Biofuel blending reduces particle emissions from aircraft engines at cruise conditions

Abstract

Aviation-related aerosol emissions contribute to the formation of contrail cirrus clouds that can alter upper tropospheric radiation and water budgets, and therefore climate. The magnitude of air-traffic-related aerosol-cloud interactions and the ways in which these interactions might change in the future remain uncertain. Modelling studies of the present and future effects of aviation on climate require detailed information about the number of aerosol particles emitted per kilogram of fuel burned and the microphysical properties of those aerosols that are relevant for cloud formation. However, previous observational data at cruise altitudes are sparse for engines burning conventional fuels, and no data have previously been reported for biofuel use in-flight. Here we report observations from research aircraft that sampled the exhaust of engines onboard a NASA DC-8 aircraft as they burned conventional Jet A fuel and a 50:50 (by volume) blend of Jet A fuel and a biofuel derived from Camelina oil. We show that, compared to using conventional fuels, biofuel blending reduces particle number and mass emissions immediately behind the aircraft by 50 to 70 per cent. Our observations quantify the impact of biofuel blending on aerosol emissions at cruise conditions and provide key microphysical parameters, which will be useful to assess the potential of biofuel use in aviation as a viable strategy to mitigate climate change.

Figure 1:

Side view of the NASA HU-25 Falcon aircraft sampling the DC-8 contrail (a). Also shown are forward views of the DC-8 contrails with the inboard engines throttled up to maximum continuous thrust (MCT) and the outboard engines throttled back (b), and the reverse conditions (c). The operational flight curve for the DC-8 is shown as the red curve in (d) assuming an average aircraft gross weight of 200,000 lbs. The blue points correspond to the ACCESS-2 engine thrust settings. Note that all EIs reported in this manuscript are for clear air (i.e., non-contrail-forming conditions).

Figure 2:

Geometric mean particle emissions indices ( one geometric standard deviation) for all thrust settings and each fuel burned on the right inboard engine (#3) at altitudes between 30,000 and 36,000 ft. The ratio of the EIs for the 50:50 biofuel blend and the medium sulfur Jet A are denoted beside each point with the number of stars denoting the statistical significance level as given in Table 1 and in the SOM.

Figure 3:

Particle number (left) and volume (right) EI size distributions for the total (top) and non-volatile fraction (bottom) measured at the high thrust condition behind the #3 engine. Points are geometric means and error bars are the geometric standard deviation (N = 4 and 6 for the High Sulfur Jet A and 50:50 Low Sulfur Jet A – HEFA blend, respectively). Solid lines are lognormal fits and the shaded area represents the difference between the two curves. The geometric mean diameter for each fit are denoted in the legend, while all fit parameters are given in the SOM.