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Environmental Chemistry (Air)

List of Questions

Prof. Dr. Dr. h.c. Reinhard Zellner

University of Duisburg-Essen

SS 2013

I. Structure of the atmosphere, terminologies, temperature gradient, barometric formula, trace substances 1. The different layers of the earth’s atmosphere are defined by the temperature profile. a) Sketch the temperature trends of the lowermost 100 km of the atmosphere. b) Name the resulting atmospheric layers and the regions dividing them. c) What is the meaning of the terms homosphere and heterosphere? 2. The pressure characteristics in the atmosphere are described by the barometric formula. a) State and explain the barometric formula. b) What is a scale height? c) Over what altitude range does the pressure drop to the 1/e fraction of 1 bar? d) What is the pressure at 12 km altitude, the maximum flying altitude of a commercial aircraft? 3. The temperature gradient in the lower atmosphere is determined by adiabatic expansion of a gas from 1 bar and 288 K to lower pressures. a) What is understood by the term “adiabatic expansion” as opposed to “isothermal expansion?” State and explain the state equations for both forms of expansion and sketch the two in a p vs. V diagram.

b) What is the physical meaning of the adiabatic exponent ? c) Why is the average vertical cooling rate in dry air larger than the one in humid air and what are their approximate numerical values (in units of °C/km)?

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4. The chemical composition of the atmosphere is determined by two main gases and a large number of trace gases. a) Name the atmospheric main gases as well as some of the trace gases and give their relative amounts. b) What are the sources of these trace gases? Differentiate natural and anthropogenic contributions. c) What is the approximate concentration of water vapour in air at 288 K? (Note: assume a relative humidity of 50%) 5. In atmospheric chemistry the concentrations of trace gases are frequently not described as mole fractions but as volumetric mixing ratios or as number densities (in units of molecules/cm3). a) Explain the terms ppmV, ppbV and pptV. b) What is the total number density at 1 bar and 288 K? c) What is the number density that corresponds to a mixing ratio of 1 ppbV?

II. Earth and planets 6. The evolution of oxygen on earth a) Sketch the approximate evolution of atmospheric oxygen since the existence of the earth. b) In which chemical species is most of the oxygen ever formed in the history of the Earth chemically bound and stored? 7. The planets of the solar system and their atmospheres a) What is the average surface temperature (°C) on the planets Venus, Earth and Mars? b) What are the dominant atmospheric species on Venus and Mars? c) Why is there essentially no molecular oxygen on our neighbouring planets? 8. The solar year a) Explain the origin of the seasons on Earth. b) What is the position of the sun at summer solstice (21.06.) and winter solstice (21.12.) relative to the equator and what are the names of the corresponding tropics? 9. The city centers of Hamburg and Munich have the geographical coordinates of N 53°33’01’’ E 09°59’32’’ and N 48°08’19’’ E 11°34’23’’, respectively. a) Calculate the exact latitudinal difference between the two. b) What is the approximate longitudinal difference in their locations? c) What is the average distance between two latitudes?

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III. Mixing and Dynamics 10. Trace substances are usually subject to transport as well as chemical decomposition. a) How are the atmospheric lifetime and the transport range linked qualitatively? b) Explain the connection between the temporal and the spatial scale using lifetimes of selected trace gases. c) Why are some trace gases mixed homogeneously over the globe and others are not? d) Why is the CO2 concentration in the Southern Hemisphere lower than that in the Northern Hemisphere? 11. The characteristic mixing times are different for different parts of the atmosphere. How long are the tropospheric mixing times for: a) vertical mixing? b) horizontal mixing? c) mixing between the hemispheres? d) mixing into the stratosphere? 12. Mixing in the atmosphere is caused by either molecular or turbulent (Eddy) diffusion. a) In which regions of the atmosphere is mixing predominantly achieved by molecular diffusion and in which regions by turbulent diffusion? b) Sketch the vertical profile of the eddy diffusion coefficient from 0 to approx. 60 km. c) Describe Fick’s first law of molecular diffusion and explain its significance in stationary diffusion. d) What are the units of the diffusion coefficient (D or Kz)? 13. The dynamic stability of an air mass towards vertical mixing and exchange depends on the local temperature gradient. a) Which form of the temperature gradient provides the strongest vertical mixing? b) How is vertical mixing effected by a “temperature inversion”? c) What is a labile and a stabile form, respectively, of layering of the lower atmosphere? 14. The circulation of the globe is represented by a characteristic pattern that is determined by a systematic distribution of high and low pressure systems. a) Describe schematically the global circulation patterns in the atmosphere in the latitudinal framework (north-south). b) What is the Hadley cell?

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c) Explain what is understood by equatorial doldrums, horse latitudes, “westerlies” and “easterlies”. d) What is the dominant wind direction in our latitudes? e) What is a jet stream and how is it utilised in aviation? 15. The very lower-most part of the atmosphere is called the planetary boundary layer (PBL). a) By what physical phenomenon is the planetary boundary layer (PBL) defined and what is its average vertical extent (height)? b) Sketch the development of the height of the planetary boundary layer during the course of one day.

IV. Trace substances and their bio-geo-chemical cycles 16. One of the most important natural trace gases in the atmosphere is water vapour. a) How is the relative humidity defined and how is it related to the water vapour partial pressure? b) State the Clausius-Clapeyron equation for the temperature dependence of the vapour pressure of water. c) What is the approximate saturation partial pressure of water at 0 °C and at 20 °C? 17. Precipitation is an important factor of the local and regional water budget. a) State the average global rainfall (in l/m2 or mm) per year. b) What is the average annual rainfall in Germany? c) Sketch the north-south distribution of the global rainfall. d) How is this distribution connected to the global circulation pattern? Explain in particular the relation of rainfall with low and high pressure systems. e) What is the average water content in clouds? 18. The total amount of water on the earth is distributed over a range of compartments (oceans, terrestrial ice, rivers and lakes, ground water, atmosphere). a) What are the approximate relative amounts of water in these compartments? b) The interplay between which compartments leads to the largest exchange of water (global average)? c) How big are the exchanged amounts of water per year?

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19. After water vapour, CO2 is the most important atmospheric trace gas which has both natural and anthropogenic sources. a) What are the dominant natural and anthropogenic sources? b) What is the current level of CO2 in the atmosphere (in ppmV und in %vol)? c) What is the average annual rate of the CO2 increase? d) Why does the atmospheric CO2 concentration show a seasonal variability? e) How large is the total atmospheric content of CO2 and what fraction of this is annually recycled with the oceans and the biosphere? f) Which fraction of the atmospheric CO2 is bound by photosynthesis per year and how much biomass is converted to CO2 by respiration? g) What is the atmospheric lifetime of CO2? 20. CO2 is the atmospheric trace gas which in its trend is most strongly affected by anthropogenic activity. a) How is the anthropogenically emitted CO2 distributed between the atmosphere, the oceans and the terrestrial biosphere? b) What is the current amount of anthropogenic CO2 released into the atmosphere (in Gt C and Gt CO2) by burning of fossil fuels and deforestation? c) What was the atmospheric CO2 concentration (in ppmV) prior to industrialization? d) Sketch the temporal evolution of CO2 since the 1970s in the northern hemisphere. How does this evolution differ from that in the southern hemisphere? Explain the reasons. 21. Anthropogenic CO2 production from mobile combustion engines (cars, airplanes) a) Calculate the amount of CO2 emitted from a car with a fuel consumption of 5 l/100 km during a 1 year operation (10 000 km) b) Calculate the amount of CO2 produced per passenger on a single flight of a B 747 jet airplane from Frankfurt to New York (Flying time 9 hours, kerosin consumption 10t/h, 300 passengers) Note: Assume in each case that the fuel used consists exclusively of carbon and neglect the hydrogen content. Also, assume that the density of the fuel is 1 g/cm3. 22. One of the most important atmospheric trace gases with a strong chemical cycle is methane. a) What are the main sources of atmospheric methane (natural and anthropogenic)? b) What is its global emission rate in Gt/year and what is its current atmospheric concentration? c) By which reaction is the atmospheric oxidation of methane induced and what products are formed during the oxidation of methane? d) What is the average atmospheric lifetime of methane and what are the decisive factors of this lifetime? e) Sketch the temporal evolution of the methane concentration since the beginning of the 1980s.

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V. Radiation and photochemistry 23. The electromagnetic spectrum spans over a wide range of wavelengths. a) Describe those regions of this spectrum which are of relevance to the chemistry of the atmosphere and assign them to wavelengths (in nm). b) Sketch the spectrum of the solar radiation outside the atmosphere and at the earth´s surface. c) Which area corresponds to the visible and UV-B radiation, respectively, and where is the maximum of the solar radiation? 24. The Beer-Lambert law a) State and explain the Beer-Lambert law.

b) What is the name of the mathematical product c d?

c) What is the unit of ? d) Which atmospheric trace gases have characteristic absorptions in the near UV / visible region of the electromagnetic spectrum and are therefore subject to photolysis? 25. The global radiation energy density (kWh m-2) is determined by the solar constant but varies along the latitudes. a) What is the value of the solar constant? b) How is the daily radiation energy (kWh m-2 d-1) distributed latitudinally (along a north-south axis) during the summer in the northern hemisphere? 26. Molecular oxygen and ozone are essential for the absorption of solar radiation in the UV-C and UV-B region, respectively. a) Schematically draw the absorption spectrum of oxygen and of ozone. b) How is the rate of photolysis calculated and how is it linked with the absorption coefficient of a trace gas and with the radiation intensity? c) How does the rate of photolysis of oxygen and ozone change as a function of altitude? 27. The blue sky and the sunset glow a) What is the physical reason for the blue sky and the sunset glow? b) Explain qualitatively the difference of light scattering by molecules and by particles.

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VI. Ground-level ozone, VOCs and photosmog 28. Ozone at ground level and in the troposphere is formed as a result of photochemical reactions. a) What is the annually averaged ground-level ozone concentration in Germany (in ppmV or μg/m3) in different regions (urban, rural, forested regions of the highlands)? b) How have these values developed since the beginning of the industrialisation? c) Sketch the ground-level ozone concentration during the course of one year. How does this progression change for different regional environments? 29. The daily variation of ozone at ground level is a result of photochemical creation and loss in a “titration reaction” with NO. a) Name the titration reaction b) Sketch the daily progression of the ground-level ozone for urban and rural regions in Germany and explain the sketch. 30. In the lower atmosphere ozone is produced by the oxidation of VOCs or CO in presence of NOx and light. a) Describe the mechanism of ozone formation in the lower troposphere. b) What is described as catalyst and what as fuel (include explanation)? c) What are the gross reactions that link the oxidation of CO and methane to the ozone formation? 31. The emission of VOCs in Germany is due to a number of anthropogenic as well as natural sources. a) How is the emission of VOCs in Germany distributed across the different anthropogenic sectors (industry, traffic, solvents)? b) Describe qualitatively to which extent the biogenic emissions of VOCs affect the formation of ground level ozone in Germany during summer and winter, respectively. c) What type of compounds is typical for biogenic emissions? 32. The atmospheric oxidation system is represented by free radicals of which OH and NO3 are the most important. a) How are OH radicals formed in the troposphere and what is their typical concentration during the summer and winter, respectively, in temperate zones? b) By which reactions are HOx radicals (OH and HO2) interconnected?

c) What is the lifetime of propane based on its oxidation by OH radicals (kOH = 1.1∙10-

12 cm3/s)? d) By which reaction are nitrate radicals (NO3) formed and why are they only important oxidants during the night?

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33. In regulation strategies each VOC is assigned a photochemical ozone creation potential (POCP). a) Describe the significance of POCP values and how they are determined. b) Why are POCP values in rural areas generally somewhat larger than in urban air? c) What types of compounds have the largest POCP values? 34. The dependence of the calculated ozone concentration on the local concentrations of VOCs and NOx is called on ozone-isopleth diagram. a) Sketch qualitatively an ozone-isopleth diagram b) Assign typical areas (street canyon, urban, rural forest) to the ozone-isopleth diagram. c) What is a NOx controlled and what is a VOC controlled region?

VII. Ozone in the stratosphere, ozone hole, UV-B change 35. The averaged ozone concentrations in the stratosphere show a typical distribution, depending on latitude and season. a) Sketch and explain this distribution. b) Sketch and explain the typical seasonal cycle of stratospheric ozone in our latitudes (ca. 50°N). c) What are the causes of this distribution? 36. The total amount of ozone above any one place on the Earth is specified as a column density. The associated unit is the Dobson unit, DU. a) How is the Dobson unit defined? b) What are typical values for the total amount of ozone (in DU) above Germany in spring and in autumn, respectively? c) By what type of measurement can the column density of ozone be determined? 37. Ozone in the stratosphere a) Which reactions between the different oxygen species (O, O(1D), O2, O3) are possible in a pure O2 atmosphere in the presence of light (Chapman reactions)? b) Why does the pure O2 atmosphere generate too much ozone compared to observations?

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38. The ozone concentration in the stratosphere is diminished by the action of catalytic cycles. a) Which reactions that are initiated by non-oxygen species have an additional effect on the stratospheric O3 concentration? b) Describe the system of catalytic cycles in the stratosphere. c) Which catalysts occur in the stratosphere and what are their source substances? d) By which reaction is the chlorine atom as a catalyst of the ClOx cycle generated from CFCs? 39. The chlorinated fluorocarbons (CFCs) are systematically named using an international nomenclature. This nomenclature yields names in the form CFC xyz. Use this nomenclature to name the following substances: a) CF2Cl2 b) CF4 c) CF3CF2Cl d) CF3CH2F e) CF2ClCF2CHCl2 f) Which substance has the name CFC 142a? 40. The ozone hole over Antarctica is a seasonal phenomenon. a) Draw schematically the chronological development of the ozone hole above the Antarctic. b) When does it reach its maximum extent and what is the typical size of this maximum area? c) What are the meteorological reasons for this temporal behaviour? d) Why is the ozone hole in the south usually much more intense than that in the north? 41. The UV-B intensity varies substantially on the globe. a) Sketch the seasonal variability of the UV-B intensity in Europe and at some place in the tropics. b) During which months is the UV-B intensity in the polar regions (Arctic and Antarctic) affected most strongly by the depletion of the ozone layer? 42. Spectral regions and their biological impact a) What are the approximate wavelengths limits of these separations? b) What is the biological reason for defining these regions? c) Sketch the radiation sensitivity for erythema and for DNA destruction and plant damage.

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43. The radiation dose a) Sketch the relation between solar intensity and the biological sensitivity curves in the UV-B region. b) What is understood by the term “dose rate”? c) When is there a non-linear relation between ozone depletion and the UV- intensity? 44. CFCs in the stratosphere are decomposed by UV radiation in the atmospheric window. a) Describe the mechanism by which CFCs are decomposed in the atmosphere? b) What are the lifetimes of CFC11 and CFC 12 based on their decomposition? 45. Polar stratospheric clouds (PSCs). a) What are polar stratospheric clouds (PSCs) and of what are they composed? b) What is the frost point and what type of PSCs form at this temperature? c) Due to which chemical reactions can the PSCs affect the existent Cl-compounds? d) Explain the term “halogen activation”. e) Describe the influence of the volcanic eruption of Mt. Pinatubo (Philippines, 1991) on the stratosphere. 46. The ozone hole occurs as a sequence of meteorological, chemical and photochemical events. a) Describe the sequence of chemical and photochemical events before, during and after the formation of an ozone hole. b) What happens in the dark and what after the return of the sunlight in early spring?

VIII. Aerosols 47. The terminology of ambient aerosols is related to the size fractions of its particles. a) What do the terms TSP, PM10, PM2.5 and UFP mean? b) Which of these terms is the binding quantity in the current environmental regulations and what is the (mass related) limiting value for regulation? 48. Environmental aerosols typically have a tri-modal size distribution. a) Sketch schematically these distributions and indicate the typical size range of each mode. b) What are the names of these different modes and what are their origins?

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49. According to their sources aerosols are referred to as primary and secondary. a) What is understood by the terms primary and secondary aerosols? b) State typical primary and secondary aerosols and their sources c) Which type of aerosol has the largest global source strength? 50. The chemical composition and mass concentration of ambient aerosols is different in different regions. a) Which are the dominant chemical species of aerosols in urban and rural regions as well as over the oceans? b) What are the typical mass concentrations of aerosols over the continents and the oceans and how do these vary with increasing altitude above the surface? 51. The dynamics of aerosols is determined by a number of different physical processes. a) What are the processes that influence the size distributions of aerosols? b) How do these processes affect the lifetimes of aerosols of different sizes? c) Which size fraction of aerosols has the longest lifetime in the lower troposphere? 52. Aerosols scatter the solar light. a) State the law of light scattering by particles. b) How does the scattering intensity depend on particle size and on scattering angle? c) How do these dependencies change for particles like mineral aerosols or soot, that not only scatter but also absorb solar radiation? d) What is the effect of aerosols in the climate system of the earth?

IX. Climate and Climate Change 53. Climate is described by the so called climate parameters (temperature, precipitation, pressure distribution, etc.). a) What is understood by the average of these parameters and what by their variance? b) How is climate defined and how does it differ from the weather? 54. The climate system is composed of a number of different components. a) Which are the components of the climate system? b) Explain the effects of the different components and how they interact with each other. c) What is understood by the terms of positive and negative feedbacks?

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55. The long term effects of climate change are connected with the change of the terrestrial orbital effects. a) Describe the so called terrestrial orbital parameters. b) Explain their long term effects on the terrestrial insolation? 56. The radiative balance between the sun and the earth is affected by a number of different parameters. a) Which parameters affect the radiative balance of the earth / atmosphere with the sun? b) What is the so-called albedo and how does it influence the amount of solar energy received by the surface? 57. The radiative energy density of a so called black-body increases with the fourth power of the temperature (Stefan-Boltzmann law). a) Describe and explain this law. b) What is the surface temperature of the earth purely based on the radiative balance with the sun (without greenhouse gases)? c) How can this temperature be calculated? 58. The maximum of the black-body radiation shifts with the temperature of the emitting body (Wien´s law). a) Describe and explain this law. b) At which wavelength is the maximum of the terrestrial infrared radiation? (Assume the solar radiation maximum to be at 550 nm for a temperature of 5700 K) 59. The greenhouse effect has a strong natural contribution but an anthropogenic contribution as well. a) How large is the natural greenhouse effect as expressed in °C of surface temperature. b) What are the individual contributions of the natural greenhouse gases H2O, CO2, O3, N2O, CH4? c) What are the individual contributions of the anthropogenic greenhouse gases CO2, CH4, O3, N2O, CFCs? 60. For an atmospheric trace gas to be a greenhouse gas a number of physical conditions must be fulfilled. a) Which trace gases affect the climate? b) Which physical reason causes this property? c) In which wavelength region are trace gases most effective and why?

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61. In regulatory issues of climate change the so called “greenhouse warming potential” (GWP) is used. a) Describe what is understood by the term “greenhouse warming potential”. b) What value does the GWP have for selected greenhouse trace gases? c) Why are the GWP values different? 62. Climate change is physically described by a change of the so called radiative forcing of the troposphere. a) Describe and explain the radiative forcing of the climate system? b) To which gases can the radiative forcing and its development over the last 150 years be attributed? c) How much is the contribution of aerosols? 63. The surface temperature of the globe has increased since the beginning of industrialization. a) By how much has the global surface temperature of the earth changed over the past 140 years? b) What are the causes for this temperature rise in the periods 1920-1940 and 1970-present? c) How large was the variability of the average global surface temperature in the past 10 000 years? 64. The temperature rise of the globe in the last decades has not been uniform. a) Describe how these changes differ for northern and southern latitudes as well as for oceans and continents. b) How strong has the temperature changed in arctic regions. c) Explain the positive feedback of sea ice extent and temperature. 65. The evolution of the future temperature depends strongly on the socio-economic evolution of the world`s population. a) Which parameters are used to describe the possible socio-economic scenarios of the 21st century? b) Which of these scenarios are the most extreme with respect to the emission of CO2? c) What predictions are being used for the development of the global population? d) Which temperature changes are predicted for the different scenarios in current climate modelling?