Low-aerosol Conditions over the Azores Occur During Marine Cold Air Outbreaks.
Wood, R., University of Washington
Warm Boundary Layer Processes
Wood R, J Stemmler, J Rémillard, and A Jefferson. 2017. "Low-CCN concentration air masses over the eastern North Atlantic: Seasonality, meteorology, and drivers." Journal of Geophysical Research: Atmospheres, 122(2), 10.1002/2016JD025557.
Visible satellite image showing low clouds associated with a cold air outbreak sweeping over the Azores. These conditions are associated with very low aerosol concentrations at the island.
ScienceCurrent 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.
ImpactA 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.