Abstract. Global models of atmospheric mercury generally assume that gas-phase OH and ozone are the main oxidants converting Hg 0 to Hg II and thus driving mercury deposition to ecosystems. However, thermodynamic considerations argue against the importance of these reactions. We demonstrate here the viability of atomic bromine (Br) as an alternative Hg 0 oxidant. We conduct a global 3-D simulation with the GEOS-Chem model assuming gas-phase Br to be the sole Hg 0 oxidant (Hg + Br model) and compare to the previous version of the model with OH and ozone as the sole oxidants (Hg + OH/O 3 model). We specify global 3-D Br concentration fields based on our best understanding of tropospheric and stratospheric Br chemistry. In both the Hg + Br and Hg + OH/O 3 models, we add an aqueous photochemical reduction of Hg II in cloud to impose a tropospheric lifetime for mercury of 6.5 months against deposition, as needed to reconcile observed total gaseous mercury (TGM) concentrations with current estimates of anthropogenic emissions. This added reduction would not be necessary in the Hg + Br model if we adjusted the Br oxidation kinetics downward within their range of uncertainty. We find that the Hg + Br and Hg + OH/O 3 models are equally capable of reproducing the spatial distribution of TGM and its seasonal cycle at northern mid-latitudes. The Hg + Br model shows a steeper decline of TGM concentrations from the tropics to southern mid-latitudes. Only the Hg + Br model can reproduce the springtime depletion and summer rebound of TGM observed at polar sites; the snowpack component of GEOS-Chem suggests that 40% of Hg II deposited to snow in the Arctic is transferred to the ocean and land reservoirs, amounting to aCorrespondence to: C. D. Holmes (cdholmes@post.harvard.edu) net deposition flux to the Arctic of 60 Mg a −1 . Summertime events of depleted Hg 0 at Antarctic sites due to subsidence are much better simulated by the Hg + Br model. Model comparisons to observed wet deposition fluxes of mercury in the US and Europe show general consistency. However the Hg + Br model does not capture the summer maximum over the southeast US because of low subtropical Br concentrations while the Hg + OH/O 3 model does. Vertical profiles measured from aircraft show a decline of Hg 0 above the tropopause that can be captured by both the Hg + Br and Hg + OH/O 3 models, except in Arctic spring where the observed decline is much steeper than simulated by either model; we speculate that oxidation by Cl species might be responsible. The Hg + Br and Hg + OH/O 3 models yield similar global budgets for the cycling of mercury between the atmosphere and surface reservoirs, but the Hg + Br model results in a much larger fraction of mercury deposited to the Southern Hemisphere oceans.

[1] Knowledge of the atmospheric chemistry of reactive greenhouse gases is needed to accurately quantify the relationship between human activities and climate, and to incorporate uncertainty in our projections of greenhouse gas abundances. We present a method for estimating the fraction of greenhouse gases attributable to human activities, both currently and for future scenarios. Key variables used to calculate the atmospheric chemistry and budgets of major non-CO 2 greenhouse gases are codified along with their uncertainties, and then used to project budgets and abundances under the new climate-change scenarios. This new approach uses our knowledge of changing abundances and lifetimes to estimate current total anthropogenic emissions, independently and possibly more accurately than inventorybased scenarios. We derive a present-day atmospheric lifetime for methane (CH 4 ) of 9.1 AE 0.9 y and anthropogenic emissions of 352 AE 45 Tg/y (64% of total emissions). For N 2 O, corresponding values are 131 AE 10 y and 6.5 AE 1.3 TgN/y (41% of total); and for HFC-134a, the lifetime is 14.2 AE 1.5 y. Citation: Prather, M. J., C. D. Holmes, and J. Hsu (2012), Reactive greenhouse gas scenarios: Systematic exploration of uncertainties and the role of atmospheric chemistry, Geophys.

Abstract. Atmospheric deposition of Hg(II) represents a major input of mercury to surface environments. The phase of Hg(II) (gas or particle) has important implications for deposition. We use long-term observations of reactive gaseous mercury (RGM, the gaseous component of Hg(II)), particle-bound mercury (PBM, the particulate component of Hg(II)), fine particulate matter (PM 2.5 ), and temperature (T ) at five sites in North America to derive an empirical gas-particle partitioning relationship log 10 (K −1 ) = (10±1)-(2500±300)/T where K = (PBM/PM 2.5 )/RGM with PBM and RGM in common mixing ratio units, PM 2.5 in µg m −3 , and T in K. This relationship is within the range of previous work but is based on far more extensive data from multiple sites. We implement this empirical relationship in the GEOS-Chem global 3-D Hg model to partition Hg(II) between the gas and particle phases. The resulting gas-phase fraction of Hg(II) ranges from over 90 % in warm air with little aerosol to less than 10 % in cold air with high aerosol. Hg deposition to high latitudes increases because of more efficient scavenging of particulate Hg(II) by precipitating snow. Model comparison to Hg observations at the North American surface sites suggests that subsidence from the free troposphere (warm air, low aerosol) is a major factor driving the seasonality of RGM, while elevated PBM is mostly associated with high aerosol loads. Simulation of RGM and PBM at these sites is improved by including fast in-plume reduction of Hg(II) emitted from coal combustion and by assuming that anthropogenic particulate Hg(p) behaves as semivolatile Hg(II) rather than as a refractory particulate component. We improve the simulation of Hg wet deposition fluxes in the US relative to a previous version of GEOS-Chem; this largely reflects independent improvement of the washout algorithm. The observed wintertime minimum in wet deposition fluxes is attributed to inefficient snow scavenging of gas-phase Hg(II).

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 Hg 0 /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. IntroductionAnthropogenic mercury (Hg) sources have enriched atmospheric Hg deposition globally by at least a factor of 3 (1).Atmospheric Hg is predominantly the gaseous elemental form (Hg 0 ), and is oxidized to Hg II , which is then rapidly deposited. It is estimated that more than 80% of the Hg deposited to oceans is reemitted to the atmosphere as Hg 0 , driving the cycle of Hg through biogeochemical reservoirs (2). Aqueous reduction of divalent inorganic mercury (Hg II ) and subsequent loss of Hg 0 reduces the potentially bioavailable Hg II pool that may be converted to monomethylmercury, the most toxic species that poses health risks to fish consuming populations and wildlife (3).Previous efforts to model Hg air-sea exchange (2) and atmospheric transport (4-6) have been unable to reproduce high atmospheric concentrations observed in the Northern Hemisphere marine boundary layer (MBL) during ocean cruises (7-9). We hypothesize that this results from subsurface seawater Hg enrichment, reflecting the legacy of past anthropogenic inputs and controlling Northern Hemisphere MBL concentrations. Previous research comparing preindustrial and contemporary Hg budgets for different ocean basins indicates that anthropogenic enrichment of Hg reservoirs in the Atlantic Ocean and Mediterranean Sea is >50% (3). Other regions such as deep waters of the Pacific Ocean have seen negligible anthropogenic impacts while Hg concentrations in intermediate waters of the North Pacific (NP) appear to be increasing (10). These gradients in subsurface Hg across ocean regions (11, 12) have not been represented in models simulating atmospheric Hg. Here we investigate the potential effects of legacy anthropogenic Hg accumulation on oceanic air-sea exchange in the GEOSChem global model (1) by including the effects of variability in subsurface ocean concentrations in our simulation.Most marine surface waters...