Overview: On the transport and transformation of pollutants in the outflow of major population centres – observational data from the EMeRGe European intensive operational period in summer 2017
Authors: Andrés Hernández M. Dolores , Hilboll Andreas , Ziereis Helmut , Förster Eric , Krüger Ovid O. , Kaiser Katharina , Schneider Johannes , Barnaba Francesca , Vrekoussis Mihalis , Schmidt Jörg , Huntrieser Heidi , Blechschmidt Anne-Marlene , George Midhun , Nenakhov Vladyslav , Harlass Theresa , Holanda Bruna A. , Wolf Jennifer , Eirenschmalz Lisa , Krebsbach Marc , Pöhlker Mira L. , Kalisz Hedegaard Anna B. , Mei Linlu , Pfeilsticker Klaus , Liu Yangzhuoran , Koppmann Ralf , Schlager Hans , Bohn Birger , Schumann Ulrich , Richter Andreas , Schreiner Benjamin , Sauer Daniel , Baumann Robert , Mertens Mariano , Jöckel Patrick , Kilian Markus , Stratmann Greta , Pöhlker Christopher , Campanelli Monica , Pandolfi Marco , Sicard Michael , Gómez-Amo José L. , Pujadas Manuel , Bigge Katja , Kluge Flora , Schwarz Anja , Daskalakis Nikos , Walter David , Zahn Andreas , Pöschl Ulrich , Bönisch Harald , Borrmann Stephan , Platt Ulrich , Burrows John P. 
Published: May 2022

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.