Abstract
Measurements of the stable isotopic composition (δD(H2) or δD) of atmospheric molecular hydrogen (H2) are a useful addition to mixing ratio (χ(H2)) measurements for understanding the atmospheric H2 cycle. δD datasets published so far consist mostly of observations at background locations. We complement these with observations from the Cabauw tall tower at the CESAR site, situated in a densely populated region of the Netherlands. Our measurements show a large anthropogenic influence on the local H2 cycle, with frequently occurring pollution events that are characterized by χ(H2) values that reach up to ≈1 ppm and low δD values. An isotopic source signature analysis yields an apparent source signature below −400‰, which is much more D-depleted than the fossil fuel combustion source signature commonly used in H2 budget studies. Two diurnal cycles that were sampled at a suburban site near London also show a more D-depleted source signature (≈−340‰), though not as extremely depleted as at Cabauw. The source signature of the Northwest European vehicle fleet may have shifted to somewhat lower values due to changes in vehicle technology and driving conditions. Even so, the surprisingly depleted apparent source signature at Cabauw requires additional explanation; microbial H2 production seems the most likely cause. The Cabauw tower site also allowed us to sample vertical profiles. We found no decrease in χ(H2) at lower sampling levels (20 and 60 m) with respect to higher sampling levels (120 and 200 m). There was a significant shift to lower median δD values at the lower levels. This confirms the limited role of soil uptake around Cabauw, and again points to microbial H2 production during an extended growing season, as well as to possible differences in average fossil fuel combustion source signature between the different footprint areas of the sampling levels. So, although knowledge of the background cycle of H2 has improved over the last decade, surprising features come to light when a non-background location is studied, revealing remaining gaps in our understanding.
Highlights
With typical background mixing ratios above 500 ppb, atmospheric molecular hydrogen (H2) is the second most abundant reduced trace gas in the atmosphere after methane (CH4)
Flask data that were collected from the EUROHYDROS station Mace Head in a similar fashion are shown for comparison, as well as an extrapolated harmonic function that was fit to the Mace Head data (Batenburg et al, 2011)
The set of c(H2) and dD data presented here shows that the H2 cycle at Cabauw is under heavy anthropogenic influence
Summary
With typical background mixing ratios above 500 ppb (nmole/ mole), atmospheric molecular hydrogen (H2) is the second most abundant reduced trace gas in the atmosphere after methane (CH4). The resulting rise in H2 levels may have consequences for stratospheric ozone chemistry and an indirect climate effect through a decrease in the oxidative capacity of the atmosphere (Schultz et al, 2003; Warwick et al, 2004; Tromp et al, 2003; Feck et al, 2008) Because of this impending disturbance of the H2 cycle, the global H2 budget has received growing attention in recent years, with several new budget estimates and simulations of future scenarios published recently by Bousquet et al (2011); Yashiro et al (2011); Yver et al (2011a); Pieterse et al (2011, 2013); Wang et al (2013) and Popa et al (2015), but considerable uncertainties remain
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