The State of the Atmosphere is affected by the Biosphere, and vice versa. This lecture will focus on the chemical composition of the atmosphere and

Transcription

1 The State of the Atmosphere is affected by the Biosphere, and vice versa. This lecture will focus on the chemical composition of the atmosphere and study how the biosphere has changed it and continues to change it. The thrust of this lecture will be on the Breathing of the Biophsere 1

2 There is humor in our science, e.g. New Yorker cartoon 2

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4 The state of the climate/weather affects the rate processes by which trace gases are produced and consumed by plants and microbes. Conversely, the amount of biogenic material, produced and consumed by plants and microbes, affects the state of the atmosphere. Here is yet another feedback! 4

5 It is big questions like this that drive scientists and their curiosity to learn more. Can you think of some additional questions? 5

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7 The atmosphere is a thin layer across the top of the planet. 99.9% of it is within 50 km, compared to the 6300 km radius of the Earth. On clear days we view the sky as blue. 7

8 Constituents in the atmosphere and their relative fractions.. The atmosphere is dominated by N 2 and inert gas. Oxygen levels remain high (21%) and are out of equilibrium, because biological production meets biological and chemical consumption, consequently O2 levels remain in steady state. 8

9 James Lovelock is the originator of the Gaia Hypothesis Recommended Reading: Gaia, a new look at life on earth / J. E. Lovelock. Lovelock, James, 1919 Oxford ; New York : Oxford University Press, LocationCall No.Status Main (Gardner) Stacks QH313.L68 c.2 AVAILABLE I got to meet Lovelock circa 1982 when I was a postdoc in Oak Ridge, TN and hear him describe his recent book and new and evolving ideas. It started to change my thinking, as I had come from an agricultural and atmospheric science background. The field of Earth System Science was brand new and evolving quickly. His idea that life affected the state and evolution of the chemical composition of the atmosphere resonated with me and made sense. 9

10 A key feature of the presence of life is that the concentrations of reactive gases like Oxygen and Methane remain higher than they would be if equilibrium chemistry was able to proceed. The emission of these gases by life sustains their elevated values. 10

11 A fundamental question we all should know the answer to. But for those who are non science majors this is a great starting point 11

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13 content/uploads/2010/12/blue sky night sky.jpg 13

14 The composition of the atmosphere has close connections with the biosphere, and vice versa, as well with poetry and literature. These connections will be explored in this lecture 14

15 In Beijing and other Megacities, humans are injecting so much pollution and pollution forming chemicals into the sky that the sky is not blue and is opaque with smog. Unless we understand the biosphere and try and rectify emissions of pollution into the sky this can be the fate for many of us and already is in much of China, India, Mexico City, etc 15

16 Most of the Atmosphere is dinitrogen and it is opaque to sunlight and longwave, terrestrial energy. Greenhouse gases are important because they absorb longwave radiation, reradiate it back to the surface and warm the surface to hospitable temperatures; longwave energy interacts with molecules like H2O, CH4, CO2, O3, causing them to resonate in the forms of vibration, rotation, stretching 16

17 There are 4 major zones: 1) the Troposphere, where we live and where there is Weather; 2) the Stratosphere, where there is the ozone layer, where jets fly and is stable, hence the narrow contrails we see. The uppermost layers are the 3) Mesosphere and the 4) Thermosphere where there is less than 0.1% of the atmosphere, based on pressure. Most important: is that 99.9% of the atmosphere is below 50 km. Temperature decreases with height in the Troposphere and it Increases with height in the Stratosphere. Zones where temperature decreases with height allows convection and mixing to occur. Zones where temperature increases with height suppress convection and mixing and are more stable. The stratosphere is stably stratified and explains why jet contrails don t dissipate fast. Why is the Temperate warm in the stratosphere? This is where the stratospheric ozone layer resides. The absorption of shortwave, high energy ultraviolet light causes the temperature of this region to be elevated. The temperature of the Thermosphere may be high, but there is so little atmosphere with enough heat capacity, I suspect you would not be able to go there shirtless. 17

18 The pressure decreases exponentially with height. One is at about one half of the atmosphere at about 5.5 km, about 18,000 feet, like many mountains in the Andes. If you climbed Denali (> 20,000 ft) in Alaska you would be above one half of the atmosphere 18

19 If we are to understand the composition of different gases in the atmosphere we must first be aware of the Law of Partial Pressures 19

20 What are the Units?? 20

21 How can we weigh the atmosphere? We can use the laws of physics to compute the mass of the atmosphere by knowing the surface area of the planet, pressure at the surface and the acceleration of gravity. Remember Pressure is Force per area and Force is Mass times acceleration. So Pressure is Mass of the Atmosphere times the acceleration of gravity (9.8 m s 2 ) divided by the Surface area of the planet. So we can solve for Mass 21

22 Here is the math kpa/9.80 *4 * pi * ( ^3)^2 = ^18 kg 22

23 So if we know the mixing ratio of a gas, like CO2, we can also compute its mass in the atmosphere. Why do these simple computations? In this lecture and those to follow I want you to be able to interconvert information on the atmospheric burden of gases like CO2 in terms of mass vs their mixing ratio. This will help you better understand if we release x Pg C in the next year or decade what that will do to the mixing ratio. With this equation, if we know the net flux of CO2 into the atmosphere we can compute future concentrations 23

24 Revisit Principle of Conservation of Mass as it defines how Concentrations in the atmosphere change with time. It is due to a Flux Convergence/Divergence. In other words how Fluxes differ across the top and bottom of the control volume. 24

25 Definition of Turnover Time 25

26 Definition of Flux and Flux Density. A visual example is the number of cars passing some reference point per day. 26

27 Here is a conceptual overview diagram of the various gases that have either biospheric sinks or sources. Also shown are the general flux density magnitudes. Different gases are associated with vegetation, with aerobic and oxic soils, wetlands and anaerobic or anoxic soils. 27

28 A resistance network, with an analogy to an electronic circuit, is used to quantify and describe the biophysical controls of trace gas exchange Fluxes tend to be a balance between demand and supply To estimate Flux rates we can define the biosphere as a networks of resistors acting across a current defined by the difference in the trace gas mixing ratio in the atmosphere and at the surface. Resistors in the network include those associated with turbulence and mixing in the atmosphere, with the development of a boundary layer at the interface between plants/soil/atmosphere. Then there are biological barriers through which gases pass. These are the pores on leaves, called stomata, which are active and open and close, depending on environmental and biological conditions 28

29 The CO2 Molecule. It is 44 g per mole. 29

30 CO2 absorbs longwave, infrared radiation. This property helps explain why and how CO2 is an effective greenhouse gas. Its major IR absorption wave numbers (2350 and 667 cm 1) or wavelengths (4.25 and um) correspond with the spectrum of terrestrial longwave energy that is emitted from the surface (~ 298 K) according to Planck s law. CO2 molecules will absorb this outgoing energy, then re radiate it back to the surface (and outward). It is this re radiation of longwave energy that affects the surface radiation balance, positively, causing the temperature of the surface to be warmer than the radiative temperature of the planet, allowing water to be liquid and life to flourish. Too much CO 2, like on Venus, can cause the surface to be so hot that it can melt lead. 30

31 The change in [CO2] is due to the imbalance between sources and sinks. Here are the major sources and sinks in action. We will cover the actual fluxes in the future lecture on the global carbon balance. 31

32 The is the Mauna Loa Station in Hawaii where Dave Keeling started the long record of CO2 observations. It is a remote location in the Pacific Ocean, where CO2 can be well mixed and produce a good representative measure of the world s CO2. Visit the web site and take a virtual tour of the observatory and look at some of the records 32

33 This is the iconic figure of how the burden of CO 2 has changed in our atmosphere over the past 50+ years. It is a direct measure of the breathing of the biosphere and provide evidence how carbon dioxide from fossil fuel emissions is accumulating in our atmosphere. Go to the NOAA web site, down load the data and plot it yourself

34 Most land is in the northern hemisphere, so during the northern hemisphere summer the sink due to photosynthesis out paces sources, so there is a seasonal draw down in CO2 34

35 Look at amplitude in the north and southern hemisphere..look at phase difference in peaks and troughs. The greatest seasonal amplitudes of CO2 are towards the North pole. Amplitudes are much smaller in the southern hemisphere because it is dominated by oceans, which experience less dramatic swings in carbon uptake and release with the change in the growing season. 35

36 Politicians are always saying there is no proof we are changing the climate through changing the composition of the CO2 in the atmosphere ; Ben Carson, a 2016 presidential candidate said it a couple of days ago. Here is the Proof. Fossil fuels are plant based so their emission reflects the isotopic signature of C 3 photosynthesis. These plants prefer molecules of 12 CO 2 over heavier isotopes consisting of 13 CO 2. Over time the plant retain an isotopic signal of about 25 parts per mill. In the preindustrial age the atmosphere maintained an isotopic signal of about 6.4 per mill. So the combustion of fossil fuel with a 25 parts per mill signal has diluted the signal of the atmosphere since the industrial age. The isotopic record is a smoky gun evidence that fossil fuel combustion is changing the CO2 burden of the atmosphere. δ13c = Rsample Rstandard x 1000 Rstandard where R = 13C/12C 36

37 PeeDee Belimnite. Example of how the stable isotope ratio changes with the delta notation, that is commonly used.. Bottom line, as del 13C becomes more negative, there is less 13 C relative to 12 C in the material, making it isotopically lighter In general there is about 1% 13 C in the air compared to 12 C. 37

38 This figure bookends 3 Eras of CO 2 change over the last 800k years. Part A is shows how CO 2 rose and fell during periods of inter glaciation and glaciation. Part B sees the rise in CO 2 as societies form after the Dark Age and transition into the Industrial Era. Part C shows the more rapid rise as our energy and economy became vibrant and carbonized We will look at each of these in more detail. 38

39 Is CO2 greater day or night?; Winter or Summer 39

40 Properties. 16 g/mole, atmospheric lifetime is about 12 years 40

41 Methane also absorbed outgoing terrestrial radiation in spectral lines associated with outgoing, longwave radiation cm 1; 3321 nm 1300 cm 1; 7692 nm A kilogram of methane is 25 times more effective than a kilogram of CO2 in absorbing IR radiation over a 100 time scale. The greenhouse warming potential considers the spectral bands of IR absorption and the lifetime of the chemical compound in the atmosphere. 41

42 Methane concentrations have increased rapidly over the past 200 years, due to human associated activities. Today, methane levels exceed 1750 ppb 42

43 ATMOSPHERIC SCIENCEMethane on the Rise AgainEuan G. Nisbet, Edward J. Dlugokencky, and Philippe Bousquet Science 31 January 2014: 343 (6170), [DOI: /science ] 43

44 Like CO2 there is a greater amplitude and more production of methane in the northern hemisphere than the southern hemisphere. The north is dominated by land, with sources like rice paddies, cows, termites and gas wells..sinks of methane are from the hydroxyl radical, OH, called the detergent of the atmosphere The amplitudes of the north and south hemisphere are out of phase due to the timing of the warmer summers that stimulate enzymatic activity and life and OH activity. 44

45 Like CO 2 there has been an ebb and flow in the methane concentration between ice ages and inter glacial periods. These reflect feedbacks among warmth and biological activity and changes in the composition of the atmosphere that reinforce or inhibit greenhouse warming of the surface by trace gases. Here Methane peaks at about 700 ppb during the interglacial periods and reaches a nadir at about 350 ppb during the depth of the ice age. In other words, methane concentrations repeatedly varied by a factor of two between glacial and interglacial periods. 45

46 Methane has various routes of transport and destruction after it is produced in anoxic sediments. Methane can be transported through slow diffusion through the water column. At night if the surface is cooler than the deeper water layers, convection will occur and this can increase transport rates and efficiencies. As methane moves through the water column the water will become oxygenated and this will lead to reactions that will destroy methane. A more effective means of transport is ebullition, a fancy word for bubbles. If you look at a methane producing pond under still conditions you will see bubbles. Plant mediated xylem transport is another effective means of methane transport. Wetland plants have large vessels, called aerenchyma. These evolved to enhance oxygen diffusion to the roots for respiration in low oxygen sediments. But doors and routes evolved to facility the uptake of one gas will also yield opportunities for losses of another (like stomata, CO2 and water vapor). Here, methane can take a preferred route of transport to the atmosphere. Finally, there is a link between plants, exudation of recent photosynthesis by the roots and the stimulation of methane by archaea. 46

47 Example of aerenchyma cross section of wetland plants 47

48 Shows alternative routes of electrons from organic matter before methane can be produced. Methane production will be inhibited if other electron acceptors, like O2, iron, nitrate and sulfate, are present in the sediments 48

49 Methane can be produced by two routes after the fermentation of organic matter. One route involves the reaction between CO2 and H2. The other involves the breakdown of acetate, CH 3 COO + H + > CO 2 + CH 4 49

50 Cows, sheep and termite have archaea in their stomachs to help them to break down and digest organic matter, releasing methane in the meantime 50

51 Cow methane emissions will increase with diet, variety and season. How to estimate cow methane emissions? One clever method is the dual tracer technique. Scientists put a sulfur dioxide permeation tube in the belly of cows that emits SO2 at a known rate at a known temperature. By measuring the ratio of SO2 and CH4 measured at the cows mouth, and knowing the SO2 emission rate, the methane emission rate can be computed. 51

52 What is a Terragram?? Methane is produced directly and indirectly from human activities. What are the direct routes?...energy production and biomass burning What are the indirect sources? Enteric fermentation from cattle production and rice cultivation Enteric fermentation occurs in the guts of cows 52

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54 We have seen in earlier lectures the co evolution of life, life forms, oxygen and evolution 54

55 Oxygen is form by the splitting of water during the first step of photosynthesis 55

56 One of the early assumptions of the homeostatic, self regulatory, power of Gaia was the near constancy of oxygen over millions of years. Watson and Lovelock argued about a feedback between oxygen and fire. They did some lab experiments which asserted that dry paper would ignite if O2 levels exceeded 25% 56

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58 Geochemical modelers infer that O2 levels weren t so constant back to the start of the Phanerozoic. Data of Berner counter Lovelock and Watson that O2 was constant Phanerozoic, period of revealed life; Maximum of O2 corresponds with evolution of vascular plants, whose lignin was difficult to decompose and resulted in significant carbon burial in swamps and sediments. We talk about multiple constraints and interconnected consistency. The elevated O2 of the carboniferous was associated with gigantism in insects. They can only breathe effectively through their surface, so they needed higher O2 to overcome the surface to volume ratio that came with large size. 58

59 The Build up of CO2 by fossil fuel combustion consumes oxygen. Hence, we are seeing a downward trend in O2. But the background is so large we don t expect to run out of air. Figure from Ralph Keeling 59

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62 There is good ozone in the stratosphere and bad ozone in the troposphere 62

63 Shows the location and implications of good and bad ozone. Good ozone is in the Stratosphere; it absorbs high energy uv light and protects life from its effects, like mutations. Bad ozone is in the troposphere, affecting the air we breathe, human and plant health 63

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65 g Ozone is not well mixed like other long lived greenhouse gases. While there are regional differences in absolute ozone, in general there is a world wide increase in its concentration. Earliest data are at the turn of the 20 th Century in Paris, where concentrations were below 20 ppm. The density of monitoring networks have improved, as so the instruments, so the modern record is viewed as more reliable. 65

66 Observed trends in ozone 66

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68 Simple set of reactions that lead to the photochemical production of ozone 68

69 The fresh smell in the woods are volatile organic hydrocarbons, like monoterpenes and isoprene. They play pivotal roles in the production of photochemcial ozone 69

70 Whether ozone is produced depends on the combination of VOX and Nox concentrations 70

71 VOCs play many roles in regards to producing aerosols that can reflect light or act as cloud condensation nucleii. Chemically, they play a central role in ozone production and for biology they help plants defend against pathogens and attract pollenators. 71

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73 High energy, and very short wave ultra violet light splits O 2 molecules and the single O molecules can combine with other O 2 molecules and form ozone, O 3 The production of ozone is not unchecked due to its destruction by CFCs and nitrogen compounds, which are leading to the ozone hole over the Antarcrtic 73

74 Uv light breaks up CFCs, releasing a chlorine atom, Cl. This cleaves O3, ozone, to form ClO. Other O molecules split ClO to enable free Cl atoms to repeat the set of reactions. 74

75 Ozone in the stratosphere is depleted from Cl from CFC and NO from N 2 O. Single O atoms cause N2O to form 2 NO molecules and the NO leads to the destruction of O3. N 2 O + O(1D) > 2NO NO + O3 > NO2 + O2 NO2 + O > NO + O2 Net: O + O3 > 2 O2 75

76 Map showing the Antarctic ozone hole in 2003 Susan Solomon, a UCB graduate, was instrumental in discovering the mechanism for ozone destruction in the Statosphere. She conducted many studies in Antarctica. 76

77 An overview of the ozone hole is produced by NASA Hole Poster_hiRes_508.pdf 77

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79 We will cover nitrogen gases later in this course. 79

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83 Here are some Key points.. Help identify others gleaned from this lecture. 83

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