Reconstructing atmospheric H₂ over the past century from bi-polar firn air records

Abstract. Historical hemispheric atmospheric H₂ levels since 1930 were reconstructed using the UCI_2 firn air model and firn air measurements from three sites in Greenland: (NEEM, Summit, and Tunu) and two sites in Antarctica (South Pole and Megadunes). A joint reconstruction based on the two Antarctic sites yields H₂ levels monotonically increasing from about 350 ppb in 1900 to 550 ppb in the late 1990’s, levelling off thereafter. These results are similar to individual reconstructions published previously (Patterson et al., 2020; 2021). Reconstruction of the Greenland data is complicated by a systematic bias between Tunu and the other sites. The Tunu reconstruction shows substantially lower historical H₂ levels than the other two sites, a difference we attribute to possible bias in the calibration of the Tunu measurements. All three reconstructions show a late 20th century maximum in H₂ levels over Greenland. A joint reconstruction of the Greenland data shows H₂ levels rising 40 % from 1930–1990, reaching a maximum of 550 ppb. After 1990, reconstructed atmospheric H₂ decrease by 6 % over the next 20 years. The reconstruction deviates by at most 4 % from the few available surface air measurements of atmospheric H₂ levels over Greenland from 1998–2004. However, the longer instrumental records from sampling sites outside of Greenland show a more rapid decrease and stabilization after 1990 compared to the reconstruction. We explore the possibility that this difference is an artefact caused by the firn air model underestimating pore close-off induced enrichment, evidenced by a mismatch between measured and modelled Ne in firn air. We developed new parameterizations which more accurately capture pore close-off induced enrichment at the Greenland sites. Incorporating those parameterizations into the UCI_2 model yields reconstructions with lower H₂ levels throughout the mid-late 20^th century and more stable H₂ levels during the 1990’s, in better agreement with the flask measurements.