Comment on egusphere-2023-670

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Pollen are hygroscopic and so have the potential to act as cloud condensation nuclei (CCN) in the atmosphere. This could have yet uncertain implications for cloud processes and climate. Previous studies have investigated the hygroscopic swelling of pollen, linked to CCN activity by the &kappa;-K&ouml;hler theory, using methods that follow observed mass increase by electrodynamic balance (EDB) or vapour sorption analyser. This study uses an acoustic levitator to levitate pollen grains in the true aerosol phase and uses a macroscope to image the pollen to investigate hygroscopic behaviour when relative humidity (RH) is changed. Two pollen species were studied in this work: <em>Lilium orientalis</em> (oriental lily) and <em>Populus deltoides</em> (eastern cottonwood). Both species were successfully levitated, however, the smaller <em>Populus deltoides</em> showed greater instability throughout experiments. The quality of images taken by the macroscope, and thus calculations of pollen area and diameter ratio, varied significantly and were sensitive to lighting conditions, as well as levitated pollen grain movement and orientation. Experiments with surface-fixed pollen grains were also conducted. They showed evidence that pollen hygroscopic swelling could be observed by the macroscope. The produced results were comparable with previously reported mass increase values. Although less accurate than methods that measure mass changes, the acoustic levitator and macroscope setup offer an attractive alternative by virtue of being commercial-off-the-shelf, low-cost, and versatile. A key advantage of this method is that it is possible to visually observe particle shape dynamics under varying environmental conditions.