1.3×0.7 μg m-3 and 3.4×2.0 μg m-3 in the wet and dry seasons, respectively. In terms of particle number concentrations a median value of 220 cm-3 in the wet season and 2,200 cm-3 in the dry season were observed. An aerosol chemical speciation monitor (ACSM) was deployed in 2013, and it shows that organic aerosol account to 81% to the non-refractory PM1 aerosol loading at TT34, while in the dry season, a high 93% content of organic particles was observed. Size distribution shows the occurrence of bursts of particles with about 20 nanometers at night time, possibly associated with biological process. Very few events of new particle formation are observed. Aerosol light scattering and absorption coefficients at the TT34 site were low during the wet season, increasing by a factor of 5, approximately, in the dry season due to long range transport of biomass burning aerosols reaching the forest site. Aerosol single scattering albedo (SSA) ranged from 0.84 in the wet season up to 0.91 in the dry, indicating a surprising high absorption in the wet season, associated with biogenic particles.