Don't Get On The Wrong Side of An Oxide

Despite contrails being the most visible indication of the impact aircraft are having on the environment, they are certainly not the only form of pollution. In my first post, I touched upon the impacts of CO₂ being released however, they are not the only emissions of environmental significance. The potent nitrogen oxides (NO_x) and sulphur oxides (SO_x) are also released, albeit in smaller quantities as CO₂ but with potentially more damaging consequences. I must mention that these pollutants contribute to the formation of contrails and the modification of natural cirrus clouds too, for the sake of being able to mention contrails again!

Today, I am going to be looking at the well documented effects of nitrogen oxides. NO_x emitted from aircraft today are 5 times more efficient at affecting the global ozone burden than emissions originating from ground transportation such as road vehicles (Hauglustaine and Koffi 2012). In spite of its higher potency as pollutant than CO₂, the radiative forcing of NO_x is relatively small as described in Fuglestvedt et al. (2009), Myhre et al. (2011), and Holmes et al. (2011). This is as a result of the opposing effects it has on ozone and methane in the atmosphere. 

NO_x from aircraft promotes the formation of ozone (O₃) in the troposphere and lower stratosphere. Ozone has a positive radiative forcing (i.e a warming effect). This enhancement of ozone is offset by the chemical alterations NO_x has on the oxidising capacity of the atmosphere. NO_x cause an enhancement of the oxidising capacity, which reduces the lifetime of atmospheric methane (CH₄). This reduction in  methane leads to a negative forcing (i.e a cooling effect). Less methane also means less ozone is produced. This renders nitrogen oxides a forcing agent that is able to affect the chemistry of the atmosphere. 

Although the effects of NO_x released by aircraft may not seem that bad, Köhler et al. (2008) investigated the disturbances to methane and ozone in the atmosphere as a result of nitrogen oxide input from aircraft. They concluded impacts varied depending on the altitude of the aircraft for example, a plane flying at 11 km would lead to an ozone increase of 200% and a reduction in methane lifetime per emitted mass of  NO_x, 40% as strong as a plane flying 5 km altitude. 

The study also found that much like how the effect of ground based NO_x emissions affect atmospheric ozone and methane differently depending on their geographic location, the same applies to aviation. The geographic distribution of NO_x emissions has consequences for flight route planning, and it is predicted that growing aviation in Asia could lead to significant impacts regarding methane and ozone within the troposphere, even if emissions were to remain constant. 

This raises important questions as to what impact future increases in air travel will have as new flight routes open and increase in frequency, especially considering the planning currently involved in reducing the effects of emissions on the atmosphere. Also how strategies accounting for these harmful types of pollution will continue to be managed. 

How to plan for more planes?