Abstract. Hydroperoxyl (HO2) and organic peroxy (RO2) radicals have an unpaired spin and are highly reactive free radicals. Measurements of the sum of HO2 and RO2 provide unique
information about the chemical processing in an air mass. This paper
describes the experimental features and capabilities of the Peroxy Radical
Chemical Enhancement and Absorption Spectrometer (PeRCEAS). This is an
instrument designed to make measurements on aircraft from the boundary layer
to the lower stratosphere. PeRCEAS combines the amplified conversion of
peroxy radicals to nitrogen dioxide (NO2) with the sensitive detection
of NO2 using cavity ring-down spectroscopy (CRDS) at 408 nm.
PeRCEAS is a dual-channel instrument, with two identical reactor–detector
lines working out of phase with one another at a constant and defined
pressure lower than ambient at the aircraft altitude. The suitability of
PeRCEAS for airborne measurements in the free troposphere was evaluated by
extensive characterisation and calibration under atmospherically
representative conditions in the laboratory. The use of alternating modes of
the two instrumental channels successfully captures short-term variations in
the sum of peroxy radicals, defined as RO2∗
(RO2∗=HO2+∑RO2+OH+∑RO, with R being an
organic chain) in ambient air. For a 60 s measurement, the RO2∗
detection limit is < 2 pptv for a minimum (2σ) NO2
detectable mixing ratio < 60 pptv, under laboratory conditions in
the range of atmospheric pressures and temperatures expected in the free
troposphere. PeRCEAS has been successfully deployed within the OMO
(Oxidation Mechanism Observations) and EMeRGe (Effect of Megacities on the
transport and transformation of pollutants on the Regional and Global
scales) missions in different airborne campaigns aboard the High Altitude LOng range
research aircraft (HALO) for the study of the composition of the free
troposphere.
Review of “Airborne measurement of peroxy radicals using chemical amplification couple with cavity ring down spectroscopy: the PeRCEAS instrument” by Midhun George et al.
