With respect to the atmospheric budget of molecular hydrogen (H₂), Kraków represents a typical central European urban agglomeration with intense traffic and a relatively high proportion of low-level emissions associated with burning of fossil fuel (mainly coal and gas) for heating. The vehicle fleet in the city still contains a relatively high fraction of cars without properly operating catalysts, which constitutes a considerable source of atmospheric hydrogen. The mixing ratios of hydrogen in near-ground atmosphere were measured quasi-continuously over two years (from 01.2007 till 12.2008) at two different locations within the urban area of Kraków: close to the city center and at the outskirts of the city. Although both measurement locations were under the influence of local traffic, they differ with respect to the structure of local terrain (proportions of buildings, roads, and area covered by vegetation), as well as by local micrometeorological conditions. A very wide range of H₂ mixing ratios was observed at both sites, with peak mixing ratios reaching 2,800 ppb. Distinct seasonality of H₂ mixing ratios was observed, with higher values recorded during winter months. Also, distinct daily variations of H₂ levels often were observed, with morning and evening H₂ maxima associated with traffic rush hours. Diurnal variation of hydrogen concentrations at both locations differs seasonally due to different micrometeorological conditions and source patterns, including car traffic intensity.
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