- 1School of Chemistry, University of Leeds, Leeds, United Kingdom of Great Britain – England, Scotland, Wales
- 2National Centre for Atmospheric Science, University of Leeds, United Kingdom of Great Britain – England, Scotland, Wales
Inlet-pre-injectors (IPIs) are now increasingly used by the HOx measurement community (e.g. Mao et al., 2012; Novelli et al., 2014; Woodward-Massey et al., 2020; Cho et al., 2021) when measuring ambient OH by laser-induced fluorescence (LIF) to provide an interference-free measurement.
Traditionally, OH is detected by LIF in a low-pressure detection cell by tuning the laser wavelength on and off an OH transition to distinguish the OH fluorescence signal from the instrument background signal which is comprised of laser and solar scatter and any detector dark counts. However, this method of OH detection, which is often known as OHWAVE, does not allow the signal contribution from any OH generated within the detection cell to be differentiated from the ambient OH signal. The injection of propane (C3H8) or perfluoropropene (C3F6) scavenger via an IPI before the OH sampling nozzle leads to rapid removal of ambient OH and provides a measure of the instrument background signal whilst tuned to the OH transition. By this method, which is often known as OHCHEM, any signal from OH generated internally within the detection cell contributes to the background signal and so is distinguished from the ambient OH signal.
The concentration of OH scavenger injected via the IPI before sampling by the LIF instrument must be high enough to rapidly remove ambient OH, but not too high such that there is also removal of any internally-generated OH. A point-source of OH, generated by a Hg lamp in a humidified zero air flow, can be used to optimise the concentration of scavenger required. Here we will show, however, that higher concentrations of scavenger are often required to fully remove ambient OH as the scavenger must out-compete the reactions occurring under ambient conditions (e.g. HO2+NO) that are continually producing OH within the IPI.
Taking examples from previous OH measurement campaigns in two contrasting environments (forested and urban), the efficiency of an IPI to remove ambient OH is investigated using a detailed chemistry box-model based on the Master Chemical Mechanism (MCMv3.3.1) and constrained to measurements made during the campaigns. Taking typical residence times between scavenger injection and sampling by the low-pressure LIF detection cell, and varying the concentration of scavenger added, we show that under certain scenarios, >25 % of ambient OH could remain and may be erroneously considered as an OH interference.
Cho et al., Atmospheric Measurement Techniques, 14, 1851 – 1877, 2021
Mao et al., Atmospheric Chemistry and Physics, 12, 8009 – 8020, 2012
Novelli et al., Atmospheric Measurement Techniques, 7, 3413 – 3430, 2014
Woodward-Massey et al., Atmospheric Measurement Techniques, 13, 3119 – 3146, 2020
How to cite: Whalley, L. and Heard, D.: A modelling study investigating the efficiency of inlet-pre-injectors in removing ambient OH under different atmospheric conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10751, https://doi.org/10.5194/egusphere-egu25-10751, 2025.
