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 (O 3) 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 O 3 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, NO 2 and NO x respectively. Linear regression analysis between NO 2, 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 O 3), all at 360 ± 10 m a.g.l.