Particle formation and particle growth
For rain, we need clouds. And for clouds, we need both water vapor and particles for this water vapor to condense on. But not just any particles. Cloud condensation nuclei need to be of a certain size, at least something around 80 nanometers (nm). There are some particles floating in the air that already are this large, like soot particles, or biological particles like fungi and certain viruses (the coronavirus has a diameter of ca. 100 nm). But most particles are much smaller, and grow larger while they are in the atmosphere. This process is called “new particle formation”. Even gas molecules can form clusters of this size through growth. Studies looking at aerosols and cloud formation even suggest that new particle formation might be the dominant source of particles contributing to cloud formation, in particular in the central Amazon.
This particle growth, however, does not happen continuously over time, but rather in defined events and under specific conditions. The details of how these processes work are still a matter of ongoing research. This is especially true for the Amazon region, where new particle formation inside the planetary boundary layer (the lowermost ≈1-2 km of the atmosphere) appears to be much less frequent than in other areas of the world. Instead, studies suggest that new particle formation takes place higher up in the atmosphere, and particles are then brought down with downdrafts in convective clouds.
Therefore, Marco A. Franco and his colleagues set out to analyze a long time series of the aerosol number size distribution of more than six years. In addition, they measured numerous meteorological parameters to identify possible connections to aerosol growth events. Understanding these processes better is important to be able to predict how climate change might impact Amazonian forests, as cloud formation is an essential process for rain and thus, for maintaining rainforests. They did this study with data from ATTO, where the atmosphere is comparably clean and largely free of anthropogenic pollution.
Growing in cycles
The research team did not detect new particle formation itself, but aerosol particle growth events. These particles were, however, not newly formed, but had already aged and grown to a medium-size between 10 and 50 nm. The subsequent growth events occurred on average only once a week, i.e. on 14 percent of analyzed days. However, this did not happen uniformly at all. Instead, the scientists found pronounced daily and seasonal cycles. Marco A. Franco and his colleagues noticed in their data that most particle formation events happened in the morning hours. The peak around 7 am emphasizes the role of sunlight and changing atmospheric conditions over the course of the day. Remarkably though, they were also able to detect particle growth events at night. Further research is needed to understand the mechanisms driving this growth.
They also discovered that the large majority of particle growth events took place during the wet season. In those months, thunderstorms are very frequent and are associated with downdrafts of air from higher atmospheric layers, which was corroborated by Machado et al., 2021. And indeed, almost three-quarters of the particle formation events took place in the presence of storm clouds. This aligns with previous theories that these downdrafts bring in medium-sized particles, which then continue to grow to a size relevant for cloud formation.
However, some 30 % of events took place in the absence of convection. Therefore, for around one-third of particle formation events, there has to be a source for particles other than downdrafts from higher atmospheric layers. To unveil those sources and processes, we need future studies, as they contribute significantly to particle growth events and thus cloud formation in the Amazon rainforest.
Marco A. Franco and his co-authors published the study “Occurrence and growth of sub-50nm aerosol particles in the Amazonian boundary layer” Open Access in the journal Atmospheric Chemistry and Physics.
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