Source of D-enriched hydrogen molecule in urban areas: photochemical decomposition of anthropogenic NMHCs?

We determined temporal variations in both the mixing ratio and δD value of atmospheric H2 in Sapporo, Japan, for every 1 hour in summer using a new CF-IRMS system that separates H2 from air with automatic multistep gas chromatograph followed by introduction to a mass spectrometer in a continuous flow of helium. The analytical precisions to determine mixing ratio and δD were ± 5 % and ± 3.6 permil, respectively. By using the correlation between δD values and reciprocal of mixing ratios, we clarified at least two major sources of H2 within the urban area. While more than 70 % of data can be explained by a simple mixing between H2 that had been derived from fossil fuel combustion (δD = -425 ± 80 permil VSMOW) and that in back ground air (δD = +120 ± 20 permil VSMOW), we have to assume additional source that has an isotopic signature around +195 ± 130 permil VSMOW to explain the other ca. 25 % data, most of which have lower H2 mixing ratios around 520 ppbv. Previous studies on δD values of urban H2 had been explained by a simple mixing between H2 in background air and that from fossil fuel combustion. Besides to fossil fuel combustion, however, photochemical decomposition of anthropogenic NMHCs could be the local source of urban H2 as well. While δD values of H2 produced through photochemical decomposition of NMHCs are not defined as yet, the D-enriched value around +190 permil VSMOW can be anticipated from past observations. We conclude that photochemical decomposition of NMHCs could be additional source of H2 in urban area at least in Sapporo. The local source strength of H2 produced through the photochemical decomposition of NMHCs can be estimated to be approximately 10 % of that from the fossil fuel combustion.