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Conference report: 17th ETH Conference on Combustion Generated Nanoparticles

23 July 2013

The 17th ETH Conference on Combustion Generated Nanoparticles was held on June 23-26, 2013 in Zurich. The Conference program included about 60 presentations and 58 posters in the poster sessions. The meeting was attended by over 360 registrants.

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Regulatory Particulate Metrics. While the traditional metric for particulate emissions has been filter based PM and Europe has recently introduced a particle number (PN) limit, neither metric is without its challenges. Filter based PM is close to the lower limit where it can reliably quantify particulate emissions in many modern diesel engines. While PN is considerably more sensitive, the definition of what constitutes regulated PN emissions is still in flux and the direct health effects of particle numbers remain uncertain.

Considering that filter-based PM has reached its limit in regulating particulate emissions, its usefulness in future regulations is sometimes questioned [Ricardo, Andersson]. Some of the challenges with filter-based PM are: it can lose its correlation with PN at about 3 mg/kWh, and legacy volatiles in CVS tunnels can make it difficult to differentiate exhaust contributions from background levels. At the Euro VI level, particle mass is at best a pass/fail indicator and cannot reliably be used to quantify relatively small differences in PM emissions. However, there are at least two situations in which relatively high levels of volatile materials can be produced that would not be fully detected by the PN-based approach and a PM measurement would be more suitable: i) DPF equipped vehicles are likely to experience elevated volatile emissions during active regeneration cycles and ii) in SCR equipped engines, incomplete decomposition and side reactions of reductants can lead to significant contributions to overall particle emissions.

While a PN-based regulatory approach would appear to be the ultimate regulatory metric due to its seemingly excellent sensitivity down to very low levels, there are challenges with this as well. When the California ARB initially proposed the LEV-III regulation, they considered incorporating a PN limit [Cackette, retired CARB]. However, the health effects data used to support the LEV III and EPA Tier 3 regulations did not show any convincing evidence that the effects on human health of ultra-fine particle (UFP) emissions are any different from PM2.5. Ambient PM2.5 has long been known to have important adverse health effects. However, it was conceded that perhaps the studies on UFP were not designed properly.

A similar message was delivered by the European Federation of Clean Air and Environmental Protection Associations (EFCA) [Van Ham]. EFCA supports a limit on Black Carbon Particle (BCP) as an additional indicator for particulate matter. BCP is the fraction of particulate matter which has the stronger health impact when compared with other indicators of PM presently used. An additional benefit of limiting BCP is that it has climate forcing properties and reductions in BCP would have an immediate benefit in slowing down global warming. While BCP is predominantly found in the ultrafine particle fraction (< 100 nm), a legitimate question is whether its small size is the property responsible for its toxicity. If so, limiting particle numbers (PN), as is currently done in some Euro 5 and Euro 6 based standards, might be sufficient to limit BCP. Unfortunately, the PN database is presently too sparse to even test this hypothesis: long-term epidemiological studies on PN have not been published and short-term studies are limited; systematic monitoring data of PN are unavailable and the secondary formation of PN in the atmosphere makes data less source-specific. While PN correlates well with traffic, the correlation with BCP is generally better. A reliable database for PN would require many years of research and studies, which suggests PN is a poor candidate for short-term policy action to improve health protection. However, an adequate database which supports BCP and its implementation in air quality policy is available. EFCA does not currently have a position on the preferred measurement technique that could be used to limit BCP emissions.

The 18th ETH Conference on Combustion Generated Nanoparticles will be held on June 22–25, 2014 at the ETH Zentrum in Zürich, Switzerland.

Conference website: www.lav.ethz.ch/nanoparticle_conf