NOx a laughing matter

Diesel engine emissions (NO_x) and laughing gas (N₂O) may be denoted similarly, but the resource implications associated with their release as pollutants are quite different.

In the wake of Volkswagen’s recent revelations, a number of nitrogenous molecules have made their way into the headlines. There appears to be some confusion. One article in the New Statesman refers to ‘nitrous oxides’ (N₂O) in the context of the emissions debacle.[1] This is likely to be a mistake as the nitrogen-containing molecules usually of interest with regards to emissions from diesel engines are nitrogen monoxide (NO) and nitrogen dioxide (NO₂). These molecules, though denoted similarly to N₂O, have quite different resource implications associated with their release into the atmo- and biospheres.

This post is an attempt to clarify the resource implications of greatest relevance to each of these compounds’ release, and explore some ways in which they are linked.

N₂O. Dinitrogen monoxide (or nitrous oxide). N₂O emissions have grave resource implications in terms their global warming potential, but have little to do with diesel engines. N₂O is one of the six most abundant greenhouse gases (GHG), the others being water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄), ozone (O₃) and chlorofluorocarbons (CFCs). When considering a 100-year residence period in the atmosphere, N₂O has about 300 times the global warming potency of CO₂, on a weight-for-weight basis. Confusingly, N₂O is also used in vehicles to enhance propulsion, but this is a niche use of the compound.

NO & NO₂. Nitric oxide (or nitrogen monoxide) and nitrogen dioxide. Collectively called ‘NO_x’ these are the compounds of greatest relevance when discussing emissions from internal combustion engines, especially of late, diesel engines. NO_x emissions are deemed harmful to human health above certain levels, and permissible levels are set differently in various car markets. Permissible levels of NO_x emitted from domestic vehicles are lower in the US, hence the much smaller share of diesel vehicle sales (about 1%) compared to Europe (about 50%).

The key distinction between N₂O and NO_x is the scope of their influence. N₂O is a global pollutant. The location of the source of its emission is almost irrelevant in terms of its global warming consequences. NO_x are largely local pollutants, affecting local air quality.

Despite being distinct, there are at least two ways in which these pollutants are linked. With regards to vehicle emissions, there is a trade-off between the energy efficiency of a diesel engine and the extent to which it emits NO_x. Diesel engines burn their fuel lean, with a high air-to-fuel ratio. This, along with higher operating temperatures and pressures results in greater thermal efficiency: and usually higher levels of NO_x emissions. Less energy-efficient engines on the other hand (such as gasoline engines) will consume more fuel per distance travelled, emitting more CO₂. As with N₂O, more CO₂ results in greater levels of global warming – all else being equal.

Furthermore, it is hoped that emissions in the transport sector will be reduced in the medium-term by large-scale electrification of domestic vehicles. With a decarbonised electricity system – a concurrent medium-term ambition – this would enable a lowering of both local pollution levels (including NO_x) and GHG emissions (including N₂O and CO₂). Currently however, many countries produce a large share of their electricity from burning coal, which releases considerably more CO₂ per unit of energy than diesel or gasoline. Therefore, with the current state of many nations’ electricity grids, transport electrification presents another trade-off mechanism for these two groups of pollutants.

Given these trade-offs, it will be interesting to see in any policy developments following the emissions scandal, how the various oxides are favoured: Whether the proximity and immediacy of harm associated with NO_x trumps the wider-reaching, latent resource consequences associated with N₂O (and other GHGs), or vice-versa.

[1] One of several examples of molecule mix-ups in the article entitled The 4 biggest misconceptions about the Volkswagen emissions scandal published online on the 25^th of September 2015, accessed 23^rd October 2015: ‘Diesel engines therefore give off more nitrous oxides than petrol engines, but because of their greater efficiency, they give off less carbon emissions per mile.’