Abstract
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.
Highlights
The greater London urban region is home to approximately 8–12 million people, making it the largest conurbation in the United Kingdom and one of a growing number of megacities throughout the world
There were usually scattered fair weather cumuli clouds over the study region with no precipitation on flight days, though cloud base was above the flight measurement altitude for all but a few brief periods, which have been removed from the analysis
One important distinction between our results and those reported by Moteki et al (2007) and Subramanian et al (2010) in addition to differences in aerosol and photochemical environment was that we considered the entire refractory black carbon (rBC) population but the previous studies restricted their analyses of the conversion rate to thickly coated rBC to particles within a specific mass/size range
Summary
The greater London urban region is home to approximately 8–12 million people, making it the largest conurbation in the United Kingdom and one of a growing number of megacities throughout the world. McMeeking et al.: Trace gases and aerosols over London megacities, and these numbers are expected to grow in the coming years. Megacities frequently have very high concentrations of observed air pollutants (Banta et al, 2005), which combined with their large populations results in serious health and visibility impacts (Garland et al, 2008; Parekh, 2001). In addition to local impacts, the large emissions of pollutants from megacities make them important contributors to air pollution on regional and global scales (Lawrence et al, 2007; Molina and Molina, 2004). Cities emit primary aerosols as well as oxides of nitrogen (NOx) and volatile organic compounds (VOCs) that drive photochemical smog formation and oxidise to secondary aerosol precursors
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