Pollen Rupture and Its Impact on Precipitation in Clean Continental Conditions

Abstract

Pollen grains emitted from vegetation can rupture, releasing subpollen particles (SPPs) as fine atmospheric particulates. Previous laboratory research demonstrates potential for SPPs as efficient cloud condensation nuclei (CCN). We develop the first model of atmospheric pollen grain rupture and implement the mechanism in regional climate model simulations over spring pollen season in the United States with a CCN-dependent moisture scheme. The source of SPPs (surface or in-atmosphere) depends on region and sometimes season, due to the distribution of relative humidity and rain. Simulated concentrations of SPPs are approximately 1–10 or 1–1,000 cm−3, depending on the number of SPPs produced per pollen grain (nspg). Lower nspg (103) produces a negligible effect on precipitation, but high nspg (106) in clean continental CCN background concentrations (100 CCN per cubic centimeter) shows that SPPs suppress average seasonal precipitation by 32% and shift rates from heavy to light while increasing dry days. This effect is smaller (2% reduction) for polluted air.