Atmospheric hydrogen variations and traffic emissions at an urban site in Finland

Atmospheric hydrogen (H2) 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 simulta- neously with high carbon monoxide (CO) values and co- incided with high traffic flow periods. Carbon monox- ide and radon ( 222 Rn) were continuously monitored at the same site and they were used in estimation of the hydro- gen emissions from traffic. The morning rush hour slope of 1H2/1CO was in average 0.43±0.03 ppb (H2)/ppb (CO). After correction due to soil deposition of H2 the slope was 0.49±0.07 ppb (H2)/ppb (CO). Using this slope and CO emission statistics, a road traffic emission of about 260 t (H2)/year was estimated for Helsinki in 2007.