Baseline measurements of inorganic trace gases over the Amazon

The Amazon rainforest is one of the last remaining wildernesses on Earth. It is thus a unique, natural laboratory for the study of surface-atmosphere interactions. Although these have been studied for a long time, there is still much to learn and significant knowledge gaps to fill. One specific need is for baseline measurements of gases and aerosol in the Amazonian atmosphere. These are important so that we can discern the impact of anthropogenic changes and climate change. Robbie Ramsay and his co-authors have addressed this in a new study of inorganic trace gases over the Amazon.

The research team made measurements at ATTO for a month in the dry season of 2017. They sampled the air above the forest canopy in 42 and 60-meter height at the 80-m walk-up tower. They made hourly measurements of several inorganic trace gases (such as ammonia and nitric acid) and aerosols. This allows them to not only measure the concentration of the gases but also their flux vertically in the air column.

GRAEGOR detector unit for measuring trace gases like nitrogen in the laboratory container. © Robbie Ramsay / University of Edinburgh
GRAEGOR detector unit for measuring trace gases like nitrogen in the laboratory container. © Robbie Ramsay / University of Edinburgh

Long-range transport of trace gases

ATTO is located very remotely in the central Amazon Rainforest. Despite that, the air masses arriving at the site are not always entirely pristine. Especially in the dry season, we see intrusions of polluted air more frequently. This is also something that Robbie and his colleagues observed. During the four weeks, there were several periods during which they noticed elevated concentrations of sulfate-containing aerosols and ammonia. Together with black carbon, these are a clear indicator of anthropogenic emissions.

Looking at the wind speed and direction, the team was able to trace the path of the air masses before they arrived at ATTO. And indeed, they had traveled over large urban areas to the South and South-East, and over areas where fires were recorded. Tracing them back even further, they found out that some of those air masses originated in South-West Africa. Biomass burning often occurs in this region during this time of year. Thus, the gases likely partly originated in Africa, and partly in other parts of the Brazilian Amazon.

Salt-ejecting fungi?

The team also estimated the deposition velocity. It describes the vertical speed at which the trace gases and aerosols measured are deposited from the atmosphere to the surface. These values are important to estimate the rate at which gases and aerosols are removed from the atmosphere through dry deposition. This is critical for modeling the lifetime of gases and particles in the atmosphere.

In addition, they found significantly more chloride ions and nitrate ions than had previously been estimated. Common sources for them might be very local biomass burning or marine air from the Atlantic containing sea salt. However, it is also possible that biogenic crustal materials such as fungal spores are the source of these ions.  For fungi especially, scientists know that they actively discharge their spores through liquid jets. Analyses in other studies have shown that 40-60% of fungal spore fragments contain Chloride ions in the form of salt.

The study is a first step in deciphering the exchange processes of inorganic trace gases between the Amazon rainforest and the atmosphere. A next step might be to replicate this study in the wet season. This would provide a more complete view of the annual pattern of inorganic trace gas and coarse aerosol biosphere-atmosphere exchange over tropical rainforests.

Ramsay et al. published the study “Concentrations and biosphere-atmosphere fluxes of inorganic trace gases and associated ionic aerosol counterparts over the Amazon rainforest” Open Access in Atmos. Chem. Phys.

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