Quantitative assessment of coagulation of atmospheric particles onto airborne birch pollen grains

The coagulation of airborne particles on the surface of allergenic pollen grains is poorly described in the literature. However, particles deposited on the surface of pollen grains could have an effect on allergy and its symptoms. Observations of pollen surface alterations reported in the literature are either qualitative or are tainted by sampling artifacts that overestimate surface particulate pollution. Birch pollen grains (BPGs) were sampled in the atmosphere during pollination to quantitatively assess the extent of particulate deposition on allergenic pollen surfaces. Airborne PGs were collected with a cascade impactor ensuring no sampling artifact unlike pollen samplers usually used in this type of study. Counting, sizing and elemental analysis of particles adhering on pollen surfaces were performed by SEM/EDX. 68% of PGs did not carry any particle on their visible surface while the remaining 32% had between 1 and 16 particles on their surface. On average, polluted BPGs carried 2 ± 1 particles, representing an average surface coverage of 0.95%. We hypothesized that the main coagulation process was particle deposition by gravitational settling. Collision velocity calculations indicate that coagulation due to gravitational settling is prevalent under typical pollination atmospheric conditions. However, the effects of electric charges carried by pollen are largely unknown and may significantly alter coagulation rates. The presence of particle-polluted allergenic pollens appears to be common in the urban atmosphere, even under low air pollution conditions. The impact of chemical pollution of BPGs due to the presence of particulate matter (including soot particles) remains relatively unknown, both on the allergy and on the reproductive capacity of the trees. • Pollen grains were sampled under moderate pollution (average PM10 = 26.3 μg m −3 ). • One third of the airborne pollen grains were observed with adhered airborne particles. • Polluted pollen carried 2 ± 1 particles with an average surface coverage of 0.95%. • Particulate coagulation on pollen has potential effects on its allergenicity and viability.