Researchers at MIT have determined that growth in aviation causes twice as much damage to air quality as to the climate. In an open-access paper in the journal Environmental Research Letters, they examine how this damage can be mitigated, and provide consistent comparative assessments of aviation emissions trade-offs, considering both climate and air quality impacts.

Aviation emissions are an increasingly significant contributor to anthropogenic climate change. They cause five per cent of global climate forcing. When you consider the full flight, which includes emissions from takeoff, cruise and landing, aircraft emissions are also responsible for around 16,000 premature deaths a year from impaired air quality. This is small compared to other sectors, being only around 0.4% of the total deaths attributed annually to global air quality degradation, but is often overlooked in policy analysis.

The challenges for aviation sector decision makers wanting to reduce these impacts are the trade-offs between different emission types, and their impacts in different locations.

—lead researcher Dr Sebastian Eastham, from the Laboratory for Aviation and the Environment in MIT's Department of Aeronautics and Astronautics

Historically, attempts to address the climate and air quality impacts from aviation have been through changes in policy, technology, and/or operations: improvements to fuel efficiency; more stringent emissions standards; market-based measures to reduce CO₂ emissions; or the introduction of sustainable aviation fuels.

But the study notes reducing one type of emission can come at the cost of increasing another, either in absolute terms or by limiting potential reductions offered by new technology.

We could decrease NO_x emissions by designing engines with lower combustor temperatures. However, the resulting loss in thermodynamic efficiency would mean we need to burn more fuel, meaning more CO₂. These are the types of trade-offs that need to be quantified, and our study offers a fast way for decision makers to do this.

We developed a set of metrics for comparing the climate and air quality impacts of aviation emissions at all flights stages, by estimating the social costs per unit of emitted pollutant. The cost metrics are broken down by flight phase—cruise, landing and take-off—and by the geographical region of emission, both per kg of emission and per kg of fuel burn.

—Dr Eastham

The research team applied the metrics to evaluate the effects of a global expansion in aviation, consistent in magnitude with its current annual growth. They then used this as a benchmark for three scenarios.

First, they considered a growth scenario with fuel efficiency increases and reductions in NO_x emissions factors consistent with 10-year goals. Second, they evaluated the trade-offs between the climate and air quality impacts of engine-based NO_x emissions reductions. Finally, they re-assessed the climate and air quality trade-offs of jet fuel desulfurization.

Our results show three components are responsible for 97% of climate and air quality damages per unit aviation fuel burn: air quality impacts of NO_x at 58%; climate impacts of CO₂ at 25%; and climate impacts of contrails at 14%. It is important to note that the vast majority, around 86% in fact, of the NO_x impacts on air quality are due to the emissions from cruise as opposed to the landing and takeoff cycle. These components—cruise NO_x emissions, CO₂ emissions, and contrails—are therefore primary targets for future strategies to reduce the atmospheric impacts of aviation emissions.

To reduce the climate impacts of aviation, measures aimed at reducing CO₂ emissions and contrails are likely to have the greatest net climate benefit. In contrast, we found that 94% of air quality impacts are driven by NO_x. This suggests that measures aimed at reducing NO_x emissions during cruise could lead to the greatest net benefits, even if they cause a small but uncertain climate NO_x disbenefit and a small decrease in fuel efficiency.

Finally, we found the air quality impacts of aviation emissions significantly exceed the climate impacts, with air quality impacts being 1.7 to 4.4 times higher than the climate impact per unit of fuel burn. This must be contrasted to ground-based industries, where post-combustion emissions control and access to cleaner fuels is widespread. For example, the climate impacts of the US power sector are of similar magnitude as the air quality impacts following significant declines in co-pollutant emissions over the past 15 years. This points towards potential political and technological opportunities for reducing the atmospheric impacts of the aviation sector.

—Dr Eastham

Resources

• Carla Grobler, Philip J Wolfe, Kingshuk Dasadhikari, Irene C Dedoussi, Florian Allroggen, Raymond L Speth, Sebastian D Eastham, Akshat Agarwal, Mark D Staples, Jayant Sabnis (2019) “Marginal climate and air quality costs of aviation emissions” Environmental Research Letters, Volume 14, Number 11 doi: 10.1088/1748-9326/ab4942