What are NOx and PM, why are they damaging and how much do diesel transport fridges emit?

Sunswap and other advocates of electrification will often emphasise the reduction of secondary pollutants – in addition to carbon dioxide – as a further benefit of switching to clean technology. Nitrogen oxides and particulate matter are secondary pollutants emitted by burning diesel, and they are produced at much higher levels than by burning petrol or […]

Sunswap and other advocates of electrification will often emphasise the reduction of secondary pollutants – in addition to carbon dioxide – as a further benefit of switching to clean technology. Nitrogen oxides and particulate matter are secondary pollutants emitted by burning diesel, and they are produced at much higher levels than by burning petrol or natural gas. It’s important to know exactly what NOx and PM are, in order to understand how and why they harm human beings and the natural ecosystem when released.

NOx stands for Nitrogen Oxides: Nitrogen oxides is a term used to cover three gases; nitric acid (HNO₃), nitrous acid, (HNO₂) and nitrogen dioxide (NO₂), formed when fossil fuels burn and mix with nitrogen gas (N₂), which makes up around 70% of the earth’s atmosphere. Diesel produces more NOx than petroleum due to the concentration of heat and pressure at which it burns, which is also the factor that makes it more energy-efficient.

PM stands for Particulate Matter: Diesel particulate matter consists of “carbonaceous solids” such as polycyclic aromatic hydrocarbons, formaldehyde, benzene, acetaldehyde, acrolein, and 1,3-butadiene. They form small particles that can easily find their way through the barriers in your nose, mouth, and throat. There are two main categories of particulate; PM10 and PM2.5. Each number denotes the category of particles’ maximum diameter in microns (1 micron = 1000th of a millimetre). PM10’s definition aligns with that of soot; it causes respiratory issues in the nose and throat, while PM2.5 goes deeper; entering the lungs and bloodstream, increasing the likelihood of serious disease.

NOx and particulate matter contribute towards the smog that can cloak big cities, even when there a blue skies in the surrounding region

Why do NOx and PM do more damage in towns and cities?

In rural areas, this pollution mainly affects roadside plants and wildlife. NOx generally dissipates quickly enough into the atmosphere to avoid direct and acute damage to humans and wildlife. However, wind conditions can concentrate NOx in urban, suburban and rural areas alike (see image below), with high concentrations potentially leading to acid rain. NOx also acts as an “ozone precursor” reacting with volatile organic compounds (VOCs) to create ozone, which at lower levels is poisonous; called “tropospheric ozone” (although it’s vital in the upper-atmosphere; in the “ozone layer”).

In urban areas, there are higher concentrations being emitted, which is coupled with less opportunity to dissipate and settle due to more buildings and movement. Levels of NOx are subsequently high enough across Europe’s urban population to seriously reduce quality of life and contribute to deadly disease. In 2013, NOx levels in Lewisham, London were so acute that they were legally attributed to the death of a 9-year-old girl.

NOx levels in the UK and north-west Europe at the time of writing, during a predominantly easterly weather system.

As our trials show, replacing a diesel or hybrid transport refrigeration unit with Sunswap’s Endurance TRU fully removes these toxic emissions from depots, roads and urban areas by eliminating tailpipe emission. In cases where our technology uses grid electricity, and where a proportion of grid electricity comes from coal-fired power stations, there is a small scope 2 NOx emission. However, this is marginal, as coal is increasingly being phased out in the areas where Endurance operates.

Key stats of the emission of NOx and PM from traditional diesel transport refrigeration units:

  • Road transport makes up 28% of the UK’s wider NOx emissions and 12-13% of particulate emissions.
  • Transport Refrigeration Units emit 2.6x more NOx than diesel tractor units (a truck’s main engine), per litre of fuel.
  • Projections by the UK government estimate that between 29,000 and 43,000 premature deaths are caused by NOx every year.

Electrification of cold chain technology like fridges and trucks is a clear route to removing these secondary air pollutants from the air we breathe, thus maintaining basic quality of life and protecting public health. Biofuels like Biodiesel and HVO (Hydrotreated Vegetable Oil) still emit both NOx and PM – in fact, a typical biodiesel emits 10% more NOx than regular “white” diesel, although additives and filters can be used to partially reduce this. Similarly, hybrid systems that use power-take-off via a diesel truck will also emit more as they are burning more diesel through increased resistance.

Switching to Endurance doesn’t just lower carbon footprint and reduce cost, it cleans up urban areas and operating hotspots, thereby protecting communities as well as staff in the logistics industry.