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

Abstract

Observations of elemental mercury (Hg(0)) at sites in North America and Europe show large decreases (∼ 1-2% y(-1)) from 1990 to present. Observations in background northern hemisphere air, including Mauna Loa Observatory (Hawaii) and CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) aircraft flights, show weaker decreases (<1% y(-1)). These decreases are inconsistent with current global emission inventories indicating flat or increasing emissions over that period. However, the inventories have three major flaws: (i) they do not account for the decline in atmospheric release of Hg from commercial products; (ii) they are biased in their estimate of artisanal and small-scale gold mining emissions; and (iii) they do not properly account for the change in Hg(0)/Hg(II) speciation of emissions from coal-fired utilities after implementation of emission controls targeted at SO2 and NOx. We construct an improved global emission inventory for the period 1990 to 2010 accounting for the above factors and find a 20% decrease in total Hg emissions and a 30% decrease in anthropogenic Hg(0) emissions, with much larger decreases in North America and Europe offsetting the effect of increasing emissions in Asia. Implementation of our inventory in a global 3D atmospheric Hg simulation [GEOS-Chem (Goddard Earth Observing System-Chemistry)] coupled to land and ocean reservoirs reproduces the observed large-scale trends in atmospheric Hg(0) concentrations and in Hg(II) wet deposition. The large trends observed in North America and Europe reflect the phase-out of Hg from commercial products as well as the cobenefit from SO2 and NOx emission controls on coal-fired utilities.

Keywords: atmosphere; emission; mercury; trend.

Fig. 1.

Major factors driving declines in Hg emission from US coal-fired utilities between 2005 and 2015. Trends were inferred from data on the implementation of different types of emission control technologies.

Fig. 2.

Trends in atmospheric Hg⁰ concentrations (A) and Hg^II wet deposition fluxes (B and C) from 1990 to present. Observations from the sites in Table 1 are shown as circles (if trends are statistically significant, P < 0.05) and diamonds (not significant). The background shows the trends computed in the GEOS-Chem model driven by our revised 1990 and 2010 anthropogenic emissions inventories from Table 2.

Fig. 3.

Regional trends for 1990 to 2013 in atmospheric Hg⁰ concentrations (A–D) and Hg^II wet deposition (E and F). Observations for individual years are shown as squares with linear regression as solid line. The dashed line is the trend from the GEOS-Chem simulation using our revised anthropogenic emissions inventory for 1990 and 2010 (Table 2). The data are averaged regionally across the sites in Table 1 for the free troposphere (A), North America (B and E), Western Europe (C and F), and high northern latitude regions (D) (vertical bars show the SDs). Regression coefficients (slope ± SE) and number of sites (n) are given (Insets). The SE of modeled trend is calculated based on the uncertainty range of the emission inventory (Table 2). North American atmospheric Hg⁰ concentrations are from the CAMNet (https://www.ec.gc.ca/natchem) and AMNet (nadp.sws.uiuc.edu/amn) networks and the Experimental Lakes Area, Canada. North American Hg^II wet deposition is from the MDN (nadp.sws.uiuc.edu/mdn). Observations in Western Europe are from the EMEP network (www.emep.int). High-latitude sites include Alert, Canada and Zeppelin, Norway, and three sites above 60° N from the EMEP network.