November 28, 2017 report
Study of European historical data suggests proportion of NO2 in NOx emissions smaller than thought
One of the types of pollutants emitted by cars and trucks is nitrogen oxides (NOx), in particular NO2 and nitric oxide (NO). These pollutants have been in the news in Europe over the past couple of decades because many truck and car owners have chosen to switch to vehicles powered by diesel fuel, which costs less than gasoline—but such vehicles also emit more NOx. To combat air pollution, officials in Europe and the U.K have enacted emissions standards that have resulted in less NOx emitted into the atmosphere. But most such emissions have applied only to NO2, which has muddied the standards. Making things ever murkier—such standards have been based on the percentage of NO2 in total NOx emissions. This is because not only do vehicles emit NO2 directly, but NO turns into NO2 over time once released into the air. Now, it appears that the percentages that officials have been using to set their standards has been wrong.
To get an accurate measurement of the true percentage of NO2 in the NOx being emitted by vehicles, the researchers analyzed data from roadside monitors across Europe. In looking at the data, they found that NO2 percentages had risen, as expected, during the period 1995 to 2010 (as people converted to diesel) but then as emissions regulations came into effect, the percentage of NO2 leveled off and has remained at those levels ever since. These levels, the researchers note, are roughly half of what is described by air quality policy, which suggests Europe may reach mandated levels sooner than thought. Gentner and Fulizi Xiong suggest the group's finding could have also implications for developing nations as well because consumers there tend to buy used diesel vehicles from Europe.
Many European countries do not meet legal air quality standards for ambient nitrogen dioxide (NO2) near roads; a problem that has been forecasted to persist to 2030. Although European air quality standards regulate NO2 concentrations, emissions standards for new vehicles instead set limits for NOx —the combination of nitric oxide (NO) and NO2. From around 1990 onwards, the total emissions of NOx declined significantly in Europe, but roadside concentrations of NO2—a regulated species—declined much less than expected. This discrepancy has been attributed largely to the increasing usage of diesel vehicles in Europe and more directly emitted tailpipe NO2. Here we apply a data-filtering technique to 130 million hourly measurements of NOx , NO2 and ozone (O3) from roadside monitoring stations across 61 urban areas in Europe over the period 1990–2015 to estimate the continent-wide trends of directly emitted NO2. We find that the ratio of NO2 to NOx emissions increased from 1995 to around 2010 but has since stabilized at a level that is substantially lower than is assumed in some key emissions inventories. The proportion of NO x now being emitted directly from road transport as NO2 is up to a factor of two smaller than the estimates used in policy projections. We therefore conclude that there may be a faster attainment of roadside NO2 air quality standards across Europe than is currently expected.
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