Diesel engine emissions: are they no longer tolerable?

Emissions from diesel engines contain several hundred chemical compounds, which are emitted partly as gas and partly as particles. The composition of diesel exhaust and its concentrations in air has changed significantly over time, to the extent that now there is a distinction between traditional and new technology diesel emissions. New technology diesel engines comply with emissions from EURO 3 vehicles and higher. Starting with EURO 5, a significant further reduction of particle emissions has been achieved by increased temperature, which, however, lead to increased emissions of NOx and exceeded the NO2 emission limit. To overcome this problem, some car manufacturers installed illegal software that detected the vehicle test bench operation, resulting in low emissions during the test cycle. Detection of such devices in 2015 led to the Diesel scandal. In 2017 the worldwide harmonized light vehicles test procedure (WLTP) was introduced for new cars, which simulates emissions under different driving conditions. It became mandatory for certification of all new vehicles by September 2018. In addition, fleet CO2 emissions have been introduced for all cars, requiring that by 2020 95% of each manufacturer's passenger car must meet the CO2 emission target of 95 mg/km, and by 2021 100% of the fleet. All these regulations significantly reduced the emissions of diesel- and gasoline-driven cars, which since the introduction of the EURO 6 regulation in 2014 are almost similar for both. Since the energy efficiency of diesel motors is up to 20% higher than that of gasoline-driven cars resulting in up to 20% lower CO2 emission, there is no reason to question the future use of diesel engines. These regulations apply for new cars. However, air concentrations at sampling points close to streets with high traffic still exceed the limit values especially for NO2. In several cities this led to restrictions for passenger cars of EURO 5 and below. Since concentrations close to streets are not relevant to evaluate the long-term exposure of the population, these measures are highly debatable.