by Fabio Caiazzo, Akshat Agarwal, Raymond L Speth and Steven R H Barrett (Environmental Research Letters) Contrails and contrail-cirrus may be the largest source of radiative forcing (RF) attributable to aviation. Biomass-derived alternative jet fuels are a potentially major way to mitigate the climate impacts of aviation by reducing lifecycle CO2 emissions. Given the up to 90% reduction in soot emissions from paraffinic biofuels, the potential for a significant impact on contrail RF due to the reduction in contrail-forming ice nuclei (IN) remains an open question. We simulate contrail formation and evolution to quantify RF over the United States under different emissions scenarios. Replacing conventional jet fuels with paraffinic biofuels generates two competing effects. First, the higher water emissions index results in an increase in contrail occurrence (~ +8%). On the other hand, these contrails are composed of larger diameter crystals (~ +58%) at lower number concentrations (~ −75%), reducing both contrail optical depth (~ −29%) and albedo (~ −32%). The net changes in contrail RF induced by switching to biofuels range from −4% to +18% among a range of assumed ice crystal habits (shapes). In comparison, cleaner burning engines (with no increase in water emissions index) result in changes to net contrail RF ranging between −13% and +5% depending on habit. Thus, we find that even 67% to 75% reductions in aircraft soot emissions are insufficient to substantially reduce warming from contrails, and that the use of biofuels may either increase or decrease contrail warming—contrary to previous expectations of a significant decrease in warming. READ MORE
Aviation biofuels may not cut contrail warming (Environmental Research Web)
Aviation biofuel pollutant emissions and contrails investigated (Biofuels International)
Excerpt from Biofuels International: NASA and DLR have been researching whether aviation biofuels are more ‘eco-friendly’ for some time. According to a statement from the University of Mainz, previous studies have shown that 50% and 70% less soot particles form from a 50% biofuel mixture with 50% normal kerosene.
“We are interested in the chemical composition of the exhaust particles, among other things, because we want to know how many soot particles the exhaust gas contains, how many metal-containing particles, if they are coated with condensable materials, and how the particles change in contrail formation,” says postdoctoral researcher Sergej Molleker from the Particle Chemistry Department of the MPI for Chemistry.
“At altitudes of eight kilometres, soot particles and water vapour form ice crystals at -50C, which can be seen in the sky as contrails. Among other things, the ice crystals prevent heat from escaping the atmosphere into space, meaning that every contrail creates its own small greenhouse effect.”
“If we find a way to reduce the soot particles in the aircraft exhaust, the climate-warming effect could be reduced by new fuel blends,” says Stephan Borrmann. “We also have the opportunity to make rare and valuable measurements of the natural ice clouds (cirrus) at this altitude, the properties and effects of which are also a key research topic.”
According to a statement, initial results are expected from the test flights within two months. READ MORE