Fresh warning on emissions from rising diesel car sales

A report by a Department of the Environment (DoE) advisory group which was heavily criticised by the motor industry earlier this year after warning that rising sales of diesel cars would slow reductions in emissions of particulates and nitrogen oxides has now received broad backing from a study commissioned by two other Government Departments.1 The research has also done nothing to support manufacturers' claims that diesel cars will help achieve major cuts in carbon dioxide emissions.

The new report was commissioned by the Department of Transport (DTp) and the Department of Trade and Industry from the Energy Technology Support Unit (ETSU). Although its completion was announced on 21 November, the DTp declined to supply a copy, saying it was due for publication in mid-December. That publication deadline was missed, but ENDS was able to secure a copy through its usual channels.

The DTp press release which announced the report opened with the claim that ETSU had confirmed that "over the next ten years, exhaust emissions of regulated pollutants are set to fall substantially as a result of increasingly stringent vehicle emissions standards." That presentation entirely glossed over the point of the study.

There were two main reasons why the research was commissioned. One was the oil industry's concern that continuing market penetration by diesel cars, particularly if encouraged by favourable rates of duty on diesel, could force it to make costly investments at refineries. The second was the growing controversy over the environmental impacts of diesels as against petrol cars.

That controversy was stoked last January by a report from the DoE's Quality of Urban Air Review Group (QUARG) which criticised motor manufacturers' attempts to promote diesels as a solution to air quality problems (ENDS Report 228, pp 9-10 ).

Although diesels emit lower levels of carbon monoxide and hydrocarbons than petrol cars with three-way catalysts, QUARG pointed out that their emissions of nitrogen oxides and particulates are much higher. Its report projected that if diesel cars were to account for half of all new car sales, NOx emissions from road transport would be several percentage points higher, and particulate emissions twice as high, by 2005 than if the present sales pattern was maintained.

The Society of Motor Manufacturers and Traders dismissed the report as "unnecessarily alarmist", arguing that any link between diesel emissions and effects on human health remained "unproven", and that any move to discourage diesel sales would make it more difficult to curb carbon dioxide emissions.

Both QUARG's and ETSU's reports come against a background of rising diesel sales. In 1990, diesels accounted for only 6.5% of new car registrations. Last year the figure was 18% - and this year it will break the 20% barrier.

Diesels still account for only 6.5% of the total car fleet. ETSU considered what the effects would be on emissions if their share of the fleet was 10%, 20%, 30% and 40% in 2005. The calculations were based on the mean rates of growth in road traffic forecast by the DTp, and took into account known changes in emission standards to 2000 - though not the effects of the increases in fuel duty announced in the last two Budgets. The study also took into account energy use and emissions at refineries in response to changes in the scale and pattern of demand for road fuels. The results were compared with 1990 emission levels.

The main findings were:

  • As the DTp's press release pointed out, emissions of "regulated" pollutants are all projected to decline by 2005. Taking road transport emissions and refineries together, and assuming no increase in vehicle fuel efficiency, emissions of carbon monoxide are projected to fall by at least 65%, hydrocarbons by at least 86%, and nitrogen oxides by 66% from the 1990 baseline.

    Emissions of particulates are projected to decline by 60% when diesels are assumed to have a 10% share of the car fleet in 2005, but by only 46% if their share is 40%.

    However, for the "unregulated" pollutant, carbon dioxide, emissions are projected to increase by 32-36% by 2005.

  • The study puts paid to any suggestion that higher diesel sales will be of much assistance in curbing the UK's emissions of CO2. Compared with a 10% diesel share of the car parc in 2005, overall CO2 emissions from road transport and refineries are projected to fall by no more than 1-3% when diesel's share of the car parc increases to 20-40%.

    In addition, with a 20-40% diesel share of the car parc, particulate emissions would be 12-35% higher, and NOx emissions 1-4% higher, in 2005 than if diesel's share was only 10%.

  • Much bigger reductions in CO2 emissions can be achieved by improving fuel efficiency than by switching to diesel cars, ETSU concludes. With only a 1% per year improvement in the fuel efficiency of cars, total CO2 emissions from road transport would rise by 26-29% by 2005, compared to 35-38% without any improvements.

  • Diesel cars will be the biggest mobile source of particulates by 2005 if their share of the car parc is 30% or more. In 1990, they accounted for just over 5% of road transport particulate emissions. By 2005, however, their contribution will rise at least four-fold in absolute terms, and to around half of total road transport emissions, if their share of the car parc reaches 40%.

    These projections may have significant implications for urban air quality - and also for diesel car producers. The signals from the Government's work on PM10, the particulates of greatest concern to human health, are that substantial reductions in concentrations of these pollutants will be needed to meet future air quality targets (ENDS Report 237, p 3 ).

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