NO2 from Passive Filters
Nitrogen Dioxide (NO2) Emissions. Diesel engines emit oxides of nitrogen (NOx), consisting of nitrogen oxide (NO) and nitrogen dioxide (NO2). The percentage of each does vary, but typically NO2 might make up 10% of the total NOx coming out of the engine. Reports have linked continuously regenerating filter systems to increased emissions of NO2 from the tailpipe. These do not always contain sufficient information to enable the reader to make an informed judgement on the issues.
Some key points are therefore set out below:
- NO2 produced on the catalyst is the key component used to burn the soot collected in the filter, and so passive filter systems using platinum group metals can lead to an increase in tailpipe NO2 over engine-out
- The amount of excess NO2 produced varies greatly according to factors such as the actual amount of particulate and NOx produced by each engine as well as the driving pattern of the vehicle and is therefore difficult to design out by optimising the filter system
- Continuously regenerating filter systems do not increase total NOx but change the balance of NO to NO2 at the tailpipe. NO reacts more slowly in the atmosphere to form NO2 in any case so direct NO2 (that emitted from the tailpipe) mostly affects air quality close to busy roads
- All of the NO emitted by an engine would ultimately be converted to NO2 in the atmosphere, irrespective of the presence of aftertreatment systems
- The health benefits from reduced PM emissions (plus HC and CO) are widely considered to greatly outweigh any disadvantage from increased NO2 formation. The World Health Organisation* and California Air Resources Board** amongst others have published the statements below about the health impacts of the two emissions
- More than 120,000 CRT systems have brought significant air quality improvement in many cities around the world, and will continue to do so
- Direct NO2 emissions from these catalysed filter systems are only part of picture - NO2 formation in the atmosphere and from other sources needs to be considered
* NO2 - 'The few long-term studies have not shown evidence for association between NO2 and mortality.'
PM - 'Studies have suggested that life expectancy is decreased by long-term exposure to PM. This is supported by new analyses of time-series studies that have shown death being advanced by periods of at least a few months, for causes of death such as cardiovascular and chronic pulmonary disease.'
** ”While it is not possible to conduct a quantitative risk assessment evaluating NO2 increases against DPM reductions, DPM-related health effects are generally considered more severe, leading to nearly 2,000 deaths in California at population-weighted exposure of 1.8 µg/m³ estimated for the year 2000, as well as significant morbidity effects (Lloyd and Cackette, 2001).”
Solutions - Regulatory
- The California Air Resources Board (CARB) recognises the importance of reducing PM, and the fact that CRT systems are highly effective in achieving this. It has therefore introduced regulations to limit the increase in NO2 emissions from filter systems from 2007, and to ensure that California will continue to benefit from the use of CRT technology. Johnson Matthey’s systems will comply with these new limits.
- Some countries, for instance within the EU, have adopted limits for atmospheric NO2 levels that will be difficult to meet in some areas due to primary NO2 emissions as well as atmospheric effects.
- It would be beneficial to introduce schemes that mandate or incentivise both PM and NOx reduction retrofit technologies
Retrofit technology is already available that will enable vehicles to meet the latest emissions limits for original equipment.
Solutions – Technology
- Optimised CRT systems that limit NO2 increases as a percentage of total NOx
- Retrofit SCRT technology to reduce total NOx and PM emissions