Abstract. Field campaigns have been carried out with the FAGE (fluorescence assay by
gas expansion) technique in remote biogenic environments in the last decade
to quantify the in situ concentrations of OH, the main oxidant in the
atmosphere. These data have revealed concentrations of OH radicals up to a
factor of 10 higher than predicted by models, whereby the disagreement
increases with decreasing NO concentration. This was interpreted as a major
lack in our understanding of the chemistry of biogenic VOCs (volatile organic
compounds), particularly isoprene, which are dominant in remote pristine
conditions. But interferences in these measurements of unknown origin have
also been discovered for some FAGE instruments: using a pre-injector, all
ambient OH is removed by fast reaction before entering the FAGE cell, and any
remaining OH signal can be attributed to an interference. This technique is
now systematically used for FAGE measurements, allowing the reliable
quantification of ambient OH concentrations along with the signal due to
interference OH. However, the disagreement between modelled and measured high
OH concentrations of earlier field campaigns as well as the origin of the
now-quantifiable background OH is still not understood. We present in this
paper the compelling idea that this interference, and thus the disagreement
between model and measurement in earlier field campaigns, might be at least
partially due to the unexpected decomposition of a new class of molecule,
ROOOH, within the FAGE instruments. This idea is based on experiments,
obtained with the FAGE set-up of the University of Lille, and supported by a
modelling study. Even though the occurrence of this interference will be
highly dependent on the design and measurement conditions of different FAGE
instruments, including ROOOH in atmospheric chemistry models might reflect a
missing piece of the puzzle in our understanding of OH in clean atmospheres.
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