Gaseous elemental mercury in the marine boundary layer: Evidence for rapid removal in anthropogenic pollution

In this study, gas-phase elemental mercury (Hg-0) and related species (including inorganic reactive gaseous mercury(RGM) and particulate mercury (PHg)) were measured at Cheeka Peak Observatory (CPO), Washington State, in the marine boundary layer during 2001 -2002. Air of continental origin containing anthropogenic pollutants from the urban areas to the east contained on average 5.3% lower Hg-0 levels as compared to the marine background. This result is difficult to reconcile since it is known that industrial emissions in our region are sources of Hg-0. The rate of removal of Hg-0 from a pollution plume necessary to account for our observations is inconsistent with the accepted view of Hg-0 as a stable atmospheric pollutant. The largest and most frequent Hg-0 loss events occurred in the presence of increased ozone (O-3) during the summer. Hg-0 and O-3 also display diurnal cycles that are out-of-phase with one another. In other seasons Hg-0 behavior is less consistent, as we observe weak positive correlations with 03 and occasional Hg-0 enhancements in local pollution. RGM and PHg concentrations are enhanced only slightly during Hg-0 loss events, comprising a small fraction of the mercury pool (similar to3%). Long-range transported pollution of Asian origin was also detected at CPO, and this contains both higher and lower levels of Hg-0 as compared to the background with maximum changes being <20%. Here, the more photochemically processed the air mass, as determined by propane/ethane ratios, the more likely we are to observe Hg-0 loss. Air from the marine background in summer displays a significant diurnal cycle with a phase that matches the diurnal cycles seen in polluted air masses. A Junge lifetime for Hg-0 in the clean marine boundary layer is calculated to be 7.1 months, which is on the low end of previous estimates (0.5-2 yr).