Flux Measurements and Chemical Characterization of Ultrafine Aerosol Particles in the Amazon

As the largest tropical forest with approximately 4.7 million km², the Amazon rainforest has a significant impact on regional and global climate. Atmospheric aerosols critically shape Earth’s climate by scattering and absorbing solar radiation and by influencing cloud formation and precipitation. Pristine regions such as the Amazon provide a glimpse of pre-industrial atmospheric conditions and are particularly important for assessing climate change. Previous studies have investigated aerosol concentrations, properties, and sources as a function of seasonality and diurnal variation [1–3]. However, the identity and interplay of natural aerosol sources, and their relevance to overall aerosol cycling, remain poorly understood.

In particular, the formation and growth of particles smaller than 100 nm is still uncertain. The very small masses of ultrafine particles present a major analytical challenge, resulting in an incomplete understanding of their origin and properties. Here, we propose two approaches that could provide new insights into the aforementioned questions. The first is size-resolved aerosol flux measurements to determine whether and when ultrafine particles are transported out of or into the canopy. The second is a chemical analysis of characteristic tracers in sub-100 nm aerosol samples.

Aerosol exchange between the forest and the atmosphere is driven by turbulence, influencing both deposition and emission of particles. To obtain turbulent fluctuations with high time resolution we applied the eddy covariance method (ECM) at 52 m on the 80 m walk-up tower at the Amazon Tall Tower Observatory (ATTO). Using 10 Hz eddy covariance measurements of 3D wind and size-resolved particle concentrations, we aim to quantify this exchange to improve our understanding of biosphere–atmosphere interactions. This approach yields a unique long-term dataset of size-resolved aerosol particle fluxes in the Amazon, enabling the investigation of biogenic aerosol exchange and the turbulent transport of nutrients. Preliminary analysis suggests pronounced diurnal cycles and seasonal variability in aerosol fluxes.

Additionally, we focus on the chemical characterization of sub-100 nm aerosol particles. Due to the major analytical challenges, we have applied a 'nanobulk' method combining spot sampler technology with scanning transmission X-ray microscopy and near-edge X-ray absorption fine structure (STXM-NEXAFS) spectroscopy. With this approach we collect and chemically characterize ultrafine particles under clean rainforest conditions. By sampling pristine background and new particle formation events, we aim to investigate potential differences in aerosol particle composition under varying atmospheric conditions.

The chemical characterization of ultrafine particles shows consistent spectroscopic signatures across all samples and deposition spots without major differences as a function of pollution and sub-100 nm events. Spectroscopic signatures suggest the predominance of secondary organic aerosols. Surprisingly, biogenic potassium salts could not be observed below 100 nm, yet they are very abundant at sizes larger than 100 nm.

  

 

[1] Artaxo et al., Tellus B: Chemical and Physical Meteorology, 74, 24–163, 2022

[2] Franco et al., ACP, 22, 3469-3492, 2022

[3] Valiati et al., ACP, 25, 14923-14944, 2025