Sarah Strode

Global 3-D land-ocean-atmosphere model for mercury: Present-day versus preindustrial cycles and anthropogenic enrichment factors for deposition

We develop a mechanistic representation of land-atmosphere cycling in a global 3-D ocean-atmosphere model of mercury (GEOS-Chem). The resulting land-ocean-atmosphere model is used to construct preindustrial and present biogeochemical cycles of mercury, to examine the legacy of past anthropogenic emissions, to map anthropogenic enrichment factors for deposition, and to attribute mercury deposition in the United States. Land emission in the model includes prompt recycling of recently deposited mercury (600 Mg a -1 for present day), soil volatilization (550 Mg a -1 ), and evapotranspiration (550 Mg a -1 ). The spatial distribution of soil concentrations is derived from local steady state between land emission and deposition in the preindustrial simulation, augmented for the present day by a 15% increase in the soil reservoir distributed following the pattern of anthropogenic deposition. Mercury deposition and hence emission are predicted to be highest in the subtropics. Our atmospheric lifetime of mercury against deposition (0.50 year) is shorter than past estimates because of our accounting of Hg(0) dry deposition, but recycling from surface reservoirs results in an effective lifetime of 1.6 years against transfer to long-lived reservoirs in the soil and deep ocean. Present-day anthropogenic enrichment of mercury deposition exceeds a factor of 5 in continental source regions. We estimate that 68% of the deposition over the United States is anthropogenic, including 20% from North American emissions (20% primary and <1% recycled through surface reservoirs), 31% from emissions outside North America (22% primary and 9% recycled), and 16% from the legacy of anthropogenic mercury accumulated in soils and the deep ocean.

An Improved Global Model for Air-Sea Exchange of Mercury: High Concentrations over the North Atlantic

We develop an improved treatment of the surface ocean in the GEOS-Chem global 3-D biogeochemical model for mercury (Hg). We replace the globally uniform subsurface ocean Hg concentrations used in the original model with basin-specific values based on measurements. Updated chemical mechanisms for Hg⁰/Hg(II) redox reactions in the surface ocean include both photochemical and biological processes, and we improved the parametrization of particle-associated Hg scavenging. Modeled aqueous Hg concentrations are consistent with limited surface water observations. Results more accurately reproduce high-observed MBL concentrations over the North Atlantic (NA) and the associated seasonal trends. High seasonal evasion in the NA is driven by inputs from Hg enriched subsurface waters through entrainment and Ekman pumping. Globally, subsurface waters account for 40% of Hg inputs to the ocean mixed layer, and 60% is from atmospheric deposition. Although globally the ocean is a net sink for 3.8 Mmol Hg y⁻¹, the NA is a net source to the atmosphere, potentially due to enrichment of subsurface waters with legacy Hg from historical anthropogenic sources.

Observations of Reactive Gaseous Mercury in the Free Troposphere at the Mount Bachelor Observatory

August 2005. The mean mercury concentrations (at standard conditions) were 1.54 ng/m 3 (GEM), 5.2 pg/m 3 (PHg), and 43 pg/m 3 (RGM). RGM enhancements, up to 600 pg/m 3 , occurred at night and were linked to a diurnal pattern of upslope and downslope flows that mixed in boundary layer air during the day and free tropospheric air at night. During the night, RGM was inversely correlated (P < 0.0001) with CO (r = � 0.36), GEM (r = � 0.73), and H2 O( r =� 0.44), was positively correlated with ozone (r = 0.38), and could not be linked to recent anthropogenic emissions from local sources or long-range transport. Principal component analysis and a composite of change in RGM versus change in GEM during RGM enhancements indicate that a nearly quantitative shift in speciation is associated with increases in ozone and decreases in water vapor and CO. This argues that high concentrations of RGM are present in the free troposphere because of in situ oxidation of GEM to RGM. A global chemical transport model reproduces the RGM mean and diurnal pattern but underestimates the magnitude of the largest observed enhancements. Since the only modeled, in situ RGM production mechanisms are oxidation of GEM by ozone and OH, this implies that there are faster reaction rates or additional RGM production mechanisms in the free troposphere.

Sources, fate and transport of atmospheric mercury from Asia

Environmental context. Mercury is a global problem and more than half of all anthropogenic emissions are from Asia. In this paper we review the sources of mercury coming from Asia, the environmental fate of these emissions and their global transport. Asian emissions of mercury are responsible for a small, but significant share of the mercury that deposits throughout the world, especially in the northern hemisphere. Abstract. Asian anthropogenic emissions of mercury to the atmosphere contribute 54% of all anthropogenic emissions. For this reason, it is important to understand how these emissions impact both the regional and global mercury cycle. The largest share of mercury emissions in Asia is due to coal combustion and smelting. Approximately half of the Asian anthropogenic emissions are emitted as Hg0. The remainder are emitted as gaseous Hg2+ compounds and particle-bound Hg (PHg). These latter forms of Hg are susceptible to relatively rapid removal from the atmosphere. The Asian emissions lead to very high concentrations of airborne Hg in urban and rural areas of Asia. Modelling the fate and transport of the Asian emissions using the Goddard Earth Observing System (GEOS)-Chem model of global tropospheric chemistry shows that these emissions are associated with high deposition of Hg within Asia, but there is little data with which to evaluate this model result. Observations downwind of Asia show that Hg0 is the dominant form in Asian plumes, with generally small, but variable amounts of other forms. Thus we conclude that most of the Asian Hg2+ and PHg have been removed in the source region. In addition, downwind observations show that total emissions are significantly larger than indicated by the anthropogenic emission inventory. This likely reflects either an underestimate of the anthropogenic emissions or emissions of Hg from land. Plumes containing enhancements in Asian Hg0, but not other species, have been detected as far away as North America. Because only Asian Hg0 is transported long distances, the deposition is distributed relatively uniformly in the northern hemisphere. In North America, Asian anthropogenic emissions account for 7–20% of all deposition, with an average of 16%.

Vertical distribution of mercury, CO, ozone, and aerosol scattering coefficient in the Pacific Northwest during the spring 2006 INTEX-B campaign

[1] In the spring of 2006, we measured the vertical distribution of gaseous elemental mercury (GEM), CO, ozone, and aerosol scattering coefficient in the Pacific Northwest concurrent with NASA’s INTEX-B campaign. Seven profiles from the surface to 6 km were conducted from 12 April to 8 May along with one flight in the Seattle-Tacoma boundary layer. Ozone had a bimodal distribution with the lower mode occurring primarily in the mixed layer and the higher mode occurring in the free troposphere. In the free troposphere, the mixing ratios (1 � s) of GEM, CO, ozone, and aerosol scattering coefficient were 1.52 (0.165) ng/m 3 , 142 (14.9) ppbv, 78 (7.7) ppbv, and 3.0 (1.8) Mm � 1 , respectively. GEM and CO were correlated in the high ozone mode (r 2 = 0.30) but were uncorrelated in the lower mode (r 2 = 0.05). Three flights observed enhancements of GEM and CO with good correlations and with regression slopes (0.0067 (±0.0027) ng/m 3 /ppbv by ordinary least squares regression and 0.0097 (±0.0018) ng/m 3 /ppbv by reduced major axis regression) slightly higher than previous observations of enhancements due to Asian industrial long-range transport (LRT). The influence of Asian LRT is supported by back trajectories and a global chemical transport model. In the SeattleTacoma boundary layer flight, CO was uncorrelated with GEM, which reflects relatively weaker local GEM sources. On three flights, pockets of air were observed with strong inverse GEM-ozone and ozone-CO correlations (in contrast to all data), which is evidence of upper tropospheric/lower stratospheric (UTLS) influence. An extrapolation of the GEM-CO and GEM-ozone slopes suggests the UTLS can be depleted of GEM.

The Geos-Chem model

We examine the response of deposition to decreases in anthropogenic emissions using the GEOS-Chem global atmosphere-ocean-land mercury simulation. Total global mercury sources in the model are 9230 Mg yr-1 (3400 Mg yr-1 anthropogenic, 650 Mg yr-1 biomass burning, 2180 Mg yr-1 land emissions, 3000 Mg yr-1 ocean emissions). Our atmospheric simulation describes the cycling of mercury through the surface ocean and land reservoirs. The model includes atmospheric oxidation of Hg0 by OH and O3, and in-cloud reduction of Hg(II). Wet and dry deposition account for 32% and 68% of the global sink, respectively. The lifetime of mercury against deposition is 0.6 years. We conduct four sensitivity simulations where anthropogenic emissions are reduced by 20% over East Asia, Europe, South Asia, and North America, leading to decreases in global deposition of -3.5, -0.9, -0.8, and -0.5% respectively. One third of the deposition decrease occurs in the source regions, and the rest is distributed globally due to decreased long-range transport of Hg0 and subsequent oxidation to Hg(II). Regional decreases in deposition within the source regions range from -12% for East Asia (60% of depositions due to local emissions) to -3% for North America (where only 15% of deposition is due to local emissions). When normalized by total emissions, we find that Hg deposition in the Arctic is more sensitive to decreases in European emissions compared to decreases in East Asian, North American, or South Asian emissions. Our estimates of the distribution of deposition and its response to decreases in anthropogenic emissions are limited by uncertainties in the speciation of anthropogenic emissions, redox chemistry of atmospheric mercury, the role of dry deposition, and the cycling efficiency of mercury in the ocean and land reservoirs.