Study uncovers role of longer-chain unburned hydrocarbon emissions from diesels in London air pollution; calls for regulatory shift
Researchers at the University of York (UK) have found that longer-chain unburned hydrocarbons released from diesel—which are not currently explicitly considered as part of air quality control strategies—represent only 20–30% of the total atmospheric hydrocarbon mixing ratio but contribute more than 50% of the total atmospheric hydrocarbon mass and are a dominant local source of secondary organic aerosols (SOA) in London—and by extension to other developed megacities.
The study found that 60% of the winter primary hydrocarbon hydroxyl radical reactivity in London is from those diesel-related hydrocarbons; the authors predicted that the longer-chain HCs contribute up to 50 % of the ozone production potential in London. The results, they said in an open access paper published in the journal Atmospheric Chemistry and Physics, suggest the need for a shift in policy focus onto gas-phase hydrocarbons released from diesels as this vehicle type continues to displace gasoline world-wide.
The primary regulated air pollutants (criteria pollutants) are particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs). However, these can react in the atmosphere to create secondary pollutants, such as ozone (O3), oxygenated VOCs (OVOCs), peroxy acyl nitrates (PANs) and SOAs, adding to the overall pollution load.
Hydrocarbons are key precursors to two priority air pollutants, ozone and particulate matter. Those with two to seven carbons have historically been straightforward to observe and have been successfully reduced in many developed cities through air quality policy interventions. Longer chain hydrocarbons released from diesel vehicles are not considered explicitly as part of air quality strategies and there are few direct measurements of their gaseous abundance in the atmosphere.
… Comparing real-world urban composition with regulatory emissions inventories in the UK and US highlights a previously unaccounted for, but very significant, under-reporting of diesel-related hydrocarbons; an underestimation of a factor ∼ 4 for C9 species rising to a factor of over 70 for C12 during winter. These observations show that hydrocarbons from diesel vehicles can dominate gas phase reactive carbon in cities with high diesel fleet fractions.—Dunmore et al.
The York team used two gas chromatography (GC) instruments to investigate the abundance and trends of diesel-related hydrocarbons in the atmosphere at a typical urban background site in London.
|Total mass by carbon number and functionality from UK 2012 (left) and US 2011 (right) emission inventories suggest little presence of the long-chain diesel hydrocarbons. Dunmore et al.
|The York study’s findings on the seasonal median values for hydrocarbon mixing ratio, mass concentration and primary hydrocarbon OH reactivity in London air grouped by carbon number and potential emission source. Dunmore et al.
Based on their findings, the researchers concluded that current inventories and emissions estimates do not adequately represent emissions of gas phase long-chain hydrocarbons from the diesel fleet under real-world conditions and in a developed urban environment. The calculated impact of these species is significant, they found, particularly in terms of OH reactivity, ozone formation potential and SOA production.
The shift to an increasingly diesel-powered fleet in many developed cities in response to energy efficiency drivers has shifted the balance of hydrocarbons in urban air from short to long chain compounds, they concluded, pointing to their results as providing direct atmospheric evidence of this effect in London.
Previous air quality assessments of diesel-related hydrocarbons in the atmosphere are few in number, and as discussed previously, have been made only in the US where geographic characteristics and vehicle fleet composition are very different to London, and Europe more widely. In many cities the impact of diesel hydrocarbons remains to be determined, but this work demonstrates that it will likely be significant in locations with substantial diesel fleets. An improvement in measurement infrastructure appears to be essential if this source is to be quantified more widely or the impacts of policy evaluated.—Dunmore et al.
Dunmore, R. E., Hopkins, J. R., Lidster, R. T., Lee, J. D., Evans, M. J., Rickard, A. R., Lewis, A. C., and Hamilton, J. F. (2015) “Diesel-related hydrocarbons can dominate gas phase reactive carbon in megacities,” Atmos. Chem. Phys., 15, 9983-9996, doi: 10.5194/acp-15-9983-2015