During the Amazon rainforest’s wet season (December to May), pristine atmospheric conditions, like those before the industrial age, are found alongside polluted air from an urban area centrally located within the forest. This makes the Amazon a unique natural laboratory to study how human activities have affected the atmosphere since preindustrial times.
A new paper analyzing data from the Green Ocean Amazon (GoAmazon2014/15) field campaign shows how urban pollution greatly alters natural aerosol processes in the otherwise pristine rainforest. The paper, which uses data from the Atmospheric Radiation Measurement (ARM) user facility, appears in the journal Nature Communications.
Aerosols—tiny particles in the air—change sunlight reaching the Earth, act as nuclei for cloud formation, and affect precipitation. Over the Amazon, natural carbon-containing particles known as biogenic secondary organic aerosols (SOA) dominate the atmosphere. These particles are formed by the oxidation of carbonaceous gases emitted from plants and trees. In the pristine air, chemical reactions involving nitrogen oxide emitted from soils and gases emitted from trees sustain oxidants that convert some of the gaseous forest carbon into SOA.
GoAmazon explored how a pollution plume from Manaus, Brazil, altered atmospheric chemistry and SOA formation within the pristine rainforest. In the Nature Communications paper, an international team of researchers reported that nitrogen oxide emitted from Manaus increased biogenic SOA formation over the forest by 60 to 200 percent on average and as much as 400 percent.
Scientists compared high-resolution chemical transport model simulations of nitrogen oxide, gaseous forest carbon, oxidants, and organic aerosols over the Amazon against ARM GoAmazon aircraft measurements taken during the daytime. Results showed that the nitrogen oxide emitted from Manaus increased the concentration of oxidants, specifically ozone and hydroxyl radicals in the presence of sunlight. This, in turn, led to the increased SOA production. The nitrogen oxide emissions in the Manaus plume were 10 to 50 times greater than those from the soils.
The increases in biogenic SOA formation were much larger than what has been seen in other locations worldwide, including the United States, although absolute SOA concentrations in the Manaus plume were often lower than in most polluted urban regions.
“The impact of pollution in creating secondary organic aerosols has been very difficult to tease out,” says lead author Manish Shrivastava of Pacific Northwest National Laboratory (PNNL) in Richland, Washington. “Our findings indicate the Earth’s atmosphere in many continental locations has already been substantially altered by human activities, and there’s a much larger and widespread impact than has been appreciated.”
For more information, read the full PNNL web release.# # #