Global modelling of H<sub>2</sub> mixing ratios and isotopic compositions with the TM5 model

Abstract

The isotopic composition of molecular hydrogen (H₂) contains independent information for constraining the global H₂ budget. To explore this, we have implemented hydrogen sources and sinks, including their stable isotopic composition and isotope fractionation constants, into the global chemistry transport model TM5. For the first time, a global model now includes a simplified but explicit isotope reaction scheme for the photochemical production of H₂. We present a comparison of modelled results for the H₂ mixing ratio and isotope composition with available measurements on seasonal to inter annual time scales for the years 2001–2007. The base model results agree well with observations for H₂ mixing ratios. For δD[H₂], modelled values are slightly lower than measurements. A detailed sensitivity study is performed to identify the most important parameters for modelling the isotopic composition of H₂. The results show that on the global scale, the discrepancy between model and measurements can be closed by adjusting the default values of the isotope effects in deposition, photochemistry and the stratosphere-troposphere exchange within the known range of uncertainty. However, the available isotope data do not provide sufficient information to uniquely constrain the global isotope budget. Therefore, additional studies focussing on the isotopic composition near the tropopause and on the isotope effects in the photochemistry and deposition are recommended

The stable isotopic composition of molecular hydrogen in the tropopause region probed by the CARIBIC aircraft

“…Together, these processes result in typical atmospheric H 2 mixing ratios (χ (H 2 )) of around 547 ppb (nmole/mole) at ground level (Novelli et al, 1999, converted to the latest χ(H 2 ) scale established by Jordan and Steinberg (2011)), with on average 3% higher values in the Southern Hemisphere. Model results indicate that χ (H 2 ) may increase slightly with height, especially in the Northern Hemisphere extratropics (Hauglustaine and Ehhalt, 2002;Price et al, 2007;Pieterse et al, 2011). Despite the qualitative agreement, the uncertainties in the estimates of the magnitudes of the different terms in the budget are large.…”

“…This may affect the atmosphere's oxidative capacity and stratospheric ozone levels (Schultz et al, 2003;Warwick et al, 2004;Tromp et al, 2003;Feck et al, 2008). A number of global H 2 budget estimates have been made (Novelli et al, 1999;Hauglustaine and Ehhalt, 2002;Sanderson et al, Bousquet et al, 2011;Pieterse et al, 2011Pieterse et al, , 2012. These estimates agree that the largest source of H 2 to the atmosphere is oxidation of hydrocarbons, followed by combustion of fossil fuels and biomass (see Table 1).…”

“…This is problematic since Stratosphere-Troposphere Exchange (STE) in the UTLS may Table 1. The global H 2 budget and isotope budget as estimated by Pieterse et al (2011) in a model study with global model TM5. The estimated magnitudes, isotopic signatures and uncertainties (superand subscript numbers) of the different surface sources were based on previous studies.…”

“…McCarthy et al (2004) presented parametrizations of δD and the mixing ratio of deuterated hydrogen (χ (HD)) relative to χ (CH 4 ), based on the data from Rahn et al (2003). Pieterse et al (2011Pieterse et al ( , 2012 used this to obtain the first results with the TM5 model, but had to apply a large correction to keep the model results close to observed tropospheric δD values. The CARIBIC data presented here allow us to calculate 4636 A. M. Batenburg et al: δD(H 2 ) in the tropopause region a new correlation between δD and χ(CH 4 ).…”

Isotopic signatures of production and uptake of H<sub>2</sub> by soil

“…In the troposphere, H 2 has a mole fraction of about 550 parts per billion (ppb = nmol mol −1 ) and a lifetime of around 2 years (Novelli et al, 1999;Price et al, 2007;Xiao et al, 2007;Pieterse et al, 2011;. H 2 can affect atmospheric chemistry and composition in several ways.…”