A Cleansing Rain Falls; a Soil-Filled Mist Arises

Rain’s reputation for cleansing the air may come with a caveat after new findings show that they play a role in generating airborne organic particles. The surprising results show that when droplets hit the dirt, particularly in grasslands and tilled fields, they launch a mist of microscopic particles into the air. The mechanism begins as organic matter in the soil dissolves in the puddles when it rains. Subsequent raindrops create air bubbles in the puddles that mix with the dissolved matter. The bubbles float upward and quickly burst, releasing into the air a mist of the soil organic compounds.

A research team led by Mary Gilles (Chemical Sciences Division, Berkeley Lab) gathered samples of airborne particles in Oklahoma to examine their uptake of water. Under the microscope, scientists noticed spherical, glassy particles. They were so anomalous that Gilles scribbled in her lab notebook, “Something is wrong with these samples.” Her collaborator, Alexander Laskin and his group (Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory) then took the baton and analyzed the particles using advanced chemical imaging tools, including scanning transmission x-ray microscopy (STXM) and near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy at ALS Beamlines 5.3.2.2 and 11.0.2. These results were combined with extensive laboratory characterization and nucleation studies by the Laskin group at PNNL. The results confirmed that the tiny particles were composed of organic matter from soil. The findings could influence how scientists model our planet’s climate and future.

Scientists have found that rain triggers the release of a mist of particles from wet soils into the air, a finding with consequences for how scientists model our planet’s climate and future. (Credit: Pacific Northwest National Laboratory.)