Two columns of air, two seasons, and two aircraft

The Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility is like a doctor who makes house calls: The state-of-the-art facility travels around the world studying global climate change in climatically important regions.

The ARM facility’s current deployment, along with two research aircraft that measure atmospheric components over land and sea, is a 12-month visit to the Highlands Center at Cape Cod National Seashore, at the request of scientists from the Pacific Northwest National Laboratory (PNNL).

The setting

Funded by DOE, ARM’s two-column aerosol project (TCAP) is designed to quantify aerosol properties, radiation, and cloud characteristics at locations that experience a variety of atmospheric conditions, including both clear and cloudy skies and both clean and polluted air. Cape Cod’s unique geography boasts a wide range of cloud regimes, which can be studied with ground-based instruments without the cost and logistics of ship-borne deployment. Additionally, although the cape is far from large local aerosol sources, it still experiences a mix of clean marine air from the Atlantic Ocean, pollutant-laden air from the ‘ozone corridor’ along the east coast of North America, and air laden with naturally occurring particles from large forested areas.

Supplementing the ARM ground-based measurements at Cape Cod are observations made from the two aircraft as they fly in stacked formation over and between two columns of air, one directly over Cape Cod and the other hundreds of kilometers out to sea. The lower-flying aircraft carried a suite of instruments to determine the aerosol optical depth, or the extinction of solar radiation by aerosols, within the columns of air. It also collected samples via a series of stairstep profiles within each column. The higher-flying craft carried a new NASA multiwavelength lidar that provides continuous remotely detected observations of optical and cloud properties throughout the column below. One set of flights by the two aircraft was completed in July, and another set is scheduled for next winter.

‘We focused on sampling the two atmospheric columns, while many other projects have attempted to follow the evolution of plumes from North America,’ says PNNL scientist Larry Berg . Results of those samplings will give scientists detailed observations with which to study the horizontal gradients of aerosol concentrations and effects on solar radiation off the coast of North America.

The aerosols

With observations from TCAP, Berg and his team will investigate how the chemical composition of manmade and natural aerosols changes their optical properties; how cloud–aerosol interactions vary with distance from the coast and time of year; and how well climate models simulate the effects.

‘We decided that the best way to address these questions was to sample within two columns of the atmosphere, one very near the coast . . . and one further out to sea,’ adds Berg.

With observations from new onboard instruments that include aerosol and single-particle mass spectrometers, TCAP scientists were able to make real-time, in situ measurements of composition, size, and mixtures of chemicals that make up individual particles. Those properties, in turn, determine optical properties and the ability of individual particles to act as cloud condensation nuclei in the atmosphere. Understanding such properties is important for assessing aerosol optical depth, which helps to determine how much solar energy reaches Earth’s surface.

Increasing the concentration of aerosols through natural or manmade activities is thought to increase the number of small droplets in a pollutant-laden cloud and thus enhance the brightness of the cloud. That so-called ‘aerosol indirect effect’ may be crucial in offsetting global warming because brighter clouds return more energy to space. The indirect effect is distinct from ‘aerosol direct effects,’ a term that describes the direct scattering and absorption of solar energy by particles.

The results?

‘During the summer we see a mix of manmade and naturally occurring particles,’ says Berg, ‘but in the winter I anticipate that the vast majority will be manmade, but with some sea-salt aerosol.’ He also expects to see a higher concentration of particles in the summer.

Data from TCAP, accessible to anyone in the atmospheric research community, will be used to build better process models for developing new parameterizations of aerosol or cloud–aerosol interactions. It can also help in the evaluation of regional- and global-scale models to predict conditions far into the future.

While the ARM mobile facility is at Cape Cod, the ‘doctors’ are in at PNNL and have plenty of data to analyze over the next months. The climate-change models await their diagnoses.