H2 with δ(D,H2)

H2 production

D-enriched H2:δ(D,H2) increase

CH4 oxidation

oxidation by OH

H2 destruction

Preferential re-moval of H over D:

δ(D,H2) increase

Correlations between δ(D,H2) and methane (CH4) in the UTLS region obtained from CARIBIC samples A.M. Batenburg1, T.J. Schuck2, A. Baker2, C.A.M. Brenninkmeijer2 and T. Röckmann1

1Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, the Netherlands 2Max Planck Institute for Chemistry, Atmospheric Chemistry Division, Mainz, Germany

Atmospheric H2 and the CARIBIC projectIf H2 comes into wide use as a fuel, its atmospheric levels may rise (due to leakage) above present-day levels (≈ 0.5 ppm). This may a�ect the atmosphere’s oxidative capacity and stratospher-ic ozone chemistry. Unfortunately, large uncertainties still exist in the global H2 budget. The di�erent sources and sinks of H2 have distinct isotope ef-fects. Therefore, measurements of isotopic composition (deuterium content, expressed as δ(D,H2)) are a promising tool to constrain the terms in the global budget.The CARIBIC project uses an automated instrument container on board of a commercial passenger aircraft to carry out in-situ measurements and collect air samples. The resulting samples are mostly from the Upper Troposphere-Lower Stratosphere (UTLS) region, an interesting, but little studied, part of the at-mosphere.

Fig.1: The aircraft (Lufthansa Airbus A340-600) and the at-tached inlet system used for CARIBIC.

Flights to CaracasIn total, 490 samples from 21 return �ights have been analyzed for H2 mixing ratios (m(H2)) and δ(D,H2) at the IMAU isotope labo-ratory. Fig. 2 shows results of 7 �ights with destination Caracas. Some samples seem a�ected by pollution (a high m(H2) and low δ(D,H2) value). This tends to occur in samples that were taken around take-o� and landing of the aircraft (e.g. �ight 286 and 266, values are o� the �gure scale).With meteorological and chemical data, sam-ples with stratospheric in�uence can be identi�ed. These samples often show a clear elevation in δ(D,H2) . Fig.2: m(H2) (a) and δ(D,H2) (b) data from �ights to Caracas, Venezuela. Stratosphere-

in�uenced samples are shown with open symbols.

Stratosphere data156 samples contain air from the lower stratosphere. Previ-ously it was found that in the stratosphere, the competing production and destruction processes for H2 balance out, but that these do cause a strong deuterium enrichment (Fig.3). δ(D,H2) shows a compact correlation with methane (CH4), which is destroyed as H2 is produced.In the CARIBIC data, δ(D,H2) is also strongly correlated with CH4, whereas m(H2) is not (Fig. 4 (a)). The correlation’s slope is very similar to the one found in a stratospheric balloon �ight (Röckmann et al., 2003) (Fig. 4(b)). That this e�ect is found in samples from di�erent geographic locations and altitudes shows that this correlation likely holds globally.Fig.3: A cartoon representation of

the the stratospheric H2 cycle

Fig.4 (a): m(H2) and δ(D,H2) plotted against methane mixing ratio (m(CH4)), with �ts through the data. (b): δ(D,H2) plotted against m(CH4) for the CARIBIC samples and literature values (Röckmann et al., 2003)

Monsoon �ights Schuck et al. (2010) found increased levels of CH4 in �ights to India during summer monsoon.No enhancement was found in m(H2), but the samples with increased methane showed a de-crease in δ(D,H2) that is cor-related to m(CH4) (Fig.5 and 6).This δ(D,H2)decrease in the `Monsoon plume’ is prob-ably caused by increased convection, bringing H2 that is depleted in D from surface sources to cruising altitude. That no simultaneous H2 increase is seen, may indi-cate a contribution from a very depleted source. In-creased microbial produc-tion in the wet season is much more strongly de-pleted than combustion sources (≈ -700‰, resp. ≈ -200‰).

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Fig.5: m(CH4) (a) and δ(D,H2)(b) measured on tropospheric samples from �ights to India south of 400 N in the summer monsoon (July-Sept,green shades) and after monsoon (Okt-Nov, reddish colors).

Fig.6: δ(D,H2) plotted against m(CH4) for the monsoon samples in Fig.5, with linear �t.

ConclusionsA large number of CARIBIC samples were analyzed for m(H2) and δ(D,H2).• For the stratosphere the number of available δ(D,H2) observations has in-creased �vefold, and they provide in-formation about the stratospheric H2 cycle.• First observations of δ(D,H2) in the summer monsoon were made. These contain an interesting correlation with the CH4 increase in this season, which may point to an as yet unstud-ied microbial H2 source. • These samples from the UTLS region are complementary to observations from ground stations and ship cruises and will help in constraining the un-certainties in the H2 budget and vali-dating models.

References- Röckmann et al., Heavy hydrogen in the strato-sphere, Atmos. Chem. Phys 3, 2015-2023, 2003- Schuck et al, Greenhouse gas relationships in the Indian summer monsoon plume measured by the CARIBIC passenger aircraft, Atmos. Chem. Phys. 11, 503-518, 2011

Questions: a.m.batenburg@uu.nl