ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-7387-2009 Atmospheric hydrogen variations and traffic emissions at an urban site in Finland Aalto T. 1 Lallo M. 1 Hatakka J. 1 Laurila T. 1 Finnish Meteorological Institute, Climate Change Research, P.O. Box 503, 00101 Helsinki, Finland 05 10 2009 9 19 7387 7396 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/9/7387/2009/acp-9-7387-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/7387/2009/acp-9-7387-2009.pdf

Atmospheric hydrogen (H<sub>2</sub>) mixing ratios were observed over a one year period from summer 2007 to 2008 in Helsinki, Finland. Relatively stable background values of hydrogen were occasionally observed at the site, with minimum in October and maximum between March and May. High hydrogen mixing ratios occurred simultaneously with high carbon monoxide (CO) values and coincided with high traffic flow periods. Carbon monoxide and radon (<sup>222</sup>Rn) were continuously monitored at the same site and they were used in estimation of the hydrogen emissions from traffic. The morning rush hour slope of ΔH<sub>2</sub>/ΔCO was in average 0.43&plusmn;0.03 ppb (H<sub>2</sub>)/ppb (CO). After correction due to soil deposition of H<sub>2</sub> the slope was 0.49&plusmn;0.07 ppb (H<sub>2</sub>)/ppb (CO). Using this slope and CO emission statistics, a road traffic emission of about 260 t (H<sub>2</sub>)/year was estimated for Helsinki in 2007.

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