Implementation of a chemical background method for atmospheric OH measurements by laser-induced fluorescence: characterisation and observations from the UK and China

Abstract

Abstract. Hydroxyl (OH) and hydroperoxy (HO2) radicals are central to the understanding of atmospheric chemistry. Owing to their short lifetimes, these species are frequently used to test the accuracy of model predictions and their underlying chemical mechanisms. In forested environments, laser-induced fluorescence–fluorescence assay by gas expansion (LIF–FAGE) measurements of OH have often shown substantial disagreement with model predictions, suggesting the presence of unknown OH sources in such environments. However, it is also possible that the measurements have been affected by instrumental artefacts, due to the presence of interfering species that cannot be discriminated using the traditional method of obtaining background signals via modulation of the laser excitation wavelength (OHwave). The interference hypothesis can be tested by using an alternative method to determine the OH background signal, via the addition of a chemical scavenging prior to sampling of ambient air (OHchem). In this work, the Leeds FAGE instrument was modified to include such a system to facilitate measurements of OHchem, in which propane was used to selectively remove OH from ambient air using an inlet pre-injector (IPI). The IPI system was characterised in detail, and it was found that the system did not reduce the instrument sensitivity towards OH ( 99 %) without the removal of OH formed inside the fluorescence cell (

OH and HO<sub>2</sub> radical chemistry at a suburban site during the EXPLORE-YRD campaign in 2018

“…Several studies conducted in forested environments indicated that OH measurements by laser-induced fluorescence technique using the wavelength modulation method might suffer from unknown internally produced interference (Mao et al, 2012;Novelli et al, 2017), and the magnitude of interference is highly dependent on the specific design of the instrument, the operating parameters, and the type of environment in which the instrument is deployed (Fuchs et al, 2016;Novelli et al, 2014;Woodward-Massey et al, 2020;Cho et al, 2021). To investigate the possible OH interference in this campaign, we performed an extended chemical modulation experiment on 7 June.…”

Elevated levels of OH observed in haze events during wintertime in central Beijing

“…The Leeds IPI was first implemented during the ICOZA campaign in Norfolk, UK, in the summer of 2015, and is described in further detail elsewhere (Woodward-Massey et al, 2019). During the APHH winter campaign the laser online (wavelength tuned to the OH transition) period lasted 300 seconds for both OHchem and OHwave data acquisition cycles.…”

“…The 185 slope of 1.05±0.07 demonstrates that within the errors in the linear fit no interference was evident during the winter campaign. OHwave data were corrected for the known interference from O3 + H2O, see (Woodward-Massey et al, 2019) for further details. All figures and calculation from now on have used OHwave as it is the most extensive time-series (12 days compared to 5 days).…”

“…OHwave data were corrected for 195 the known interference from O3 + H2O. Taken from (Woodward-Massey et al, 2019) where further details can be found.…”