Emissions from aviation often result in jet trails, a specific type of cloud formation. These clouds can influence climate through alteration of water vapor and heat retention due to how they reflect radiation. Jet emissions also contain aerosol particles that can influence atmospheric behavior. Direct emissions of carbon dioxide also influence the climate. For all these reasons, governments are considering ways to curb aviation-related emissions.

Recently, a team of researchers has investigated the impact of biofuels on aviation emissions. Sustainable bio jet fuels offer a potential route to offsetting greenhouse gas emissions, but issues of cost and sustainability continue to hamper biofuels' widespread adoption. Use of bio jet fuels also offers other advantages, including a potential reduction in harmful emissions due to near-zero levels of sulfur and aromatic chemicals common in petroleum-based jet fuels. These types of emissions are particularly problematic because they can induce further cloud formation.

Cruise condition evaluation of jet engine exhaust

The studies were conducted using NASA DC-8 turbofan engines on flights originating at NASA's Armstrong Flight Research Center in Palmdale, California. The aircraft contained four wing-mounted engines that could be fed fuel separately. The researchers took measurements of jet engine exhaust at cruise conditions when burning either a blended biofuel or a conventional petroleum jet fuel. The petroleum jet fuels explored were either medium- or low-sulfur Jet A fuel. The biofuel was an approximately 50:50 volume blend of low-sulfur content Jet A fuel and a Camelina-based EFA biojet fuel.

Sampling of the left and right inboard DC-8 engine exhaust plumes was conducted by research aircraft flying in a trailing formation at a distance of roughly 30 to 150 meters. The researchers investigated three different engine thrust conditions to model the range of realistic flight behavior.

The team obtained data on trace gases (carbon dioxide, carbon monoxide, and nitrogen oxides) as well as particulates (number and mass concentrations). They generated something called a species emissions index, which represents the number of particles or mass of a given chemical emitted per kilogram of fuel burned.

There were a couple of trends that held regardless of the fuel mixture in use. A first-pass examination revealed a noticeable decrease in the concentration of ultrafine particles under jet cloud-forming conditions versus clear air conditions. The researchers think this decrease is likely due to ultrafine particles colliding with larger, cloud-forming ice particles. Further investigation was limited to clear-air exhaust plumes.

The team found that changes in sulfur content from 22 ± 13 ppm to 416 ± 37 ppm did not have a noticeable effect on emission of particles with diameters exceeding 5 to 10 nanometers, though we already knew that sulfur mostly ends up in smaller particles. In any case, the aviation-related sulfur contribution to pollution is relatively small compared to sources on the ground.

But the key comparison was of the emissions indices for the 50/50 biofuel blend and the low-sulfur-content Jet A fuel. This comparison revealed that use of the biofuel blend decreased both volatile and non-volatile emissions by roughly half. Additionally, black-carbon equivalent mass levels were reduced by 50 to 70 percent when the biofuel blend was used compared to the petroleum-based Jet A.

Analysis of the particle distribution revealed that the biofuel blend results in fewer and smaller particles. The scientists think this shift is likely caused by a reduction in the number of larger, sooty aerosols. However, they found that the emissions indices for soot particle numbers remained high (around 1014 kg-1) with the biofuel blend.

This investigation provides critical quantitative information on the impact of biofuel blending on aviation-related aerosol emissions. This information can now be incorporated when we asses the ability of biofuels to reduce the impact of aviation-related emissions on climate change.

Nature, 2017. DOI: 10.1038/nature21420 (About DOIs).