H<sub>2</sub> vertical profiles in the continental boundary layer: measurements at the Cabauw tall tower in the Netherlands

Abstract

In-situ, quasi-continuous measurements of atmospheric hydrogen (H₂) have been performed since 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. ²²²Rn measurements are performed in air sampled at 20 and 200 m. <br><br> This H₂ dataset represents the first in-situ, quasi-continuous measurement series of vertical profiles of H₂ in the lower continental boundary layer. From the three-year long time series, we characterize the main features and variability patterns of H₂ and CO on various time scales; the time series is too short to justify an attempt to determine multi-annual trends. Seasonal cycles are present 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. <br><br> 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 behaviour which differentiates it from all the other species measured, due to its particular distribution of sources and sinks, that is, with the main source in our area (anthropogenic emissions) and the main sink (soil uptake) both near ground level. <br><br> The local to regional soil sink of H₂ is observable as H₂ depletion at the lower sampling levels in some of the stable nights, although the signals at Cabauw are smaller than observed at other stations. Positive vertical gradients are another consequence of the soil uptake. Our estimation for the regional H₂ soil uptake flux, using the radon tracer method, is (&minus;1.89 &plusmn; 0.26) &times; 10^&minus;5 g/(m²h), significantly smaller than other recent results from Europe. Local soil and weather characteristics might be responsible for the very low soil uptake of H₂. Our result could also be biased by the absence of radon flux estimates that could reliably approximate the fluxes during the relevant time intervals in our study domain. <br><br> H₂/CO ratios of the traffic emissions computed from our data, with an average of 0.54 &plusmn; 0.07 mol:mol, are larger and more scattered 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. In contrast, the H₂/CO ratios of the large scale pollution events, with an average of 0.36 &plusmn; 0.05 mol:mol, are very similar to results of previous studies; these ratios were observed to slightly increase with sampling height, possibly due to a stronger influence of soil uptake at the lower sampling heights

Reassessing the variability in atmospheric H2 using the two‐way nested TM5 model

“…The measurements performed at the tall tower station near Cabauw in The Netherlands are strongly influenced by urban activity [ Popa and Vermeulen , ]. Contrary to the station near London, the station at Cabauw is located in a grid cell with much less urban influence than representative for this site.…”

“…Model values for the H 2 mixing ratios are compared with available data from a subset of stations from the EuroHydros project [ Engel and EUROHYDROS PIs , ] within the high‐resolution zoom region over Europe, namely Mace Head (Ireland) [ Grant et al ., ], London (United Kingdom) [ Fowler et al ., ], Weybourne (United Kingdom), Cabauw (The Netherlands) [ Popa et al ., ], Gif‐sur‐Yvette (France) [, ], Taunus (Germany), Heidelberg (Germany) [ Hammer and Levin , ], Jungfraujoch (Switzerland) [ Bond et al ., ], and Bialystok (Poland); see Figure . The global scale performance for the H 2 mixing ratios is evaluated using flask sampling data from the CSIRO network measured at Alert (Canada), Cape Ferguson (Australia), Cape Grim (Australia), Casey Station (Antarctica), Macquarie Island (Australia), Mauna Loa (United States), Mawson (Antarctica), and the South Pole.…”

Net CO2 surface emissions at Bern, Switzerland inferred from ambient observations of CO2, δ(O2/N2), and 222Rn using a customized radon tracer inversion

“…If the 222 Rn surface flux is known, its ratio to a measured 222 Rn concentration difference over time at a certain observation height can be applied to calculate the surface flux of another constituent (e.g., CO 2 ) from its concurrently observed concentration difference at the same height. The method has been successfully applied for a wide range of gases such as CO 2 [ Levin , ; Schmidt et al ., ], CH 4 [ Schmidt et al ., ; van der Laan et al ., ], N 2 O [ van der Laan et al ., ; Wilson et al ., ], CFCs [ Biraud et al ., ], H 2 [ Popa et al ., ; Yver et al ., ], and fossil fuel‐based CO 2 [ van der Laan et al ., ]. Generally, a constant 222 Rn soil emission rate is assumed and multiplied by the ratio of observed CO 2 and 222 Rn concentrations.…”

Inferring &lt;sup&gt;222&lt;/sup&gt;Radon soil fluxes from ambient &lt;sup&gt;222&lt;/sup&gt;Radon activity and eddy covariance measurements of CO&lt;sub&gt;2&lt;/sub&gt;

“…In principle it also detects 220 Rn (half-life of 55.6 s); however this is prevented by the relatively long residence time (∼ 10 half-lives) of the air sample from the tower inlet to the detector. The total measurement uncertainty is about 11 % of the measured value at both sites (at an activity concentration of 1 Bq m −3 ) including measurement precision resulting from counting statistics (∼ 3-4 %), accuracy of the source (∼ 4 %), the coefficient of variability of valid monthly calibration coefficients (∼ 2 %) and the background count vari-ability (∼ 10 mBq m −3 ) (van der Laan et al, 2010;Popa et al, 2011;Schmithüsen et al, 2016). Ambient observations of CO 2 mole fractions, 222 Rn activity concentration and CO 2 surface fluxes for the period of November 2007-April 2010 at LUT are shown in Fig.…”

“…The water table is generally ∼ 1 m below the surface. Ambient CO 2 mole fractions are measured with a LiCor-7000 non-dispersive infrared analyser sampled from heights of 20 m (used in this study), 60, 120 and 200 m (Popa et al, 2011;Vermeulen et al, 2011). The measurement precision is generally < ± 0.1 ppm.…”