C A M P
The Chlorofluorocarbon Alternatives Monitoring Project
In the early 1990's, the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration expanded the HATS flask sampling program to monitor atmospheric levels of compounds used as replacements for chlorofluorocarbons (CFCs), methyl chloroform, and carbon tetrachloride (production of CFCs, methyl chloroform, and carbon tetrachloride will be prohibited throughout the globe as a result of the Montreal Protocol on Substances that Deplete the Ozone layer and subsequent Amendments). This research effort, made possible in part by funds from the Atmospheric Chemistry Project of NOAA's Global Climate and Global Change Program, is focused on replacements that have long atmospheric residence times and physical and chemical characteristics indicating they could influence levels of ozone in the stratosphere and contribute to radiative heating of the atmosphere.
Two classes of compounds encompassed by this description are the
and hydrofluorocarbons (HFCs).
These chemicals are currently used by industry in applications formerly requiring the use of CFCs, methyl chloroform, carbon tetrachloride and halons. These 'CFC-alternatives' are considered by industry and much of the scientific community as suitable replacements, at least on a temporary basis, owing to their reduced potential to deplete stratospheric ozone. It is clear that the availability of these compounds along with the development of additional alternative processes and chemicals over the past few years has allowed for the timely elimination of CFCs, methyl chloroform, and carbon tetrachloride by industry.
There is concern about long term use of these alternatives, however, as HCFCs contain chlorine and this chlorine contributes to stratospheric ozone depletion. Because of this, Amendments to the Montreal Protocol have mandated a phase-out schedule for production of HCFCs, thereby relegating HCFCs to the role of interim replacements only. Nations that have ratified the fully adjusted and amended Montreal Protocol have agreed to fully eliminate HCFC production by 2020-2040. In the case of the non-ozone-depleting HFCs, it has been postulated that extensive use of these chemicals in the future could contribute significantly to enhanced radiative atmospheric heating. Also, a number of the HFCs, for example HFC-134a, are expected to decompose in the atmosphere and produce a long-lived chemical called trifluoroacetic acid (or TFA) that is known to have adverse effects on certain biota. The amounts and adverse impacts of this byproduct are predicted to be small for many years into the future as long as mixing ratios of these TFA-precursors remain small. However, it has been postulated that the effects could be magnified in certain ecosystems where evaporation and containment can act to enhance levels of non-volatile solutes by many orders of magnitude (such as in seasonal wetland areas).
The Chlorofluorocarbon Alternatives Monitoring Project allows measurements of the global atmospheric distribution of these compounds over time. Samples are collected at seven remote locations on a regular basis throughout the year as part of the HATS flask sampling program, and are analyzed with customized state-of-the-art instrumentation in our Boulder Laboratory. This instrumentation allows for separation and detection of components in air samples with gas chromatography and mass spectrometry techniques (GC-MS). By analyzing comparatively large samples (0.2 - 0.8 L of air), we can precisely determine the abundance of these compounds in air at levels as low as 1 part per 10,000,000,000,000 (0.1 part per trillion). Accurate results are obtained by comparing responses from samples to calibration gas standards prepared within the standards lab of the HATS/CMDL group. The data acquired has in the past and will continue to provide a wealth of information regarding atmospheric chemistry and dynamics, the activities of industry during this transition period, and the year to year variability of the natural atmospheric environment.
Furthermore, along with measurements of alternatives to ozone-depleting substances, GC-MS analysis of flasks in this program has provided a wealth of unique data for other important trace gases on a global scale. Data for CFCs and other ozone-depleting substances from GC-MS analysis of flasks are semi-independent from results obtained with electron capture instruments located in the field. Although flasks allow for only a much lower sampling frequency than on-site instrumentation, flask samples are collected at many more sites than currently have on-site instrumentation. As of 2001, flasks were being analyzed from 12 stations across the globe; HATS on-site instruments are collecting data at 5 remote sites for the major ozone depleting gases, HCFC-22, the methyl halides, N2O, and SF6.
The flask sampling program also enables measurements of global distributions and trends for methyl halides, chlorinated gases with shorter lifetimes, benzene, COS, and additional brominated gases such as CH2Br2 and CHBr3. Some of these gases play important roles in regulating stratospheric ozone; our flask measurements provide a wealth of unique information regarding chemical and transport processes on broad scales. This type of information will enhance our predictive capacity to understand the chemical and radiative state of the atmosphere and how it changes over time.
Relevance of findings to date:
Data collected in this program has been published in numerous peer-reviewed articles in the scientific literature. Data from both flask measurements and on-site instrumentation have been used to assess changes in the total concentration of ozone-depleting halogen [Montzka et al., 1996; Montzka et al., 1999]. As of 1994, the concentration of total ozone-depleting halogen has decreased in the lower atmosphere. This change has been brought about through nations reducing their production of ozone-depleting gases in accord with the Montreal Protocol on Substances that Deplete the Ozone Layer. Most of the decline observed in the overall ozone-depleting potential of the atmosphere has resulted from the rapid decline observed for atmospheric CH3CCl3. Halons, however, continue to increase in the atmosphere and are delaying the decline more than other ozone-depleting gases (Butler et al., 1999). Concentrations of HCFCs are increasing relatively rapidly in the atmosphere, but these increases have not offset the gains realized through the declines in the major ozone-depleting gases.
Flask measurements of CH3CCl3 were used to infer concentrations of OH on global and hemispheric scales [Montzka et al., 2000]. It was shown that because industrial emissions of CH3CCl3 have declined dramatically in recent years, unique and more certain estimates of lifetimes on global and hemispheric scales were possible. The hydroxyl radical (OH) is the agent that is responsible for cleansing the atmosphere of nearly all chemically-reduced gases. As a result, accurate estimates of its concentration are extremely important for enhancing our understanding of atmospheric chemistry.
Flask data for HCFC-22, CH2Cl2, and C2Cl4 were used to estimate the accuracy of a 3-d global atmospheric chemistry model [Spivakovsky et al., 2000]. The data were found to be reasonably consistent with the model calculation, although some important uncertainties remain.
The global tropospheric abundance and distribution for HFC-134a were reported [Montzka et al., 1996]. This study was the first to report measurements of this important non-chlorinated substitute for ozone-depleting gases in the atmosphere. In 1994, the first global tropospheric distributions of HCFC-142b, HCFC-141b, were reported from measurements made in this group [Montzka et al., 1994]. All three of these "CFC-alternative" compounds are increasing rapidly throughout the lower atmosphere. As of 2001, however, they contributed only a small amount to the chlorine burden of the atmosphere (just over 5%).
In an earlier study, the global abundance and distribution of HCFC-22 was reported [Montzka et al., 1993]. Our results for this important HCFCs resolved a significant disparity observed for atmospheric levels of this compound by investigators using different techniques. This disparity had placed some uncertainty on the rate at which this class of compounds (and also HFCs) are removed from the atmosphere. However, from our current understanding of atmospheric levels, emission estimates, and laboratory experiments concerning HCFC-22, a consistent picture develops regarding atmospheric removal rates that is supported by similar studies of other industrial compounds (methyl chloroform).
HATS GC-MS flask instrumentation has been used to measure halogenated gases in air extracted from the snowpack, or firn, in Antarctica and Greenland (Butler et al., 1999). These studies confirm the purely anthropogenic origin of the ozone-depleting CFCs, HCFCs, methyl chloroform and halons.
HATS GC-MS flask instrumentation has been used to improve our understanding of the role the ocean plays in regulating atmospheric mixing ratios of some trace gases, in particular methyl bromide (Lobert et al., 1997; King et al., 2000). Results from the analysis of flasks sampled onboard ships in different oceans of the world confirm the conclusions obtained from our ship-board instrumentation and allow insights into the atmosphere-ocean fluxes of other trace gases as well.
Data from the HATS GC-MS flask sampling program has also appeared in WMO Scientific Assessments of Ozone Depletion (Kurylo et al., 1999; Montzka et al., 2002).
List of compounds for which data are currently being obtained from the HATS GC-MS flask sampling program:
- tetrafluoroethane (HFC-134a)
- methyl chloroform (CH3CCl3)
- carbon tetrachloride (CCl4)
- dichloromethane (CH2Cl2)
- chloroform (CHCl3)
- tetrachloroethylene (C2Cl4)
- Halon 1211
- Halon 2402
- Bromoform (CHBr3)
- Benzene (C6H6)
Links to measurement data and post-script figures from NOAA/CMDL made by CAMP and other programs at NOAA/CMDL:
(The Chlorofluorocarbon Alternatives Monitoring Project is part of the Flask Sampling Program of the Halocarbons and other Atmospheric Trace Species Group (HATS) of NOAA/CMDL)
Project Leader: Dr. Stephen A.
Montzka; phone: (303)-497-6657.