Quantification of the kinetics of ozone uptake by tree pollen and the extent of modification

Understanding the interaction between ozone and pollen is crucial, as it may influence pollen allergenicity and reproductive viability. This study investigates the kinetic uptake of ozone by pollen, and the its resulting modification, revealing significant variability among 12 tree pollen species.

Exposure of pollen to moderate ozone levels (130 – 150 ppb) in small air chambers facilitated the measurement of ozone uptake until total surface saturation. This yielded initial uptake coefficients (0.6 – 6.4 x 10^-5) and total adsorbed ozone (9.4 – 2200 ng O₃ per mg of pollen). Using this information, we calculate the number of reactive sites on pollen surfaces (10¹³ – 10¹⁶ sites cm^-2) and observed rate coefficients (k_obs) ranging from 10^-15 – 10^-17 cm³ sites^-1 s^-1. Notably, Ash pollen, similar in size to Birch pollen, exhibited 25 times greater ozone uptake, indicating a dependence on pollen type. Molecular modifications induced by ozone were explored using optical spectroscopy, identifying potential chemical markers and emphasizing molecular diversity among pollen species. Antioxidant molecules, like carotenoids in Ash, indicated a protective role for the pollen coat, suggesting that significant ozone uptake may not harm pollination.

Using a box model with plausible rates for pollen emission and deposition, we were able to calculate the extent of pollen modification by ozone in airborne pollen under environmentally relevant conditions. Depending on the pollen species, the extent of oxidation in airborne pollen ranged from 24% - 97%, and was highest in the late afternoon. Sensitivity analysis shows that the extent of oxidation is sensitive to uptake kinetics, ozone concentrations, and assumptions about mixing height. Under typical conditions, tree pollen grains are airborne for long enough that they will experience a significant proportion of the total potential modifications induced by ozone exposure.