Ice Nucleation Ability of Tree Pollen Altered by Atmospheric Processing
Ellen Gute, Robert O. David, Zamin A. Kanji, Jonathan P. D. Abbatt
Abstract
Atmospheric subpollen particles (SPPs) released from pollen grains can act as ice nucleating particles. Using a droplet–freezing approach to assess immersion freezing–ice nucleating (IN) activity, we investigate the effects of simulated atmospheric processing on IN ability in aqueous suspensions for SPPs exposed to (A) light arising from artificial solar radiation and 310 nm UV radiation, (B) acidic water conditions, and (C) oxidation by hydrogen peroxide and a hydroxyl radical. The most studied IN–active SPP, gray alder, loses IN activity upon exposure to all processing modes, with less active IN sites more deactivated relative to their more active counterparts. The two other investigated SPPs, silver birch and red mulberry, are less responsive to atmospheric aging. The most significant impact arose from simulated solar radiation, where a 24 h exposure lowered the IN-activation temperature of gray alder SPPs by up to 5 °C. Similar changes observed after 4 h of exposure to 310 nm light indicate that UV radiation is the most important aging pathway for active-site deactivation. Additionally, we found that water acidity (pH 4–3) negatively impacts the IN activity of all SPPs. Oxidation induced by hydrogen peroxide at high concentrations of 10–500 mM reduced IN activity, consistent with chemical processing altering a specific active-site configuration. Changes in IN activity via aqueous OH radical oxidation were not observable above the effects arising from hydrogen peroxide when present as the OH precursor. Overall, these results imply that the role SPPs play in cloud ice formation may be reduced with longer atmospheric processing times.