Upgraded aerosol inlets will revolutionise the way the FAAM aircraft samples aerosol particles from outside the aircraft as it flies and delivers them to the instruments inside. The project is led by Dr Paul I. Williams, National Centre for Atmospheric Science and University of Manchester.
What are aerosols?
Aerosols are tiny particles suspended in the atmosphere. They can be natural e.g. fog or dust, or caused by human activity e.g. combustion sources (smoke). Aerosol particles can range from a few nanometres (nm) to several microns (μm) in diameter, making them far too small to see with the naked eye. Aerosols in the atmosphere can have a big effect on the Earth’s climate, inducing both artificial warming and/or cooling, depending on their composition. Studying aerosols gives scientists a better understanding of how these tiny particles affect our environment.
Aerosol particles are notoriously hard to collect and quantify. Sampling down pipes and around bends in tubes often means aerosol particles are lost or broken up, particularly when they’re collected by something moving very fast. This is an an extra challenge when sampling with an aircraft, which is often the only way to reach aerosols in the atmosphere high above the ground.
Inlets on the FAAM aircraft similar to the ones that will be used for the upgraded aerosol inlets.
Sampling losses depend on the size of the particles, the number of bends in the sample pipes and how long the aerosol spends in the sampling system. All these factors are temperature- and pressure-dependent, so with FAAM’s atmospheric research aircraft these effects are different at sea level compared with those at 35,000 feet.
New instrument will enable FAAM to sample the smallest aerosol particles
A project to upgrade the aerosol sampling capabilities of the aircraft is being delivered through the FAAM Airborne Laboratory’s Mid-Life Upgrade.
“It will give us the ability to sample aerosols across a large size range, with high transmission efficiency with known losses, across the flight envelope of the aircraft”, says Paul Williams, Senior Research Fellow at the National Centre for Atmospheric Science and lead scientist behind the project.
“No one instrument on the FAAM aircraft can count or size over this size range, similarly no one inlet would be able to deliver aerosol across this entire size range. Therefore, based on the needs of the scientific community, we’re proposing three separate designs.”
- Standard Aerosol Inlet (SAI)
Most of the aerosol sampling on the aircraft is sub-1μm. The SAI will be the workhorse of the FAAM aircraft, providing high transmission efficiency in the size range of 10nm to ~3μm, for instruments such as the aerosol mass spectrometer and particle counters. This inlet will be permanently fitted to the aircraft.
- Super Micron Particle Inlet (SMPI)
Sampling large particles presents a different challenge to the sub-micron particles. These particles are generally heavier and are often lost at bends in sample pipes. This inlet will deliver particles up to at least 10μm in diameter, and is designed for instruments such as the bioaerosol detector. This will be fitted for specific projects.
- UltraFine Particle Inlet (UFPI)
A completely new design for sampling sub-10nm particles is being developed. This will be the first of its kind for any research aircraft, making the FAAM aircraft unique among research aircraft globally. Particles this small are able to diffuse out of the walls of the sampling lines, so time in the sampling system is key. Keeping lines as short as possible will reduce the time the aerosol spends in the system. The UFPI will have three particle counters strapped to the inside skin of the aircraft where the inlet is fitted. One counter will measure the total aerosol particle concentration. Another will have a catalytic stripper on the inlet to evaporate all the volatile material and leave and count the non-volatile particles. This will enable us to calculate the number of volatile particles in the sample. The third will count particles greater than 10nm, allowing us to determine the number of particles between ~5nm and 10nm.
A specialised sampling inlet slows aerosols down as it collects them
The challenge of designing the inlets falls to enviscope GmbH, who provide and develop measurement systems for atmospheric research on aircraft. They have designed inlets for both aerosol and trace gases, including the CARIBIC Air Probe for an Airbus A350.
A CARIBIC air probe, similar to the inlet in development for the FAAM Airborne Laboratory
Aside from the wide range of temperatures and pressures, the other big challenge in designing these inlets is decelerating air from an aircraft moving at 120m/s. The instruments on board the aircraft need the aerosols to be moving much more slowly. A series of nested diffusors reduce the aerosols’ velocity as they reach the sample inlet.
A schematic showing the system of nested diffusors that will slow air down enough to be measured by sensitive instruments aboard the research aircraft.
“The aircraft’s wide operational range across different altitudes makes the design of such air inlets challenging, especially considering the high demand for sampled air to supply numerous experiments within the cabin.” said Timo Röschenthaler, leading design engineer at enviscope. “For the BAE-146, a groundbreaking air inlet system for aerosols in the nanometer range is envisioned, requiring the development of an entirely new measurement principle.”
The project is now in the detailed design phase after a feasibility study considering the requirements of all stakeholders. Production is expected in 2026, with installation in 2027.
To contact Dr. Paul I. Williams: paul.i.williams@ncas.ac.uk
To contact enviscope: Dr. Stefan Hofmann (project lead), s.hofmann@enviscope.de (https://www.enviscope.de/).