European legislation sets stringent limits on nitrogen oxides (NOx) and fine particle number (PN) emitted from vehicle exhausts as both are a major source of air pollution and hazardous to human health. Real-driving emissions tests have recently been implemented in the EU to get a more realistic measurement of these emissions from vehicles. Such tests are performed using portable emission measurement systems (PEMS) which enable real-world testing of the emissions, but these currently lack full performance and metrological characterisation. VSL is partner of the MetroPEMS project ((2020-2023), https://lnkd.in/exPVpDmc), which addresses the need for PEMS calibration standards and guidelines.
VSL is developing reliable gas standards for testing and calibrating PEMS instruments. Nitrogen dioxide (NO2), at high amount fractions (> 500 µmol/mol), is one of the main components in such gas standards. If low NO2 amount fractions (< 10 µmol/mol) gas standards pose analytical challenges due to the inevitable presence of nitric acid (HNO3), as recently demonstrated in the key-comparison report (https://lnkd.in/dYGbvhBB), high NO2 amount fractions, as relevant for PEMS, are even more challenging. This is due to the formation of N2O4, the NO2 dimer.
VSL has prepared several NO2 gas standards in the range 500-2500 µmol/mol, including binary gas standards of NO2 in synthetic air and multicomponent mixtures containing nitrogen monoxide, carbon monoxide, carbon dioxide and NO2, in nitrogen.
The stability of the developed gas standards is currently being monitored using several analytical techniques including our VSL’s new FTIR spectrometer facility, which enables accurate quantification of both nitric acid and the NO2 dimer. Results of the first 6 months of stability testing are promising and this study will continue to obtain 1 year stability data. The graph depicts the VSL experimental data for the quantification of N2O4 in the binary NO2 gas standards and for comparison a simulation based on an equilibration model. The outcome of this development study will be published.