H2 vertical profiles in the continental boundary layer: measurements at the Cabauw tall tower in The Netherlands

In-situ, quasi-continuous measurements of atmospheric hydrogen (H2 ) have been performed since October 2007 at the Cabauw tall tower station in the Netherlands. Mole fractions of H2 , CO and several greenhouse gases are determined simultaneously in air sampled successively at four heights, between 20 and 200 m above ground level. 222 Rn measurements are performed in air sampled at 20 and 200 m. This H2 dataset represents the first in-situ, quasi-continuous long-term measurement series of vertical profiles of H2 in the lower continental boundary layer. Seasonal cycles are present at all heights in both H2 and CO, and their amplitude varies with the sampling height. The seasonality is evident in both the "baseline" values and in the short term (diurnal to synoptic time scales) variability, the latter being significantly larger during winter. The observed H2 short term signals and vertical gradients are in many cases well correlated to other species, especially to CO. On the other hand, H2 has at times a unique behaviour, due to its particular distribution of sources and sinks. Our estimation for the regional H2 soil uptake flux, using the radon tracer method, is (-1.89 ± 0.26) × 10-5 g/(m2 h), significantly smaller than other recent results from Europe. H2 /CO ratios of the traffic emissions computed from our data, with an average of 0.54 ± 0.07 mol:mol, are larger and more variable than estimated in some of the previous studies in Europe. This difference can be explained by a different driving regime, due to the frequent traffic jams in the influence area of Cabauw. The H2 /CO ratios of the large scale pollution events have an average of 0.36 ± 0.05 mol:mol; these ratios were observed to slightly increase with sampling height, possibly due to a stronger influence of soil uptake at the lower sampling heights.