Sub-micrometre Particulate Matter Is Primarily in Liquid Form over Amazon Rainforest

Martin, S. T., Harvard University

Aerosol Properties

Aerosol Processes

Bateman AP, Z Gong, P Liu, B Sato, G Cirino, Y Zhang, P Artaxo, AK Bertram, AO Manzi, LV Rizzo, RA Souza, RA Zaveri, and ST Martin. 2016. "Sub-micrometre particulate matter is primarily in liquid form over Amazon rainforest." Nature Geoscience, 9(1), 10.1038/ngeo2599.

Measured rebound fractions

Measured rebound fractions


The rebound of submicrometer PM was studied in central Amazonia during the wet and dry seasons using a particle-impaction apparatus equipped with RH adjustment. Rebound can indicate whether a particle is liquid or non-liquid. Physical state can affect particle growth and reactivity and hence particle number, size, and composition, with important tie-ins to the connections between aerosol particles and climate change/prediction.


The results of the present study highlight a biome-dependent distribution of liquid and non-liquid PM over forested regions. This study shows liquid particles over tropical forest whereas an earlier study showed non-liquid particles over boreal forests. These differences arise both because of intrinsic differences related to BVOC emissions and oxidation pathways as well as extrinsic differences in climatology of RH and temperature, among other possible factors.


The measured particle-rebound fractions as a function of relative humidity are plotted in the figure. Green (wet season) and blue (dry season) coloring correspond to the background conditions; red coloring corresponds to polluted conditions. The probability density functions of RH measured at the T3 site during the 2014 wet and dry seasons are plotted in the lower panel. An absence of rebound implies liquid PM, as observed at higher RH values. Conversely, a rebound fraction of 0.8 to 1.0 implies non-liquid PM, as observed at lower RH values. The coloring of data points in the figure associated with background compared to polluted back trajectories. The data show that the transition to liquid PM was complete by 80% RH for clean conditions during both the dry and wet seasons. This value of 80% RH can be compared to Amazonian climatology. The probability density functions of RH and temperature are shown in the inset of the figure. The threshold of 80% RH was exceeded 70% and 80% of the time for the dry and wet seasons, respectively. The implication is that the prevailing submicron PM in Amazonia is liquid most of the time, at least near the Earth’s surface.