The Atmosphere

1. Reading: Chap 1.1, 1.2, 1.4 The Atmosphere Characterized by • Chemical composition • Major components: N2, O2, Ar • Trace gases • Aerosols • Physical phenomena • Solar radiation, terrestrial thermal radiation and energy balance • Atmospheric zones • Atmospheric density • Atmospheric pressure

2. Chemical Composition • Atmosphere is a mixture of gases and particulate-phase substances • Most abundant • Nitrogen (78 %) • Oxygen (21 %) • Trace gases and aerosols make up approximately 1 % (Table 1.1) • Some are present in “constant” concentrations • N2, O2 and noble gases

3. Chemical Composition • Others vary temporally and spatially: • Water vapor (H2O) • Carbon dioxide (CO2), Carbon monoxide (CO) • Ozone (O3) • Methane (CH4) • Nitrogen oxides (nitrous oxide (N2O); nitric oxide (NO); nitrogen dioxide (NO2)) • Ammonia (NH3) • Formaldehyde (HCHO) • Sulfur dioxide (SO2) • Reduced sulfur compounds (H2S, COS, CS2, (CH3)2S) • Odd hydrogen species (OH,HO2,H2O2) • Particulate-phase species • Nitrate (NO3-), Ammonium (NH4+), Sulfate (SO4 2+) Why do they vary? How do they vary?

4. Nitrogen (N2) • Most abundant atmospheric gas • Limited direct role in atmospheric and life processes • Precursor for the formation of nitrate used by plants to synthesize proteins • Results from atmospheric and symbiotic biological processes • Nitrous oxide (N2O) • Nitric oxide (NO) • Nitrogen dioxide (NO2) • Dintrogen pentoxide (N2O5) • Nitrate radical (NO3·) What are these atmospheric and biological processes?

5. Oxygen (O2) • Essential for metabolism; Required for the evolution of life • Precursor for the production of stratospheric O3; formation of the O3 layer made life possible • O3: • background surface levels (~ 20 ppbv); peak levels (8-10 ppmv) occur in middle stratosphere • Absorbs UV and thermal energy Figure 1.1

6. Stratospheric O3 Figure 1.2 Total column O3 (vertical sum) • Varies seasonally and latitudinally • Highest production occurs in the tropics • Highest concentrations at poles • Significant transport Why?

7. Carbon Dioxide (CO2) • Atmospheric concentration of 0.037% (370 ppmv) • Raw material for photosynthesis • Thermal absorber and major greenhouse gas Noble Gases (Ar, Ne, He, Kr, Xe) • Ar concentration of 0.934% • Not participating in atmospheric reactions

8. Water (H2O) • Present as a solid, liquid and gas; phase changes are a major factor in weather phenomenon • Has significant atmospheric effects • Water vapor concentrations highly variable (0.1- 30,000 ppmv) • Water vapor is a thermal absorber and major greenhouse gas; characterized by phase changes at ambient temperatures www.raindropimage.com www.edholden.com www.ux1.eiu.edu

9. Clouds • Form as warm, moist air rises and condenses • Large air mass of tiny water droplets • Droplets must grow a million fold to produce rain • Light scattering appears to give these form • By reflecting sunlight are primarily responsible for the earth’s albedo • Absorb thermal energy and retard its flow to space • Associated air mass an agent of water and energy movement in the atmosphere

10. Where did all the O2 come from? How did the earth's atmosphere get the way it is? (a theory) • Earth formed (4.6 billions years ago) with no atmosphere or lost whatever atmosphere it might have had very early in its history • Some geophysical process is needed that emits gases so that an atmosphere can form • Source of gaseous emissions is volcanism. Likely only a very small fraction of these molecules lost to space. • Paradox - composition of volcanic emissions is very different from that of present atmosphere; volcanic emissions: 85% H2O vapor, 10% CO2, a few percent of nitrogen and sulfur compounds, traces of noble gases and other species, O2 is absent.

11. burial of carbon Conceptualization of UV absorption allowing development of photosynthesis • As the surface cooled, water condensed • CO2 dissolved in the newly formed ocean and precipitated (limestone, dolomite rock) • Evolution of life was preceded by formation of organic species • Organic matter produced by photosynthesis may be removed before re-oxidation, i.e. fossilized. • Certain life-forms in the ocean developed the ability to photo-synthetically produce organic molecules and free O2.

12. Build-up of Atmospheric Oxygen • Each fossilized C-atom originating from CO2 that is removed from the atmosphere is essentially replaced by a molecule of O2 • The fraction of CO2 in the atmosphere would be reduced and the fraction of O2 would increase • Initially O2 increase is slow since all of the surface minerals would be slowly oxidized, gradually O2 builds up • N2 is not very reactive; hence, it accumulated and became a larger fraction of the earth's gas composition

13. Are we in danger of depleting oxygen in the atmosphere if we continue to burn fossil fuels? Importance of Stratospheric Ozone • As O2 builds in the atmosphere, O3 forms and reduces the penetration of UV light to the surface • Photosynthesis can then take place on land O2 + hn 2 O O2 + O + M  O3 O3 +hn O2 + O burial of carbon

14. What is “black body”? What happened to the difference? Figure 1.4 What are the features of the spectrum? Solar Radiation • Source of most of the earth’s energy • Emitted by the sun at an effective black body temperature of 6000 oK • Received by the earth at a constant rate • External to the earth (1370 W/m2/s) • Per unit earth surface area (343 W/m2/s)

15. Solar Spectrum • Most energy in the 0.15- 4 m region • Half of this energy in visible light spectrum; peaks in the green at 0.49 m • Partially absorbed by atmospheric gases • UV radiation < 0.18 m by O2 at 100 km; 0.2-0.3 m by O3 below 60 km • Infrared radiation H2O, CO2 Earth’s Albedo • Ability of the atmosphere and earth surfaces to reflect sunlight; varies regionally/seasonally • Averages ~ 30%; primarily due to clouds (55%), the cloud-free atmosphere (23%), and earth surfaces (22%)

16. Figure 1.5 What are the features of the spectrum? Thermal Radiation • Earth absorbs solar radiation and re-emits longer infra-red wavelengths • Earth radiates as black body at 290oK

17. Thermal Emissions • Emission is primarily in the 1-30 m spectral region • Peak emission at 11 m • Significant absorption by H2O, CO2, and other greenhouse gases • Significant transmission through the “atmospheric window”(~ 7-13 m) www.atmosphere.mpg.de/enid/252.html

18. /s /s /s What happened to the difference? What if there is no absorption? 390 W/m2/s Earth’s Energy Balance

19. Surface Air Temperatures • Average surface air temperature (~ 15oC or 59 oF) • Vary regionally due to the unequal distribution of solar radiation • The difference results in energy transport by atmosphere and ocean currents Vertical Temperature Distribution • Significant temperature changes with height occur • Vertical temperature patterns describe atmospheric zones or layers

20. Vertical Temperatures and Zones • Troposphere • Lowest layer of atmosphere • Temperature decreases with height on average – 6.5 oC/km • Depth varies from 8-18 km • Characterized by vertical and horizontal air motion; Location of all "weather" phenomena • Characterized by 2 regions • Planetary boundary layer(~ 1 km depth) • Free troposphere

21. Vertical Temperatures and Zones • Stratopause • Isothermal conditions • Forms boundary between stratosphere & mesosphere • Tropopause • Layer of air immediately above troposphere • Temperature is isothermal • Varies in depth

22. Vertical Temperatures and Zones • Stratosphere • Temperature increases with height to altitude of 45-55 km • Very stable region with little vertical mixing of air • Few clouds/no weather • Warmer temperatures due to absorption of UV radiation • Complex chemistry involving NO, OH., NH3, O, O2, O3, Cl + other species.   • Chapman Reactions: • O+ O2 + M  O3 + M • O + O+ M  O2 + M • O3 + h O2 + O + Heat • for h corresponding to 0.24  • 0.30 m

23. Vertical Temperatures and Zones • Mesosphere • Temperature decreases with height up to an altitude of 85 km • Coldest region of atmosphere • Rapid vertical mixing • Photo dissociation of oxygen:   • O2 + h 2O + Heat   • for h corresponding to 0.18  0.24 m • Chapman Reactions: • O+ O2 + M  O3 + M • O + O+ M  O2 + M • O3 + h O2 + O + Heat • for h corresponding to 0.24  0.30 ~m

24. Thermosphere • Extends from 90-95 km to ~ 1000 km • High thermodynamic temperatures (~1200 oC) • Solar energy absorbed by N2 & O2 • Results in photo-ionization • Ionized layer called the ionosphere • Forms auroras • Reflects radio signals Vertical Temperatures and Zones

25. Aurora in Fairbanks, Alaska Why different colors? http://www.geo.mtu.edu/weather/aurora/ Aurora Borealis

26. How does density change wrt elevation? What’s the role of density in air pollution? Atmospheric Density • Mass of atmospheric molecules per unit volume of air • Decreases exponentially with height • Most (80-90%) atmospheric mass below 12 km; 99% below 33 km Figure 1.7

27. What are the parameters that affect pressure? How does it change wrt height? Why does it change in that way? Atmospheric Pressure • Force applied to a surface as a result of the collision of molecules with it • Maximum values at sea level • 760 mmHg, 29.92 inHg, 14.7 PSI, 1.012325 x 105 Pa, 1013 mbars • Small surface differences occur as a result of temperature and density differences

28. Quick Reflection