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Life ’ s control of the atmosphere and the role of the atmosphere in affecting the surficial Earth

Atmosphere. Life ’ s control of the atmosphere and the role of the atmosphere in affecting the surficial Earth. Topics. Structure (vertical) Air movement Vertical Horizontal Composition Residence time and reactivity of constituents Aerosols Trace gases Deposition. Isabel 2003.

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Life ’ s control of the atmosphere and the role of the atmosphere in affecting the surficial Earth

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  1. Atmosphere Life’s control of the atmosphere and the role of the atmosphere in affecting the surficial Earth

  2. Topics • Structure (vertical) • Air movement • Vertical • Horizontal • Composition • Residence time and reactivity of constituents • Aerosols • Trace gases • Deposition Isabel 2003

  3. Vertical Structure • Most gas is in the troposphere, which is fairly well mixed and affects most life • Pressure from weight of gas • Heating from bottom up • Stratosphere heating due to ozone • Vertical mixing due to rising tropical air and large-scale winds

  4. Jet Stream

  5. Sunlight Sunlight Equator Vernal equinox (sun aims directly at equator) 60° 60° 60° 60° Winter solstice (northern hemisphere tilts away from the sun) 30° 30° 30° 30° Equator Equator Equator Equator 30° 30° 30° 30° Sun Summer solstice (northern hemisphere tilts toward the sun) Autumnal equinox (sun aims directly at equator) 29.05 Earth’s Orbit, Rotation, and Climate 23.5°

  6. Global Air Circulation • Direct cells due to rising (equator) and sinking air (poles) • Indirect cells driven by direct cells • Rising warm air at equator cools, dropping moisture • Dry air sinks at ~30° latitude (locations of large deserts) • Spinning of Earth moves air right (N) and left (S) leading to easterly and westerly flows

  7. Potential Surface winds

  8. Potential Surface winds on a spinning earth

  9. Resulting Surface Winds • Coriolis due to Earth spinning leads to general wind patterns moving from East and West. • Troposphere completely mixes quickly (few months), so gases with hemisphere differences must have local sources or sinks • Primary winds affecting climate (and ocean circulation • Why do storms circulate?

  10. Surface Winds

  11. Composition of the Atmosphere • Major (%) • N2 (78.1%); O2 (20.9%); Ar (0.93%) • ppm (10-6) • CO2 (360+); Ne (18); He (5.2); CH4 (1.8); Kr (1.1) • ppb (10-9) • H2, N2O, Xe • ppt (10-12) • Other traces gases such a chlorofluorocarbons • Total mass of atmosphere = 514 x 1019 g • Gases at high concentrations are least reactive

  12. Residence Time (RT) • RT refers to the average amount of time that a constituent remains in the atmosphere • Examples (approximate) • Water - 9 days • CO2 - 5 years • O2 - 4000 years • N2 - 20 million years • Since the atmosphere mixes in about 1-2 years, some gases are well mixed (long RT), whereas others are not (short RT)

  13. Residence Time (RT) Mean RT=Mass/Flux =g/(g/time) =time • EXAMPLE • CO2 in atmosphere = 2.8 x 1018 g • CO2 consumption by plants = 220 x 1015 g yr-1 • CO2 consumption by ocean = 330 x 1015 g yr-1 • Total consumption = 550 x 1015 g yr-1 • MRT = 2.8 ÷ 0.55 = 5.1 years • All based on ‘Steady State’ Conditions

  14. Residence Time (RT) Reactive gases exhibit heterogeneous distributions (highly variable). They also remain at low concentrations in the atmosphere.

  15. Aerosols • Particles in the atmosphere • Primary aerosols are typical particles • Soil and volcanic dust, seasalt, forest fires ash • Secondary aerosols are produced from gases that react in the atmosphere • SO2 H2SO4 • 2NH3 + H2SO4  (NH4)2SO4 • Gas-gas reactions - homogeneous • Gas-particle reactions - heterogenous • Anthropogenic aerosols are important (10-20%) • Soot, industrial particles, sulfates, and nitrates

  16. Aerosols (con’t) • Small particles last longer and are more numerous, but make up small mass • Land: soil dust and anthropogenic particles • Sea: seasalt & small dust particles • Therefore, can use chemical composition to deduce source

  17. Atmospheric Deposition • Deposition patterns reflect source

  18. Atmospheric Deposition Calcium reflects desert dust Sodium reflects sea sat

  19. Aerosols (con’t) • Dust from the Sahara is “choking” Caribbean corals (with fungal infections) and fertilizing algae that overgrow corals (Shinn, et al. 2000. Geophys. Res. Lett., 27: 3029-3033).

  20. Aerosols (con’t) • Aerosols increase albedo (cooling) Clouds with aerosols have many small droplets that reflect light Clouds with a few large droplets allow light to pass. • 2003 study predicts large decrease in air-borne particles by 2090 leading to much more warming

  21. Anthropogenic Loading SO2 Emissions Constituent Human:Natural Lead 333 Cadmium 20 Copper 14 Arsenic 4 Sulfur 2 Dust 125 Petroleum (oceans) 6

  22. Tropospheric Reactions • Major constituents are relatively unreactive • N2 is almost inert (20 million year RT) • Even N molecules are poorly reactive (N2, N2O) • Odd N molecules are more reactive (NH3 and NO) • N2 is removed by N2 fixation1 and returned by denitrification2, which are similar in rate (it would take N2 fixation 20 million years to remove N2) 1 N2 + 8H+ + 8e- +  2NH3 + H2 (requires 16ATP) 2 5CH2O + 4H+ + 4NO3-  2N2 + 5CO2 + 7H2O

  23. Tropospheric Reactions • Major constituents (con’t) • O2 controlled by long-term burial of OM (4000 year RT) • If O2 is produced by photosynthesis, then is removed by respiration • If plants never decompose, then net gain in O2 • At current rate of geologic exposure of reduced materials, O2 would disappear in 2 million years • CO2 exhibits seasonal behavior (5 year RT) • Also is non steady state with sources exceeding sinks

  24. Trace Gases • Trace gases (usually reduced) are produced by microbes in wetlands and soils, and by anthropogenic activities • Methane (CH4) - anaerobic wetlands • Nitrous oxide (N2O) - aerobic soils and wetlands • Ammonia (NH3) - wetlands and feedlots • NOx (NO, NO2) - soils and combustion products • Reduced S species • (DMS)-oceans and wetlands • Hydrogen sulfide (H2S) - wetlands

  25. Ozone and Hydroxyl Radical • Trace gases are mostly removed by oxidation reactions within the troposphere • O2 is not very reactive, but trace gases react with ozone (O3) and hydroxyl radical (OH) -Ozone Production in the Stratosphere O2 + hv(ultraviolet light)  O + O O + O2 O3 -Ozone Production in the Troposphere NO2(nitrogen dioxide) + O2 + hv  NO(nitric oxide) + O3

  26. Ozone and Hydroxyl Radical • Hydroxyl radicals are produced from ozone O3 + hv O2 + O(1D)(excited oxygen atom) O(1D) + H2O  2OH • Hydroxyl radicals can form other short-lived oxidizing molecules • Hydrogen peroxide (H2O2) • Hydrogen dioxide (HO2)

  27. Stratospheric Ozone • Ultra-violet light not only aids in the formation of ozone, but also destroys ozone (uv destroyed in formation and destruction; barrier to uv) O3 + hv O2 + O O + O3 O2 + O2 • Chlorofluorocarbons (freons) catalyze O3 loss South Pole (ozone hole)

  28. Atmospheric Deposition • Atmospheric constituents removed to Earth surface • Modes • Rainfall (wetfall when referring to nutrient deposition) • Rainout - nucleation processes in clouds • Washout - below clouds, i.e., scavenging of aerosols and dissolution of gases as drops fall • Dryfall (gravitational settling of particles) • Direct absorption (plants consume some gases directly) • CO2, NH3, S gases

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