Rainfall-Particle Feedback in the Amazon Forest

 Atmospheric aerosol particles are essential for the formation of clouds and precipitation, thereby impacting
the global energy budget and the water cycle. However, particle production in pristine environments like the
Amazon rainforests are not yet well understood. New particle formation can occur in the outflow of high
convective clouds and be transported downward (top-down) or they may form and grow from oxidized
biogenic volatile organic compounds at low altitudes before entering the largescale vertical circulation
(bottom-up). In this study, we examine a comprehensive measurement dataset of aerosols, trace gases,
and meteorological parameters to examine new particle formation and growth in the Amazon rainforest.
The results reveal that rain events are important not only for the downdraft of particles from aloft but
also for the injection of ozone into the forest canopy. While near-ground particle enhancements by
downdrafts were modest, .another less frequent but more efficient process acted to increase sub-40 nm
particle concentrations nearly one hour after the maximum precipitation. This phenomenon occurs due to
downdrafts of ozone-rich air entering the canopy containing reactive organic species and initiating particle
production. Particularly on days when the previous night has a high ozone concentration, these particles
grow in the early morning to form cloud condensation nuclei in the early afternoon