Terrestrial Mercury Dynamics

Transcription

1 Terrestrial Mercury Dynamics Plus: New Atmospheric Mercury Redox Chemistry 6 th GEOS- Chem Mee.ng May 7 th, 213 Bess Sturges Corbi@ corbi@@seas.harvard.edu Co- authors and collaborators: Elsie Sunderland, Daniel Jacob, Hannah Horowitz, Nicole Smith Downey, Daniel Obrist, Charley Driscoll, and David Streets

2 Global Terrestrial Mercury Model Coupled to GEOS- Chem

3 Atmosphere- Terrestrial Mercury Exchange Terrestrial emissions account for an es.mated 2-3% of total emissions to the atmosphere in the present- day. Much of the mercury in present- day soils is legacy mercury from historical emissions. Dynamic bidirec.onal exchange occurs at the atmosphere- terrestrial interface: Present- day soil Hg storage Smith- Downey et al. 21 Previous modeling work has shown that terrestrial soils are important long- term reservoirs of mercury, but the magnitude is poorly constrained. Amos et al. 213

4 New Developments to the Global Terrestrial Mercury Model Mercury binds strongly to reduced sulfur groups in soil organic Here we use soil pool sulfur content from the CENTURY model to scale the affinity of mercury binding to SOM. 5, Mg C R R C S Hg S R S Based on Skyllberg et al. 28 The GTMM is based on the CASA carbon model, which defines soil pools based on characteris.c overturning.me (Smith- Downey et al. 21, PoOer et al. 1993). GEOS- Chem runs to an equilibrium natural state then to the present using historical anthropogenic emissions (Streets et al. 211). Monthly Hg atmospheric deposi.on fluxes and climate data drive the GTMM. A frac.on of mercury is released to atmosphere during microbial respira.on. 15 7, Mg 25, Mg 5, Mg CorbiO et al. in prep.

5 Evalua.on with recent observa.ons of throughfall Hg and leaf, and soil Hg:C Leaves ng Hg g - 1 C , 1,5 1, 5 1, 7,5 5, *Preliminary results Hg:C increases as more C than Hg is released during microbial respira.on of SOM (~5% Hg per C li@er pools, 3% soil pools ref. Pokharel et al. 211, Obrist et al. 21). Con.nuing work to run a fully coupled spin up from natural steady- state condi.ons to the present. Further evalua.on with deposi.on measurements. Different soil storage pa@erns from previous modeling work have implica.ons for long- term fate of Hg in soils. Soil 2,5 CorbiO et al. in prep. Data from Obrist et al. 211, Yu et al. in prep.

6 Modeling the Atmospheric Redox Chemistry of Mercury

7 Atmospheric Oxida.on and Reduc.on Chemistry Modified from Holmes et al. 21 aqueous photoreduc6on NO 2, HO 2, BrO, ClO Atmospheric models have not previously considered reac.on of HgBr with more abundant radicals like NO 2 and HO 2. Upward revision of the dissocia.on of HgBr works in the opposite direc.on, but the overall effect is more oxida.on. An increase in the (poorly constrained) photoreduc.on rate results in model atmospheric Hg concentra.ons comparable to observa.ons. HgBr 2 HgCl 2 HgBrHO 2 Hg Oxidant Op.ons in GEOS- Chem: Hg + OH à HgOH Hg + O 3 à HgO 3 Hg + Br + X à HgBrX, X = Br, OH (recommended) Hg + Br + X à HgBrX, X =OH, BrO, ClO, NO 2, and HO 2 (in development) HgBrNO 2 HgBrNO 2 Dibble et al. 212

8 Preliminary Model Results for New Redox Chemistry Dibble et al. 212 Redox Chemistry faster NO 2, HO 2, BrO, ClO Modeled atmospheric concentra.ons using faster redox are consistent with observa.ons. Net oxida.on (and consequently deposi.on) shiks more toward the poles in the model. aqueous photoreduc6on rate constant x5 Surface Total Gaseous Mercury Hg II Dry Deposi.on Wet Deposi.on Hg II Surface Concentra.on TGM [ng m - 3 ]

9 UT/LS Atmospheric Mercury Trend Ozone [ppbv] Lower Stratosphere Hg CARIBIC Hg Standard Chem Hg New Chem Hg Al.tude [km] Zonal Hg : New Chemistry Hg [ng m - 3 ] W 9W 9E 18E New Standard Chemistry Hg CorbiO et al. in prep. Hg [ng m - 3 ] Addi.onal chemical mechanisms are needed to explain the sharp decline in total measured atmospheric mercury above the tropopause. Al.tude [km] W 9W 9E 18E Difference [%] +1% +5% - 5% - 1%