MIDTERM EXAM: THURSDAY, OCTOBER 6

1. MIDTERM EXAM: THURSDAY, OCTOBER 6 • You will have 1 hour (start @ 8:45): Work efficiently!! • Closed notes, closed book (bring calculator) • 3 parts • 3 short answer (30 points) • 3 short calculation (45 points) • 1 problem (25 points) • You will have a copy of the periodic table, plus the following constants: USEFUL NUMBERS: Avogadro’s number = 6.02 x 1023 mol-1 Gas constant = 8.31 J/mol/K Stefan-Boltzmann constant = 5.67 x 10-8 W/m2/K4 Rearth = 6400 km Scale height of the atmosphere = 7.4 km Solar constant = 1370 W/m2 ma = 5.2 x 1018 kg Nair = 1.8 x 1020 moles

2. TOPICS COVERED ON THE MIDTERM Measures of Concentration Atmospheric Pressure Simple box models Atmospheric Transport Biogeochemical Cycles (including Carbon Cycle) Radiation & The Greenhouse Effect Stratospheric Chemistry (including concepts from photochemistry)

3. Chapman Mechanism: Source of ozone (O2 + hv) Sink of ozone (O+O3)  predicts too much ozone! REVIEWING STRATOSPHERIC CHEMISTRY…BASIC MECHANISM Other ozone sinks: catalytic loss cycles: 1. HOx 2. NOx 3. ClOx slow H2O OH HO2 fast slow Closes the overall stratospheric ozone budget: sinks balance source

4. REVIEWING STRATOSPHERIC CHEMISTRY…WHY DOESN’T THIS EXPLAIN THE OZONE HOLE? Antarctic ozone hole observed in austral SPRING! Catalytic NOx and ClOx cycles all depend on [O]  The source of O is photolysis (peaks in summer not spring!) Need another mechanism to explain ozone depletion in spring: ClO + ClO + M  ClOOCl + M ClOOCl + hv  ClOO + Cl ClOO + M  Cl + O2 2 x [Cl + O3  ClO + O2] NET: 2O3  3O2 Perfect! Key here is high [ClO] Where from?

5. REVIEWING STRATOSPHERIC CHEMISTRY…THE IMPORTANCE OF PSCs Polar stratospheric clouds: conversion of ClOx reservoirs to Cl2 ClNO3 + HCl –PSCCl2 + HNO3 Cl2 + hv  2Cl • Once sun comes up the Cl goes on to react with O3 • At first ClO builds up (remember not enough [O]), eventually ClO + ClO cycle takes over • ClOOCl photolyzed back to Cl  more ozone depletion • Then stratosphere warms up, no more PSCs, no more conversion of reservoir species, slows down depletion PSC formation Key to polar O3 depletion is cold T + sunlight Frost point of water

6. CHLORINE CYCLING & THE OZONE HOLE HCl ClNO3 Standard ClOx cycle PSC Antarctica: early Winter Antarctica: as Winter progresses into early Spring… Cl2 OH NO2 CH4 hv hv Antarctica: Spring! O3 ClO CFCs Cl ClOOCl O hv ClOO M

7. REVIEWING STRATOSPHERIC CHEMISTRY…N2O5 HYDROLYSIS: INCREASING SENSITIVITY TO Cl N2O5 + H2O –aerosol2HNO3 • Effects on catalytic cycles (for ozone loss): • NOx cycle: moves from one reservoir (N2O5) to a longer-lived reservoir (HNO3)  longer for NOx to be recycled • ↓ importance of NOx-catalysis • ClOx cycle: NOx important for termination of ClOx cycling. A reduction in NOx reduces efficiency of ClO + NO2 + M  ClNO3 + M • ↑ importance of ClOx-catalysis • HOx cycle: When HNO3 eventually returned to NOx will also produce HOx (HNO3 + h NO2 + OH) • ↑ importance of HOx-catalysis Mt. Pinatubo Overall: generally has little effect on TOTAL ozone loss, but does make ozone loss more sensitive to Cl levels Also: will enhance ozone loss in the presence of aerosols (eg. volcano)