On the understanding of tropospheric fast photochemistry: airborne observations of peroxy radicals during the EMeRGe-Europe campaign.

In this study, airborne measurements of the sum of hydroperoxyl (HO₂) and organic peroxy (RO₂) radicals that react with NO to produce NO₂, i.e. RO₂^*, 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 (MPCs). All 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 Long range research aircraft (HALO). The on-board measurements of RO₂* were made using the in-situ instrument Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). RO₂ mixing ratios up to 120 pptv were observed in air masses of different origins and composition under different local actinometrical conditions during seven HALO research flights in July 2017 over Europe. The range and variability of the RO₂^* measurements agree reasonably well with radical production rates estimated using photolysis frequencies and RO₂* precursor concentrations measured on-board. RO₂* is primarily produced following the photolysis of ozone (O3), formaldehyde (HCHO), glyoxal (CHOCHO), and nitrous acid (HONO) in the airmasses investigated. The suitability of photostationary steady-state (PSS) assumptions to estimate the mixing ratios and the variability of RO₂* during the airborne observations is investigated. The PSS assumption is robust enough to calculate RO₂ mixing rations for most conditions encountered in air masses measured. The similarities and discrepancies between measured and calculated RO₂^* mixing ratios are analysed stepwise. The parameters, which predominantly control the RO₂^* mixing ratios under different chemical and physical regimes, are identified during the analysis. The dominant removal processes of RO₂* in the airmasses measured up to 2000 m are the loss of OH and RO through the reaction with NO_x during the radical interconversion. Above 2000 m, HO₂ -- HO₂ and HO₂ -- RO₂ reactions dominate RO₂ loss reactions. RO₂ calculations underestimated (< 20 %) the measurements by the analytical expression inside the pollution plumes probed. The underestimation is attributed to the limitations of the PSS analysis to take into account the production of RO₂* through oxidation and photolysis of the OVOCs not measured during EMeRGe. [ABSTRACT FROM AUTHOR]