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MACAO: the quest for cleaner cabin air

Despite the fact that air in the aircraft passenger cabin is of a very high grade compared to that of other modes of transport, the quest for ever cleaner air is ongoing. Clean Sky's MACAO project is developing the tools to monitor and ultimately address issues of cabin air contamination, raising the standards of air quality for European aviation.

In a typical airliner air conditioning system, air supplied to the cabin is a mix of around 50% from outside air – often ‘bleed air’ from the engine – which is combined with approximately 50% of filtered, re-circulated air. Volatile Organic Compounds (VOCs) and ozone can also find their way into the cabin, especially ozone, when the aircraft is flying high near the ozone layer. Unwanted by-products of engine combustion, such as NOx, can also pollute the cabin, while other pollutants emanate from within the cabin – ethanol in alcoholic beverages can evaporate, for example. Furthermore, aircraft interior furnishings manufactured using plastics and coatings can also release formaldehyde. This cumulative cocktail of gases presents concerns for the health and comfort of passengers and crew, as prolonged exposure to elevated ozone and VOCs levels are linked to respiratory and cardiovascular issues.

But accurately detecting and understanding the complex make-up and specific origin of these various pollutants, and formulating preventative measures to cope with contaminants in the cockpit and the passenger cabin is a complex undertaking, as Clean Sky explains:

"The MACAO (Development of VOCs and ozone Micro-analysers based on microfluidic devices for Aircraft Cabin Air mOnitoring) project kicked off in early 2016 (and runs until December 2020) with the mandate of focusing on the development of efficient and accurate micro-analysers for detecting, analysing and preventing the circulation of various gaseous pollutants encountered in the aircraft cabin air conditioning systems. What is particularly innovative in this project is that up to now there haven't been any suitable instruments that have been specifically designed to measure the VOCs and ozone concentrations that circulate in the cabin environment. Being able to analyse very small quantities of these gases using microfluidic devices is relatively new, and that is where the expertise of the MACAO consortium is quite relevant to this project".

Clean Sky colleague and Project Officer Michel Goulain adds that "Especially as we head towards the introduction of more electric aircraft, electrical air conditioning systems require better filtration systems and sensors, so MACAO is an important project for European aviation as it will contribute to increasing the overall efficiency of more electrical aircraft by reducing their overall weight and power consumption through increased miniaturization compared to existing systems".

The project is tasked with developing two analytical instruments based on microfluidic devices for the measurement of major air pollutants concentrations in airplanes. One micro-analyser will measure concentrations of various VOCs whereas the second will measure ozone concentrations. And by measuring VOCs and ozone concentrations, regardless of their origin, the project will seek solutions for the removal of these substances and thus, in the long term, improve cabin comfort and health standards for both passengers and crew.

"Some pollutants can come from outside (engine oil) but can also be emitted or formed by reactions inside the cabin (seats, alcoholic beverages). These pollutants can cause potential hazards to the crew and passengers" says Dr. Stéphane Le Calvé, Director of Research at the French National Center for Scientific Research (CNRS) in Strasbourg and the coordinator of the MACAO project. 

CNRS and the University of Strasbourg jointly operate the Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé (ICPEES), a research unit which forms a key player in the MACAO consortium. Other organizations in the consortium include INSA Toulouse, a public Engineering School, part of the Federal University of Toulouse Midi-Pyrénées, whose "Modelling of Mechanical Systems and Microsystems" group has developed a high-level of expertise in modelling, simulation, and experimental analysis of gas flows in microsystems. Also supporting the MACAO project are Tronico, headquartered in Nantes, specialising in electronic assembly for small production runs, and In'Air Solutions, an innovative start-up that focuses on the development and commercialization of micro-analysers for air quality diagnosis.

To date, scientific and technical progress on the VOCs micro-analyser is promising. A preconcentrator has been successfully developed and validated under laboratory conditions, demonstrating the capability of the prototype to adsorb and to desorb the targeted organic species, and In'Air Solutions has already investigated the preferred configuration of the micro-analyser in respect of production costs.
In terms of demonstrations and measurable results, Dr. Le Calvé reports that "prototypes have already been successfully tested under laboratory-controlled conditions. Their responses are perfectly proportional to the concentrations of the targeted pollutants (benzene and its derivatives, ozone) in the air. Other tests will have to be performed under more representative conditions such as in simulators or on board flights, either at the end of the project or after the project. So far, we have two laboratory prototypes: one benzene analyser equipped with a new pre-concentration unit in order to improve its analytical sensitivity, and one ozone analyser. For the ozone micro-analyser, we had to abandon a first development that did not allow us to reach the required sensitivity. Then, we considered a second possible technical development, which led to some delays". However, the MACAO team are now "seeking to make the micro-analysers evolve to make them fully automatic, which is essential for a demonstration in real conditions of embedded flight".

As for the bottom line – the potential to improve the quality of air in the passenger cabin – Dr. Le Calvé says that "the integration of this type of instrument into aircraft will make it possible to assess pollution levels in aircraft and potentially alert the crew in case of accidental or excessive pollution. Ultimately, modifications can also be taken to improve air quality in aircraft, such as the integration of an air purifier".