June 2008

Measuring the Arctic’s haze and smoke

NCAR researchers investigate air pollution, climate change

The Arctic is often perceived as a pristine place, located as it is far from the world’s smokestacks. And yet its atmosphere serves as a receptor for air pollution from the industrial regions of North America, Europe, and Asia.

The reddish-brown soup of pollution that peaks in late winter and early spring even has a name—Arctic haze. During the summer, smoke from wildfires joins the mix.

This spring and summer, NCAR researchers from ESSL/ACD and EOL are supporting a NASA field project to investigate the chemistry of the Arctic’s lower atmosphere. Their objective is to identify how air pollution contributes to climate change in the region and learn more about why the Arctic’s climate is changing so rapidly.

“The Arctic is a beacon of global change, having warmed more rapidly than anywhere on the planet over the past 100 years,” says Guy Brasseur, director of ESSL. “There’s an urgent need for research to better understand changes in the atmospheric composition and climate of this vulnerable place.”

Called ARCTAS (Arctic Research on the Composition of the Troposphere from Aircraft and Satellites), the field project is associated with International Polar Year. It combines atmospheric measurements taken aboard research aircraft with data from NASA satellites and computer modeling. It includes two aircraft deployments, using NASA’s DC-8, each three weeks in duration.

During the spring deployment, which took place in April in Fairbanks, Alaska, scientists gathered information about the effects of Arctic haze, stratosphere-troposphere exchange, and sunrise photochemistry (chemical reactions that occur when sunlight returns to the Arctic in spring). During summer deployments scheduled for June 18–25 in Palmdale, California, and June 26–July 12 in Cold Lake, Alberta, the team will investigate how emissions from northern wildfires affect the Arctic’s atmosphere.

Arctic haze and climate change

Atmospheric circulation carries air pollution and wildfire emissions from Earth’s northern midlatitudes to the Arctic, where they mix and react with sunlight, producing the ozone and aerosols that compose Arctic haze.

The haze affects the highly reflective Arctic ice sheet in ways that can increase temperatures both in the atmosphere and on Earth’s surface. Particles deposited on the surface darken the snow, reducing its albedo (reflectivity) and causing it to absorb more sunlight, warming the surface. Particles may also impact the radiative characteristics of Arctic clouds, making them more effective insulators. Such disruptions to the environment trigger responses that include melting permafrost and ice sheets.

To learn more about the interaction between Arctic pollution and climate change, the ARCTAS team is focusing on four major scientific themes:

• long-range transport of pollution to the Arctic

• the impacts of boreal wildfires on atmosphere and climate

• aerosol radiative forcing from Arctic haze and other air pollution

• chemical processes (ozone, mercury, aerosols, and halogen) related to haze

Chemical weather forecasting

Several ACD researchers are forecasting chemical weather in support of ARCTAS—that is, using satellite ­observations and model simulations to predict distributions of the aerosols and trace gases that make up air pollution above the Arctic. Their forecasts will show the aircraft team where to fly to sample the pollution.

To produce the forecasts, the ­researchers use the Data Assimilation Research Testbed (DART) with the NCAR Community Atmosphere Model with Chemistry (CAM-Chem). ­CAM-Chem couples NCAR’s MOZART (Model for OZone And Related chemical Tracers) with the Community Atmosphere Model for an inter­active look at chemistry and climate.

The researchers use near real-time observations to improve the model, including meteorological observations and measurements from MOPITT and MODIS, instruments aboard NASA satellites. MOPITT (Measurements Of Pollution In The Troposphere) retrieves carbon monoxide measurements, while MODIS (Medium Resolution Imaging Spectroradiometer) retrieves measurements of aerosol optical depth.

“One of the nice things about participating in ARCTAS, as well as other field experiments, is that it’s a good test for our models,” says Louisa Emmons, who’s leading ACD’s chemical forecasting efforts. “We predict where a pollution plume is going to be and then the aircraft goes off and samples it, so right away we know whether our model was accurate.”

After the operational phase of ARCTAS is over, the researchers will switch their focus away from chemical forecasting toward questions related to chemistry and climate.

Airborne measurements

A number of NCAR researchers, in conjunction with university colleagues, are operating instruments during ARCTAS flights aboard the DC-8, measuring formaldehyde, hydrogen oxide radicals, nitrogen oxides, ozone, volatile organic compounds, and atmospheric radiation. The comprehensive suite of instruments portrays Arctic pollution in more detail than ever before.

From ACD, Andrew Weinheimer, David Knapp, and Denise Montzka are measuring nitrogen oxides and ozone; Eric Apel and Alan Hills are looking at a suite of more than 30 volatile organic compounds; and Rick Shetter, Sam Hall, and Kirk Ullmann are observing solar radiation. (Rick is also director of the University of North Dakota’s National Suborbital Education and Research Center, which is managing the engineering, data systems, and science on the DC-8.) Chris Cantrell, Lee Mauldin, Becky Anderson, and Ed Kosciuch are measuring hydrogen oxide and peroxy radicals.

From EOL, Alan Fried, Dirk Richter, Petter Weibring, and Jim Walega are deploying a difference frequency generation absorption spectrometer to measure formaldehyde.

A trace gas that plays a role in ozone production, formaldehyde is an example of one gas that is important to ARCTAS, for several reasons. “It’s a moderate- to short-lived gas with a lifetime of several hours that reflects localized chemistry, not long-range transport, and as such is one means of deducing how fresh the sampled air is,” Alan explains. “If we’re seeing high levels of formaldehyde, it’s not from long-range transport.”

Formaldehyde measurements also offer insight into the unique halogen chemistry that occurs near Arctic ice, which results in significant depletion of the ozone in the Arctic’s boundary layer. Sea salt deposited onto the ice surface produces bromine and chlorine compounds in both the gas and aerosol ­phases, through mechanisms that scientists don’t fully understand. Formaldehyde measurements, in conjunction with those of other gases, give clues as to which of these two halogens is dominant.

The large collection of instruments on the DC-8 provides an opportunity for researchers to compare measurements made by different instruments. Chris Cantrell and colleagues, for example, are comparing their measurements of hydrogen oxides, made by a mass spectrometer, to measurements from an instrument from Pennsylvania State University that uses a different technique (laser-induced fluorescence) to measure the same radicals. The two techniques have never been used during the same airborne mission.

“This gives us a chance to compare how the instruments are working,” Chris says. It might also help the team address the fact that other research flights have found discrepancies between observed hydrogen oxide concentrations and ­values simulated by models.

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ARCTAS

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