CFC Emissions and Stratospheric Ozone Depletion Assignment

Questions? Ask: Prof. Carmichael and Sarika Kulkarni

Objectives

To learn about atmospheric fate and transport of air pollutants. To model emissions, transport and chemical destruction of chlorofluorocarbons in the atmosphere. To evaluate potential of industrial chemicals to deplete stratospheric ozone. To evaluate strategies for the monitoring and reduction of stratospheric ozone depletion.

Date Available

Tuesday, August 31, 2004

Date Due

Tuesday, September 21, 2004

Background

Stratospheric Ozone Depletion was one of the most politically charged environmental issues in the 1970s and 1980s. As more scientific evidence surfaced throughout the 1980s and 1990s, antics by publicity seekers were replaced by international agreements and national legislation to deal with this issue. Nonetheless, it is informative to review the commentary from the left (e.g. Greenpeace) and right (e.g. Rush Limbaugh) of the poltical spectrum. Both have a similar mission, only differ in audiences. Some selected offerings:

Greenpeace: Ozone Crisis

Rush Investigates the Ozone Layer

We have been discussing the role of CFCs in the destruction of stratospheric ozone. As a result of the strong evidence linking CFCs to the Antarctic Ozone Hole, international conventions are in place which call for the phaseout of CFCs by the end of the decade. The CFCs are being replaced initially by HCFCs. The HCFCs however still contain chlorine. Ultimately, the HCFCs will also be replaced with chlorine-free chemicals. In this problem we are going to use the STELLA II software to explore various aspects of the CFC issue.

Tools

You will need the following tools to complete this module:

A computer with Internet connection
A Web browser and linked Helper Applications
Imagination
Paper and Pencil (and eraser)
Calculator
STELLA - On ICAEN Machine

Task 1: Descriptions of Ozone and Ozone Hole

An excellent description of the main issues related to stratospheric ozone is given on the website solcomhouse(Ozone hole). Read carefully this material.

Task 2: Ozone Hole for 2004

The ozone hole in 2000 is back and is apparently growing at a rate greater than any prior year. You can get up-to-date information on what's happening at the TOMS (Total Ozone Mapping Spectrometer) web site and on the solcomhouse (South American )site.

Describe the details of the 2000 Antarctic hole and how it compares to other years.

Task 3: Ozone Depletion Potential of Industrial Chemicals

Consider the ozone depletion potential of five industrial chemicals released to the atmosphere.

A convenient way to estimate potential ozone impacts is to estimate a Ozone Depletion Potential (ODP) for chemicals (hopefully before release). The ODP of a chemical is based on the reactivity of the chemical in the ozone cycle and the atmospheric lifetime. This is typically normalized by the ozone depletion potential of CFC-11. The ODP is given in Equation 1:

(Eq. 1)

where
k is the rate constant for the reaction of chemical i with atomic oxygen [cm3/molecule/s]
tau is the atmospheric lifetime of chemical i [years]
The values for reaction rate and recent production are presented below in Tables 1.1 and 1.2.

Table 1.1. Parameters to compute ODP for major chlorinated solvents

             Rate Constant for Reaction
             with Atomic Oxygen at          Atmospheric
             298 K, (cm3/molecule/s)        Lifetime (yrs)
 

methylene chloride      2.12E-10               0.6

1,1,1-trichloroethane   3.18E-10               6.3

trichloroethylene       NA (use 0)             0.1

tetrachloroethylene     NA (use 0)             0.6

CFC-113                 2.0E-10                90.0

CFC-11                  2.3E-10                60.0
(CFCl3)

Table 1.2. Recent production of major chlorinated solvents (thousands of metric tons)

                     1,1,1                              
        Methylene    Trichloro       Trichloro  Tetrachloro             
Year    Chloride     ethane          ethylene   ethylene     CFC-113      TOTAL
1979    287          325             145          351        47           1155
1980    256          314             121          347        50           1088
1981    269          279             117          313        52           1031
1982    241          270             120          265        56           952
1983    265          266             100          248        60           939
1984    275          306             86           260        68           995
1985    263          268             73           224        73           901
1986    257          296             82           188        73           896
1987    234          315             82           215        78           924
1988    229          328             82           226        78           943
1989    213          353             82           215        78           941

With Equation 1 and the information in Tables 1.1 and 1.2:

assume that the 1979 and 1989 atmospheric emissions of each solvent amounted to 80% of production. Using the ODP, rank the five chemicals by their impact to ozone destruction for 1979 and 1989. Compare the overall ODP of the chemicals (i.e. the sum) in 1979 versus 1989. What conclusions can be drawn?
suppose that an international governmental body decided that all CFCs must be replaced by an equal mass of 1,1,1-trichloroethane. Repeat the above calculations using the 1989 data. Compare the overall ODP for this case to the overall ODP above.

Task 4: CFC Fate and Transport Modeling

One of the primary CFCs is CFC-11 (CCl3F). Examine the background information on the uses and emissions of CFCs since 1930. In this part of the assignment the behavior of CFC-11 in both the troposphere and the stratosphere is investigated. As discussed in class, the transport of CFCs in the atmosphere and the differing chemical behavior of CFCs in the troposphere and stratosphere must be considered by the model.

The STELLA model schematic shown considers the atmosphere to be divided into two compartments (a troposphere and a stratosphere). It considers that CFC-11 is emitted into the tropopshere where it is mixed and removed by transport into the stratosphere. In the stratosphere CFC-11 reacts chemically to release chlorine atoms. The troposphere compartment yields the mixing ratio of CFC-11. The stratospheric compartment yields the mixing ratio of CFC-11 and all free chlorine molecules. Thus while specific information regarding CFC-11 is used, the stratospheric chlorine is scaled to reflect the compounded chlorine levels coming from all the various CFCs emitted. The STELLA model begins the simulation in 1930 and uses the historical yearly emissions for CFC-11.

* Equation is a GIF Image *

Calculate and plot the CFC-11 emissions, mixing ratios in the troposphere and stratosphere, and chlorine in the stratosphere from year 1930 to 1989. Plot mixing ratios in units of parts per trillion. Comment on how the mixing ratios in the troposphere and stratosphere respond to the changing emissions.

Some needed constants:
Avogadro's Number = 6.0238E23 molecules/mole
Chlorine atoms = 3 (per CFC-11 molecule)
MW CFC-11 = 137.36 (g/mol)
MW Cl = 35.45 (g/mol)
Stratospheric air molecules = 1.95E43 (total number in stratosphere)
Stratospheric lifetime = 55 years (for CFC-11)
Stratospheric lifetiime = 1 year (for Cl atoms)
Total Cl ratio = 14.5 (average ratio between total chlorine and that from CFC-11)
Tropospheric air molecules = 8.69E43 (total number in troposphere)
Adjustment factor = 3.68
Tropospheric residence time = 5 years (average time for molecule to transport to stratosphere)

1E9 g = 1 Mkg
1E12 = 1 trillion

Task 5: Ozone Hole Predictions

The antarctic ozone hole first formed when the chlorine levels in the stratosphere reached ~ 2 ppbv. Based on the strong evidence linking the free chlorine levels to the ozone hole formation, an international agreement was struck to phaseout the use of CFCs. The Montreal Protocol spelled out a time scale for the replacement of CFCs by the year 2000. This phase-out is to be accomplished by a reduction in the need for these chemicals (conservation, new designs, etc.) and the development of replacement chemicals which have lower ODPs (ozone depletion potentials). The emissions for CFC-11 to meet the Montreal Protocol are as follows (Table 3.1):

Table 3.1. Emissions of CFC-11 for meeting Montreal Protocol. Emissions are give as ratios relative to emissions in 1989.

        Year    Emissions
        1990    1.042
        1991    0.833
        1992    0.833
        1993    0.666
        1994    0.666
        1995    0.417
        1996    0.417
        1997    0.125
        1998    0.125
        1999    0.125
        2000    0.0

A). Using the STELLA model calculate the mixing ratio of CFC-11 in the troposphere and stratosphere and free chlorine in the stratosphere until year 2150 using the emissions given in Table 1. What year do chlorine levels in the stratosphere drop below 2 ppbv?

B). Figures from a recent article in Naturesummarize measurements of CFC-11 from 1978 to 1992. Compare your model results to the observations. Using the data from 1989 to 1993 what conclusion can you draw regarding the present compliance with the Montreal Protocol. (Are we ahead of schedule or behind? Can we use such measurements as a means of varifying compliance?)

Task 6: Impact of Replacement Chemicals

The first generation of replacement chemicals are called the HCFCs (hydrochlorofluorocarbons). These are made by inserting a hydrogen atom into the CFC. The reason for doing this is to reduce the atmospheric lifetime of the HCFC. The HCFCs react in the troposphere with the OH radical. One of the new replacement HCFCs (HCFC-123) has a tropospheric lifetime of 2 years, a stratospheric lifetime of 50 years, and has 2 chlorine atoms. Modify the STELLA model to handle this HCFC (don't forget to adjust total chlorine ratio value - see model, re: CFC-11 has 3 Cl, HCFC-123 has 2). Note: the tropospheric residence time is unchanged; you must now add a HCFC sink in the troposphere due to the chemical reaction.

Assume that we could have replaced (instantaneously) all CFC-11 in 1989 by the equivalent emissions of HCFC-123 (CF3CHCl2 - molecular weight 152.9 g/mol) and that the emissions of HCFC for all years after 1989 were equal to those in 1989. Under this assumption, calculate the mixing ratio of HCFC in the troposphere and free chlorine in the stratosphere for the next 150 years. Under this scenario, when will the chlorine levels in the stratosphere fall below 2 ppbv? How does this compare with that value calculated in Task 3, part A?

The task of replacing HCFC's is not easy as discussed in C&EN, August 17, 1998, p17-18 Page 1 Page 2. Please comment on what you learned from this article regarding the challenges to industry of the mandated phaseout.

Task 7: Phase-Out of Ozone Depleting Chemicals

Based on the above information what are your recommendations regarding the present CFC phase-out schedule? Is phase-out by 2000 sufficient? or should we accelerate? How quickly would you advocate the elimination of the HCFCs? How well will we do if international compliance with the Montreal Protocol is 90%? or 50%? Keep in mind that the production of stratospheric ozone is rather slow so that the recovery of the stratospheric ozone concentrations will lag behind the chlorine levels by several decades.

Task 8: Is the battle won?

So far we have focused solely on chlorine compounds. However, other halogen compounds (for example, Br) containing compounds are also strong ozone depleting compounds. Find an article or web site (C&EN, March 2, 1998, p27) related to these compounds and comment on:

What these compounds are important for?
What are their major sources?
What is being done to deal with these compounds?

Task 9: The Links with Global Warming

Stratospheric Ozone loss and global warming are interlinked. For example, CFC's are strong greenhouse gases. But, why hasn't the ozone hole appeared in the Northern hemisphere? Describe in your own words why this is the case. But there is a growing concern that increasing levels of gases may change this (C&EN, April 13, 1998, p12). Can you find additional discussions of this issue on the web? Please comment on what you found and your thoughts on whether we have the ozone hole "linked".

Task 10: Low Ozone event in Arctic

It is interesting to think about why the ozone hole appeared in the Antractic and not the Arctic.

Discussion what is the similarities and differences between these 2 regions. But interesting things are indeed happening to Artic ozone(solcomhouse-Arctic Ozone and European Ozone Event).
Comment on what is happening and what the consequenes may be.
What are your overall thoughts on the Stratospheric Ozone problem and whether the problem is solved?

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