Pollen Rupture and Its Impact on Precipitation in Clean Continental Conditions

• Published in: Geophysical research letters Vol. 45; no. 14; pp. 7156 - 7164
• Main Authors: Wozniak, M. C., Solmon, F., Steiner, A. L.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.07.2018
• Subjects: Air pollution; Anthropogenic factors; Atmosphere; Atmospheric models; Atmospheric particulates; Atmospheric pollution deposition; Atmospheric precipitations; biological aerosols; CCN; Climate models; Cloud condensation nuclei; Cloud droplets; Clouds; Computer simulation; Condensation; Condensation nuclei; Geographical distribution; Human influences; Humidity; Interfaces; Internal pressure; Laboratories; Laboratory experimentation; Nucleus; Numerical models; Particulates; Pollen; Pollution control; Precipitation; Rain; Rainfall simulators; Regional climate models; Regional climates; Relative humidity; Rupture; Rupturing; Seasonal distribution; Seasonal precipitation; Seasons; Vertical distribution; Water availability; Water pollution; Water vapor; Water vapour
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL077692

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. Plain Language Summary Pollen grains emitted by wind from a variety of plants can swell from exposure to high levels of humidity, creating internal pressure that may cause the grains to rupture. Particles that are 10 to a thousand times smaller than pollen grains are released in the process. These subpollen particles (SPPs) have been found in laboratory studies to efficiently collect water on their surfaces, making them potential cloud condensation nuclei (i.e., particles that may grow into cloud droplets). We have developed a numerical model of pollen rupture that interfaces with an atmosphere model to determine (1) how many SPPs are produced during the pollen season from two different sources: rupture of pollen at the surface and rupture of airborne pollen grains; (2) the geographic and vertical distribution of SPPs seasonally; and (3) the impact of SPPs on regional precipitation. We find that the strength of either source in any region or phase of season depends on rain and relative humidity. We also find that SPPs have the potential to suppress seasonal precipitation in clean conditions when anthropogenic pollution is not present depending on how many are released for each pollen grain that ruptures. The magnitude of suppression regionally is dependent on source magnitude of SPPs, as well as the availability of water vapor. Key Points The first model of moisture‐induced pollen rupture and release of subpollen particles (SPPs) is coupled to a regional climate model During peak pollen season in the United States, simulated SPPs range from 1 to 1,000 cm−3, depending on the number produced per pollen grain ruptured SPP may have the ability to suppress precipitation regionally in clean continental CCN conditions and induce a negative feedback to SPP production