Revisi&ng atmospheric Hg oxida&on mechanisms in GEOS- Chem: constraints from observa&ons

Transcription

1 Revisi&ng atmospheric Hg oxida&on mechanisms in GEOS- Chem: constraints from observa&ons Hannah M. Horowitz Daniel J. Jacob, Helen M. Amos, Theodore S. Dibble, Franz Slemr, Johan A. Schmidt, Daniel A. Jaffe, Elizabeth S. CorbiI, and Elsie M. Sunderland IGC7 6 May 2015 Contact: hmhorow@fas.harvard.edu

2 Reconciling advances in our scien&fic understanding with observa&onal constraints Robust observa?onal constraints for atmospheric Hg redox cycling: Atmospheric burden: Mg of total Hg Hg 0 life?me against oxida?on + deposi?on: years Example of prior GEOS- Chem Hg model results, within constraints: Hg 0 (4.0 Gg) Hg 0 +Br 8000 Hg II (1.0 Gg) Burden: 5000 Mg Hg 0 life?me: 10.4 months Geogenic Anthropogenic photoreduction Biomass burning 1500 Vegetation, soil, snow Recent research suggests these fluxes are incorrect or incomplete How should we improve the model, and what will happen? Fluxes in Mg a -1 Soil (1000 Gg) Ocean surface (6.3 Gg) Holmes et al., 2010

3 Proposing new standard atmospheric chemical mechanism for Hg in GEOS- Chem Standard v9-02 GC Br fields from Parrella et al. (2012) Proposed changes GC Br fields from Schmidt et al. in prep. Al?tude Change in Br concentra?ons v9-02 annual zonal mean [Br] New annual zonal mean [Br] Schmidt et al. tropospheric Br is on average 70% higher La&tude La&tude Burden? Life?me? E6 2.33E6 3.5E6 molec cm E6 2.33E6 3.5E6 molec cm - 3 New [Br] standard v9-02 % difference New standard v9-02 Al?tude E6-1.25E E6 2.5E6 molec cm %

4 Proposing new standard atmospheric chemical mechanism for Hg in GEOS- Chem Standard v9-02 GC Br fields from Parrella et al. (2012) Strong in- plume reduc&on (75% of Hg II ) Proposed changes GC Br fields from Schmidt et al. in prep. No in- plume reduc&on Fossil fuel emissions specia?on Hg II Hg 0 Strong IPR No IPR More wet deposi?on Burden? Life?me?

5 Proposing new standard atmospheric chemical mechanism for Hg in GEOS- Chem Standard v9-02 GC Br fields from Parrella et al. (2012) Strong in- plume reduc&on (75% of Hg II ) In- cloud Hg(II) photoreduc&on Proposed changes GC Br fields from Schmidt et al. in prep. No in- plume reduc&on No in- cloud Hg(II) reduc&on If in- cloud, why not also in sulfate aerosol in UT/LS? Reduc&on here is not compa&ble with understood ver&cal profile of Hg 20 O 3 Ozone Geopoten&al height (km) Geopotential height (km) Hg II Hg(II) Hg(0) Hg 0 Tropopause THg THg Lyman and Jaffe, 2011 Based on aircra7 observa:ons ,000 1,500 Hg (pg m 3 ) or ozone (ppb)

6 Proposing new standard atmospheric chemical mechanism for Hg in GEOS- Chem Standard v9-02 GC Br fields from Parrella et al. (2012) Strong in- plume reduc&on (75% of Hg II ) In- cloud Hg(II) photoreduc&on Oxida&on ini&ated by Br atom, completed by Br & OH Proposed changes GC Br fields from Schmidt et al. in prep. No in- plume reduc&on No in- cloud Hg(II) reduc&on Addi&onal oxida&on reac&ons and updated rate coefficients based on literature review 1) Hg 0 + X + M HgX + M 2) HgX + M Hg 0 + X + M 2a) HgX + X Hg 0 + X 2 3) HgX + Y HgXY k 1 = (T / 298) 1.86 cm 6 molec 2 s (T / 298) k 2 = (T / 298) 0.9 exp( 7292 / T ) cm 3 molec -9 (T 1 / s298) exp(-7600/t) k 2a = cm 3 molec 1 s k 3 = (T / 298) 0.57 cm 3 molec 1 s X = Br, Cl Y = Br, OH, HO 2, NO 2, BrO, ClO + in- cloud aqueous oxida&on by O 3, OH, HOCl/OCl - smaller rate coefficients + more oxidants: effects on burden and life?me not immediately clear New reac:ons and rates from Dibble et al., 2012, 2014; Wilcox, 2009; Balabanov et al., 2005; Donohoue et al., 2005; Wang and Pehkonen, 2004; Lin and Pehkonen, 1997, 1998; Munthe 1992; Buxton et al., 1988

7 Importance of new oxida&on pathways Frac&on of total annual oxida&on of Hg 0 : Zonal total Hg II produc&on per year Br atom (all blue segments) is s&ll dominant ini&ator of oxida&on Ini&al second- step oxidants (Br, OH) outcompeted by more abundant radicals Altitude (km) Latitude Latitude kg m -3 a -1 Oxida&on now at lower al&tudes, due to NO 2, HO 2 New Br fields affect polar oxida&on

8 Observa&onal comparison: reasonable life&mes, seasonality, spa&al distribu&on, but low bias in concentra&ons Burden: Hg 0 life?me against ox. + dep.: ng m -3 TGM ng m -3 TGM Obs Std v9-02 New model Mid latitudes Old Model Version J F M A M J J A S O N D Arctic Improved Br? J F M A M J J A S O N D New model // Std v Mg // 4500 Mg 6.4 months // 11.2 months TGM [ng/m 3 ] Land based stations Temme et al Laurier et al Lamborg et al Sorenson et al New Model Version Zonal Mean Old Model Version Zonal Mean Combined effect of differences in oxida&on and reduc&on Surface TGM Latitude