Concentrations of Ozone-Depleting Substances

Ozone, a gas present throughout the Earth’s atmosphere, is a pollutant at the Earth’s surface but forms a protective layer in the stratosphere, helping shield the Earth from the sun’s ultraviolet (UV) radiation. Exposure to UV radiation is associated with skin cancer, cataracts, and other human health and ecological problems (U.S. EPA, 2006).

Starting in the late 1970s, stratospheric ozone levels were observed to be declining due to worldwide releases of various human-produced chemicals referred to as ozone-depleting substances (ODSs), particularly halocarbons such as the long-lived chlorofluorocarbons (CFCs), bromine-containing halons, and methyl bromide. Through rapid catalytic reactions with ozone, the chlorine and bromine from these chemicals have depleted the protective ozone layer (see the Stratospheric Ozone Levels indicator).

Worldwide production and consumption of ODSs are being progressively eliminated under the provisions of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. Over time, reducing the atmospheric loading of ODSs is expected to result in global increases in stratospheric ozone. However, because some ODS gases have long atmospheric lifetimes, and because of pre-phaseout ODS stockpiling for post-phaseout use, ambient concentrations of ODSs have only recently begun to stabilize and in some cases begun to decline. While some gases, like methyl chloroform, decay quickly in the atmosphere, other gases, like CFCs and halons, have atmospheric lifetimes on the order of hundreds or thousands of years.

Measures of effective equivalent troposphere chlorine (EECl) and effective equivalent stratospheric chlorine (EESC) are commonly used to represent atmospheric concentrations of ODSs. Both represent ODS concentrations weighted by their potential to catalyze the destruction of stratospheric ozone relative to the ability of chlorine to do so. (EESC is typically derived by adding a 3-year time lag to EECl to account for the time it takes for emissions of ODSs at the Earth’s surface to migrate from the troposphere to the stratosphere and cause stratospheric ozone depletion.)

This indicator presents trends in concentrations of tropospheric ODSs as EECl. The EECl trend is based on measurements from the National Oceanic and Atmospheric Association (NOAA) Climate Monitoring and Diagnostics Laboratory and estimates of halocarbon emissions from industrial and international sources from 1995 to 2013. Concentrations of EECl are presented as weighted averages based on ground-based measurements of mixing ratios¹ since 1995 at the following remote locations: Alert, Northwest Territories, Canada; Point Barrow, Alaska; Niwot Ridge, Colorado; Mauna Loa, Hawaii; American Samoa; Cape Grim, Tasmania, Australia; and the South Pole (Montzka et al., 1999). Data on total EECl are also available for 1992 through 1994, but these years of monitoring are only presented in the chemical-specific graphs because the monitoring did not include methyl bromide, a quantitatively important ODS. Because most ODSs have long atmospheric half-lives, the ODS concentrations shown in this indicator reflect past and recent contributions from emissions sources within the U.S. and worldwide.

¹ The mixing ratio is the ratio of the partial pressure of a gas to the total atmospheric pressure.

Total EECl resulting from ODS reached its peak concentration in the mid-1990s at slightly over 2,700 parts per trillion of air by volume and has declined by approximately 18 percent between the mid-1990s and 2013 (Exhibit 1). Although tropospheric concentrations of CFCs and several other individual ODS compounds have begun to decline, concentrations of halons and selected hydrochlorofluorocarbons (HCFCs) have not yet stabilized.

Declines in EECl abundances of several ODSs in the troposphere between 1992 and 2013 have contributed to the decline in total EECl (Exhibit 2). EECl attributed to methyl chloroform has decreased by 97 percent over this period due to decreased emissions as well as its short atmospheric lifetime. EECl associated with CFCs has decreased more slowly: 2013 levels are approximately 9 percent lower than the peak tropospheric concentration that occurred between 1995 and 1997. The slow decline of CFCs is a result of continued emissions of CFCs from stockpiles in developed countries, continued production and use in developing countries, and their long atmospheric lifetimes. EECl from methyl bromide has decreased approximately 32 percent from its peak in 1998 to 2013; however, continued use of methyl bromide in developing countries and in developed countries through critical use exemptions slows the decrease in EECl associated with this compound. EECl from methyl bromide exhibits seasonal variations, which likely results from the seasonal use of this chemical as a soil fumigant.

Although the concentrations of some tropospheric ODSs have declined, concentrations of others, including certain halons and HCFCs, have increased (Exhibit 2). In 2013, EECl attributed to halons was 42 percent higher than 1992 levels, which reflects continued emissions of halons from stockpiles. The EECl attributed to HCFCs in 2013 was more than three times higher than in 1992. This is due to continued production and consumption of HCFCs in both developed and developing countries. 

• The calculation of EECl depends on the understanding of the interactions and atmospheric residence times of many different gases; incorrect knowledge about these factors could affect trends in the EECl.
• EECl is calculated by weighting each ODS’s concentration by the substance’s ability to catalyze destruction of stratospheric ozone, or the ozone destruction potential. The ozone destruction potentials used to transform the data have inherent uncertainties, which can affect the trend analyses.
• Factors additional to trends in halocarbons affect trends in stratospheric ozone. These factors include changes in climate (e.g., temperature, winds), changes in emissions and concentrations of trace gases like nitrous oxide and methane, and changes in aerosol loading such as occurs after an explosive volcanic eruption.
• The trend for halons is based only on halon 1211 and halon 1301. According to NOAA, these are two of the three most commonly used halons. The third—halon 2402—is not included in this indicator because it was not measured over the entire period of record. Current measurements indicate that halon 2402 accounts for less than 10 percent of the total EECl for these three chemicals. 

Tropospheric concentrations of ODSs presented in this indicator are based on measurements made by NOAA’s Global Monitoring Division. The 1992-2013 data are taken from a file (“2014 update”) found at the Division’s online data repository (NOAA, 2014) (ftp://ftp.cmdl.noaa.gov/hats/Total_Cl_Br/). The file updates tropospheric ODS concentrations previously reported in the peer-reviewed literature (Montzka et al., 1999, 2003).