[1] The objective of this study has been to investigate the origin of the water vapor effect on the chain length (CL) of peroxy radical chemical amplifiers (PERCA). Results of the investigation of the water interference in the determination of peroxy radicals by using the PERCA technique are presented. The experimental conditions have been analyzed and modeled. A nonlinear dependence of the CL on the relative humidity (RH) has been accurately determined. The combined analysis of experimental and simulated results rules out wall loses as a single explanation of the CL variation observed and indicates three reactions, which possibly account for this water effect:À À À À À! nonradical products (e.g., HNO 3 ). Assuming a mechanism involving the formation of a HO 2 -nH 2 O complex, the corresponding rate coefficients and their water dependence have been estimated. The quadratic dependence of these rate coefficients upon the RH implies the participation of two H 2 O molecules in the proposed reactions. The study has shown that at a RH of 80% an effective secondorder rate coefficient of 10 À15 cm 3 molecule À1 s À1 for the reaction of CO with HO 2 , or 10À13 cm 3 molecule À1 s À1 for the reaction of HO 2 with NO, explains the observed behavior. Both these complex reactions have potential significance for the chemistry of the marine boundary layer (MBL) and their atmospheric implications are discussed.

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Abstract. Development of an airborne instrument for the determination of peroxy radicals (PeRCEAS -peroxy radical chemical enhancement and absorption spectroscopy) is reported. Ambient peroxy radicals (HO 2 and RO 2 , R being an organic chain) are converted to NO 2 in a reactor using a chain reaction involving NO and CO. Provided that the amplification factor, called effective chain length (eCL), is known, the concentration of NO 2 can be used as a proxy for the peroxy radical concentration in the sampled air. The eCL depends on radical surface losses and must thus be determined experimentally for each individual setup. NO 2 is detected by continuous-wave cavity ring-down spectroscopy (cw-CRDS) using an extended cavity diode laser (ECDL) at 408.9 nm. Optical feedback from a V-shaped resonator maximizes transmission and allows for a simple detector setup. CRDS directly yields absorption coefficients, thus providing NO 2 concentrations without additional calibration. The optimum 1σ detection limit is 0.3 ppbv at an averaging time of 40 s and an inlet pressure of 300 hPa. Effective chain lengths were determined for HO 2 and CH 3 O 2 at different inlet pressures. The 1σ detection limit at an inlet pressure of 300 hPa for HO 2 is 3 pptv for an averaging time of 120 s.

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Abstract. Megacities and other major population centres (MPCs) worldwide are major sources of air pollution, both locally as well as downwind. The overall assessment and prediction of the impact of MPC pollution on tropospheric chemistry are challenging. The present work provides an overview of the highlights of a major new contribution to the understanding of this issue based on the data and analysis of the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) international project. EMeRGe focuses on atmospheric chemistry, dynamics, and transport of local and regional pollution originating in MPCs. Airborne measurements, taking advantage of the long range capabilities of the High Altitude and LOng Range Research Aircraft (HALO, https://www.halo-spp.de, last access: 22 March 2022), are a central part of the project. The synergistic use and consistent interpretation of observational data sets of different spatial and temporal resolution (e.g. from ground-based networks, airborne campaigns, and satellite measurements) supported by modelling within EMeRGe provide unique insight to test the current understanding of MPC pollution outflows. In order to obtain an adequate set of measurements at different spatial scales, two field experiments were positioned in time and space to contrast situations when the photochemical transformation of plumes emerging from MPCs is large. These experiments were conducted in summer 2017 over Europe and in the inter-monsoon period over Asia in spring 2018. The intensive observational periods (IOPs) involved HALO airborne measurements of ozone and its precursors, volatile organic compounds, aerosol particles, and related species as well as coordinated ground-based ancillary observations at different sites. Perfluorocarbon (PFC) tracer releases and model forecasts supported the flight planning, the identification of pollution plumes, and the analysis of chemical transformations during transport. This paper describes the experimental deployment and scientific questions of the IOP in Europe. The MPC targets – London (United Kingdom; UK), the Benelux/Ruhr area (Belgium, the Netherlands, Luxembourg and Germany), Paris (France), Rome and the Po Valley (Italy), and Madrid and Barcelona (Spain) – were investigated during seven HALO research flights with an aircraft base in Germany for a total of 53 flight hours. An in-flight comparison of HALO with the collaborating UK-airborne platform Facility for Airborne Atmospheric Measurements (FAAM) took place to assure accuracy and comparability of the instrumentation on board. Overall, EMeRGe unites measurements of near- and far-field emissions and hence deals with complex air masses of local and distant sources. Regional transport of several European MPC outflows was successfully identified and measured. Chemical processing of the MPC emissions was inferred from airborne observations of primary and secondary pollutants and the ratios between species having different chemical lifetimes. Photochemical processing of aerosol and secondary formation or organic acids was evident during the transport of MPC plumes. Urban plumes mix efficiently with natural sources as mineral dust and with biomass burning emissions from vegetation and forest fires. This confirms the importance of wildland fire emissions in Europe and indicates an important but discontinuous contribution to the European emission budget that might be of relevance in the design of efficient mitigation strategies. The present work provides an overview of the most salient results in the European context, with these being addressed in more detail within additional dedicated EMeRGe studies. The deployment and results obtained in Asia will be the subject of separate publications.