The number and size of Major Population Centers (MPC) is increasing worldwide and the impact of their emissions on the local, regional and hemispheric atmospheric composition is inadequately understood. In the past two decades, the growing importance of emissions from MPC for air pollution and climate change has been recognized. A series of European (e.g., MEGAPOLI, CITYZEN, ICARTT, etc.), American (e.g., EAST ASIA, MILAGRO, INTEX, ADAPTE, etc) and Asian (e.g., IMPACT, PRIDE, CAREBeijing etc.) projects have attempted the first quantification of the emissions from targeted MPC, and study their transport and transformation. Asia is of particular importance, because of its rapid industrialization and economic growth over the past 40 years. Our current understanding and the key scientific issues have recently been summarized in the Global Atmosphere Watch (GAW) report number 205: Impacts of Megacities on Air Pollution and Climate.

The impact of MPCs is not only dependent on the amount and type of emissions but also on the regional geography, meteorology, atmospheric photochemistry and chemistry. The dependence on these essential parameters results in large uncertainties when extrapolating locally quantified sources for example that determined in a particular megacity, to regional and global impacts on chemical composition and atmospheric pollution. Further vertical mixing, deposition processes and synergetic effects of the emissions in areas having clusters of megacities lead to unexpected non-linear effects at local and regional levels. In order to reduce the uncertainties and improve predictive capability, measurement campaigns, and theoretical studies, which focus on the long range transport of plumes from MPC in different regions of the world, are required. These need to be complementary to the long term data from ground–based and satellite borne instrumentation. Such activities may further serve to investigate, test and validate existing hypotheses, which explain the transport and transformation pathways, the dispersion and distribution of pollutants from megacities/ large conurbations, and their impact on regional, hemispheric and global air pollution. The latter is a prerequisite for improving current assessments of atmospheric pollution and climate change and the improvement of the accuracy of the prediction of future changes.

EMeRGe aims to investigate experimentally the patterns of atmospheric transport and transformation of pollution plumes originating from Eurasia and both tropical and subtropical Asian megacities and MPCs. To meet this objective airborne measurements of key reactive gases and aerosols in optimized flight patterns exploiting the capabilities of the HALO aircraft are proposed. Utilising its long-range altitude endurance and its potential to carry large payloads, the HALO provides a unique possibility to undertake both transects and Lagrangian experiments of the outflow from the target regions, given the necessary flight permissions. These measurements provide novel information on the source strengths, atmospheric transport, and transformation of pollution plumes of varying chemical composition emitted by different large urban agglomerates having different emission characteristics. The investigation of the vertical and horizontal chemical evolution of the outflow will provide essential information required to improve our understanding and to quantify the impacts of megacities on regional and global effects of their air pollution. The information content of the HALO EMeRGe campaign data will be enhanced synergistically by combining the EMeRGe measurements with satellite data products from the measurements of the instrumentation on EUMETSAT Metop A and B platforms and the Sentinel 5 Precursor. Interpretation will use state-of-the-art chemical transport models. As a spin off, EMerGe also yields validation of some key satellite data products, which is an original goal of HALO. The participation of state-of-the-art atmospheric modeling groups in the analysis of data will optimize and maximize the scientific outcomes. EMeRGe builds on and benefits from the results obtained during the OMO HALO mission in 2015, which will provide important new information about long range transport of pollutants in pre-monsoon and monsoon conditions over South Asia.

EMeRGe will focus on following scientific key issues, highlighted by the GAW report:

• • The photochemical O3 formation and the oxidizing capacity of the MPC plumes on a regional scale. This will be achieved by making measurements of O3, its precursors, peroxy radicals (RO2*) as well as the nitrogen oxides (NOx, NOy), CO, VOCs, UV radiation and the relevant photolysis frequencies upwind and downwind of selected megacities.
  • The vertical transport of O3 and its precursors to assess transport pattern differences between European and Asian megacities by measuring vertical concentration profiles of some key species downwind of the MPC. The dynamical and temporal evolution of air plumes from megacities by using tracers that can subsequently be detected over a period of several days. (Tagging plumes greatly facilitates the investigation of plume transport as well as of photochemical and microphysical transformation processes).
  • The long range transport of air pollution from megacities by measuring horizontal and vertical distribution of long lived greenhouse gases such as carbon dioxide, CO2, methane CH4, and nitrous oxide, N2O.
  • The evolution of radiative properties of emissions of megacities by comparative measurement of aerosol particle properties and composition upwind and downwind of selected megacities.

The EMeRGe mission is proposed and led by the Institute of Environmental Physics of the University of Bremen but is a community–based partnership including the University research teams and those from the large research facilities. HALO will be equipped with a comprehensive suite of instruments to provide the chemical, physical, and optical measurements required to achieve the scientific goals described above. These will include measurements of optical and meteorological parameters, long-lived greenhouse gases, O3 and its precursors, aerosols and their precursors and chemical tracers of sources and transport of pollutants. EMeRGe has selected in the first instance to study Eurasian and SE Asian megacities and MPCs having different atmospheric dispersion conditions to assess the commonality, identify the differences in the outflow transport and transformation patterns and determine their critical physical and chemical pathways. Atmospheric composition, dispersion, and chemical processing within the plumes from major population centers will be characterized, analyzed, and assessed. The optimized research flights and patterns have been selected for periods, where active photochemistry is mainly driven by insolation and the reduced precipitation favors mixing, transport and chemical transformation. During two intensive measurement periods in July 2017 (three weeks) and March 2018 (four weeks), the HALO EMeRGe measurements will be performed under both non-precipitating and precipitating conditions. This approach delivers an optimal and distinct observation of the regional impacts along dominant atmospheric transport pathways coming from European and Asian MPCs.