1. Oxidant chemistry in the tropical troposphere: role of oxygenated VOCs and halogens, and implications for mercury Daniel J. Jacob with Kevin J. Wecht, Lee T. Murray, Emily V. Fischer, Justin P. Parrella, Anne L. Soerensen, Helen M. Amos

2. Radical cycle controlling tropospheric ozone and OH O3O3 O2O2 h O3O3 OH HO 2,RO 2 h, H 2 O Deposition NO H 2 O 2 ROOH CO, VOC NO 2 h STRATOSPHERE TROPOSPHERE SURFACE h biosphere combustion industry lightning combustion soils

3. Latitude [°] 03060-60-30 monthly methane oxidation (GEOS-Chem) 4.0 8.0 12.0 10 8 kg CH 4 month -1 Annual Average January July Oxidation of long-lived gases by OH is mostly in tropics Kevin Wecht, Harvard

4. Ozone distribution in tropical troposphere OMI-MLS tropospheric ozone columns, 2004-2005 NO x from lightning open fires soils fuel combustion Murray et al. [2012] Jacob et al. [1996] Ozone budget schematic (Walker circulation): DJF MAM JJA SON

5. Volatile organic compounds (VOCs) in the atmosphere: carbon oxidation chain VOCRO 2 NO 2 O3O3 organic peroxy radicals NO h carbonyls R’O 2 h OH + products organic aerosol ROOH organic peroxides OH HO 2 OH, h OH products EARTH SURFACE biosphere combustion industry deposition Increasing functionality & cleavage sources of organic aerosol sources/sinks of oxidants (ozone, OH) OVOCs

6. Volatile organic compounds (VOCs) in the atmosphere: effect on nitrogen cycle NO x CH 3 C(O)OO OH EARTH SURFACE combustion soils deposition Reservoirs for long-range transport of NO x lightning deposition HNO 3 peroxyacetylnitrate (PAN) other organic nitrates NO x OH deposition HNO 3 Long-range atmospheric transport RO 2 hours

7. Distributions of NOx, HNO 3, and PAN over Pacific PEM-Tropics B aircraft campaign (Mar-Apr 1999): latitude-altitude x-sections NO x below 6 km over Pacific is mainly from PAN decomposition Staudt et al. [2003]

8. Terrestrial Marine Open fires Anthropogenic Atmospheric formation methyl- glyoxal acetone acetal- dehyde isoprene propane >C 2 alkenes ethanol ethane CH 3 COO 2 + NO 2 + M  PAN + M xylene toluene >C 3 alkanes Other Global sources of PAN 124 88 100 Emily Fischer, Harvard sources in Tg C a -1

9. 0 – 3 km Above 3 km Methylglyoxal Other (isoprene) Acetone Acetaldehyde 427 PAN precursors over Pacific Acetone and acetaldehyde are the main precursors Emily Fischer, Harvard January mean GEOS-Chem results

10. Global budget of acetone Vegetation 32 Ocean uptake 82 Open fires 3 Anthropogenic <1 Acetone lifetime 14 days (37 days vs. OH, hv) OH 19 33 propane i-butane OH biogenic VOCs OH, O 3 h Ocean evasion 80 26 5 12 deposition to land Acetone 15 nM production loss Observations indicate 15 (10-20) nM acetone in ocean  Implies that ocean acetone is internally controlled  Implies that ocean is dominant source to the atmosphere Fischer et al. [2012] Rates in Tg a -1

11. Global distribution of acetone and net air-sea fluxes Circles: mean obs from aircraft campaigns Background: GEOS-Chem model GEOS-Chem annual mean sea-to-air fluxes Ocean is net source in tropics (except coastal), sink in northern extratropics Remote atmospheric background is determined by ocean control, long photochemical lifetime Fischer et al. [2012]

12. Halogen radical chemistry in troposphere: sink for ozone, NO x, VOCs, mercury X ≡ Cl, Br, I sea salt source organohalogen source radical cycling non-radical reservoir formation heterogeneous recycling

13. Bromine chemistry in the atmosphere Tropopause (8-18 km) Troposphere Stratosphere Halons CH 3 Br CHBr 3 CH 2 Br 2 Sea salt BrBrO BrNO 3 HOBrHBr O3O3 hv, NO hv OH Inorganic bromine (Br y ) Br y OH, h debromination deposition industryplankton Stratospheric BrO: 2-10 ppt Tropospheric BrO: 0.5-2 ppt Thule GOME-2 BrO columns Satellite residual [Theys et al., 2011] BrO column, 10 13 cm -2 VSLS

14. Mean vertical profiles of CHBr 3 and CH 2 Br 2 From NASA aircraft campaigns over Pacific in April-June Vertical profiles steeper for CHBr 3 (mean lifetime 21 days) than for CH 2 Br (91 days), steeper in extratropics than in tropics Parrella et al. [2012]

15. Global tropospheric Br y budget in GEOS-Chem (Gg Br a -1 ) SURFACE CHBr 3 407 CH 2 Br 2 57 CH 3 Br Marine biosphere Sea-salt debromination (50% of 1-10 µm particles) STRATOSPHERE TROPOSPHERE 7-9 ppt Liang et al. [2010] stratospheric Bry model (upper boundary conditions) 56 36 Br y 3.2 ppt Volcanoes (5-15) Deposition lifetime 7 days 1420 Sea salt is the dominant global source but is released in marine boundary layer where lifetime against deposition is short; CHBr 3 is major source in the free troposphere Parrella et al. [2012]

16. Tropospheric Br y cycling in GEOS-Chem Gg Br (ppt) Global annual mean concentrations in Gg Br (ppt), rates in Gg Br s -1 Model includes HOBr+HBr in aq aerosols with  = 0.2, ice with  = 0.1 Mean daytime BrO = 0.6 ppt; would be 0.3 ppt without HOBr+HBr reaction Parrella et al. [2012]

17. Zonal annual mean concentrations (ppt) in GEOS-Chem Br BrO Br y Br y is 2-4 ppt, highest over Southern Ocean (sea salt) BrO increases with latitude (photochemical sink) Br increases with altitude (BrO photolysis) Parrella et al. [2012]

18. Comparison to seasonal satellite data for tropospheric BrO [Theys et al., 2011] model TOMCAT has lower  =0.02 for HOBr+HBr than GEOS-Chem, large polar spring source from blowing snow HOBr+HBr reaction critical for increasing BrO with latitude, winter/spring NH max in GEOS-Chem (9:30 am) Parrella et al. [2012]

19. Effect of Br chemistry on tropospheric ozone Zonal mean ozone decreases (ppb) in GEOS-Chem Two processes: catalytic ozone loss via HOBr, NO x loss via BrNO 3 Global OH also decreases by 4% due to decreases in ozone and NO x Parrella et al. [2012]

20. Bromine chemistry improves simulation of 19 th century surface ozone Standard models without bromine are too high, peak in winter-spring; bromine chemistry corrects these biases Model BrO is similar in pre-industrial and present atmosphere (canceling effects) Parrella et al. [2012]

21. Atmospheric lifetime of Hg(0) against oxidation to Hg(II) by Br Hg(0) + Br ↔ Hg(I) → Hg(II) Br,OH 2-step Hg(0) oxidation (Goodsite et al., 2004; Donohoue et al., 2006) Emission Deposition GEOS-Chem Br yields Hg(0) global mean tropospheric lifetime of 4 months, consistent with observational constraints Br in pre-industrial atmosphere was 40% higher than in present-day (less ozone), implying a pre-industrial Hg(0) lifetime of only 2 months  Hg could have been more efficiently deposited to northern mid-latitude oceans in the past Parrella et al. [2012]

22. Mechanism for uptake of atmospheric Hg by ocean thermocline Marine boundary layer Free troposphere Ocean mixed layer 0-100 m Subsurface ocean 100-1500 m Deep ocean Hg(0) Hg(II) gas  particle Hg(0) Hg(II) sea-salt Hg(II) dissolved Hg(II) particulate Hg(0) burial water exchange Br Br ?well-mixed pool most of total Hg deposition as implemented in GEOS-Chem model

23. Hg(0) decreasing trend in North Atlantic surface air Large decrease observed since 1990 in N Atlantic, not in S Atlantic Model can reproduce this decrease based on 80% observed decrease of dissolved Hg in subsurface N Atlantic since 1990 Why this large subsurface ocean decrease? Increasing MBL ozone, decreasing coastal inputs from rivers/wastewater, missing historical Hg sources? Soerensen et al. [2012] Ensemble of cruise data, 1977-present

24. Historical inventory of global anthropogenic Hg emissions Large legacy contribution from N. American and European emissions; Asian dominance is a recent phenomenon Pre-1850 releases from mining account for 40% of all-time anthropogenic emissions Streets et al., 2012

25. Global biogeochemical model for mercury Primary emissions 7-box model with 7 coupled ODEs dm/dt = s(t) – km where s is primary emission (atmosphere only) Model is initialized at natural steady state, forced with historical anthropogenic emissions for 2000 BC – present; % present-day enrichments are indicated Helen Amos, Harvard

26. Contribution of old anthropogenic (legacy) mercury to global atmospheric deposition and surface ocean GEOS-Chem based global biogeochemical model of mercury cycling Mercury pollution is mainly a legacy problem that will take centuries to fix; all we can do in short term is prevent it from getting worse Helen Amos, Harvard