[1] This work reassesses the global atmospheric budget of H 2 with the TM5 model. The recent adjustment of the calibration scale for H 2 translates into a change in the tropospheric burden. Furthermore, the ECMWF Reanalysis-Interim (ERA-Interim) data from the European Centre for Medium-Range Weather Forecasts (ECMWF) used in this study show slower vertical transport than the operational data used before. Consequently, more H 2 is removed by deposition. The deposition parametrization is updated because significant deposition fluxes for snow, water, and vegetation surfaces were calculated in our previous study. Timescales of 1-2 h are asserted for the transport of H 2 through the canopies of densely vegetated regions. The global scale variability of H 2 and ıD[H 2 ] is well represented by the updated model. H 2 is slightly overestimated in the Southern Hemisphere because too little H 2 is removed by dry deposition to rainforests and savannahs. The variability in H 2 over Europe is further investigated using a high-resolution model subdomain. It is shown that discrepancies between the model and the observations are mainly caused by the finite model resolution. The tropospheric burden is estimated at 165˙8 Tg H 2 . The removal rates of H 2 by deposition and photochemical oxidation are estimated at 53˙4 and 23˙2 Tg H 2 /yr, resulting in a tropospheric lifetime of 2.2˙0.2 year.Citation: Pieterse, G., et al. (2013), Reassessing the variability in atmospheric H 2 using the two-way nested TM5 model,

South Asian air is among the most polluted in the world, causing premature death of millions and asserting a strong perturbation of the regional climate. A central component is carbon monoxide (CO), which is a key modulator of the oxidizing capacity of the atmosphere and a potent indirect greenhouse gas. While CO concentrations are declining elsewhere, South Asia exhibits an increasing trend for unresolved reasons. In this paper, we use dual-isotope (δ 13 C and δ 18 O) fingerprinting of CO intercepted in the South Asian outflow to constrain the relative contributions from primary and secondary CO sources. Results show that combustion-derived primary sources dominate the wintertime continental CO fingerprint ( f primary ∼ 79 ± 4%), significantly higher than the global estimate ( f primary ∼ 55 ± 5%). Satellite-based inventory estimates match isotope-constrained f primary -CO, suggesting observational convergence in source characterization and a prospect for model–observation reconciliation. This “ground-truthing” emphasizes the pressing need to mitigate incomplete combustion activities for climate/air quality benefits in South Asia.

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.

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.

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.

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 (−1.89 ± 0.26) × 10⁻⁵ 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.

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 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 ± 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

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