Multi‐decadal decline of mercury in the North Atlantic atmosphere explained by changing subsurface seawater concentrations

Abstract

) over the NA (steeper than at Northern Hemispheric land sites) but no significant decline over the SA. Surface water Hg 0 measurements in the NA show a decline of À5.7% a À1 since 1999, and limited subsurface ocean data show an $80% decline from 1980 to present. We use a coupled global atmosphere-ocean model to show that the decline in NA atmospheric concentrations can be explained by decreasing oceanic evasion from the NA driven by declining subsurface water Hg concentrations. We speculate that this large historical decline of Hg in the NA Ocean could have been caused by decreasing Hg inputs from rivers and wastewater and by changes in the oxidant chemistry of the atmospheric marine boundary layer.

Observed decrease in atmospheric mercury explained by global decline in anthropogenic emissions

“…). Model increases are limited to East Asia, consistent with preliminary observations and previous modeling studies (6,11,27).…”

“…The concentrations of major oxidants for atmospheric Hg 0 , including OH, O 3 , and Br, have remained relatively steady or slightly decreased since the mid-1990s and are thus not an important driver for the observed decline (5). Decreasing riverine discharges, which was previously speculated to drive the decline in North Atlantic Ocean Hg concentration and subsequent reemission flux (6), are also insufficient for forcing the global atmospheric trend (31,33).…”

“…By contrast, global inventories suggest flat or increasing Hg emissions over the last two decades (1,10). Decreasing reemission of Hg from oceans and soils has been speculated (5,6). Here we show that the declining atmospheric concentrations can be explained by the phase-out of Hg from commercial products and by shifts in the speciation of Hg emissions driven by air pollution control technologies.…”

Comparison of mercury concentrations measured at several sites in the Southern Hemisphere

“…This number can be viewed as a preliminary threshold for judging how representative the trends observed at any background site in the Southern Hemisphere are. With this threshold, much smaller trends at shorter time periods can be detected by long-term measurements at several sites when compared to shipboard measurements as reviewed by Soerensen et al (2012). Figure 4 shows an overall tendency of annual average mercury concentrations for Cape Point to increase with time.…”

“…It also makes statistical tests for the differences of averages impossible. It is not surprising that, using such data, Soerensen et al (2012) concluded that no significant trend in the Southern Hemisphere could be detected so far. While we agree with this conclusion, a qualification is required: the quality of the data used by Soerensen et al (2012) does not allow detection of trends smaller than their variability, i.e.…”