Airborne observations of peroxy radicals during the EMeRGe campaign in Europe

In this study, airborne measurements of the sum of hydroperoxyl (HO$_2$) and organic peroxy (RO$_2$) radicals that react with nitrogen monoxide (NO) to produce nitrogen dioxide (NO$_2$), 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 RO$^∗_2$ on HALO were made using the in situ instrument Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). RO$^∗_2$ is to a good approximation the sum of peroxy radicals reacting with NO to produce NO$_2$. RO$^∗_2$ 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 RO$^∗_2$ precursor concentrations measured on board, as well as the relevant rate coefficients. ... mehrGenerally, high RO$^∗_2$ concentrations were measured in air masses with high production rates. In the air masses investigated, RO$^∗_2$ is primarily produced by the reaction of O$^1$D 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 RO$^∗_2$ concentrations are expected to be in a photostationary steady state (PSS), i.e. a balance between production and loss rates is assumed. The RO$^∗_2$ production and loss rates and the suitability of PSS assumptions to estimate the RO$^∗_2$ mixing ratios and variability during the airborne observations are discussed. The PSS assumption for RO$^∗_2$ is considered robust enough to calculate RO$^∗_2$ mixing ratios for most conditions encountered in the air masses measured. The similarities and discrepancies between measured and PSS calculated RO$^∗_2$ mixing ratios are discussed. The dominant terminating processes for RO$^∗_2$ in the pollution plumes measured up to 2000 m are the formation of nitrous acid, nitric acid, and organic nitrates. Above2000 m, HO$_2$–HO$_2$ and HO$_2$–RO$_2$ reactions dominate the RO$^∗_2$ removal. RO$^∗_2$ 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 RO$^∗_2$ 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 RO$^∗_2$ measured is overestimated by the analytical expression. This may be caused by the formation of H$_2$O and O$_2$ from OH and HO$_2$, being about 4 times faster than the rate of the OH oxidation reaction of the dominant OVOCs considered.