Stratospheric Chemistry HS 2017 Solution to Homework Problem Set 3

Transcription

1 Stratospheric Chemistry HS 2017 Solution to Homework Problem Set 3 For questions: andrea.stenke@env.ethz.ch (CHN P14)

2 Problem 1: The Montreal Protocol and Climate (a) Chemical Formulas CFC-11 : CFCl 3 (=Trichlorofluoromethane, Freon-11) CFC-12 : CF 2 Cl 2 (=Dichlorodifluoromethane, Freon-12) CFC-113 : CF 2 ClCFCl 2 (=Trichlorotrifluoroethane, Freon-113) Methyl chloroform : CH 3 CCl 3 (=Trichloroethane) HCFC-22 : CHF 2 Cl (=Chlorodifluoromethane) HCFC-142a : CH 2 FCHFCl (=Chlorodifluoroethane, Freon-142a) HCFC-142b : CH 3 CF 2 Cl (=Chlorodifluoroethane, Freon-142b) HFC-23 : CHF 3 (=Fluoroform) HFC-134a : CF 3 CH 2 F (=Tetrafluoroethane, Freon-134a) HFC-152a : CH 3 CHF 2 (=Difluoroethane, Freon-152a)

3 Problem 1: The Montreal Protocol and Climate (b) Why is the atmospheric lifetime so different between CFCs on the one hand, and HCFCs and HFCs on the other hand? HFCs and HCFCs are partly destroyed by the OH radical in the troposphere (via the C-H bond), shortening their lifetime. CFCs do not have a C-H bond, therefore this reaction is not possible for CFCs. For HCFC-22 : CHClF 2 + OH à H 2 O + CClF 2 (Difluoromethyl radical) CClF 2 + O 2 + M à CClF 2 O 2 (peroxy radical) CClF 2 O 2 + NO à CClF 2 O + NO 2 (alkoxy radical) RO

4 Problem 1: The Montreal Protocol and Climate (b) Why is the atmospheric lifetime so different between CFCs on the one hand, and HCFCs and HFCs on the other hand? Seinfeld & Pandis, 2006

5 Problem 1: The Montreal Protocol and Climate (c) In Figure 1, what are the reasons for the characteristic shapes of the CFC production curves (black lines): steep increase until the mid- 1970s, slight decrease during the late 1970s, increase (notable in CFC-11) in the 2nd half of the 1980s, sharp decrease in the 1990s? (A) Use as aerosol propellants and refrigerants, introduction of new applications including solvents and farms. (B) 1974 Molina and Rowland early warning : USA, Canada, Netherlands and Sweden banned or discouraged use of ODS in most personal care aerosols products. (A) (B) (C) (D) (C) Continued growth in refrigerant uses; rapid growth in applications of solvents, foam-blowing and fire protection agents; continued use in aerosols products in Europe and Asia. (D) Montreal Protocol 1987

6 Problem 1: The Montreal Protocol and Climate (d) In figure 1, why does CFC-12 production decrease sharply after 1988 (center panel, black line), but its atmospheric mixing ratio starts reducing only some 15 years later (right panel)? 1. Production emission. There might be a delay between the production and usage (=emission) of the CFCcontaining product. 2. Emission mixing ratio. Because of its long lifetime of about 102 years, CFC-12, once emitted, stays for along time in the atmosphere.

7 Problem 1: The Montreal Protocol and Climate (e) Explain briefly why ozone depletion from ODSs counteracts their climate warming effect. Whose radiative forcing is more important, the direct forcing by the ODS increase or the indirect forcing via stratospheric ozone loss? Ozone is a greenhouse gas Stratospheric ozone depletion due to ODSs leads to a negative radiative forcing response Velders et al. (2007): Ozone depletion offset W/m 2 20% of direct positive ODS radiative forcing => RF from ODSs is more important

8 Problem 1: The Montreal Protocol and Climate (f) Definition of ozone depletion potential (ODP)

9 Problem 1: The Montreal Protocol and Climate (g) Definition of global warming potential (GWP) TH: time horizon over which the calculation is considered RF: Radiative Forcing. It is defined as the net change (in Wm -2 ) in the energy balance due to some imposed perturbation. More information about RF can be found here (p.664) NAL.pdf

10 Problem 1: The Montreal Protocol and Climate (h) A doubling of atmospheric CFC-11 mixing ratios leads basically to a doubling of its global net warming effect. What does this tell you about the atmospheric optical depth at the wavelengths where CFC-11 absorbs most efficiently? What does this mean for its GWP? CFC-11 absorbs at 11.8 µm atmosphere at this wavelength optically thin GWP 100 (CFC-11) = 5160 strong greenhouse gas GWP is independent of concentration, it is calculated for a unit mass (1 kg)

11 Altitude (kilometers) Problem 2: Family Concept (a) Chlorine family abundances Measurements of Chlorine Gases from Space Annual mean 2006 (30 70 N) ClO ClONO 2 Other chlorine gases Hydrogen chloride (HCl) Chlorine source gases (CFCs, HCFCs, CCl 4, CH 3 Cl, etc.) Total available chlorine Chlorine (parts per trillion) Altitude (miles) ClO undergoes reactions until stable reservoir species HCl is produced ClONO 2 photolyzes to ClO and NO 2 in upper stratosphere Reactive Cl is deactivated to produce stable reservoir species (ClONO 2 /HCl) Source gases photolyze in stratosphere, producing reactive Cl Source gases are stable in the troposphere

12 Problem 2: Family Concept (b) Should the total available chlorine be altitude independent? Measurements of Chlorine Gases from Space Annual mean 2006 (30 70 N) Altitude (kilometers) ClO ClONO 2 Other chlorine gases Hydrogen chloride (HCl) Total available chlorine Altitude (miles) YES! Chlorine source gases (CFCs, HCFCs, CCl 4, CH 3 Cl, etc.) Chlorine (parts per trillion)

13 Problem 2: Family Concept (c) Abundances over Antarctica between fall and spring HCl (g) + H 2 O (l) à Cl - (aq) + H 3 O + (aq) ClONO 2(aq) + Cl - (aq) à NO 3 - (aq) + Cl 2(g) ClO + NO 2 + M à ClONO 2 + M Cl + CH 4 à HCl + CH 3

14 Problem 3: Global ozone trend Equivalent latitude The polar vortex meanders around the pole along a streamline of the same potential vorticity. Equivalent latitude means that the area this streamline encloses will be transformed to a circle of the same area and centred to yield new geographical latitude for each transformed point. Equivalent latitude filters out the meander due to dynamical effects, separating these effects from chemical effects. Therefore, changing ozone concentration for a certain equivalent latitude indicates a chemical effect.

15 Problem 3: Global ozone trend (a) Why is ozone loss stronger in the South polar region? 1. The polar vortex is much stronger in the South than in the North, for which reason the Antarctic does not get new ozone via mixing with ozone-rich air from mid-latitudes for a longer time during the winter season in South than in North polar region. 2. As well the Arctic is warmer than Antarctic in winter, leading to less PSC formation, and thus less ozone depletion. 3. The year-to-year variability is much larger in the North than in the South. Polar vortex is stronger in the South because of a different distribution of land masses in the Northern hemisphere leading to the formation of waves which contribute to the variability and instability of the polar vortex.

16 Problem 3: Global ozone trend (b) Wintertime Arctic ozone trends show an increase from the period (panel A) to the period (panel B), while Antarctic wintertime ozone trends show a reduction. Speculate what the potential reasons for the differences in Arctic and Antarctic ozone trends between panel (A) and panel (B) are!

17 Problem 3: Global ozone trend (b) What are the potential reasons for the differences in Arctic and Antarctic ozone trends between panel A ( ) and panel B ( )? Arctic Bodeker et al. (2001): The increase in Arctic ozone trends from the period to the period results from severe wintertime ozone depletions observed in the period. Why? Pawson and Naujokat (1999): Cold temperatures (below T NAT ) and long polar vortex lead to the formation of PSCs in the stratosphere: more ozone depletion in the early spring. Therefore, an ozone trend as presented in this exercise strongly depends on the time period taken into account.

18 Problem 3: Global ozone trend Antarctic Bodeker et al., 2001: The reduction in Antarctic ozone trends from the to the period most likely results from saturation where almost all of the ozone between 15 and 19km inside the Antarctic vortex has been destroyed in recent years or from leveling off of the stratospheric chlorine loading. Furthermore, enhanced stratospheric aerosols were present throughout much of the 1980s, potentially steepening ozone trends before the Mount Pinatubo eruption. 1) From Figure Q12-3, Antarctic ozone is totally removed between 15 and 19km. There is a saturation in ozone trend. 2) CFC emissions should not have an impact on ozone depletion for the period Based on Figure Q20-2, the decrease in ESC is expected after ) In 1982 : El-Chicon volcanic eruption. In 1991: Pinatubo eruption. The stratospheric atmosphere is cleaned from aerosols after Less stratospheric aerosols lead to less Antarctic ozone depletion.

19 Problem 3: Global ozone trend Antarctic Figure Q20-2 Figure Q12-3