QUESTIONS Atmospheric CO2 has a lifetime (turnover time) of 5 years against photosynthesis and a lifetime of 10 years against dissolution in the oceans. What is its overall atmospheric lifetime? 2. Are the following loss processes first-order (linear with concentration?    A. Uptake of CO2 by the biosphere    B. Photolysis of gases in the stratosphere    C. Scavenging of aerosol particles by precipitation 3. The Montreal protocol banned worldwide production of CFC-12 as of 1996. CFC-12 is removed from the atmosphere by photolysis in the stratosphere and the resulting CFC-12 atmospheric lifetime is 100 years. Assuming full compliance with the Montreal protocol, how long will it take for CFC-12 concentrations to drop to half of present-day values? 4. Consider a 2-box model for the atmosphere where one box is the troposphere (1000-150 hPa) and the other the stratosphere (150-1 hPa).  The lifetime of air in the stratosphere is 2 years.  What is the lifetime of air in the troposphere? 5. . Of the simple models we discussed in chapter 3, which one would be best suited for addressing the following problems?   a. Is the observed rise of atmospheric CO2 concentrations consistent with the known rate of CO2 emission from fossil fuel combustion?  b. Large amounts of radioactive particles are released to the atmosphere in a nuclear power plant accident. Which areas will be affected by this radioactive plume?  c. An air pollution monitoring station in Fort Collins suddenly detects high concentrations of a toxic gas. Where is this gas coming from?  

CHAPTER 6: GEOCHEMICAL CYCLES THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS 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

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

HISTORY OF EARTH’S ATMOSPHERE N2 CO2 H2O O2 O2 reaches current levels; life invades continents oceans form CO2 dissolves Life forms in oceans Onset of photosynthesis Outgassing 4.5 Gy B.P 4 Gy B.P. 3.5 Gy B.P. 0.4 Gy B.P. present

COMPARING THE ATMOSPHERES OF EARTH, VENUS, AND MARS Radius (km) 6100 6400 3400 Surface pressure (atm) 91 1 0.007 CO2 (mol/mol) 0.96 3x10-4 0.95 N2 (mol/mol) 3.4x10-2 0.78 2.7x10-2 O2 (mol/mol) 6.9x10-5 0.21 1.3x10-3 H2O (atm, mol/mol) 3x10-3 1x10-2

EVOLUTION OF O2 AND O3 IN EARTH’S ATMOSPHERE

Atmospheric Composition (average) 1 ppm= 1x10-6 red = increased by human activity Gas Mole fraction Nitrogen (N2) 0.78 Oxygen (O2) 0.21 Water (H2O) 0.04 to < 5x10-3; 4x10-6 -strat Argon (Ar) 0.0093 Carbon Dioxide (CO2) 370x10-6 (date: 2000) Neon (Ne) 18.2x10-6 Ozone (O3) ¶ 0.02x10-6 to 10x10 –6 Helium (He) 5.2x10-6 Methane (CH4) 1.7x10-6 Krypton (Kr) 1.1x10-6 Hydrogen (H2) 0.55x10-6 Nitrous Oxide (N2O) 0.32x10-6 Carbon Monoxide (CO) 0.03x10-6 to 0.3x10-6 Chlorofluorocarbons 3.0x10-9 Carbonyl Sulfide (COS) 0.1x10-9 ¶ Ozone has increased in the troposphere, but decreased in the stratosphere.

NOAA Greenhouse Gas records Montreal Protocol- 1996

Nitrogen: Nitrogen is a major component of the atmosphere, but an essential nutrient in short supply to living organisms. Why is "fixed" nitrogen in short supply? Why does it stay in the atmosphere at all? OXIDATION STATES OF NITROGEN N has 5 electrons in valence shell a9 oxidation states from –3 to +5 Increasing oxidation number (oxidation reactions) Fixed N = odd N -3 +1 +2 +3 +4 +5 NH3 Ammonia NH4+ Ammonium R1N(R2)R3 Organic N N2 N2O Nitrous oxide NO Nitric oxide HONO Nitrous acid NO2- Nitrite NO2 Nitrogen dioxide HNO3 Nitric acid NO3- Nitrate N2O5 Nitrogen pentoxide free radical free radical Decreasing oxidation number (reduction reactions)

THE NITROGEN CYCLE: MAJOR PROCESSES combustion lightning free radical ATMOSPHERE N2 NO oxidation HNO3 denitri- fication biofixation deposition orgN decay NH3/NH4+ NO3- BIOSPHERE assimilation nitrification weathering burial LITHOSPHERE "fixed" or "odd" N is less stable globally than N2

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

N2O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION Important as source of NOx radicals in stratosphere greenhouse gas IPCC [2001]

N2O versus depth in the Greenland Ice sheet. Constraints on N2O budget changes since pre-industrial time from new firn air and ice core isotope measurements S. Bernard, T. R¨ockmann, J. Kaiser, J.-M. Barnola, H. Fischer, T. Blunier, and J. Chappellaz, Atmos. Chem. Phys., 6, 493–503, 2006 N2O in the atmosphere

PRESENT-DAY GLOBAL BUDGET OF ATMOSPHERIC N2O SOURCES (Tg N yr-1) 18 (7 – 37) Natural 10 (5 – 16) Ocean 3 (1 - 5) Tropical soils 4 (3 – 6) Temperate soils 2 (1 – 4) Anthropogenic 8 (2 – 21) Agricultural soils 4 (1 – 15) Livestock 2 (1 – 3) Industrial 1 (1 – 2) SINK (Tg N yr-1) Photolysis and oxidation in stratosphere 12 (9 – 16) ACCUMULATION (Tg N yr-1) 4 (3 – 5) IPCC [2001] Although a closed budget can be constructed, uncertainties in sources are large! (N2O atm mass = 5.13 1018 kg x 3.1 10-7 x28/29 = 1535 Tg )

BOX MODEL OF THE N2O CYCLE Inventories in Tg N Flows in Tg N yr-1 3 6 8 1.53 103 N2O

SULFUR CYCLE Most sulfur is tied up in sediments and soils. There are large fluxes to the atmosphere, but with short atmospheric lifetimes, the atmospheric S burden is small. SO2: Anthropogenic (fossil fuel combustion) source comparable to natural sources (soils, sediments, volcanoes) Sulfur is oxidized in the atmosphere: SO2 ---- > H2SO4 S(+IV) S(+VI) Sulfate is an important contributor to acidity of precipitation. Sulfuric acid has a low Pvap and thus partitions primarily to aerosol/aqueous phase Strongly perturbed by human activities!

FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE Source of O2: photosynthesis nCO2 + nH2O g (CH2O)n + nO2 Sink: respiration/decay (CH2O)n + nO2 g nCO2 + nH2O CO2 O2 lifetime: 5000 years Photosynthesis less respiration O2 orgC orgC decay litter

…however, abundance of organic carbon in biosphere/soil/ocean reservoirs is too small to control atmospheric O2 levels

SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE O2 lifetime: 3 million years O2: 1.2x106 Pg O CO2 O2 Photosynthesis decay Fe2O3 H2SO4 runoff weathering CO2 O2 FeS2 orgC OCEAN CONTINENT orgC Uplift burial CO2 orgC: 1x107 Pg C FeS2: 5x106 Pg S microbes orgC FeS2 SEDIMENTS Compression subduction

ATMOSPHERIC CARBON Unreactive Carbon: CO2: GHG (more to follow…) (tATM~20 yrs) Reactive Carbon: CH4: GHG, important in oxidant chemistry (tATM~9 yrs) CO: important in oxidant chemistry (later…) (tATM~2 mos) NMHCs: source of CO, oxidant chemistry (tATM~sec-mos) Black Carbon: radiatively important (tATM~days) CH4 (C= -IV) CO (C= +II) CO2 (C= +IV) Oxidation

ATMOSPHERIC METHANE (CH4) Important: as a GHG and as it affects the oxidizing capacity of troposphere

SOURCES OF ATMOSPHERIC METHANE BIOMASS BURNING 20 ANIMALS 90 WETLANDS 180 LANDFILLS 50 GLOBAL METHANE SOURCES (Tg CH4 yr-1) GAS 60 TERMITES 25 COAL 40 RICE 85

SINKS OF ATMOSPHERIC METHANE Transport to the Stratosphere Only a few percent, rapidly destroyed  BUT the most important source of water vapour in the dry stratosphere II. Oxidation CH4 + OH  CH3O2CO + other products O2 Lifetime ~ 9 years

IPCC [2001] Projections of Future CH4 Emissions (Tg CH4) to 2050 ATMOSPHERIC CH4: PAST TRENDS, FUTURE PREDICTIONS IPCC [2001] Projections of Future CH4 Emissions (Tg CH4) to 2050 Variations of CH4 Concentration (ppbv) Over the Past 1000 years [Etheridge et al., 1998] Scenarios 900 A1B A1T A1F1 A2 B1 B2 IS92a 1600 1400 800 1200 700 1000 800 600 1000 1500 2000 2000 2020 2040 Year Year

Pinatubo Fires Low T / precip