Evaluating the variability and consistency of NOx emission regulation between sectors†

The emissions of nitrogen oxides (NO_x) from combustion have been regulated for several decades with substantial reductions in national totals being reported in high-income countries since the 1990s. Most technical regulation on emissions is sectoral, appliance specific, and uses metrics aligned to activity data, for example grams of NO_x per kilometre driven or grams per kilonewton thrust. It is not straightforward therefore to compare the relative stringency of emission regulation between sectors. Here we undertake a regulatory assessment placing all the key NO_x emitting sectors onto a common grams of NO_x per kilowatt hour (g_[NOx] kWh⁻¹) baseline, covering appliances as small as 1 kW to greater than 2 GW. This common scale facilitates meaningful regulatory comparisons and may help to inform future policy decisions. We find little regulatory consistency between sectors when viewed on a per kWh output basis, with non-road mobile machinery (NRMM), medium combustion plant (MCP), maritime and civil aviation having more permissive regulatory limits when compared to emissions from passenger cars and domestic boilers. This difference can be large for appliances with the same nominal power rating; for example, the allowable NO_x emissions for a backhoe loader are 4.3 times higher than those for a passenger car. Transparency in pollutant emissions varies considerably between sectors. Data from MCPs and the Industrial Emissions Directive (IED) are less accessible due to commercial sensitivities and the use of less definitively defined principles of ‘Best Available Techniques’. Whilst electrification is likely in the long-term to eliminate some NO_x sources, it is notable that this will be in sectors that currently have more stringent regulatory limits (e.g. road transport, domestic heating). More permissively regulated sectors such as NRMM, MCPs and aviation are likely to retain combustion systems and will continue to emit substantial NO_x unless the adoption of low carbon fuel is accompanied by revision of NO_x emission standards.