Continuous high-frequency observations of hydrogen at the Mace Head baseline atmospheric monitoring station over the 1994-1998 period

Continuous high-frequency (every 40-min) automatic measurements of hydrogen have been made at the Mace Head atmospheric research station on the Atlant ic Ocean coast of Ireland throughout 1994-1998. These observations represen t one the most comprehensive in situ records of a trace gas that has receiv ed comparatively little attention. Individual measurements have been sorted by four independent methods to separate clean, maritime air masses from re gionally polluted European air masses. Hydrogen concentrations in midlatitu de Northern Hemisphere baseline air show a distinct seasonal cycle with hig hest concentrations during spring and lowest concentrations during late aut umn, with a peak-to-trough amplitude of 38 +/- 6 ppb, averaged over the obs erved seasonal cycles from 1994 to 1998. The mean hydrogen concentration in midlatitude Northern Hemisphere baseline air on January 1, 1995, was estim ated as 496.5 ppb with an upward trend of 1.2 +/- 0.8 ppb yr(-1). Evidence has also been obtained for European pollution sources with source strength of about 0.8 Tg yr(-1) and for deposition of hydrogen to soils. The observa tion of slightly elevated hydrogen concentrations relative to baseline leve ls in tropical maritime air masses points to a latitudinal gradient in hydr ogen with higher concentrations in lower latitudes of the Northern Hemisphe re and in the Southern Hemisphere. This is confirmed by comparable hydrogen observations at Cape Grim, Tasmania, which are consistently higher than me asurements recorded at Mace Head. Mean hemispheric concentrations of 504 an d 520 ppb have been estimated for the Northern and Southern Hemispheres, re spectively, for January 1, 1996, corresponding to a total atmospheric hydro gen burden of 182 Tg.