Decreasing trends in total gaseous mercury observations in baseline air at Mace Head, Ireland from 1996 to 2009

In this study, the concentrations of total gaseous mercury in baseline air masses arriving at Mace Head, Ireland after having traversed the thousands of kilometres uninterrupted fetch of the North Atlantic Ocean, have been used for the assessment of possible trends in the atmospheric mercury background concentration over a 14-year period (i.e., 1996–2009), a statistically significant negative (downwards) trend of −0.028 ± 0.01 ng m−3 yr−1, representing a trend of 1.6–2.0% per year, has been detected in the total gaseous mercury levels in these baseline air masses. These findings are set in the context of the available literature studies of atmospheric Hg trends.

► Our data set is the longest existing time series for Hg in temperate background air.
► More than 40 000 concentration measurements could be attributed to NH background air.
► We find a downward trend of 1.6–2.0%/yr over the 14 years measurement period.
► This decline is large in comparison to that seen in other trace gases.
► This decline contradicts current global emission inventories for mercury.