Live pine pollen in rainwater: reconstructing its long-range transport

摘要

Raindrops brim with pollen even when there is no ambient local pollen. How does this nonlocal pollen get inside rain? The likely answer is long-range transport beneath or inside clouds. To test this hypothesis, we captured rain-delivered pollen on Ocracoke Island, NC, USA over a 12-day interval before local pine pollen release then reconstructed its trajectory and its atmospheric exposure conditions. Findings were as follows: four rain episodes yielded a total of 632 pollen grains of which 6.7% germinated. To find pollen sources, we first identified pollen-releasing forested areas using a predictive heat sum equation for each rain episode. Next, we constructed the backward trajectory for air parcels carrying rain-delivered pollen from those forests using the MLDP atmospheric transport and dispersion model. Nonlocal sources were located at distances up to 300 km from Ocracoke Island and distances lessened with each successive episode. Below-cloud transport time was 8 and 17 h for Episodes A and B, respectively. Pollen grains were exposed to harsh atmospheric conditions during below-cloud transport, yet some grains still germinated. Atmospheric turbulence patterns changed for each episode, so distance from pollen source was poorly correlated with pollen transport time. Pollen germination was not closely correlated with either distances or transport time. In-cloud transport was more likely for pollen sampled during Episodes C and D. Pine pollen, although rarely allergenic, brings fresh insights into how atmospheric events can trigger human respiratory distress.