1Centre for Atmospheric Science, University of Manchester, Manchester, UK
2School of Earth and Environment, University of Leeds, Leeds, UK
3Laboratoire des Sciences du Climat et de l'Environnement, Laboratoire mixte CEA-CNRS-UVSQ, CEA Saclay, Gif-sur-Yvette, France
4Observation Based Research, Met Office, Exeter, UK
5College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK
6Department of Chemistry, University of York, York, UK
7Laboratoire Atmosphères, Milieux et Observations Spatiales, Laboratoire mixte CNRS-UVSQ-UPMC, Université Paris, Paris, France
8Department of Meteorology, University of Reading, Reading, UK
*now at: Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
**now at: Aeroqual, 109 Valley Road, Auckland 1024, New Zealand
Received: 21 Oct 2011 – Discussion started: 16 Nov 2011
Abstract. The Emissions around the M25 motorway (EM25) campaign took place over the megacity of London in the United Kingdom in June 2009 with the aim of characterising trace gas and aerosol composition and properties entering and emitted from the urban region. It featured two mobile platforms, the UK BAe-146 Facility for Airborne Atmospheric Measurements (FAAM) research aircraft and a ground-based mobile lidar van, both travelling in circuits around London, roughly following the path of the M25 motorway circling the city. We present an overview of findings from the project, which took place during typical UK summertime pollution conditions. Emission ratios of volatile organic compounds (VOCs) to acetylene and carbon monoxide emitted from the London region were consistent with measurements in and downwind of other large urban areas and indicated traffic and associated fuel evaporation were major sources. Sub-micron aerosol composition was dominated by secondary species including sulphate (24% of sub-micron mass in the London plume and 29% in the non-plume regional aerosol), nitrate (24% plume; 20% regional) and organic aerosol (29% plume; 31% regional). The primary sub-micron aerosol emissions from London were minor compared to the larger regional background, with only limited increases in aerosol mass in the urban plume compared to the background (~12% mass increase on average). Black carbon mass was the major exception and more than doubled in the urban plume, leading to a decrease in the single scattering albedo from 0.91 in the regional aerosol to 0.86 in the London plume, on average. Our observations indicated that regional aerosol plays a major role on aerosol concentrations around London, at least during typical summertime conditions, meaning future efforts to reduce PM levels in London must account for regional as well as local aerosol sources.
Revised: 09 Apr 2012 – Accepted: 09 May 2012 – Published: 13 Jun 2012
McMeeking, G. R., Bart, M., Chazette, P., Haywood, J. M., Hopkins, J. R., McQuaid, J. B., Morgan, W. T., Raut, J.-C., Ryder, C. L., Savage, N., Turnbull, K., and Coe, H.: Airborne measurements of trace gases and aerosols over the London metropolitan region, Atmos. Chem. Phys., 12, 5163-5187, doi:10.5194/acp-12-5163-2012, 2012.