Data from Remote Regions Provide Insight Into Aerosol and Cloud Interactions

ARM data from a remote site in the Azores provides clues to aerosol and cloud interactions in the pre-industrial area — important information for quantifying the effects of atmospheric pollution on clouds.

The Science

Current estimates suggest that between one-quarter and two-thirds of all particles on which cloud droplets form (known as cloud condensation nuclei) in the atmosphere may be a direct result of human activities. Climate models suggest that this increase in particle concentrations has led to brighter clouds, which cool the earth and may be masking a significant fraction of the global warming that the Earth would be experiencing if particle concentrations had not increased. 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 these particles (known as aerosol) remain in the atmosphere and how quickly they are removed, primarily by clouds and rain. It can be difficult to find locations and times in the Northern Hemisphere where current concentrations of aerosol particles have not been increased since pre-industrial times, but studying these regions is important for understanding what the atmosphere may have been like in pre-industrial conditions.

The 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.  Observations show that concentrations of cloud-forming aerosol particles at the site can vary by two orders of magnitude.  Many of the cases with high aerosol concentrations can be traced back to plumes of aerosol particles from the North American continent.  At other times, there are cases of very low aerosol concentration, more similar to pre-industrial conditions.  Several previous studies explored how continental pollution is transported to the Azores by the prevailing eastward-moving winds. In contrast, there have been very few studies that have examined the low end of the range of aerosol concentrations. This study focused on cases where the island experienced very low concentrations of aerosol particles, providing clues to the processes that remove particles from the atmosphere and helping scientists understand what conditions were like in the pre-industrial environment.


The researchers use a long record of aerosol particles measured at the Atmospheric Radiation Measurement (ARM) site on Graciosa Island in the Azores to characterize air masses with very low concentrations. Additional surface, satellite, and weather model data are used to explore the meteorological and cloud context occurring during low-aerosol-concentration events. These events occur in all seasons, but their frequency was three times higher in December-May than during June-November. 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.

Principal Investigator(s)

Robert Wood
University of Washington


This work was supported by DOE grants DE SC0006865MOD0002 and DE-SC0013489.  The CAP-MBL deployment of the ARM Mobile Facility was supported by the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program Climate Research Facility and the DOE Atmospheric Sciences Program.


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.