Megacity and local contributions to regional air pollution: an aircraft case study over London
Authors: Ashworth Kirsti , Bucci Silvia , Gallimore Peter J. , Lee Junghwa , Nelson Beth S. , Sanchez-Marroquín Alberto , Schimpf Marina B. , Smith Paul D. , Drysdale Will S. , Hopkins Jim R. , Lee James D. , Pitt Joe R. , Di Carlo Piero , Krejci Radovan , McQuaid James B. 
Published: June 2020

Abstract. In July 2017 three research flights circumnavigating the megacity
of London were conducted as a part of the STANCO training school for
students and early career researchers organised by EUFAR (European Facility
for Airborne Research). Measurements were made from the UK's Facility for
Airborne Atmospheric Measurements (FAAM) BAe-146-301 atmospheric research
aircraft with the aim to sample, characterise and quantify the impact of
megacity outflow pollution on air quality in the surrounding region.
Conditions were extremely favourable for airborne measurements, and all three
flights were able to observe clear pollution events along the flight path. A
small change in wind direction provided sufficiently different air mass
origins over the 2 d such that a distinct pollution plume from London,
attributable marine emissions and a double-peaked dispersed area of
pollution resulting from a combination of local and transported emissions
were measured. We were able to analyse the effect of London emissions on air
quality in the wider region and the extent to which local sources contribute
to pollution events. The background air upwind of London was relatively clean during both days;
concentrations of CO were 88–95 ppbv, total (measured) volatile organic
compounds (VOCs) were 1.6–1.8 ppbv and NOx was 0.7–0.8 ppbv. Downwind
of London, we encountered elevations in all species with CO>100 ppbv, VOCs 2.8–3.8 ppbv, CH4>2080 ppbv and NOx>4 ppbv, and peak concentrations in individual pollution events were
higher still. Levels of O3 were inversely correlated with NOx
during the first flight, with O3 concentrations of 37 ppbv upwind
falling to ∼26 ppbv in the well-defined London plume. Total
pollutant fluxes from London were estimated through a vertical plane
downwind of the city. Our calculated CO2 fluxes are within the combined
uncertainty of those estimated previously, but there was a greater disparity
in our estimates of CH4 and CO. On the second day, winds were lighter and downwind O3 concentrations
were elevated to ∼39–43 ppbv (from ∼32 to 35 ppbv
upwind), reflecting the contribution of more aged pollution to the regional
background. Elevations in pollutant concentrations were dispersed over a
wider area than the first day, although we also encountered a number of
clear transient enhancements from local sources. This series of flights demonstrated that even in a region of megacity
outflow, such as the south-east of the UK, local fresh emissions and more
distant UK sources of pollution can all contribute substantially to
pollution events. In the highly complex atmosphere around a megacity where a
high background level of pollution mixes with a variety of local sources at
a range of spatial and temporal scales and atmospheric dynamics are further
complicated by the urban heat island, the use of pollutant ratios to track
and determine the ageing of air masses may not be valid. The individual
sources must therefore all be well-characterised and constrained to
understand air quality around megacities such as London. Research aircraft
offer that capability through targeted sampling of specific sources and
longitudinal studies monitoring trends in emission strength and profiles
over time.