Airborne determination of the temporo-spatial distribution of benzene, toluene, nitrogen oxides and ozone in the boundary layer across Greater London, UK

Abstract. Highly spatially resolved mixing ratios of benzene and toluene, nitrogen oxides (NO_x) 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), NO_x by dual-channel NO_x 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 NO_x 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 NO_x, benzene and toluene mixing ratios were observed in the morning (~ 40 ppbv NO_x, ~ 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.