Regulators have assumed that anything emitted above 3,000 feet would be deposited into a part of the atmosphere that has significantly smoother air, meaning pollutants wouldn’t be affected by turbulent air that could mix them toward the ground. Thus, even though 90% of aircraft fuel is burned at cruise altitudes, only those pollutants that are emitted during takeoff and landing are regulated by measuring emissions during tests of newly manufactured engines in simulated takeoff and landing conditions.

However, recent regional modeling work indicates that cruise emissions may contribute a significant fraction of aircraft-accountable ground-level pollutant concentrations on a regional scale.

We report simulation results indicating that aircraft cruise emissions are implicated in increased premature mortality on a hemispheric scale. Furthermore, meridional and zonal circulation patterns at cruise altitudes displace impacts from flight paths by several thousand kilometers. Our approach is to use a recent aircraft emissions inventory, a global chemistry-transport model, population density and disease statistics, and concentration-response functions derived from epidemiological studies to assess the impact of aircraft emissions globally on premature mortality. Parametric uncertainties in aircraft emissions and concentration-response functions are propagated throughout the analysis, along with estimates of modeling uncertainty.

—Barrett et al.

The researchers also note that the impact of landing and takeoff emissions is likely to be under-resolved. Secondary H₂SO₄-HNO₃-NH₃ aerosols are found to dominate mortality impacts.

Due to the altitude and region of the atmosphere at which aircraft emissions at cruise are deposited, the extent of transboundary air pollution is particularly strong. The authors describe how strong zonal westerly winds aloft, the mean meridional circulation around 30-60°N, interaction of aircraft-attributable aerosol precursors with background ammonia, and high population densities in combination give rise to an estimated 3,500 premature mortalities per year in China and India combined, despite their relatively small current share of aircraft emissions.

Subsidence of aviation-attributable aerosol and aerosol precursors occurs predominantly around the dry subtropical ridge, which results in reduced wet removal of aviation-attributable aerosol.

It is also found that aircraft NO_x emissions serve to increase oxidation of non-aviation SO₂, thereby further increasing the air quality impacts of aviation. We recommend that cruise emissions be explicitly considered in the development of policies, technologies and operational procedures designed to mitigate the air quality impacts of air transportation.

—Barrett et al.

The study was funded by the UK Research Councils with help from the US Department of Transportation.

Steve Lott, a spokesman for the International Air Transport Association, a trade group that represents 230 airlines, says that aviation is “a small part of a big problem,” particularly when compared to other transportation sources of emissions, such as those caused by shipping, which a 2007 study linked to 60,000 premature deaths per year.

Lourdes Maurice, the chief scientific and technical adviser for environment at the Federal Aviation Administration, says that if the agency can confirm the findings through additional research, then it will work with the Environmental Protection Agency and the International Civil Aviation Organization to consider appropriate regulatory action. The FAA will continue to fund research to address uncertainties highlighted by the work, she adds.

Lead author Steven Barrett from MIT notes that there are many uncertainties, including how accurately the model reflects how air travels vertically from high altitudes to low altitudes. To address this, he is collaborating with researchers at Harvard to study an isotope of the element beryllium that is produced naturally at high altitudes and attaches to atmospheric particles that eventually reach the ground through air or rain. Researchers have a general idea of how much beryllium is concentrated in the atmosphere, and Barrett and his colleagues are currently analyzing ground measurements of the element to quantify the extent to which the model “gets vertical transport right.”

Resources

• Steven R. H. Barrett, Rex E. Britter and Ian A. Waitz (2010) Global Mortality Attributable to Aircraft Cruise Emissions. Environ. Sci. Technol., Article ASAP. doi: 10.1021/es101325r