This paper examines the potential unintended impact of a specific CO2 mitigation measure on NOx emissions in Ireland over the period 2008–2020. This measure was introduced in 2008 and linked vehicle registration and motor taxes to CO2 emissions. While the policy has led to a significant decline in new-car specific emissions, it would appear that this is due to a switch in purchasing behaviour to diesel vehicles rather than a move to smaller and less powerful vehicles. Diesel cars represented 73% of new car purchases in 2011, compared with 28% in 2007. Daly and Ó Gallachóir (2011b) simulated this taxation measure and project a CO2 saving of 7% in 2020. Using the same methodology and approach as Daly and Ó Gallachóir (2011b), this paper simulates the impact of the change in taxation policy on NOx emissions over the period 2008–2020.
The results indicate a significant share of the savings in NOx emissions as a result of EURO standards will be eroded due to dieselization: While there is a reduction in absolute NOx emissions in the post-tax scenario (36% reduction on 2008 emissions in 2020), this compares with a 50% reduction for the pre-tax scenario. This improvement in simulated emissions is due to the turnover of the car stock and improved Euro standards for air pollutants. The current NEC ceiling, 65 kt for NOx for Ireland, was exceeded by 10.7 kt in 2010 and continues to apply for future years. This will increase the effort required to meet the NOx emission targets.
This paper has shown that climate mitigation policies, which are generally associated with the co-benefit of reduced air pollution, can have negative impacts on air pollution, particularly when CO2 savings come from fuel switching. This switch towards diesel use will likely have a similar effect on other pollutants such as particulate matter (PM). Further, recent studies have pointed to the failure of EURO 5 and 6 standards for meeting real-life emission reductions: Weiss et al. (2012) tested a range of Euro 4–6 cars and found that their emissions exceeded standards on the road by 260±130%, therefore the negative consequences of promoting diesel cars are likely to be more pronounced than the results in this paper indicate. We recommend that further work should focus on using improved real-world measurements of emissions and on the impacts to air quality of vehicle electrification and biofuel blends.
This paper points to important interdependencies in emissions targets: While a reduction in air pollution is often a consequence of energy efficiency and climate mitigation policies, fuel switching can lead to negative effects. While the current focus on policy action is on climate change, rather than air pollution, it is important for policy-makers to be cognisant of possible negative side effects of climate change policies on air pollutants.