Sub-micrometre particulate matter is primarily in liquid form over Amazon rainforest

Atmospheric particulate matter influences the Earth's energy balance directly, by altering or absorbing solar radiation, and indirectly by influencing cloud formation(1). Whether organic particulate matter exists in a liquid, semi-solid, or solid state can affect particle growth and reactivity(2,3), and hence particle number, size and composition. The properties and abundance of particles, in turn, influence their direct and indirecte effects on energy balance(4). Non-liquid particulate matter was identified over a boreal forest of Northern Europe(5), but laboratory studies suggest that, at higher relative humidity levels, particles can be liquid(6,7). Here we measure the physical state of particulate matter with diameters smaller than 1 mu m over the tropical rainforest of central Amazonia in 2013. A real-time particle rebound technique shows that the particulate matter was liquid for relative humidity greater than 80% for temperatures between 296 and 300 K during both the wet and dry seasons. Combining these findings with the distributions of relative humidity and temperature in Amazonia, we conclude that near-surface sub-micrometre particulate matter in Amazonia is liquid most of the time during both the wet and the dry seasons.