Measuring VOCs under real driving emission conditions by means of PTR-MS with active countermeasures for the humidity dependence

In this contribution, we present a dedicated instrument for monitoring volatile organic compounds (VOCs) under real driving emissions (RDE) conditions in line with the objectives of the EU and UKRI funded AEROSOLS project. This instrument fulfils the necessary criteria to be installed and operated in an SUV passenger car and to measure the emitted VOCs in a time-resolved manner. The VOCs are ionised and analysed using the proton transfer reaction (PTR) in combination with a time-of-flight (TOF) mass spectrometer. PTR mass-spectrometry (PTR-MS) has proven for many years to be a versatile ionisation technique for the quantification of VOCs, provided that the ion chemistry within the drift tube is well defined. However, for a number of compounds, the ion yield of the detected VOCs is dependent on humidity, making quantification difficult. There have been several attempts to solve this problem. One strategy is a labour- and time-intensive calibration at different humidity levels prior to the actual measurement and correction of the derived concentration after the measurement. Another strategy is to flood the drift tube with large amounts of water vapour, which contradicts the well-defined ion chemistry. Here we present the results of a study using a novel method that eliminates any influence of changing sample humidity on the measurements and has virtually no drawbacks. By introducing a controlled flow of water vapour directly into the PTR reaction region, the humidity is always kept constant. We present both laboratory-based studies on compounds of known humidity dependence and a long-time measurement of the outside air. In the former, we found a signal variation of about a factor of five between dry and 23 g m^-3 absolute sample humidity for hydrogen sulphide, for example. By automatically injecting between 0.4 and 1.2 sccm of water vapour, the ion yield intensities for all compounds were decoupled from the sample humidity. In the latter study, we present a measurement during summertime, and despite the change from dry to humid conditions, the humidity in the PTR reaction region remained constant. Therefore, all changes in ion yield intensities represent true concentration changes and not artefacts due to varying water concentration in the air.

Acknowledgement: This research was co-funded by the European Union’s Horizon Europe research and innovation programme within the AEROSOLS project under grant agreement No. 101096912 and UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee [grant numbers 10092043 and 10100997].