How rain captures air pollution

Human activities and natural sources pollute the air we breathe, harming our health and the environment, and marring the beauty of our skies. One of nature’s processes for cleaning the air is rain. A falling rain droplet sweeps through the air colliding with suspended pollution particles. These particle-droplet collisions have been presumed to be capture events, but the details of what occurs during collision remains unclear. We investigate these collision events and show that rain droplets capture pollution particles internally and on their outer surfaces with multiple collision behaviors, that include: cavity-forming droplet entries, ricochets, and more. Rain drop diameter and free fall velocity, in addition to pollution particle characteristics determine which capture or escape behavior occurs. Our findings reveals that rain does not capture all particulate matter upon collision nor does it capture all airborne pollutants equally. Hence, some pollutants may be more difficult to clean out of the air than others and environmental models on rain scavenging efficiencies should take into account both rain and pollution characteristics to more accurately describe pollution fluxes in the environment.