Airborne observations of peroxy radicals during the EMeRGe campaign in Europe
Authors: George Midhun , Andrés Hernández Maria Dolores , Nenakhov Vladyslav , Liu Yangzhuoran , Burrows John Philip , Bohn Birger , Förster Eric , Obersteiner Florian , Zahn Andreas , Harlaß Theresa , Ziereis Helmut , Schlager Hans , Schreiner Benjamin , Kluge Flora , Bigge Katja , Pfeilsticker Klaus 
Published: July 2023

Abstract. In this study, airborne measurements of the sum of hydroperoxyl
(HO2) and organic peroxy (RO2) radicals that react with nitrogen
monoxide (NO) to produce nitrogen dioxide (NO2), coupled with
actinometry and other key trace gases measurements, have been used to test
the current understanding of the fast photochemistry in the outflow of major
population centres. The measurements were made during the airborne campaign
of the EMeRGe (Effect of Megacities on the transport and transformation of
pollutants on the Regional to Global scales) project in Europe on board the
High Altitude and Long Range Research Aircraft (HALO). The measurements of
RO2∗ on HALO were made using the in situ instrument Peroxy
Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS).
RO2∗ is to a good approximation the sum of peroxy radicals
reacting with NO to produce NO2. RO2∗ mixing ratios up to
120 pptv were observed in air masses of different origins and composition
under different local actinometric conditions during seven HALO research
flights in July 2017 over Europe. Radical production rates were estimated using knowledge of the photolysis
frequencies and the RO2∗ precursor concentrations measured
on board, as well as the relevant rate coefficients. Generally, high
RO2∗ concentrations were measured in air masses with high
production rates. In the air masses investigated, RO2∗ is
primarily produced by the reaction of O1D with water vapour and the
photolysis of nitrous acid (HONO) and of the oxygenated volatile organic
compounds (OVOCs, e.g. formaldehyde (HCHO) and glyoxal (CHOCHO)). Due to
their short lifetime in most environments, the RO2∗ concentrations are expected to be in a photostationary steady state (PSS), i.e. a balance between production and loss rates is assumed. The
RO2∗ production and loss rates and the suitability of PSS
assumptions to estimate the RO2∗ mixing ratios and variability
during the airborne observations are discussed. The PSS assumption for
RO2∗ is considered robust enough to calculate RO2∗ mixing ratios for most conditions encountered in the air masses measured.
The similarities and discrepancies between measured and PSS calculated
RO2∗ mixing ratios are discussed. The dominant terminating
processes for RO2∗ in the pollution plumes measured up to 2000
m are the formation of nitrous acid, nitric acid, and organic nitrates. Above
2000 m, HO2–HO2 and HO2–RO2 reactions dominate the
RO2∗ removal. RO2∗ calculations by the PSS
analytical expression inside the pollution plumes probed often
underestimated the measurements. The underestimation is attributed to the
limitations of the PSS equation used for the analysis. In particular, this
expression does not account for the yields of RO2∗ from the
oxidation and photolysis of volatile organic compounds, VOCs, and OVOCs
other than those measured during the EMeRGe research flights in Europe. In
air masses with NO mixing ratios ≤50 pptv and low VOC/NO ratios, the
RO2∗ measured is overestimated by the analytical expression.
This may be caused by the formation of H2O and O2 from OH and
HO2, being about 4 times faster than the rate of the OH oxidation
reaction of the dominant OVOCs considered.