High-frequency, continuous hydrogen observations at Mace Head, Ireland from 1994 to 2022: Baselines, pollution events and ‘missing’ sources

This study analysed just under three hundred thousand 40-min hydrogen observations taken at the Mace Head baseline station on the Atlantic Ocean coastline of Ireland between 1994 and 2022. Advection of European polluted air masses to Mace Head brought elevated hydrogen mixing ratios. Annual mean European excess mixing ratios were between 2 and 4 ppb above baseline in the 1990s but have steadily declined to about 0.2 ppb in 2021, with an e-folding time of 10 years. European excess carbon monoxide mixing ratios declined in an analogous manner, confirming the cause of the declines in both hydrogen and carbon monoxide as the fitting of exhaust gas catalysts to petrol-engined motor vehicles. The annual mean baseline hydrogen mixing ratios were constant over the 1994–2010 period, averaging about 513 ppb, before an upwards trend set in bringing the annual mean up to 532 ppb in 2021. Monthly baseline mixing ratios exhibited a seasonal cycle with an average spring maximum of 531 ppb and an autumn minimum of 497 ppb. Differences between the monthly means and the 1995–2010 averages were constant up to December 2015 when an anomalous growth trend of 2–4 ppb yr⁻¹ set in. A two-hemisphere two-box model was used to describe the hydrogen budget over the 1995–2022 period. This model highlighted that a ‘missing’ source was required to explain the anomalous growth in the Mace Head observations from 2010 onwards. Six potential candidates for this ‘missing’ source were characterised. However, there is a dearth of hydrogen observations with which to identify the real ‘missing’ source with any certainty. The anomalous growth in hydrogen since 2010 reported here may feasibly be the first evidence of the widespread seepage of natural hydrogen into the atmosphere.