1. Questions 1. A sea-breeze circulation often produces a temperature inversion. Explain why. 2. A well known air pollution problem is "fumigation" where surface sites downwind of a major pollution source with elevated smokestacks experience sudden bursts of very high pollutant concentrations in mid- morning. Can you explain this observation on the basis of atmospheric stability? 3. A persistent mystery in atmospheric chemistry is why the stratosphere is so dry (3-5 ppmv H 2 O). Based on water vapor concentrations observed just below the tropopause, one would expect the air entering the stratosphere to be moister, One theory is that very strong thunderstorms piercing through the tropopause can act as a "cold finger" for condensation of water and thereby remove water from the lower stratosphere. How would this work?

2. CHAPTER 6: GEOCHEMICAL CYCLES Most abundant elements: oxygen (in solid earth!), iron (core), silicon (mantle), hydrogen (oceans), nitrogen, carbon, sulfur… The elemental composition of the Earth has remained essentially unchanged over its 4.5 Gyr history –Extraterrestrial inputs (e.g., from meteorites, cometary material) have been relatively unimportant –Escape to space has been restricted by gravity Biogeochemical cycling of these elements between the different reservoirs of the Earth system determines the composition of the Earth’s atmosphere and oceans, and the evolution of life THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS

3. BIOGEOCHEMICAL CYCLING OF ELEMENTS: examples of major processes Physical exchange, redox chemistry, biochemistry are involved Surface reservoirs

4. HISTORY OF EARTH’S ATMOSPHERE Outgassing N 2 CO 2 H 2 O oceans form CO 2 dissolves Life forms in oceans Onset of photosynthesis O2O2 O 2 reaches current levels; life invades continents 4.5 Gy B.P 4 Gy B.P. 3.5 Gy B.P. 0.4 Gy B.P. present

5. EVOLUTION OF O 2 AND O 3 IN EARTH’S ATMOSPHERE

6. COMPARING THE ATMOSPHERES OF EARTH, VENUS, AND MARS VenusEarthMars Radius (km)610064003400 Surface pressure (atm)9110.007 CO 2 (mol/mol)0.963x10 -4 0.95 N 2 (mol/mol)3.4x10 -2 0.782.7x10 -2 O 2 (mol/mol)6.9x10 -5 0.211.3x10 -3 H 2 O (mol/mol)3x10 -3 1x10 -2 3x10 -4

7. OXIDATION STATES OF NITROGEN N has 5 electrons in valence shell  9 oxidation states from –3 to +5 -30+1+2+3+4+5 NH 3 Ammonia NH 4 + Ammonium R 1 N(R 2 )R 3 Organic N N2N2 N 2 O Nitrous oxide NO Nitric oxide HONO Nitrous acid NO 2 - Nitrite NO 2 Nitrogen dioxide HNO 3 Nitric acid NO 3 - Nitrate Decreasing oxidation number (reduction reactions) Increasing oxidation number (oxidation reactions)

8. THE NITROGEN CYCLE: MAJOR PROCESSES ATMOSPHERE N2N2 NO HNO 3 NH 3 /NH 4 + NO 3 - orgN BIOSPHERE LITHOSPHERE combustion lightning oxidation deposition assimilation decay nitrification denitri- fication biofixation burial weathering

9. Questions 1. Denitrification seems at first glance to be a terrible waste for the biosphere, jettisoning precious fixed nitrogen back to the atmospheric N 2 reservoir. In fact, denitrification is essential for maintaining life in the interior of continents. Can you see why? 2. Although volcanoes don't emit O 2 they do emit a lot of oxygen (as H 2 O and CO 2 ). Both H 2 O and CO 2 photolyze in the upper atmosphere. Photolysis of H 2 O results in production of atmospheric O 2 but photolysis of CO 2 does not. Why this difference?

10. BOX MODEL OF THE NITROGEN CYCLE Inventories in Tg N Flows in Tg N yr -1

11. N 2 O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION Important as source of NO x radicals in stratosphere greenhouse gas IPCC [2014] Main anthropogenic source: agriculture

12. FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE Source of O 2 : photosynthesis nCO 2 + nH 2 O  (CH 2 O) n + nO 2 Sink: respiration/decay (CH 2 O) n + nO 2  nCO 2 + nH 2 O O2O2 CO 2 orgC litter Photosynthesis less respiration decay O 2 lifetime: 6000 years vs photosynthesis ~200 PgO/yr 1.2x10 6 Pg 4x10 3 Pg 8x10 2 Pg

13. …however, abundance of organic carbon in biosphere/soil/ocean reservoirs is too small to control atmospheric O 2 levels

14. SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE O2O2 CO 2 Compression subduction Uplift CONTINENT OCEAN FeS 2 orgC weathering Fe 2 O 3 H 2 SO 4 runoff O2O2 CO 2 Photosynthesis decay orgC burial SEDIMENTS microbes FeS 2 orgC CO 2 orgC: 1x10 7 Pg C FeS 2 : 5x10 6 Pg S O 2 : 1.2x10 6 Pg O O 2 lifetime: 3 million years

16. On average, only 60% of emitted CO 2 remains in the atmosphere – but there is large interannual variability in this fraction INTERANNUAL TREND IN CO 2 INCREASE Pg C yr -1

17. ATMOSPHERIC CO 2 INCREASE OVER PAST 1000 YEARS Intergovernmental Panel on Climate Change (IPCC), 2007

18. ANTARCTIC ICE CORE RECORD OF TEMPERATURE AND CO 2 CO 2 and temperature are strongly correlated through glacial-interglacial cycles

19. Questions 1.Comparison of the rates of CO 2 atmospheric accumulation vs. global fossil fuel emission indicates that only 60% of the CO 2 emitted by fossil fuel combustion remains in the atmosphere. Does this mean that atmospheric CO 2 has a lifetime of only a few years? Does this mean that CO 2 would start declining if fossil fuel emissions were to stop tomorrow? 2. A famous politician suggested sarcastically that "we all quit breathing" to reduce the source of CO 2 to the atmosphere. Would that work? Briefly explain.

20. UPTAKE OF CO 2 BY THE OCEANS CO 2 (g) CO 2. H 2 O HCO 3 - + H + HCO 3 - CO 3 2- + H + K H = 3x10 -2 M atm -1 K 1 = 9x10 -7 M K 2 = 7x10 -10 M pK 1 Ocean pH = 8.2 pK 2 CO 2. H 2 O HCO 3 - CO 3 2- OCEAN ATMOSPHERE

21. EQUILIBRIUM PARTITIONING OF CO 2 BETWEEN ATMOSPHERE AND GLOBAL OCEAN Equilibrium for present-day ocean:  only 3% of total inorganic carbon is currently in the atmosphere But CO 2 (g)  [H + ]  F  … positive feedback to increasing CO 2 Pose problem differently: how does a CO 2 addition dN partition between the atmosphere and ocean at equilibrium (whole ocean)?  28% of added CO 2 remains in atmosphere!

22. FURTHER LIMITATION OF CO 2 UPTAKE: SLOW OCEAN TURNOVER (~ 200 years) Inventories in 10 15 m 3 water Flows in 10 15 m 3 yr -1 Uptake by oceanic mixed layer only (V OC = 3.6x10 16 m 3 ) would give f = 0.94 (94% of added CO 2 remains in atmosphere)

23. Observed penetration of fossil fuel CO 2 into the oceans

24. MEAN COMPOSITION OF SEAWATER

25. LIMIT ON OCEAN UPTAKE OF CO 2 : CONSERVATION OF ALKALINITY Equilibrium calculation for [Alk] = 2.3x10 -3 M pCO 2, ppm 100 200 300 400 500 8.6 8.4 8.2 2 3 4 1.4 1.6 1.8 1.9 2.0 2.1 Ocean pH [CO 3 2- ], 10 -4 M [HCO 3 - ], 10 -3 M [CO 2. H 2 O]+[HCO 3 - ] +[CO 3 2- ], 10 -3 M Charge balance in the ocean: [HCO 3 - ] + 2[CO 3 2- ] = [Na + ] + [K + ] + 2[Mg 2+ ] + 2[Ca 2+ ] - [Cl - ] – 2[SO 4 2- ] – [Br - ] The alkalinity [Alk] ≈ [HCO 3 - ] + 2[CO 3 2- ] = 2.3x10 -3 M is the excess base relative to the CO 2 -H 2 O system It is conserved upon addition of CO 2  uptake of CO 2 is limited by the existing supply of CO 3 2- : Increasing Alk requires dissolution of sediments: …which takes place over a time scale of thousands of years CO 2 (g) + CO 3 2 + H 2 O 2HCO 3 - Ca 2+ + CO 3 2- CaCO 3

26. LAND-ATMOSPHERE CARBON CYCLING: MAJOR PROCESSES

27. LAND-ATMOSPHERE CARBON CYCLING: BOX MODEL Inventories in PgC Flows in PgC a -1

28. Reforestation in action: Harvard Forest in Petersham, central Mass. – then and now

29. EVIDENCE FOR LAND UPTAKE OF CO 2 FROM TRENDS IN O 2, 1990-2000

30. Questions 1.The present-day fossil fuel source of CO 2 to the atmosphere is 8 Pg C a -1. 30% of that is removed by uptake by the ocean every year. Assume that this uptake is restricted to the surface ocean, 100-m deep and covering a global area of 3x10 14 m 2. The present-day CO 3 2- concentration in the surface ocean is 2x10 -4 M. What fraction of that CO 3 2- is consumed in a single year of fossil fuel input? 2. Melting of polar icecaps is expected to reduce deep water formation and hence the transfer of CO 2 to the deep ocean. Why? 3. From the standpoint of controlling atmospheric CO 2, is it better to heat your home with a wood stove or by natural gas? 4. You wish to fly from Boston to California on a commercial flight that consumes 100,000 lbs of fuel for the trip. The company offers - as an extra charge on your ticket - to make your personal trip carbon-neutral by planting trees. Does this seem practical, in terms of the number of trees that would need to be planted? And is this a reasonable long-term proposition for mitigating your personal "carbon footprint"?

31. NET UPTAKE OF CO 2 BY TERRESTRIAL BIOSPHERE (1.4 Pg C yr -1 in the 1990s; IPCC [2001]) is a small residual of large atm-bio exchange Gross primary production (GPP): GPP = CO 2 uptake by photosynthesis = 120 PgC yr -1 Net primary production (NPP): NPP = GPP – “autotrophic” respiration by green plants = 60 PgC yr -1 Net ecosystem production (NEP): NEP = NPP – “heterotrophic” respiration by decomposers = 10 PgC yr -1 Net biome production (NBP) NBP = NEP – fires/erosion/harvesting = 1.4 PgC yr -1

32. NET PRIMARY PRODUCTIVITY (NPP): The tropics dominate

33. Observed latitudinal gradient of atmospheric CO 2 shows that the “missing sink” is at northern mid-latitudes …but we still don’t know what continent

34. Improved system of atmospheric observations holds the key to pinpointing where the terrestrial sink is located Combustion sources, fast longitudinal transport complicate sink attribution We need satellite observations of atmospheric CO 2 !

35. FAILED LAUNCH OF OCO-1 - Feb 24, 2009 OCO-2 to launch in July 2014

36. HUMAN INFLUENCE ON THE CARBON CYCLE Natural fluxes in black; anthropogenic contribution (1990s) in red

37. Present-day CO 2 emissions per country… and per capita:

38. Per Capita Fossil Fuel Use since 1950, selected countries USA China India UK

39. So far, CO 2 uptake by land+ocean has increased as emissions have increased land+ ocean atm Ballantyne et al., 2012 Will this continue in the future?

40. PROJECTIONS OF FUTURE CO 2 CONCENTRATIONS [IPCC, 2001]

41. PROJECTED FUTURE TRENDS IN CO 2 UPTAKE BY OCEANS AND TERRESTRIAL BIOSPHERE

42. CO 2 emissions over past decade have followed the most pessimistic scenario actual

43. Questions 1. Dead organisms sedimenting on the ocean floor have calcium carbonate (CaCO 3 ) shells. The sediments contain 9x10 7 Pg C, mostly as CaCO 3. Does the burial of the oxygen in these shells affect atmospheric oxygen? 2. Above-ground nuclear tests in the 1950s injected large amounts of 14 CO 2 in the atmosphere, but atmospheric observations following the nuclear test ban in 1962 showed an exponential decay of 14 CO 2 back to background values on a time scale of 5 years. This shows, according to skeptics, that if we were to shut down fossil fuel emissions then CO 2 would return to natural background values within 5 years. What do you think of this reasoning?