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.
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.
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.
Direct measurements of OH radicals are rare and difficult to achieve. However, since they react with BVOCs, Ringsdorf et al. inferred them from isoprene measurements at ATTO. To do so, they applied a technique called ‘Dynamical Time Warping’ from the field of speech recognition. Akima Ringsdorf et al. published the study “Inferring the diurnal variability of OH radical concentrations over the Amazon from BVOC measurements” Open Access in Nature Scientific Reports.
Bioaerosols influence the dynamics of the biosphere underneath. In a new study, Sylvia Mota de Oliveira and her colleagues used the ATTO site to collect air samples at 300 m above the forest. Then, they used DNA sequencing to analyze the biological components that were present and figure out what species of plant or fungi they belong to. One of the most striking new insights is the stark contrast between the species composition in the near-pristine Amazonian atmosphere compared to urban areas.
In a new study, Marco A. Franco and his colleagues analyzed when and under what conditions aerosols grow to a size relevant for cloud formation. Such growth events are relatively rare in the Amazon rainforest and follow and pronounced diurnal and seasonal cycles. The majority take place during the daytime, and during the wet season. But the team also discovered a few remarkable exceptions.
It is long known that aerosols, directly and indirectly, affect clouds and precipitation. But very few studies have focused on the opposite: the question of how clouds modify aerosol properties. Therefore, Luiz Machado and his colleagues looked into this process at ATTO. Specifically, they studied how weather events influenced the size distribution of aerosol particles.
High-quality atmospheric CO2 measurements are sparse across the Amazon rainforest. Yet they are important to better understand the variability of sources and sinks of CO2. And indeed, one of the reasons ATTO was built was to obtain long-term measurements in such a critical region. Santiago Botía and his colleagues now published the first 6 years of continuous, high-precision measurements of atmospheric CO2 at ATTO.
The Amazon rain forest plays a major role in global hydrological cycling. Biogenic aerosols, such as pollen, fungi, and spores likely influence the formation of clouds and precipitation. However, there are many different types of bioaerosols. The particles vary considerably in size, morphology, mixing state, as well as behavior like hygroscopicity (how much particles attract water) and metabolic activity. Therefore, it is likely that not only the amount of bioaerosols affects the hydrological cycle, but also the types of aerosols present.
Ramsay et al. developed a new model to assess nitrogen exchange between atmosphere and biosphere based on observations at ATTO. This model includes parameters controlling both nitrogen deposition and emissions in tropical forests.
Soot and other aerosols from biomass burning can influence regional and global weather and climate. Lixia Liu and her colleagues studied how this affects the Amazon Basin during the dry season. While there are many different interactions between biomass burning aerosols and climate, they found that they overall lead to fewer and weaker rain events in the Amazon rainforest.
When forests burn those fires produce a lot of smoke. And that smoke usually contains soot, also called “black carbon”. Black carbon particles are aerosols that absorb radiation and as such can warm the Earth’s atmosphere and climate. But we still have much to learn about aerosols, their properties, and distribution in the atmosphere. One of those things is the question of how black carbon emitted from biomass burning in Africa (i.e. forests, grasslands, savannas etc.) is transported across the Atlantic and into the Amazon basin, and what role it plays there. Bruna Holanda and her co-authors tackled this in their new study published in ACP.