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The instrumentation of the High Altitude and Long Range (HALO) research aircraft is extended by the new Broadband AirCrAft RaDiometer Instrumentation (BACARDI) to quantify the radiative energy budget. Two sets of pyranometers and pyrgeometers are mounted to measure upward and downward solar (0.3–3 µm) and thermal–infrared (3–100 µm) irradiances. The radiometers are installed in a passively ventilated fairing to reduce the effects of the dynamic environment, e.g., fast changes in altitude and temperature. The remaining thermal effects range up to 20 W m−2 for the pyranometers and 10 W m−2 for the pyrgeometers. Using data collected by BACARDI during a night flight, it is demonstrated that the dynamic components of the offsets can be parameterized by the rate of change of the radiometer sensor temperatures, providing a greatly simplifying correction of the dynamic thermal effects. The parameterization provides a linear correction function (200–500 W m−2 K−1 s) that depends on the radiometer type and the mounting position of the radiometer on HALO. Furthermore, BACARDI measurements from the EUREC4A (Elucidating the Role of Clouds—Circulation Coupling in Climate) field campaign are analyzed to characterize the performance of the radiometers and to evaluate all corrections applied in the data processing. Vertical profiles of irradiance measurements up to 10 km altitude show that the thermal offset correction limits the bias due to temperature changes to values below 10 W m−2. Measurements with BACARDI during horizontal, circular flight patterns in cloud-free conditions demonstrate that the common geometric attitude correction of the solar downward irradiance provides reliable measurements in this typical flight section of EUREC4A, even without active stabilization of the radiometer.

The instrumentation of the High Altitude and Long Range (HALO) research aircraft is extended by the new Broadband AirCrAft RaDiometer Instrumentation (BACARDI) to quantify the radiative energy budget. Two sets of pyranometers and pyrgeometers are mounted to measure upward and downward solar (0.3–3 μm) and thermal-infrared (3–100 μm) irradiances. The radiometers are installed in a passively ventilated fairing to reduce the effects of the dynamic environment, e.g., fast changes of altitude and temperature. The remaining thermal effects range up to 20 W m-2 for the pyranometers and 10 W m-2 for the pyrgeometers; they are corrected using an new efficient method that is introduced in this paper. Using data collected by BACARDI during a night flight, the thermal offsets are parameterized by the rate of change of the radiometer sensor temperatures. Applying the sensor temperatures instead of ambient air temperature for the parameterization provides a linear correction function (200–600 W m-2 K-1 s), that depends on the mounting position of the radiometer on HALO. Furthermore, BACARDI measurements from the EUREC4A (Elucidating the role of clouds-circulation coupling in climate) field campaign are analyzed to characterize the performance of the radiometers and to evaluate all corrections applied in the data processing. Vertical profiles of irradiance measurements up to 10 km altitude show that the thermal offset correction limits the bias due to temperature changes to values below 10 W m-2. Measurements with BACARDI during horizontal, circular flight patterns in cloud-free conditions demonstrate that the common geometric attitude correction of the solar downward irradiance provides reliable measurements in this typical flight sections of EUREC4A, even without active stabilization of the radiometer.

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., The HALO deployment during EMeRGe was funded by a consortium comprising the German Research Foundation (DFG) Priority Program HALO-SPP 1294, the Institute of Atmospheric Physics of DLR, the Max Planck Society (MPG), and the Helmholtz Association. Flora Kluge, Benjamin Schreiner, and Klaus Pfeilsticker acknowledge the support given by the DFG through the project nos. PF 384-16, PF 384-17, and PG 385-19. Ralf Koppmann and Marc Krebsbach acknowledge DFG funding through project no. KR3861_1-1. Katja Bigge acknowledges additional funding from the Heidelberg Graduate School for Physics. Johannes Schneider, Katharina Kaiser, and Stephan Borrmann acknowledge funding through the DFG (project no. 316589531). Lisa Eirenschmalz and Hans Schlager acknowledge support by DFG through project MEPOLL (SCHL1857/4-1). Anna B. Kalisz Hedegaard would like to thank DAAD and DLR for a Research Fellowship. Hans Schlager acknowledge financial support by the DLR TraK (Transport and Climate) project. Michael Sicard acknowledges support from the EU (GA nos. 654109, 778349, 871115, and 101008004) and the Spanish Government (ref. nos. CGL2017-90884-REDT, PID2019-103886RB-I00, RTI2018-096548-B-I00, and MDM-2016-0600). Midhun George, Yangzhuoran Liu, M. Dolores Andrés Hernández, and John Phillip Burrows acknowledge financial support from the University of Bremen. FLEXPART simulations were performed on the HPC cluster Aether at the University of Bremen, financed by DFG within the scope of the Excellence Initiative. Anne-Marlene Blechschmidt was partly funded through the CAMS-84 project. Jennifer Wolf acknowledges support from the German Federal Ministry for Economic Affairs and Energy – BMWi (project Digitally optimized Engineering for Services – DoEfS; contract no. 20X1701B). Theresa Harlass thanks DLR VO-R for funding the young investigator research group “Greenhouse Gases”. Mariano Mertens, Patrick Jöckel, and Markus Kilian acknowledge resources of the Deutsches Klimarechenzentrum (DKRZ) granted by the WLA project ID bd0617 for the MECO(n) simulations and the financial support from the DLR projects TraK (Transport und Klima) and the Initiative and Networking Fund of the Helmholtz Association through the project “Advanced Earth System Modelling Capacity” (ESM). Bruna A. Holanda acknowledges the funding from Brazilian CNPq (process 200723/2015-4). The article processing charges for this open-access publication were covered by the University of Bremen.

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.

During the EMeRGe campaign we employed a cloud condensation nuclei counter (CCNC) on board the research aircraft HALO. The instrument was located in the CCN-Rack probing together with a single particle soot photometer (SP2) and a multi-impactor for sampling different air masses on silica nitrate substrates. The aerosol particles were sampled through the HALO Aerosol Submicrometer Inlet (HASI). The measurements have been performed with a two column continuous-flow longitudinal thermal-gradient instrument (CCN-200) manufactured by DMT. The CCN-200 measures the CCN number concentration as a function of water vapor supersaturation (S). These measurements are performed by changing S within one column from 0.10 % up to 1.00 % using 12 different supersaturations and keeping S constant within the second column (S = 0.38 %) to ensure baseline data with 1 Hz time resolution. The different supersaturations are created by changing the flow while setting a fixed temperature difference.Purpose of EMeRGe is to quantify and qualify outflows of megacities, as well as their transport and transformation in the atmosphere. Therefore, measurement flights were performed in the European airspace in 2017, probing aerosol properties over cities like London, Barcelona and Rome. In March 2018, the same set of instruments was probing the outflows of Asian megacities like Taipei, Manila and aged pollution from China Mainland. Furthermore, Japanese and South Korean outflows could be probed. The measurements took place in altitudes between 0.3 km and 13 km ASL. The scientific objective is to investigate the effect of different pollution states on aerosol and CCN properties.

Since peroxy radicals are closely involved in a number of tropospheric chemical processes like O3 budget, hydrocarbon oxidation and acid formation, the accurate measurement of these radicals can provide essential information to improve our understanding of processing and transformation of polluted outflows from megacities and Major Population Centres (MPCs).Airborne measurements of the total sum of peroxy radicals, RO2* = HO2 + ∑ RO2, where R is an organic group, were conducted in Europe in summer 2017 and in East Asia in spring 2018 within the EMeRGe (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global Scales) project by using the PeRCEAS instrument (Peroxy Radical Chemical Enhancement and Absorption Spectrometer), on board of the HALO research aircraft (www.halo.dlr.de).Over the course of both measurement campaigns different MPC outflows were investigated including among others, London, Rome, Manila and Taipei. Polluted air masses of different origin and composition were probed. Overall the peroxy radical mixing ratios were of the same order of magnitude in the air masses probed in Europe and in East Asia. The variations in the photochemical activity were studied by taking into account simultaneous observations of radical precursors and photolysis rates, while applying known oxidation mechanisms. Radical precursors, photolysis rates and aerosol load were generally higher in Asia, which might indicate higher radical loss reactions on the aerosol surface than in Europe. Moreover this study shows a clear deviation in the photostationary state for MPC outflows close to the emission sources. Based on this information, this presentation will focus on the actual understanding of the photochemical processing in the probed air masses.

Observations of tropospheric peroxy radicals are a key point for interpretation of the processing and transformation of polluted outflows from major populated centres (MPCs). A series of European MPCs are investigated by the project EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional and Global scales). With this objective two airborne campaigns using the research platform HALO (High Altitude and LOng range aircraft) were carried out over Europe in summer 2017 and over east Asia in the intermonsoon period in 2018. The Institute of Environmental Physics (IUP) in Bremen (Germany) participated in both EMeRGe campaigns with the airborne measurement of the total sum of peroxy radicals, RO2*, by using the home made PeRCEAS instrument based on the combination of the PERCA (peroxy radical chemical amplification) and CRDS (cavity ring down spectroscopy) techniques. One of the main purposes of the campaigns was the investigation of the characteristics and chemical transformation of MPC outflows at the local and regional scales.During the EMeRGe campaign in Europe, air masses of different photochemical activity were measured, where RO2* mixing ratios up to 100pptv being observed. In the present study the RO2* observations for six measurement flights of EMeRGe in Europe have been compared with RO2 (here defined as the sum of HO2 + CH3O2 + ISOOH + CH3CO3 + CH3COCH2O2) simulated by using the MECO(n) model.MECO(n) (MESSy-fied ECHAM and COSMO models nested n times), is a global/regional chemistry-climate model developed by the MESSy consortium, which couples on-line the global chemistry-climate model EMAC with the regional chemistry-climate model COSMO-CLM/MESSy. The same anthropogenic emission inventory (EDGAR 4.3.1) as well as the same solver for chemical kinetics, involving complex tropospheric and stratospheric chemistry, are applied in EMAC and COSMO-CLM/MESSy.Overall, the agreement between the measurements and model is reasonable for RO2* observations below 40 pptv. Events with higher mixing ratios seem not to be well reproduced by the model but underestimated. Further details on the modelling and the result of the comparison will be presented.

Hydroperoxyl, HO2, and organic peroxy, RO2, radicals are reactive short lived species, which play a key role in tropospheric chemistry. Measurements of the sum of HO2 and RO2, known as RO2*, provide unique information about the chemical processing in an air mass. This paper describes the experimental features and capabilities of Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). This is an instrument designed for use on aircraft from the boundary layer to the lower stratosphere. PeRCEAS combines the amplified conversion of peroxy radicals to nitrogen dioxide, NO2, with the sensitive detection of NO2 using absorption spectroscopy (CRD) at 408 nm. PeRCEAS is a dual channel instrument, with two identical reactor-detector lines working out of phase with one another at a constant and defined pressure lower than ambient. The suitability of PeRCEAS for the airborne measurement in the free troposphere was evaluated by extensive characterisation and calibration under atmospherically representative conditions in the laboratory. The use of the alternating modes of the two instrumental channels captures successfully short term variations in the composition of RO2* in ambient air. For 60s measurement, the RO2* detection limit for a minimum (2σ) NO2 detectable mixing ratio

Development of an airborne instrument for the determination of peroxy radicals (PeRCEAS – Peroxy Radical Cavity Enhanced Absorption Spectroscopy) is reported. Ambient peroxy radicals (HO2 and RO2, R being an organic chain) are converted to NO2 by adding NO, and are recycled through subsequent reaction with CO and O2, thus forming a chain reaction with an amplification factor called chain length. The concentration of NO2 is measured by continuous-wave cavity ring-down spectroscopy (CRDS) using an extended cavity diode laser at 409 nm. Optical feedback from a V-shaped cavity optimizes resonator transmission and allows for a simple detector set-up. CRDS directly yields absorption coefficients, thus providing NO2 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 mbar, corresponding to a concentration of 2 × 109 molecules cm−3. The calibration of the PeRCEAS chain length at an inlet pressure of 300 mbar yields a value of 120 ± 7. The peroxy radical 1σ detection limit for an averaging time of 120 s and a chain length of 120 is ~3 pptv.