“Our analysis suggests that the ozone production in this region is largely NOx limited, thus the introduction of additional NOx emissions to a relatively ‘clean’ atmosphere yields a larger ozone response than in other parts of the world,” said Barrett. “Aviation is growing the fastest in Asia, so the environmental impacts of future growth in aviation will be different to past growth dominated by North America and Europe.”

The team also found that NOx emitted by flights in October led to 40% more tropospheric ozone than in April. Ozone is produced more efficiently at cruise altitudes (9–12 km) than at ground-level.

“The highest ozone-impacting flights per unit fuel burn have origins or destinations in Australia or New Zealand,” said Barrett. “Our findings imply that there is potential in reducing the climate impact due to aircraft NOx emissions by mitigating flight paths in ozone ‘hotspots’ – but much more research would be needed before such a measure would be advisable.”

The top-ranked ozone-producing flights normalized by fuel burn caused 157 times more ozone formation than the flights at the bottom of the table. Routing flights away from sensitive regions could cause a trade-off with increasing flight distance and the consequent higher fuel use and carbon-dioxide emissions.

The team employed an adjoint sensitivity approach using the global chemistry-transport model GEOS-Chem to examine ozone production from 83,000 flights that were more than 200 km long and contained more than 70 passengers. “As far as we know, we are the first to apply adjoint sensitivities to this problem, whereas previous studies used more cumbersome sensitivity techniques that do not allow per-flight impact determination,” said Barrett. “Improvements occur one design decision at a time, one operational decision at a time, and one flight at a time – so it's important to be able to understand the impact individual decisions have on the environment.”

Now, the researchers intend to apply this sensitivity approach to aviation’s other climate impacts, including aerosol formation, and the longer-term effects associated with NOx emissions, which increase the production rate of OH, decreasing the lifetime of methane and decreasing ozone concentrations on longer timescales. “This will give us a ‘heat map’ of the environmental impacts of aircraft emissions as a function of where and when they occur,” said Barrett.

The team reported the study in Environmental Research Letters (ERL).

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