Low-aerosol conditions over the Azores occur during marine cold air outbreaks

Science

Current estimates suggest that between one-quarter and two-thirds of all cloud condensation nuclei in the atmosphere may be a direct result of human activities. Climate models suggest that brighter clouds are masking a significant fraction of the global warming that the Earth would be experiencing if aerosol particle concentrations had not increased. These aerosol-cloud interactions are, however, much more complex and uncertain than are processes causing greenhouse warming. The strength of this cooling effect is strongly dependent upon how many particles were present in clouds before large-scale industrialization commenced in the 1750s. The cooling effect also depends upon how long aerosol particles remain in the atmosphere and how quickly they are removed, primarily by clouds and rain. Models tell us that it can be difficult to find many locations and times in the Northern Hemisphere where current concentrations of aerosol particles have not been increased since pre-industrial times.

Impact

A study led by researchers at the University of Washington provided new insights into processes controlling aerosol concentrations at a remote island site in the Azores archipelago situated in the eastern North Atlantic Ocean. The site is far from continental sources of aerosol particles, and yet there is clear evidence of continental aerosol particles reaching there from North America. Graciosa Island experiences concentrations of cloud-forming aerosol particles that vary by two orders of magnitude from 10 cm-3 to 1000 cm-3. Many of the high-aerosol cases can be traced back to plumes of aerosol particles from the North American continent, and the Azores has been the site of several studies exploring how continental pollution is transported to remote regions by the prevailing eastward-moving winds.

In contrast, very few studies have tried to examine and understand the low end of the wide range of aerosol concentrations experience in remote regions. Our focus in this study is cases where the island experienced very low concentrations of aerosol particles, as these provide clues to the processes that remove particles from the atmosphere and may help us understand what conditions were like in the pre-industrial environment.

Summary

We use a long record of aerosol particles measured at Graciosa Island in the Azores to characterize air masses with very low concentrations. Surface, satellite, and weather model data are used to explore the meteorological and cloud context occurring during low-aerosol-concentration events, which were also associated with low concentrations of cloud droplets. These events occur in all seasons, but their frequency was three times higher in December-May than during June-November, and many of the low-aerosol events had a common meteorological basis that involves the transport of cold air from the north and west of Graciosa, a weather phenomenon known as a marine cold air outbreak. Low-aerosol events were associated with low concentrations of cloud droplets. Satellite data are consistent with the hypothesis that observed low-aerosol conditions are often formed by aerosol removed by precipitation in thick warm clouds that occurs during the early stages of cold air outbreaks.