Luciana Rizzo - Secondary Organic Aerosol Formation in the Amazon Rainforest
The term 'aerosol' is defined as a suspension of solid or liquid particles in a gas. In remote regions such as the Amazon rainforest, most of the atmospheric aerosols arise from biogenic sources, directly emitted as pollen, bacteria, leaf and insect fragments, and secondarily emitted as a result of gas-to-particle conversion. Once released in the atmosphere, these micrometric particles interact with solar radiation by absorption and scattering of light, and indirectly through the formation of cloud condensation nuclei (CCN) (Fig.1). The radiation balance at the surface is linked with photosynthesis and biomass carbon assimilation. Dry and wet deposition processes remove aerosols from the atmosphere, playing a role on the forest nutrient cycling. Therefore, the biosphere and the atmosphere are closely related, and the biogenic aerosols are an integrant part of the ecosystem dynamics.
Figure 1: Biosphere-atmosphere interactions: aerosols emitted through the metabolism of the forest interact with solar radiation, which in turn affects photosynthesis, photochemistry and nutrient cycling.
A fraction of the aerosols in the Amazonian atmosphere may result from secondary organic aerosol formation. In this process, gas-phase volatile organic compounds (VOC) naturally emitted by the vegetation are oxidized and eventually result in low vapor pressure compounds that can partition to the particle phase. The new particles arise as thermodynamically stable clusters 1-3 nm in size, which can eventually grow if there is enough condensable vapor. This phenomenon of new particle formation and growth has been observed in a variety of locations around the world, through the observation of concentration peaks of nanometric particles. As the Amazon forest is a natural source of VOC to the global atmosphere, one could expect to observe frequent events of new particle formation in the area. Nevertheless, the events observed in the Amazon differ from the ones frequently observed in boreal forests.
Figure 2 shows a time series of aerosol size distribution recently measured at a pristine forest site in Amazon, during the AMAZE-08 (Amazonian Aerosol Characterization Experiment). The color scale represents the relative concentration of particles in each diameter range. One can see an event of new particle formation at 10:00 local time, and the subsequent growth of this particles to diameters around 70-80 nm. While in the Amazon the newly formed particles are initially 20-40 nm in size, in boreal forests they are usually 3-10 nm in size. Figure 3 shows another time series of aerosol size distribution in which one can see nocturnal bursts of particles in the size range of 20-40 nm. The new particle formation events reported all over the world are observed during the day, since they are related to photochemical reactions of VOC.
Figure 2: Time series of aerosol size distribution in a pristine forest site in Amazon. There is a new particle formation event starting around 10:00 local time.
Figure 3: Another example of aerosol size distributions in a pristine forest site in Amazon. There are bursts of particles with 20-40 nm in diameter after the sunset.
Thus, the process of gas-to-particle conversion in the Amazon rainforest tags along different rules and mechanisms than the ones observed in other areas. The observed nocturnal events of new particle formation may be related to VOC oxidation by the nitrate radical (NO3), present in the atmosphere only at nighttime. To explain the fact that new particle formation in the Amazon starts with larger particles (20-40 nm in diameter) than the ones observed elsewhere, one could hypothesize that some atmospheric conditions in the Amazon may favor low volatility organic compounds to condense over pre-existent particles instead of forming new particles. Recent aerosol chamber studies suggest that high humidity levels affect the production of secondary organic aerosol. Also, the Amazon forest is very heterogeneous in terms of tree species, and therefore the VOC emissions may vary from region to region. VOC measurements need improvements, specially concerning to sesquiterpenes, a class of VOC very efficient to form new particles, and that could help to explain the events observed in the Amazon. The chemical processing of VOC in the Amazon follows different paths, and more studies concerning to organic aerosol composition and VOC oxidation mechanisms are needed for a better understanding.
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