Pollen as atmospheric cloud condensation nuclei

Abstract

Anemophilous (wind-dispersed) pollen grains are emitted in large quantities by vegetation in the midlatitudes for reproduction. Pollen grains are coarse particles (5-150μm) that can rupture when wet to form submicron subpollen particles (SPP) that may have a climatic role. Laboratory CCN experiments of six fresh pollen samples show that SPP activate as CCN at a range of sizes, requiring supersaturations from 0.81 (±0.07)% for 50 nm particles, 0.26 (±0.03)% for 100nm particles, and 0.12 (±0.00)% for 200nm particles. Compositional analyses indicate that SPP contain carbohydrates and proteins. The SPP contribution to global CCN is uncertain but could be important depending on pollen concentrations outside the surface layer and the number of SPP generated from a single pollen grain. The production of hygroscopic SPP from pollen represents a novel, biologically driven cloud formation pathway that may influence cloud optical properties and lifetimes, thereby influencing climate. Key Points Pollen grains can rupture when wet to form submicron subpollen particles (SPP) Laboratory experiments show that SPP are hygroscopic and can act as CCN Pollen grains may contribute to CCN in northern midlatitudes