Ice-nucleating particles and cloud formation
To form clouds and ultimately enable precipitation, tiny particles called aerosols are crucial. Sometimes, water condensates directly on an aerosol, which functions as cloud condensation nuclei. Other times aerosols will act as ice-nucleating particles (INPs), which means they are seeds for tiny ice crystals instead of water droplets. INPs may be dust or bioaerosols like bacteria or fungal spores, but they are a rare subclass of these aerosols with special properties.
Despite the fact that only few aerosols can act as INPs, the majority of global precipitation is formed through the pathway of ice nucleation. Therefore, this process is extremely important for understanding Earth’s climate. Unfortunately, monitoring INPs in the atmosphere is very complex and labor-intensive. As a consequence, we’re facing large knowledge gaps that include their distribution, seasonal variations and sources. Moreover, we lack conclusive measurements in near-pristine environments. Data from such regions may inform us about pre-industrial conditions. Such a baseline is vital to estimate the anthropogenic effects on INPs and figure out how they might be affected by climate change.
To fill some of those knowledge gaps, Jann Schrod and his co-authors produced a record of long-term measurements of INPs. They collected data for nearly two years at four different locations. One of those sites was ATTO, which can be considered pristine for at least part of the year. The other locations were Martinique in the Caribbean, the Taunus mountain range in Germany and the Norwegian Arctic.
Their results came as somewhat of a surprise. The research team did not find significant differences in INP concentrations between the four sites. Instead, the measurements usually fell within the same order of magnitude. Moreover, they could not detect any seasonal or interannual trends either. Short-term variably dominated in all of those locations. Furthermore, they could not identify a single physical or chemical parameter that continuously co-varied with INPs. They could also not find evidence for a strong anthropogenic effect on the concentration of INPs at the studied locations, although that might be different at urban sites. Overall, it is important to note, that the study only covered one specific nucleation mode, and the team cannot rule out differences and trends that might exist that they didn’t capture with this particular approach.
In their new study, Jann Schrod and his team couldn’t unravel all the mysteries surrounding ice-nucleating particles, and even ended up with some more questions – something that happens often in scientific research. Crucially though, their findings underline the complex nature of ice nucleation. By compiling this unique dataset, they laid the groundwork for further studies. They highlight how important it is to gather more long-term data of global INPs at highly equipped research stations, which might shed some more light on this important process.
Schrod et al. published the study “Long-term deposition and condensation ice-nucleating particle measurements from four stations across the globe” Open Access in Atmos. Chem. Phys.
Polari Corrêa and his co-authors analyzed the atmospheric dynamics in and above the forest canopy during one particular night at ATTO. Those conditions changed throughout the night. Turbulence was followed by the formation of a gravity wave and a low-level jet. It was likely formed due to the breeze from the Uatumã River and the hilly terrain. The study highlights the complex dynamics and mechanisms in the atmosphere above a dense forest.
Bioaerosols may act as cloud condensation nuclei and ice nuclei, thereby influencing the formation of clouds and precipitation. But so far there is less knowledge about the ice nucleation activity of each bioaerosol group and atmospheric models hitherto have not differentiated between them. Patade et al. created a new empirical parameterization for five groups of bioaerosols, based on analysis of the characteristics of bioaerosols at ATTO: fungal spores, bacteria, pollen, plant/animal/viral detritus, and algae. This makes it possible for any cloud model to access the role of an individual group of bioaerosols in altering cloud properties and precipitation formation.
Biogenic volatile organic compounds remove OH from the atmosphere through chemical reactions, which affects processes such as cloud formation. In a new study, Pfannerstill et al. reveal the important contributions of previously not-considered BVOCs species and underestimated OVOCs to the total OH reactivity.
Although located in the tropics, the Amazon sporadically experiences incursions of cold waves called friagem events. They significantly impact the weather patterns during the time they occur, causing for example a temperature drop and increased cloudiness. Guilherme Camarinha-Neto and his colleagues now found that they also affect atmospheric chemistry.
Convective storms often occur in the tropics and have the potential to disturb the lower part of the atmosphere. They might even improve the venting of trace gases out of the forest canopy into the atmosphere above. To better understand these processes, Maurício Oliveira and co-authors used the infrastructure at ATTO to study storm outflows during nighttime. They published the results in a new paper in the Open Access Journal Atmospheric Chemistry and Physics.
Christopher Pöhlker and co-authors published an extensive new paper, characterizing the footprint region of ATTO. They hope that fellow researchers in the Amazon region can use this publication as resource and reference work to embed ATTO observations into a larger context of Amazonian deforestation and land-use change. Pöhlker et al. published the paper Open Access in Atmospheric Chemistry and Physics Volume 19.
The Amazon rainforest interacts with the atmosphere by exchanging many substances. Many of these, such as carbon dioxide, methane, ozone, and organic compounds, are produced by the vegetation. They are very influential in both the regional and global climates. Until now, the estimates of their emission and absorption rates are based on classical theories. But those were developed over relatively short vegetation and are valid for the so-called “inertial sublayer.”
Aquino et al. published a new study in Agricultural and Forest Meteorology about the characteristics of turbulence within the forest canopy at two Amazonian sites. They found that the air layer close to ground is largly decouples from the air layer in the upper canopy and above.
Mira Pöhlker and her team continuously measured aerosols and their properties in the atmosphere at the 80 m tower at ATTO, thereby created the first such long-term record in the Amazon. They analyzed the data in two subsequent paper. The second was now published in ACP.