Abstract
Abstract. Highly spatially resolved mixing ratios of benzene and toluene, nitrogen oxides (NOx) and ozone (O3) were measured in the atmospheric boundary layer above Greater London during the period 24 June to 9 July 2013 using a Dornier 228 aircraft. Toluene and benzene were determined in situ using a proton transfer reaction mass spectrometer (PTR-MS), NOx by dual-channel NOx chemiluminescence and O3 mixing ratios by UV absorption. Average mixing ratios observed over inner London at 360 ± 10 m a.g.l. were 0.20 ± 0.05, 0.28 ± 0.07, 13.2 ± 8.6, 21.0 ± 7.3 and 34.3 ± 15.2 ppbv for benzene, toluene, NO, NO2 and NOx respectively. Linear regression analysis between NO2, benzene and toluene mixing ratios yields a strong covariance, indicating that these compounds predominantly share the same or co-located sources within the city. Average mixing ratios measured at 360 ± 10 m a.g.l. over outer London were always lower than over inner London. Where traffic densities were highest, the toluene / benzene (T / B) concentration ratios were highest (average of 1.8 ± 0.5 ppbv ppbv-1), indicative of strong local sources. Daytime maxima in NOx, benzene and toluene mixing ratios were observed in the morning (~ 40 ppbv NOx, ~ 350 pptv toluene and ~ 200 pptv benzene) and in the mid-afternoon for ozone (~ 40 ppbv O3), all at 360 ± 10 m a.g.l.
Highlights
Ground-level ozone (O3) is a secondary pollutant, produced from photochemical reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx = NO + NO2)
We investigate the mixing ratios of O3, benzene, toluene, NO, NO2 and NOx across the Greater London region during several flights using the Natural Environment Research Council (NERC) Atmospheric Research and Survey Facility Dornier 228 aircraft between 24 June and 9 July 2013
To compare the volume mixing ratios obtained with the onboard proton transfer reaction mass spectrometer (PTR-MS) with those measured by gas chromatography with flame ionisation detection (GC-FID), whole-air canister sampling (WAS) was conducted twice per flight using silica coated stainless-steel cans (Thames Restek, UK) with subsequent GC-FID analysis for benzene and toluene (Hopkins et al, 2003, 2009)
Summary
In urban areas the dominant anthropogenic sources of VOCs are vehicular exhaust, fuel evaporation and emissions from the commercial and industrial use of solvents (Karl et al, 2009; Langford et al, 2010). Within Greater London, these sites form part of the London Air Quality Network (LAQN). Measurements from these networks suffer from the limitations of being made at relatively few sites, and so may not be representative of mixing ratios on larger spatial scales. The advantages of in situ aircraft measurements are that they provide information on the horizontal and vertical distributions of air pollutants over a large spatial area, allowing continuous gradients of mixing ratios to be observed across cities and their surrounding rural areas
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