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.
Abstract. EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) is an international project focusing on atmospheric chemistry, dynamics and transport of local and regional pollution originating in megacities and other major population centres (MPCs). Airborne measurements, taking advantage of the long range capabilities of the HALO research platform (High Altitude and Long range research aircraft, www.halo-spp.de), are a central part of the research project. In order to provide 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 (IOP) 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 and the identification of pollution plumes. This paper describes the experimental deployment of the IOP in Europe, which comprised 7 HALO research flights with aircraft base in Oberpfaffenhofen (Germany) for a total of 53 flight hours. The MPC targets London (Great Britain), Benelux/Ruhr area (Belgium, The Netherlands, Luxembourg and Germany), Paris (France), Rome and Po Valley (Italy), Madrid and Barcelona (Spain) were investigated. An in-flight comparison of HALO with the collaborating UK-airborne platform FAAM took place to assure accuracy and comparability of the instrumentation on-board. Generally, significant enhancement of trace gases and aerosol particles are attributed to emissions originating in MPCs at distances of hundreds of kilometres from the sources. The proximity of different MPCs over Europe favours the mixing of plumes of different origin and level of processing and hampers the unambiguous attribution of the MPC sources. Similarly, urban plumes mix efficiently with natural sources as desert 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 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, provides a unique insight to test the current understanding of MPC pollution outflows. The present work provides an overview of the most salient results and scientific questions in the European context, 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.
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.
<pre>Remote sensing is an important measurement technique when probing the atmosphere as it is rather flexible and <br />allows for the measurement of numerous variables. For example, spectrometers are commonly used to quantify the <br />concentrations of trace gases by recording spectra of direct or scattered sunlight. Due to scattering, the light <br />paths between sun and detector can be rather complicated and radiative transfer models are necessary to retrieve <br />the information contained in the spectra. <br />Since these spectrometer measurements have to be made in spectral regions with strong absorption, it is necessary <br />to model many wavelengths to reach a sufficiently accurate representation of the absorption lines and their effects <br />on the light paths. <br />Thus, 1D models are often implemented for a fast analysis of the recorded spectra. However, this approach assumes <br />horizontal homogeneity and local sources cannot be resolved. In contrast, 3D Monte Carlo models are more realistic <br />and are able to represent this inhomogeneity, but they are computationally expensive and are not suitable for <br />operational use.</pre> <pre>We improve an existing Monte Carlo model by implementing efficient algorithms for the simultaneous calculation of <br />several wavelengths to decrease the required computation time. <br />Furthermore, we examine to which scatter order the 3D model provides more detailed results while maintaining a <br />reasonable run time.<br />This finally leads to a coupling of these two types of radiative transfer models via the scatter order into one <br />efficient model which performs realistic simulations at a computational cost comparable to 1D models.<br />So we are able to detect sources along the line of sight of ground-based measurements of scattered sunlight.<br />Here, we present our objective and first results.</pre>
PANACEA -Panhellenic infrastructure for Atmospheric Composition and Climate Change panacea-ri.gr University of Crete www.uoc.gr Ireland UCC -University College of Cork www.ucc.ie Italy CNR-ISAC -National Research Council of Italy, Institute of Atmospheric Sciences and Climate www.isac.cnr.it CNR-IMAA -National Research Council of Italy Istituto di Metodologie per l'Analisi Ambientale www.imaa.cnr.
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