High-frequency urban measurements of molecular hydrogen and carbon monoxide in the United Kingdom

High-frequency measurements of atmospheric molecular hydrogen (H2) and carbon monoxide (CO) were made at an urban site in the United Kingdom (UK) from mid-December, 2008 until early March, 2009. Very few measurements of H2 exist in the urban environment, particularly within the UK, but are an essential component in the assessment of anthropogenic emissions of H2 and to a certain extent CO. These data provide detailed information on urban time-series, diurnal cycles as well as sources and sinks of both H2 and CO at urban locations. High-frequency data were found to be strongly influenced by local meteorological conditions of wind speed and temperature. Diurnal cycles were found to follow transport frequency very closely due to the sites proximity to major carriageways, consequently a strong correlation was found between H2 and CO mole fractions. Background subtracted mean and rush hour molar H2/CO emission ratios of 0.53±0.08 and 0.57±0.06 respectively, were calculated from linear fitting of data. The scatter plot of all H2 and CO data displayed an unusual two population pattern, thought to be associated with a large industrial area 85 km to the west of the site. However, the definitive source of this two branch pattern could not be fully elucidated. H2 emissions from transport in the UK were estimated to be 188±39 Gg H2/yr, with 8.1±2.3 Tg/yr of H2 produced from vehicle emissions globally. H2 and CO deposition velocities were calculated during stable night-time inversion events when a clear decay of both species was observed. CO was found to have a much higher deposition velocity than H2, 1.3±0.8×10−3 and 2.2±1.5×10−4 m s−1 (1σ) respectively, going against the law of molecular diffusivity. The source of this unusual result was investigated, however no conclusive explanation was found for increased loss of CO over H2 during stable night time inversion events. Correspondence to: A. Grant ([email protected])