How weather affects atmospheric aerosols

Aerosols and clouds

Scientists have worked for decades on improving our understanding of atmospheric processes. They have made huge strides, which has allowed us to predict future developments of Earth’s climate. But we still have some blind spots, which are responsible for variations within the different model predictions. Clouds and aerosols contribute the largest uncertainty. It is long known that aerosols, directly and indirectly, affect clouds, weather and precipitation. One major factor is that aerosols provide condensation nuclei for cloud formation. 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. They wanted to learn more about the daily and seasonal cycles of aerosol particle sizes. Additionally, they wanted to look into the effects of weather events like lightning storms.

A dark cloud sits directly behind the ATTO tall tower, with heavy rain falling to the groud.
Luiz Machado and his colleagues investigated how weather events like thunderstorms affect aerosols in the atmosphere. © Nabig Alberto Castro Souza / INPA

Cyclic variability

Luiz and his team found a pronounced seasonality in their aerosol measurements. In agreement with prior studies, they observed a much higher concentration of aerosol particles in the dry season than in the wet season. This is likely due to both an increase in fire-related aerosols (natural and anthropogenic), and a shift in wind direction, bringing more such polluted air to the ATTO site.

In addition, they also found a typical daily cycle in the particle sizes. The maximum concentration in ultrafine particles (smaller than 50 nanometers) occurs at sunrise. For comparison, that’s smaller than a single coronavirus, which size ranges from 50 to 140 nanometers. As the sun rises, the particles grow. Consequently, the concentration of ultrafine particles decreases gradually, and that of larger ones increases. This trend lasts until the afternoon. This is the time of day when it often rains in the tropics, and the rainfall, among other processes, removes the larger particles from the atmosphere and increases the concentration of small particles.

Weather: thunderstorms and aerosols

Even though most rain falls during the wet season, thunderstorms occur most frequently during the transition from wet to dry season and vice versa. And these thunderstorms with lots of lightning activity appear to significantly affect the aerosol particles in the atmosphere. The scientists observed that the number of ultrafine particles increases starting approximately with the onset of a storm, 100 minutes before lightning activity at ATTO reaches its maximum. Larger particles on the other hand decrease, dropping to a minimum during the height of the storm. Two to three hours after afterward, this trend reverses again.

The figure shows the abundance of ultrafine particles (top graph) and larger particles (lower graphs) across a thunderstorm event. 0 minutes marks the time of maximum lightning intensity.
Figure from Machado et al. (2021). It shows the abudance of ultrafine particles (top graph) and larger particles (lower graphs) across a thunderstorm event. 0 minutes marks the time of maximum lightning intensity.

The reason for this behavior is still unclear and the team cannot explain it with the setup of this study. Nevertheless, Luiz and his colleagues hypothesize that the convective storm might bring down air from higher layers in the atmosphere with downdrafts. This air contains relatively more of these ultrafine particles and fewer larger-sized ones. Another or additional possibility is that the lightning discharges themselves might produce ultrafine particles. And since thunderstorms are associated with heavy rain, this likely washes out particles from the atmosphere, and in particular the larger, heavier ones.

Further studies will however be needed to fully explain what Luiz and his team found in their observational data.

Luiz Machado et al. published the study “How weather events modify aerosol particle size distributions in the Amazon boundary layer“ Open Access in Atmospheric Chemistry and Physics.

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