Liquid Particulate Matter above Tropical Rain Forests

Particulate matter (PM) in the Earth's atmosphere occurs as liquids, semisolids, and solids. The physical state of organic PM can constrain the available mechanisms of growth and reactivity, ultimately affecting the number, size, and composition of the particle population, especially over Earth's forested regions. Non-liquid PM was recently reported over a boreal forest of Northern Europe. Herein, the physical state, including the response to relative humidity (RH), was investigated for PM (< 1 µm) over the tropical rain forest of central Amazonia during the GoAmazon2014/5 campaign. The results using a real-time particle rebound technique show that the organic PM was liquid for RH > 80% across 296 to 300 K. These results, coupled to the distributions of RH and temperature in Amazonia, imply that near-surface submicron PM in Amazonia is liquid most of the time. Because of the liquid state there should be no diffusion constraint on semi-volatile organic partitioning between the gas and particle phases in this biome, simplifying implementation in chemical transport models (CTMs) that predict number and mass concentrations of PM. During certain time periods, however, a large fraction (10 – 40%) of particles are found in a non-liquid state. The role of anthropogenic and biogenic factors is discussed to decipher what controls the liquid particulate matter above tropical forests. Differences among Earth's biomes should be taken into account in CTMs, and an approach for prognostication of liquid organic PM is presented.