Abstract

We present a year-long high precision time series of atmospheric molecular hydrogen (H-2) measured at the UK North Sea coast from March 2008 to February 2009. We observed a pronounced seasonal cycle in H-2 with mean values in late winter/early spring similar to 40 ppb higher than those in late summer/early autumn. Background-subtracted molar H-2/CO ratios (Delta H-2/DCO) averaged 0.35 perpendicular to 0.002 for all data combined and 0.25 perpendicular to 0.002 when Delta H-2 was above 10 ppb. The DH2/DCO ratio was highest in summer, possibly as a result of larger photochemical production. Using simultaneous measurements of ozone, we estimated the deposition velocity of H-2 during nocturnal inversion events to average 3.5 +/- 0.7 x 10(-4) m s(-1) for June 2008 and 1.9 +/- 1 x 10(-4) m s(-1) for July 2008, in good agreement with other reported estimates. In May 2008, we observed an episode of exceptionally clean air being transported from the tropics but arriving from the north, in which H-2 was slightly elevated indicating minimal surface loss. On another occasion with south-easterly winds, we believe we detected emissions from H-2 production facilities in the near-continent characterised by H-2 mixing ratios reaching 1450 ppb.