Most abundant elements: oxygen (in solid earth), iron (core), silicon (mantle), hydrogen (oceans), nitrogen, carbon, sulfur

Transcription

1 Most abundant elements: oxygen (in solid earth), iron (core), silicon (mantle), hydrogen (oceans), nitrogen, carbon, sulfur The elemental compostion 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 (small amounts, mainly H, He) Biogeochemical cycling of these elements between the different reservoirs of the Earth system determines the composition of the Earthʼs atmosphere and the evolution of life 1

2 Physical exchange, redox chemistry, biochemistry are involved Outer space meteorites escape gas-water exchange Atmosphere decay photosynthesis Hydrosphere (oceans, rivers, lakes, groundwater) erosion Biosphere assimilation assimilation decay decay Soils burial Lithosphere (Earths crust) run-off subduction Deep Earth (mantle, core) volcanoes Some important nitrogen species in the air: N 2 : main constituent of atmosphere, chemically inert. Collision partner (M) in many atmospheric chemical reactions. N 2 O: strong greenhouse gas, photolysis reactions in stratosphere. NH 3, amines: only small amounts in atmosphere but implicated in nucleation (aerosol particle formation), fertilizers. One of very few basic (as opposed to acidic) compound classes in the atmosphere. NO y = reactive odd nitrogen NO X = NO + NO 2 (lumped together because of rapid interconversion): important in tropospheric oxidation, pollution, ozone chemistry. NO 3 radical: night-time oxidizer in the troposphere. HONO (nitrous acid): important e.g. for OH formation HNO 3 (nitric acid): acid rain, stratospheric clouds. N 2 O 5 : reservoir species. PAN: peroxyacetyl nitrate, important in air pollution. In the bio- and hydrosphere, (and e.g in cloud droplets) nitrate (NO 3- ), nitrite (NO 2- ) and ammonium (NH 4+ ) ions are important N species. Nitrogen content of ecosystems are often computed from carbon content via average C/N mass ratios: 7.9 for terrestial plants 5.7 for marine plants 2

3 ATMOSPHERE N 2 combustion lightning NO oxidation HNO 3 organic N BIOSPHERE assimilation biofixation decay NH 3 /NH 4+ denitri- fication nitrification deposition NO 3- burial weathering LITHOSPHERE 3

4 R. Delmas et al., Nutrient Cycling in Agroecosystems, 48, 51 60, 1997 Main sources of NO x (in Tg N / yr) are fossil fuel combustion 22.0 (15 29) fires 6.7 (3 10) microbial soil emissions 5.5 ( ) lightning 2.0 (1 4) oxidation of biogenic NH ( ) aircraft 0.5 ( ) Stratosphere (from N 2 O) 0.5 ( ) 4

5 Inventories in Tg N Flows in Tg N yr -1 N 2 is very stable (binding energy ca 945 kj/mol). Most nitrogen exists as N 2 in the atmosphere; biosphere and hydrosphere reservoirs are much smaller. Litosphere and atmosphere N stocks are of the same order of magnitude. N cycling within the biosphere and hydrosphere is much larger than the exchange with the atmosphere. Atmospheric lifetime of N is quite long. Note that human activity has significantly increased the rate of N transfer from the atmosphere to the biosphere. Exhange with the lithosphere is slower still. Atmosphere biosphere hydrosphere combination is almost a closed system with regard to nitrogen. Common nitrate (NO 3- ) minerals more soluble in water than carbonate (CO 3 2- ) minerals. partly explains why p(n 2 ) >> p(co 2 ) in the atmosphere. 5

6 Important as source of NO x radicals in stratosphere greenhouse gas IPCC [2001] 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) IPCC [2001] 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) Although a closed budget can be constructed, uncertainties in sources are large! 6

7 Oxygen-only species O( 3 P), O( 1 D), O 2, O 3 Term symbols 3 P and 1 D are related to electronic energy states (quantum mechanics). O( 3 P) = ground-state atomic oxygen and O( 1 D) = electronically excited atomic oxygen (sometimes written as O*). Hydrogen oxygen species H 2 O, OH radical, H 2 O 2 Carbon oxygen species CO CO 2 Carbon-oxygen-hydrogen species H 2 CO 3 Several million different organic compounds ( reduced carbon ), e.g.: CH 4 terpenes 7

8 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 CO 2 O 2 lifetime due to fast cycle: 5000 years Net carbon sink/oxygen source: photosynthesis minus ( less ) respiration orgc O 2 litter orgc decay 8

9 O 2 : 1.2x10 6 Pg O OCEAN O 2 Photosynthesis decay orgc CO 2 runoff Fe 2 O 3 H 2 SO 4 O 2 lifetime due to slow cycle: 3 million years FeS 2 O 2 weathering orgc CO 2 CONTINENT SEDIMENTS burial orgc CO 2 microbes FeS 2 Compression subduction Uplift orgc: 1x10 7 Pg C FeS 2 : 5x10 6 Pg S 9

10 Most of the carbon in the atmosphere is in the form of CO 2. The carbon content of the oceans is much, much larger than that of the atmosphere and biosphere. The carbon content of the litosphere is much larger still. (Recall the low solubility of carbonate minerals.) only a small fraction of the total C is in the atmosphere. The carbon exchange between the atmosphere and biosphere is large. Atmospheric lifetime of C is rather short (a few years), however the effective lifetime of added fossil C can be long ( 100s of years). In a simple box model, the carbon content of the atmosphere is determined only by the total amount of carbon in the atmospherebiosphere system. Exhange with the lithosphere is slower still. Atmosphere biosphere hydrosphere combination is almost a closed system with regard to carbon (or would be in the absence of fossil fuel emissions). Atmosphere partitioning Oceans surface middle deep sediments respiration, photosynthesis soil anthropogenic emission (e.g. burning of fossil fuel) 10

11 Petit et al.,

12 Notice: atmospheric increase is ~50% of fossil fuel emissions large interannual variability due to differing photosynthesis rates Arrows indicate El Nino events IPCC [2001] IPCC [2001] 12

13 ATMOSPHERE OCEAN CO 2 (g) K H = 3x10-2 M atm -1 CO 2. H 2 O K 1 = 9x10-7 M CO 2. H 2 O K 2 = 7x10-10 M HCO 3- HCO H + CO H + pk 1 pk 2 Ocean ph CO 2. H 2 O HCO 3- CO 3 2- Net uptake: CO 2 (g) + CO 3 2-2HCO 3- Atmospheric fraction: varies roughly as [H + ] only 3% of total inorganic carbon is in the atmosphere But CO 2 (g) [H + ] F " positive feedback to increasing CO 2 due to the change in acidity and the contribution of the different dissolved compounds Pose problem differently: how does a CO 2 addition dn partition between the atmosphere and ocean at equilibrium? varies roughly as [H + ] 2 28% of added CO 2 remains in atmosphere!# 13

14 Equilibrium calculation for Alk = 2.25x10-3 M [CO 2. H 2 O]+[HCO 3- ] +[CO 3 2- ], 10-3 M [HCO 3- ], 10-3 M [CO 3 2- ], 10-4 M Ocean ph pco 2, ppm The alkalinity is the excess positive charge in the ocean to be balanced by carbon: Alk = [Na + ] + [K + ] + 2[Mg 2+ ] + 2[Ca 2+ ] - [Cl - ] 2[SO 4 2- ] [Br - ] = [HCO 3- ] + 2[CO 3 2- ] 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: CaCO 3 Ca 2+ + CO 3 2- which takes place over a time scale of thousands of years Inventories in m 3 water Flows in 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) 14

15 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 Atmospheric CO 2 observations show that the net uptake is at northern midlatitudes but cannot resolve American vs. Eurasian contributions 15

16 Inventories in PgC Flows in PgC yr -1 Time scales are short net uptake from reforestation is transitory Inventories in PgC Flows in PgC yr -1 16

17 IPCC [2001] 17

18 H 2 S, (CH 3 ) 2 S, (CH 3 ) 2 S 2 : emitted by algae + the decay of organic matter in the biosphere OCS (in some books: COS), CS 2 : emitted by oceans, volcanoes SO 2 : emitted by volcanoes and industrial processes such as fossil fuel burning SO 3, H 2 SO 4 : oxidation products of SO 2, cause acid rain and needed for nucleation (formation of aerosol particles from gas-phase molecules). Note: SO 4 refers to all sulfur(vi) compounds 18

19 Inventories in Tg(S). Flow magnitudes not shown. 19

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