From Forest to Sky: Air Chemistry over the Amazon

A multitude of Volatile Organic Compounds (VOC) are present in the air we breathe. These airborne chemicals make up the familiar scents of flowers, fuels, and firesmoke. On a global scale, however, the single largest VOC source is the Amazon rainforest.

Each day, as the giant Amazonian ecosystem takes up carbon dioxide through photosynthesis, it also releases a fascinating cocktail of reactive chemicals into the air. These trace compounds play several roles within the forest, including protecting leaves from oxidative damage and mediating chemical communication between plants and insects. Above the canopy, they shape global atmospheric chemistry by influencing the atmospheric oxidation capacity, particle formation and the radiative budget, as well as regional clouds and precipitation.

Since 2012 we have been using sensitive mass spectrometers to characterize VOC from the 325m ATTO measurement tower in the pristine Brazilian rainforest. Initial work focused on isoprene (C5H8) and monoterpenes (C10H16), whose emissions vary between wet and dry seasons in response to light and temperature. Parallel measurements of total OH reactivity showed that many additional reactive compounds must be present, and the search for these species revealed new VOC sources and sinks, from soil, mosses and lichen.

In 2022-2023, the CAFÉ-BRAZIL airborne campaign extended these VOC measurements up to 14 km altitude across the entire Amazon basin. Even at these heights, the forest imprint is clear. Nocturnal deep convection transports substantial amounts of VOC to the upper troposphere, where they can accumulate overnight and prime the atmosphere for complex organic photochemistry at dawn. These natural chemical processes will be disrupted by continued deforestation.

 Climate models predict that the Amazon rainforest will suffer more, severe drought periods in future. Our recent measurements over the Amazon and within the BIOSPHERE 2 rainforest facility show that chiral VOCs can serve as sensitive indicators of how the forest responds to drought stress, including the extreme 2023/2024 El Niño event. Measurements of VOC in air hold the key to unlocking the complex chemical processes operating within and above the Amazon rainforest ecosystem.