18 Feb 2022
 | 18 Feb 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

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

Midhun George, Maria Dolores Andrés Hernández, Vladyslav Nenakhov, Yangzhuoran Liu, John Philip Burrows, Birger Bohn, Eric Förster, Florian Obersteiner, Andreas Zahn, Theresa Harlaß, Helmut Ziereis, Hans Schlager, Benjamin Schreiner, Flora Kluge, Katja Bigge, and Klaus Pfeilsticker

Abstract. 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 NOx 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.

Midhun George et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-119', Anonymous Referee #1, 07 Mar 2022
    • AC4: 'Reply on RC1', Midhun George, 23 Feb 2023
  • RC2: 'Comment on acp-2022-119', Anonymous Referee #2, 08 Mar 2022
    • AC1: 'Reply on RC2', Midhun George, 23 Feb 2023
  • RC3: 'Comment on acp-2022-119', Anonymous Referee #3, 09 Mar 2022
    • AC2: 'Reply on RC3', Midhun George, 23 Feb 2023
  • RC4: 'Comment on acp-2022-119', Anonymous Referee #4, 14 Mar 2022
    • AC3: 'Reply on RC4', Midhun George, 23 Feb 2023

Midhun George et al.

Midhun George et al.


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Short summary
The applicability photostationary steady-state (PSS) assumptions to estimate the amount of the sum of peroxy radicals (RO2*) during the airborne observations from the known radical chemistry and on-board measurements of RO2* precursors, photolysis frequencies, and other trace gases such as NOx and O3 was investigated. The comparison of the calculated RO2* with the actual measurements provides an insight into the main processes controlling their concentration in the airmasses measured.