Estimating CFC Air Curves
When atmospheric measurements are not available (early end of CFC dating ranges), estimates of production and release must be made using information from the Chemical Manufacturers Association (CMA) and the Alternate Fluorocarbon Environmental Acceptability Study (AFEAS).
The quality of production data is deteriorating due to unreported data and under-reporting. Calculations indicate significant under-reporting after the Montreal Protocol.
The release of CFCs into the atmosphere must account for the following:
- Fugitive emissions: 2.5% of CFC-12, 1.5% of CFC-11 and 1% of CFC-113 lost during manufacturing. Also, 0.5% of CFC-113 is lost during packaging.
- Aerosols: 6 months is needed for sale- 50% lost in the first year and 50% in the following year.
- Long-lived refrigeration: Average lifetime is 12 years with 2% loss during filling. CFC-113 used in large centrifugal chillers- lifetime is 12 years.
- Short-lived refrigeration: Average refrigerant charge life is 4 years. CFC-113 not used as a refrigerant.
- Open cell foam: 2 month release delay.
- Closed cell foam (rigid): CFC-12 lost within 2 years. CFC-11 and CFC-113 can be released slowly from this foam into the soil atmosphere after burial- lost over 12 years.
- Cleaning, drying, and dry-cleaning (CFC-113): Release is 6 months following sale.
- Heat transfer fluid (CFC-113): Lifetime is about 12 years.
- Other uses: 6 month delay in release.
- See Gamlen et al., 1986; McCarthy et al., 1975; Fisher and Midgeley, 1993; AFEAS, 1993 in reference list of ‘Background’ section.
- Atmospheric lifetime (residence time) can be estimated from Atmospheric Lifetime Experiment (ALE) measurements, and from Global Atmospheric Gases Experiment (GAGE) measurements. Estimates change frequently (see Elkins et al., 1993 in reference list of ‘Background’ section for CFCs).
- Atmospheric concentrations for CFC-11 and CFC-12 can be reconstructed for the period 1940-1975. Reliable atmospheric measurements exist after this time period.
- Atmospheric concentrations for CFC-113 can be reconstructed for the period 1953-early 1980s. Reliable atmospheric measurements exist after this time period.
Dating capabilities for CFCs
- CFC-12 -- post 1940
- CFC-11 -- post 1945
- CFC-113 -- post 1953
95% of the CFCs are released in the northern hemisphere with interhemispheric exchange occurring within 1.1-1.4 years. Seasonal variations also occur, with the highest concentrations in the spring.
CFC concentrations are higher over cities and urban areas. When sampling in urban or industrial areas, local atmospheric measurements may be more useful for CFC reconstructions than global atmospheric CFC measurements that do not account for the local anomalies.