Atmosphere and Environment Lecture Notes: Ozone, Acid Rain, and Photochemical Smog, Study notes of Chemistry

Download Atmosphere and Environment Lecture Notes: Ozone, Acid Rain, and Photochemical Smog and more Study notes Chemistry in PDF only on Docsity! Sheets Page 1 Lecture 25 Lecture 25: Atmosphere & environment Read: BLB 18.1–4 HW: BLB 18:9,11,15,29,69 Sup 18:1–3 Know: • ozone chemistry • chemistry of the lower atmosphere • sulfer compounds & acid rain • nitrogen oxides & smog check out the environmental & climate change folder under “Lessons” on our Angel site for more links & movies, fyi only Need help?? Get help!! TAs in CRC (211 Whitmore) and SI—hours on Chem 110 website; my office hours (Mon 12:30-2 & Tues 10:30-12 in 324 Chem Bldg [or 326 Chem]) Bonus deadline for BST #8: Intermolecular forces, March 26 Bonus deadline for BST #9: Solutions & dilutions, April 2 Exam 3: Monday, April 6 @ 6:30 Sheets Page 2 Lecture 25 Pressure & altitude • atmospheric gases: 78% N2, 21% O2, some Ar, CO2 & other gases • pressure at a given altitude depends upon weight of gas above it • pressure decreases exponentially as altitude ↑ altitude P collision frequency (km) (atm) (s–1) 0 1 1010 50 10–3 107 100 10–6 103 200 10–13 1 Sheets Page 5 Lecture 25 Photochemistry in the atmosphere • photoexcitation: electronic excitation (Chap 6) hν •NO2 → •NO2* (* = excited state) • photodissociation: bond broken by absorption of a photon hν O2 → •O• + •O• bond E O2 = 495 kJ/mol need sufficient energy: λ < 242 nm; verify this for yourself! recall, E = hν & c = λν (Chap 6) • O2 and •O• concentrations vary with altitude • photoionization: removal of a valence e– from a molecule by absorption of a photon hν N2 → N2+ + e– • need energy ≥ the ionization potential (1495 kJ/mol), λ < 80.1 nm Sheets Page 6 Lecture 25 Solar radiation penetration in the atmosphere higher energy lower energy < 100nm 170–300nm > 330nm _______________________________________ thermosphere NO+ photo- O2+ ionization O+ e– mesophere NO+ O2+ stratosphere O2 O photodissociation NO O3 troposphere O2 photoexcitation N2 earth Sheets Page 7 Lecture 25 Ozone • resonance: MG = , ° (VSEPR) • bond length: 1.28 Å (O2 is 1.21 Å) • light blue gas; BP = –111.3°C; pungent odor (electrical discharges) • ΔHf° = 142.3 kJ/mol (reactive, less stable than O2) (see ~Lecture 39) • in troposphere: O3 is an irritant (see smog) • in stratosphere: O3 is essential; peak of [O3] is at ~25 km; [O3] ~ 10 ppm • λ < 350 nm (UV) induces photochemistry in many organic molecules (skin cancer) • ozone in stratosphere most radiation with λ = 240–320 nm O OO O OO Sheets Page 10 Lecture 25 Ozone hole • see BLB Fig. 18.5 • special conditions at the South Pole create dramatic seasonal loss of ozone • complex reactions related to presence of polar stratospheric clouds explain the seasonal nature of the loss • nearly complete loss of ozone at some altitudes near the poles, and ~50% total reduction at other latitudes near the poles • away from the poles, depletion is not as great and is seasonal • global O3 concentration has declined since 1980 • you can burn in <7 min in Chile & Argentina; Australia 6% lethal skin cancer vs. 0.3% general world population ⇒ bring your sunblock Sheets Page 11 Lecture 25 Ozone hole (cont.) http://www.nasa.gov/vision/earth/environment/ozone_resource_page.html Ozone hole on 7 Oct 2008 http://www.atm.ch.cam.ac.uk/tour/part2.html Sheets Page 12 Lecture 25 Ozone hole (cont.) the good news: 1987: Montreal Protocol called for virtual elimination of ozone depleting substances; signed by 160 nations; 1996: production and use of CFCs banned worldwide; recovery by 2100?? See http://video.google.com/videoplay?docid=- 8206266264728754527&q=ozone+hole&total=212&start=0&num=10& so=0&type=search&plindex=0 the bad news: CFCs are stable & will remain in troposphere and continue to diffuse up to stratosphere if CFC production (or release into the atmosphere) DID stop, ozone depletion would continue for at least several decades effects include 2–10% increase in skin cancers per 1% decrease in ozone, possible effects on crop yields and ??? • EPA estimates that ozone levels will return to normal by 2050 if full compliance with protocol is achieved. The Greenhouse effect Mostradialion is absorbed by the Earth's surface and warms it, http://www.gcrio.org/ocp96/figs /fig20.gif http://atschool.eduweb.co.uk/kingworc/departments/geography/nottingham/atmosphere/pages/gfx/ greenhouseeffect.gif ry i 5 8 3] Incoming solar < Radiation Earth-Radiated 5 jpeck 0.5um Radiation > (peak 12um) 5 S| xrays Radio Waves Ultra Violet fi infra Red a4 404 40008 40um 04cm 40cm 40m Sheets Page 15 Lecture 25 Sheets Page 16 Lecture 25 Greenhouse gases & global warming (cont.) • the bad: greenhouse gas levels keep increasing  so… global temperature keeps increasing CO2—fossil fuel, biomass combustion, volcanic eruptions (but plants help take up excess CO2) CH4—animal gas [stinky!], swamp & natural gas venting, incomplete combustion, coal mining H2O vapor—evaporation/condensation http://en.wikipedia.org/wiki/Image:Global_Warming_Map.jpg www.carbuyersnotebook.com/archives/Cow.jpg Sheets Page 17 Lecture 25 Acid rain: sulfur in atmosphere • sources of SO2: bacterial decay of organic matter volcanic gases forest fires fossil fuel combustion industrial process http://media.allrefer.com/s4/l/p0013033-acid-rain.gif • in atmosphere 2SO2(g) + O2(g) → 2SO3(g) SO3(g) + H2O(l) → H2SO4(aq) http://www.epa.gov/airmarkets/cmap/mapgallery/mg_so2.html