While falling rain washes pollen grains from the atmosphere, rain can also induce pollen rupturing into submicrometer pollen fragments. Compared to intact pollen, such fragments can penetrate deeper into the human respiratory system and persist longer in the atmosphere. Herein, we provide the first online characterization of pollen fragments during thunderstorms and rain events. For the first time, we combine single-particle fluorescence spectroscopy with offline measurements of chemical tracers to provide direct evidence of atmospheric pollen fragments. We show a significant increase in pollen fragments with diameters of 0.25−1.0 μm coincident with precipitation. Pollen fragment concentrations peak during convective thunderstorms with strong downdrafts, high rates of rainfall, and numerous lightning strikes, although lightning is not required for the fragments' release. After storms, pollen fragments persist in the atmosphere for several hours. Our results show that while pollen grains decrease substantially during rain, peak concentrations of submicrometer pollen fragments occur during rain events and then persist for several hours. This study provides new information on the abundance, size distribution, and meteorological drivers of pollen fragments in the atmosphere. Because pollen fragments potentially carry allergens and thunderstorms are predicted to increase in frequency and strength, understanding human exposures to pollen fragments is increasingly important.

Ragweed pollen is a prevalent allergen in late summer and autumn, worsening seasonal allergic rhinitis and asthma symptoms. In the atmosphere, pollen can osmotically rupture to produce sub-pollen particles (SPP). Because of their smaller size, SPP can penetrate deeper into the respiratory tract than intact pollen grains and may trigger severe cases of asthma. Here we characterize airborne SPP forming from rupturing giant ragweed (Ambrosia trifida) pollen for the first time, using scanning electron microscopy and single-particle fluorescence spectroscopy. SPP ranged in diameter from 20 nm to 6.5 μm. Most SPP are capable of penetrating into the lower respiratory tract, with 82% of SPP < 1.0 μm, and are potential cloud condensation nuclei, with 50% of SPP < 0.20 μm. To support predictions of the health and environmental effects of SPP, we have developed a quantitative method to estimate the number of SPP generated per pollen grain ($${n}_{\mathrm{f}}$$ n f ) based upon the principle of mass conservation. We estimate that one giant ragweed pollen grain generates 1400 SPP across the observed size range. The new measurements and method presented herein support more accurate predictions of SPP occurrence, concentration, and air quality impacts that can help to reduce the health burden of allergic airway diseases. Graphic abstract Rupturing ragweed pollen releasing cellular components (right), viewed by an inverted light microscope.

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