High-frequency urban measurements of hydrogen and carbon monoxide in the UK

High-frequency measurements of atmospheric hydrogen (H2) and carbon monoxide (CO) were made at an urban site in the UK from mid-December 2008 until early March 2009. Very few measurements of these trace gases exist in the urban environment, particularly within the United Kingdom, but are an essential component in the assessment of anthropogenic emissions of H2 and 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 H2/CO emission ratios of 0.50 and 0.53 were calculated, the scatter plot of which displayed an unusual two population pattern, the source of which could not be elucidated. H2 emissions from transport in the UK were estimated at 175 Gg/yr, with 7.8 Tg/yr of H2 produced from vehicle emissions globally. H2 and CO deposition velocities were calculated over stable periods when a clear decay of both species was observed. CO was found to have a much higher deposition velocity than H2, 1.3×10−3 and 2.2×10−4 m s−1, respectively, going against the law of molecular diffusivity. The source of this unusual result was investigated, however no conclusive evidence was found for increased loss of CO over H2 during stable night time inversion events.