Simulating the Transport and Rupture of Pollen in the Atmosphere

Abstract

Pollen, one type of primary biological aerosol particle (PBAP), is emitted from the terrestrial biosphere and can undergo physical changes in the atmosphere via particle rupture. To examine the fate of pollen and its atmospheric processing, a pollen emission and transport scheme is coupled to the Weather Research and Forecasting Model with Chemistry (WRF‐Chem). We simulate the emission of pollen and its impacts on the cloud properties and precipitation in the Southern Great Plains from 12 to 19 April 2013, a period with both high pollen emissions and convective activity. We conduct a suite of ensemble runs that simulate primary pollen and three different pollen rupture mechanisms that generate subpollen particles, including (a) high humidity‐induced surface rupture, (b) high humidity‐induced in‐atmosphere plus surface rupture, and (c) lightning‐induced rupture, where in‐cloud and cloud‐to‐ground lightning strikes trigger pollen rupture events. When relative humidity is high (>80%), coarse primary pollen (∼1 μg m−3) is converted into fine subpollen particles (∼1.2e−4 μg m−3), which produces 80% more subpollen particles than lightning‐induced rupture. The in‐atmosphere humidity‐driven rupture predominantly produces subpollen particles, which is further enhanced during a frontal thunderstorm. During strong convection, vertical updrafts lift primary pollen and subpollen particles (∼0.5e−4 μg m−3) to the upper troposphere (∼12 km) and laterally transports the ruptured pollen in the anvil top outflow. In regions of high pollen and strong convection, ruptured pollen can influence warm cloud formation by decreasing low cloud (<4 km) cloud water mixing ratios and increasing ice phase hydrometeors aloft (>10 km).

Effects of Pollen on Hydrometeors and Precipitation in a Convective System

“…The loss in cloud droplets is offset by the increased cloud droplet condensation (Figure S4a in Supporting Information ), resulting in small changes in cloud droplet mass mixing ratio, with a decrease of 4% and 10% in the lower atmosphere and an increase of 25% and 35% around −20°C for “M23” and “M23 + rup,” respectively (Figure 9b). These findings regarding changes in cloud and rain mass mixing ratio are consistent with previous studies that only included pollen and SPP as CCN (e.g., Subba et al., 2023), which also found higher rain drop mass mixing ratio and lower cloud droplet mass mixing ratio in the lower atmosphere (with ∼10% decrease in cloud drop mass mixing ratio and 1% increase in rain drop mass mixing ratio). After adding pollen and SPPs as INPs here, the changes in rain drop mass mixing ratio increase compared to the prior study, up to 10% in “M23” (with the same rupture rate of SPPs as Subba et al., 2023) (Figure 9d), further increasing precipitation rates (Figure 10).…”

“…These findings regarding changes in cloud and rain mass mixing ratio are consistent with previous studies that only included pollen and SPP as CCN (e.g., Subba et al., 2023), which also found higher rain drop mass mixing ratio and lower cloud droplet mass mixing ratio in the lower atmosphere (with ∼10% decrease in cloud drop mass mixing ratio and 1% increase in rain drop mass mixing ratio). After adding pollen and SPPs as INPs here, the changes in rain drop mass mixing ratio increase compared to the prior study, up to 10% in “M23” (with the same rupture rate of SPPs as Subba et al., 2023) (Figure 9d), further increasing precipitation rates (Figure 10). Overall, our findings indicate that the addition of pollen and SPPs increases the deep convection in the system, enhance the collision and coalescence processes in the cloud, and increase rain drop number in the lower atmosphere compared to simulations considering the effects of pollen and SPPs effects as CCNs alone.…”

“…The coupled pollen-WRF model accounts for the impact of meteorology, including wind, precipitation, and relative humidity on the daily total pollen emission potential (Subba et al, 2023;Wozniak et al, 2018). Once in the atmosphere, pollen rupture occurs when relative humidity exceeds 80%, and a large amount (70%) of whole pollen grains will be converted into SPPs (e.g., EXP4 in Subba et al, 2023). The default rupture rate is estimated to be 1,000 SPPs grain 1 based on previous studies (Stone et al, 2021;Suphioglu et al, 1992), and we increase this rate to 1.25 × 10 5 SPPs grain 1 for a sensitivity analysis based on laboratory experiments (Matthews et al, 2023).…”

“…Zhang & Steiner, 2022c). Subba et al (2023) coupled this model with the Chemistry version of Weather Research and Forecast model (WRF-Chem 3.9.1) (Grell et al, 2005), enabling the simulation of atmospheric pollen and SPPs concentrations with meteorology and providing a foundation for future studies on pollen-cloud interactions.…”

“…In this work, we present new simulations of PBAPs and their interaction with clouds using pollen-WRF-Chem coupled modeling framework, including comprehensive pollen emissions (PECMv2.0; Wozniak & Steiner, 2017;Y. Zhang & Steiner, 2022c) and the transport, rupture, and fate of pollen in the atmosphere (Subba et al, 2023). The pollen rupture rate is measured by a realistic laboratory experiment using an environmental chamber (Matthews et al, 2023).…”