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Daniel J. Jacob

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.

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Daniel J. Jacob

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  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 O2 hn O3 STRATOSPHERE TROPOSPHERE lightning combustion soils hn NO2 NO O3 H2O2 ROOH HO2,RO2 OH hn, H2O Deposition hn CO, VOC biosphere combustion industry SURFACE

  3. Oxidation of long-lived gases by OH is mostly in tropics monthly methane oxidation (GEOS-Chem) 12.0 Annual Average January July 8.0 108 kg CH4 month-1 4.0 -60 -30 0 30 60 Latitude [°] Kevin Wecht, Harvard

  4. OMI-MLS tropospheric ozone columns, 2004-2005 DJF JJA Ozone distribution in tropical troposphere MAM SON Murray et al. [2012] Ozone budget schematic (Walker circulation): • NOx from • lightning • open fires • soils • fuel combustion Jacob et al. [1996]

  5. Volatile organic compounds (VOCs) in the atmosphere:carbon oxidation chain • sources of organic aerosol • sources/sinks of oxidants (ozone, OH) Increasing functionality & cleavage h O3 OH + products NO2 h OH R’O2 carbonyls NO organic aerosol OH VOC RO2 HO2 organic peroxy radicals OH,h ROOH products organic peroxides biosphere combustion industry OVOCs deposition EARTH SURFACE

  6. Volatile organic compounds (VOCs) in the atmosphere:effect on nitrogen cycle Reservoirs for long-range transport of NOx Long-range atmospheric transport peroxyacetylnitrate (PAN) CH3C(O)OO lightning NOx NOx RO2 OH OH other organic nitrates hours HNO3 HNO3 combustion soils deposition deposition deposition EARTH SURFACE

  7. Distributions of NOx, HNO3, and PAN over Pacific PEM-Tropics B aircraft campaign (Mar-Apr 1999): latitude-altitude x-sections NOx below 6 km over Pacific is mainly from PAN decomposition Staudt et al. [2003]

  8. Global sources of PAN Terrestrial Marine Open fires Anthropogenic Atmospheric formation ethane >C2 alkenes ethanol isoprene >C3 alkanes propane Other acetal- dehyde acetone sources in Tg C a-1 toluene 88 124 methyl- glyoxal 100 xylene CH3COO2 + NO2 + M  PAN + M Emily Fischer, Harvard

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

  10. hn propane i-butane OH Acetone lifetime 14 days (37 days vs. OH, hv) 19 26 Rates in Tg a-1 Global budget of acetone OH, O3 OH biogenic VOCs 5 33 deposition to land 12 Ocean evasion 80 Ocean uptake 82 Open fires 3 Anthropogenic <1 Vegetation 32 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]

  11. Global distribution of acetone and net air-sea fluxes 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 Circles: mean obs from aircraft campaigns Background: GEOS-Chem model Fischer et al. [2012]

  12. X ≡ Cl, Br, I organohalogen source radical cycling Halogen radical chemistry in troposphere: sink for ozone, NOx, VOCs, mercury sea salt source non-radical reservoir formation heterogeneous recycling

  13. GOME-2 BrO columns Bromine chemistry in the atmosphere Inorganic bromine (Bry) O3 hv BrNO3 Br BrO Halons hv, NO OH HBr HOBr Stratospheric BrO: 2-10 ppt CH3Br Thule Stratosphere VSLS Tropopause (8-18 km) Troposphere TroposphericBrO: 0.5-2 ppt CHBr3 CH2Br2 OH, h Bry Satellite residual [Theys et al., 2011] debromination BrO column, 1013 cm-2 deposition Sea salt industry plankton

  14. Mean vertical profiles of CHBr3 and CH2Br2 From NASA aircraft campaigns over Pacific in April-June Vertical profiles steeper for CHBr3 (mean lifetime 21 days) than for CH2Br(91 days), steeper in extratropics than in tropics Parrella et al. [2012]

  15. Liang et al. [2010] stratospheric Bry model (upper boundary conditions) STRATOSPHERE 36 TROPOSPHERE Global tropospheric Bry budget in GEOS-Chem (Gg Br a-1) 56 Bry 3.2 ppt CH3Br Deposition 57 CH2Br2 lifetime 7 days 407 CHBr3 1420 (5-15) 7-9 ppt Marine biosphere Volcanoes Sea-salt debromination (50% of 1-10 µm particles) SURFACE Sea salt is the dominant global source but is released in marine boundary layer where lifetime against deposition is short; CHBr3 is major source in the free troposphere Parrella et al. [2012]

  16. Tropospheric Bry cycling in GEOS-Chem Global annual mean concentrations in Gg Br (ppt), rates in Gg Br s-1 Gg Br (ppt) • 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 BrO • Bry is 2-4 ppt, highest over Southern Ocean (sea salt) • BrO increases with latitude(photochemical sink) • Br increases with altitude(BrO photolysis) Br Bry Parrella et al. [2012]

  18. Comparison to seasonal satellite data for tropospheric BrO[Theys et al., 2011] (9:30 am) model model • TOMCAT has lower =0.02 for HOBr+HBrthan 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 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, NOx loss via BrNO3 • Global OH also decreases by 4% due to decreases in ozone and NOx Parrella et al. [2012]

  20. Bromine chemistry improves simulation of 19th 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. 2-step Hg(0) oxidation (Goodsite et al., 2004; Donohoue et al., 2006) Br,OH Hg(0) + Br ↔ Hg(I) → Hg(II) Atmospheric lifetime of Hg(0) against oxidation to Hg(II) by Br Deposition Emission • 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. as implemented in GEOS-Chem model well-mixed pool Br ? Hg(II) gas  particle Hg(0) Mechanism for uptake of atmospheric Hg by ocean Free troposphere Br Hg(II) Marine boundary layer Hg(0) most of total Hg deposition sea-salt Ocean mixed layer 0-100 m Hg(II) particulate Hg(II) dissolved Hg(0) Subsurface ocean 100-1500 m water exchange burial thermocline Deep ocean

  23. Ensemble of cruise data, 1977-present 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]

  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. 7-box model with 7 coupled ODEs dm/dt= s(t) – km where s is primary emission (atmosphere only) Global biogeochemical model for mercury Primary emissions 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) mercuryto 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

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