Sources, properties, and atmospheric effects of Amazonian biogenic aerosol
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Abstract
tmospheric aerosol plays an important role in the Amazon rain forest as it influ ences ecological processes, clouds, precipitation, and climate. Over several decades,
progress has been made in understanding the Amazonian aerosol life cycle, with its
interplay of natural and anthropogenic emissions, transport, atmospheric processing,
and effects. However, fundamental questions, such as on the roles and relevance of
biogenic aerosol sources, have remained open. This dissertation provides experimen tal results and new insights into the sources, composition, and atmospheric cycling
of natural and anthropogenic aerosol in the Amazon rain forest.
Aerosol samples were collected at the Amazon Tall Tower Observatory for single par ticle elemental analysis in combination with multi-year online atmospheric observa tions. It was found that the inorganic elemental signature of the wet season aerosol
was defined by alternating episodes of pristine and long-range transport conditions.
During long-range transport, African dust and smoke as well as sea spray predomi nated. Under pristine conditions, where rain forest emissions prevail, the inorganic
element signature revealed a significant contribution of biogenic salts to the total
aerosol population. Based on improved statistical validity, the data set reveals an
intense biosphere-atmosphere exchange involving large parts of the aerosol size dis tribution. The results confirm the important role of biogenic salts in the Amazonian
aerosol cycle and provide a basis for studying their influence on cloud microphysical
processes.
In addition, multi-year measurements of aerosol particle number size distributions
characterized the high seasonal, diel, and meteorology-driven variability of the aerosol
population. This study focused on the effects of rain-related mixing on the occur rence and properties of particles smaller than 50 nm, originating from new particle
formation in the upper troposphere and subsequent convective downdrafts. Not all
events could be explained by this process, however, which emphasizes the potential
involvement of biogenic sources for ultrafine particles in the forest. In a further study,
the multi-year aerosol size distributions were linked to satellite-derived cloud micro physical parameters. The results showed a clear relation between aerosol abundance
and size as well as cloud microphysics and, thereby, revealed that cloud droplet for mation and growth processes in the Amazon are sensitive to changes in the aerosol
population, even under polluted conditions in the dry season.
This dissertation emphasizes the diversity and source of Amazonian aerosol, their
influence on aerosol-cloud-precipitation interactions, and subsequent effects on the
rain forest ecosystem. It can advance our knowledge on pre-industrial aerosol pro cesses and human impacts, and serve as baseline data for future studies.