Implementation of a chemical background method for atmospheric OH measurements by laser-induced fluorescence: characterisation and observations from the UK and China
Authors: Woodward-Massey Robert , Slater Eloise J. , Alen Jake , Ingham Trevor , Cryer Danny R. , Stimpson Leanne M. , Ye Chunxiang , Seakins Paul W. , Whalley Lisa K. , Heard Dwayne E. 
Published: June 2020

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 scavenger 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
(< 5 % difference to conventional sampling) and was able to
efficiently scavenge external OH (> 99 %) without the removal
of OH formed inside the fluorescence cell (< 5 %). Tests of the
photolytic interference from ozone in the presence of water vapour revealed
a small but potentially significant interference, equivalent to an OH
concentration of ∼4×105 molec. cm−3
under typical atmospheric conditions of [O3] =50 ppbv and [H2O] =1 %. Laboratory experiments to investigate potential interferences
from products of isoprene ozonolysis did result in interference signals, but
these were negligible when extrapolated down to ambient ozone and isoprene
levels. The interference from NO3 radicals was also tested but was
found to be insignificant in our system. The Leeds IPI module was deployed
during three separate field intensives that took place in summer at a
coastal site in the UK and both in summer and winter in the megacity of
Beijing, China, allowing for investigations of ambient OH interferences
under a wide range of chemical and meteorological conditions. Comparisons of
ambient OHchem measurements to the traditional OHwave method showed
excellent agreement, with OHwave vs OHchem slopes of 1.05–1.16 and
identical behaviour on a diel basis, consistent with laboratory interference
tests. The difference between OHwave and OHchem (“OHint”) was found to
scale non-linearly with OHchem, resulting in an upper limit interference of
(5.0±1.4) ×106 molec. cm−3 at the very highest
OHchem concentrations measured (23×106 molec. cm−3),
accounting for ∼14 %–21 % of the total OHwave signal.