Airport air quality: Impact of emissions from aircraft in ground run and flight

If rising demands for national and international air travel are to be accommodated, mean air-traffic movements will likely have to rise. With only limited scope for reducing emissions from jet engines, it appears quite possible that airport capacity may at some locations become constrained by air-quality requirements. During takeoff and climb-out, NO₂ - the pollutant of principal concern - constitutes some 5-10% of NO_x at the engine nozzle exit, and a fraction equilibrating at several times this downstream. Assessing associated impacts on local air quality is complicated by the action not just of transport processes of the ambient atmosphere, but also of those forced by the passage of the aircraft itself. The latter are not well understood, and not represented satisfactorily in current airport air-quality models.

To better characterise emission plumes from aircraft, and relate characteristics to determining variables, an experimental campaign has recently been undertaken at two international civil airports (Heathrow and Manchester). A backscatter lidar was deployed (RASCAL, as operated by Dr M. Bennett, University of Manchester), incorporating a frequency-tripled Nd-YAG laser operating in the near UV (l=355 nm), with a beam that was swept rapidly in the elevation. This allowed near-instantaneous spatial maps of the aerosol scattering field to be derived behind aircraft in ground run and flight. Passive and active spectroscopic measurements of NO and NO₂ concentrations, as integrated along a path through plumes, were also made. Supporting data on aircraft, derived using video camera, airport radar and flight data recorder, include time series of aircraft position and velocity; flap angle; and mass and thermal fluxes from engines. Profiles of the ambient surface-layer wind and turbulence were obtained.

The start of the ground run during departure constitutes one dynamical extreme studied, when the evolution of the aircraft wake is governed by exhaust momentum and to some degree buoyancy. Exhaust streams issuing from different locations on the airframe may tend to converge and approach the ground through the agency of a Coanda effect. The other dynamical extreme has also been investigated, when the aircraft is more than half a wingspan aloft, and emissions disperse in a manner governed by the action of streamwise vortices rolling up in the wake. Such vortices arise as a result of a shedding of the circulation about wings inducing the lift, and through forcings at flap edges if flaps are near-fully extended (as during approach).