Airborne Bioaerosol Observations Imply a Strong Terrestrial Source in the Summertime Arctic

Primary biological aerosol (PBA) is a component of coarse mode aerosol which may affect climate and health. The possible climate impacts arise from interactions between PBA and water vapor, especially since some PBA nucleate ice at warm temperatures. The health impacts span from seasonal allergies to transmission of pathogens. Despite their potential importance, the emissions, transport, and atmospheric distribution of PBA are poorly understood, especially at high latitudes where cloud effects could be pronounced. Here we report summertime measurements of fluorescent aerosol (a proxy for PBA) over the Bering and Chukchi Seas using a Wide-Band Integrated Bioaerosol Sensor aboard a Twin Otter, alongside a lidar which detected water column productivity. Most observations occurred at 300 m over the ocean with periodic excursions to 60 and 900 m. Loadings were always low in the marine boundary layer, despite the presence of subsurface plankton layers and in contrast to other recent reports, likely indicating low oceanic emissions during our study. Large variability was observed in PBA aloft, with higher concentrations approaching those observed over the Continental US. Back trajectory analysis showed that high loadings were associated with recent transit through the continental boundary layer and we estimate PBA emissions from the Arctic tundra of up to 300 m(-2) s(-1) at the warmest observed temperatures. On days with strong transport from land (similar to 50% of our flights), PBA accounts for 12% of supermicron number and 64% of supermicron volume, indicating potentially significant effects on the albedo, glaciation and lifetime of Arctic clouds. Plain Language Summary Cloud cover affects Earth's temperature because clouds reflect sunlight back to space. Biological particles can cause warm clouds to freeze sooner than most other materials. This makes the clouds go away faster and reflect less sunlight. In the warming Arctic, ice, and snow (which also reflect sunlight) are melting, exposing water. Water absorbs sunlight, which makes cloud cover even more important. At the same time, Arctic ecosystems are changing, and might emit more biological particles in response. We measured biological particles over the Arctic Ocean. We found more than we were expecting and we think most of them came from land. This could change how Arctic clouds behave in important ways. As the Arctic continues to warm there might be even more of these particles in the future.