Scientific Assessment of Ozone Depletion: 2010

Chapter 1 Scientific Summary

Scientific Summary Chapter 1: Ozone-Depleting Substances (ODSs) and Related Chemicals

The amended and adjusted Montreal Protocol continues to be successful at reducing emissions and atmospheric abundances of most controlled ozone-depleting substances (ODSs).

Tropospheric Chlorine

• Total tropospheric chlorine from long-lived chemicals (~3.4 parts per billion (ppb) in 2008) continued to decrease between 2005 and 2008. Recent decreases in tropospheric chlorine (Cl) have been at a slower rate than in earlier years (decreasing at 14 parts per trillion per year (ppt/yr) during 2007-2008 compared to a decline of 21 ppt/yr during 2003-2004) and were slower than the decline of 23 ppt/yr projected in the A1 (most likely, or baseline) scenario of the 2006 Assessment. The tropospheric Cl decline has recently been slower than projected in the A1 scenario because chlorofluorocarbon-11 (CFC-11) and CFC-12 did not decline as rapidly as projected and because increases in hydrochlorofluorocarbons (HCFCs) were larger than projected.
• The contributions of specific substances or groups of substances to the decline in tropospheric Cl have changed since the previous Assessment. Compared to 2004, by 2008 observed declines in Cl from methyl chloroform (CH₃CCl₃) had become smaller, declines in Cl from CFCs had become larger (particularly CFC-12), and increases in Cl from HCFCs had accelerated. Thus, the observed change in total tropospheric Cl of -14 ppt/yr during 2007-2008 arose from:
• -13.2 ppt Cl/yr from changes observed for CFCs
• -6.2 ppt Cl/yr from changes observed for methyl chloroform
• -5.1 ppt Cl/yr from changes observed for carbon tetrachloride
• -0.1 ppt Cl/yr from changes observed for halon-1211
• +10.6 ppt Cl/yr from changes observed for HCFCs
• Chlorofluorocarbons (CFCs), consisting primarily of CFC-11, -12, and -113, accounted for 2.08 ppb (about 62%) of total tropospheric Cl in 2008. The global atmospheric mixing ratio of CFC-12, which accounts for about one-third of the current atmospheric chlorine loading, decreased for the first time during 2005-2008 and by mid-2008 had declined by 1.3% (7.1 ± 0.2 parts per trillion, ppt) from peak levels observed during 2000-2004.
• Hydrochlorofluorocarbons (HCFCs), which are substitutes for long-lived ozone-depleting substances, accounted for 251 ppt (7.5%) of total tropospheric Cl in 2008. HCFC-22, the most abundant of the HCFCs, increased at a rate of about 8 ppt/yr (4.3%/yr) during 2007-2008, more than 50% faster than observed in 2003-2004 but comparable to the 7 ppt/yr projected in the A1 scenario of the 2006 Assessment for 2007-2008. HCFC-142b mixing ratios increased by 1.1 ppt/yr (6%/yr) during 2007-2008, about twice as fast as was observed during 2003-2004 and substantially faster than the 0.2 ppt/yr projected in the 2006 Assessment A1 scenario for 2007-2008. HCFC-141b mixing ratios increased by 0.6 ppt/yr (3%/yr) during 2007-2008, which is a similar rate observed in 2003-2004 and projected in the 2006 Assessment A1 scenario.
• Methyl chloroform (CH₃CCl₃) accounted for only 32 ppt (1%) of total tropospheric Cl in 2008, down from a mean contribution of about 10% during the 1980s.
• Carbon tetrachloride (CCl₄) accounted for 359 ppt (about 11%) of total tropospheric Cl in 2008. Mixing ratios of CCl₄ declined slightly less than projected in the A1 scenario of the 2006 Assessment during 2005-2008.

Stratospheric Chlorine and Fluorine

• The stratospheric chlorine burden derived by ground-based total column and space-based measurements of inorganic chlorine continued to decline during 2005-2008. This burden agrees within ±0.3 ppb (±8%) with the amounts expected from surface data when the delay due to transport is considered. The uncertainty in this burden is large relative to the expected chlorine contributions from shorter-lived source gases and product gases of 80 (40-130) ppt. Declines since 1996 in total column and stratospheric abundances of inorganic chlorine compounds are reasonably consistent with the observed trends in long-lived source gases over this period.
• Measured column abundances of hydrogen fluoride increased during 2005-2008 at a smaller rate than in earlier years. This is qualitatively consistent with observed changes in tropospheric fluorine (F) from CFCs, HCFCs, hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs) that increased at a mean annual rate of 40 ± 4 ppt/yr (1.6 ± 0.1%/yr) since late 1996, which is reduced from 60-100 ppt/yr observed during the 1980s and early 1990s.

Tropospheric Bromine

• Total organic bromine from controlled ODSs continued to decrease in the troposphere and by mid-2008 was 15.7 ± 0.2 ppt, approximately 1 ppt below peak levels observed in 1998. This decrease was close to that expected in the A1 scenario of the 2006 Assessment and was driven by declines observed for methyl bromide (CH₃Br) that more than offset increased bromine (Br) from halons.
• Bromine from halons stopped increasing during 2005-2008. Mixing ratios of halon-1211 decreased for the first time during 2005-2008 and by mid-2008 were 0.1 ppt below levels observed in 2004. Halon-1301 continued to increase in the atmosphere during 2005-2008 but at a slower rate than observed during 2003-2004. The mean rate of increase was 0.03-0.04 ppt/yr during 2007-2008. A decrease of 0.01 ppt/yr was observed for halon-2402 in the global troposphere during 2007-2008.
• Tropospheric methyl bromide (CH₃Br) mixing ratios continued to decline during 2005-2008, and by 2008 had declined by 1.9 ppt (about 20%) from peak levels measured during 1996-1998. Evidence continues to suggest that this decline is the result of reduced industrial production, consumption, and emission. This industry-derived emission is estimated to have accounted for 25-35% of total global CH₃Br emissions during 1996-1998, before industrial production and consumption were reduced. Uncertainties in the variability of natural emissions and in the magnitude of methyl bromide stockpiles in recent years limit our understanding of this anthropogenic emissions fraction, which is derived by comparing the observed atmospheric changes to emission changes derived from reported production and consumption.
• By 2008, nearly 50% of total methyl bromide consumption was for uses not controlled by the Montreal Protocol (quarantine and pre-shipment applications). From peak levels in 1996-1998, industrial consumption in 2008 for controlled and non-controlled uses of CH₃Br had declined by about 70%. Sulfuryl fluoride (SO₂F₂) is used increasingly as a fumigant to replace methyl bromide for controlled uses because it does not directly cause ozone depletion, but it has a calculated direct, 100-year Global Warming Potential (GWP100) of 4740. The SO₂F₂ global background mixing ratio increased during recent decades and had reached about 1.5 ppt by 2008.

Stratospheric Bromine

• Total bromine in the stratosphere was 22.5 (19.5-24.5) ppt in 2008. It is no longer increasing and by some measures has decreased slightly during recent years. Multiple measures of stratospheric bromine monoxide (BrO) show changes consistent with tropospheric Br trends derived from observed atmospheric changes in CH₃Br and the halons. Slightly less than half of the stratospheric bromine derived from these BrO observations is from controlled uses of halons and methyl bromide. The remainder comes from natural sources of methyl bromide and other bromocarbons, and from quarantine and pre-shipment uses of methyl bromide not controlled by the Montreal Protocol.

Very Short-Lived Halogenated Substances (VSLS)

VSLS are defined as trace gases whose local lifetimes are comparable to, or shorter than, tropospheric transport timescales and that have non-uniform tropospheric abundances. In practice, VSLS are considered to be those compounds having atmospheric lifetimes of less than 6 months.

• The amount of halogen from a very short-lived source substance that reaches the stratosphere depends on the location of the VSLS emissions, as well as atmospheric removal and transport processes. Substantial uncertainties remain in quantifying the full impact of chlorine- and bromine-containing VSLS on stratospheric ozone. Updated results continue to suggest that brominated VSLS contribute to stratospheric ozone depletion, particularly under enhanced aerosol loading. It is unlikely that iodinated gases are important for stratospheric ozone loss in the present-day atmosphere.
• Based on a limited number of observations, very short-lived source gases account for 55 (38-80) ppt chlorine in the middle of the tropical tropopause layer (TTL). From observations of hydrogen chloride (HCl) and carbonyl chloride (COCl₂) in this region, an additional ~25 (0-50) ppt chlorine is estimated to arise from VSLS degradation. The sum of contributions from source gases and these product gases amounts to ~80 (40-130) ppt chlorine from VSLS that potentially reaches the stratosphere. About 40 ppt of the 55 ppt of chlorine in the TTL from source gases is from anthropogenic VSLS emissions (e.g., methylene chloride, CH₂Cl₂; chloroform, CHCl₃; 1,2 dichloroethane, CH₂ClCH₂Cl; perchloroethylene, CCl₂CCl₂), but their contribution to stratospheric chlorine loading is not well quantified.
• Two independent approaches suggest that VSLS contribute significantly to stratospheric bromine. Stratospheric bromine derived from observations of BrO implies a contribution of 6 (3-8) ppt of bromine from VSLS. Observed, very short-lived source gases account for 2.7 (1.4-4.6) ppt Br in the middle of the tropical tropopause layer. By including modeled estimates of product gas injection into the stratosphere, the total contribution of VSLS to stratospheric bromine is estimated to be 1-8 ppt.
• Future climate changes could affect the contribution of VSLS to stratospheric halogen and its influence on stratospheric ozone. Future potential use of anthropogenic halogenated VSLS may contribute to stratospheric halogen in a similar way as do present-day natural VSLS. Future environmental changes could influence both anthropogenic and natural VSLS contributions to stratospheric halogens.

Equivalent Effective Stratospheric Chlorine (EESC)

Figure S1-1. Stratospheric EESC Relative to Peak Abundances Versus Time

EESC is a sum of chlorine and bromine derived from ODS tropospheric abundances weighted to reflect their potential influence on ozone in different parts of the stratosphere. The growth and decline in EESC varies in different regions of the atmosphere because a given tropospheric abundance propagates to the stratosphere with varying time lags associated with transport. Thus the EESC abundance, when it peaks, and how much it has declined from its peak vary in different regions of the atmosphere.

• EESC has decreased throughout the stratosphere.
• By the end of 2008, midlatitude EESC had decreased by about 11% from its peak value in 1997. This drop is 28% of the decrease required for EESC in midlatitudes (red curve in Figure S1-1) to return to the 1980 benchmark level.
• By the end of 2008, polar EESC had decreased by about 5% from its peak value in 2002. This drop is 10% of the decrease required for EESC in polar regions (blue curve in Figure S1-1) to return to the 1980 benchmark level.
• During the past four years, no specific substance or group of substances dominated the decline in the total combined abundance of ozone-depleting halogen in the troposphere. In contrast to earlier years, the long-lived CFCs now contribute similarly to the decline as do the short-lived CH₃CCl₃ and CH₃Br. Other substances contributed less to this decline, and HCFCs added to this halogen burden over this period.

Emission Estimates and Lifetimes

• While global emissions of CFC-12 derived from atmospheric observations decreased during 2005-2008, those for CFC-11 did not change significantly over this period. Emissions from banks account for a substantial fraction of current emissions of the CFCs, halons, and HCFCs. Emissions inferred for CFCs from global observed changes did not decline during 2005-2008 as rapidly as projected in the A1 scenario of the 2006 Assessment, most likely because of underestimates of bank emissions.
• Global emissions of CCl₄ have declined only slowly over the past decade.
• These emissions, when inferred from observed global trends, were between 40 and 80 gigagrams per year (Gg/yr) during 2005-2008 given a range for the global CCl₄ lifetime of 33-23 years. By contrast, CCl₄ emissions derived with a number of assumptions from data reported to the United Nations Environment Programme (UNEP) ranged from 0-30 Gg/yr over this same period.
• In addition, there is a large variability in CCl₄ emissions derived from data reported to UNEP that is not reflected in emissions derived from measured global mixing ratio changes. This additional discrepancy cannot be explained by scaling the lifetime or by uncertainties in the atmospheric trends. If the analysis of data reported to UNEP is correct, unknown anthropogenic sources may be partly responsible for these observed discrepancies.
• Global emissions of HCFC-22 and HCFC-142b derived from observed atmospheric trends increased during 2005-2008. HCFC-142b global emissions increased appreciably over this period, compared to a projected emissions decline of 23% from 2004 to 2008. By 2008, emissions for HCFC-142b were two times larger than had been projected in the A1 scenario of the 2006 Assessment. These emission increases were coincident with increasing production of HCFCs in developing countries in general and in East Asia particularly. It is too soon to discern any influence of the 2007 Adjustments to the Montreal Protocol on the abundance and emissions of HCFCs.
• The sum of CFC emissions (weighted by direct, 100-year GWPs) has decreased on average by 8 ± 1%/yr from 2004 to 2008, and by 2008 amounted to 1.1 ± 0.3 gigatonnes of carbon dioxide-equivalent per year (GtCO₂-eq/yr). The sum of GWP-weighted emissions of HCFCs increased by 5 ± 2%/yr from 2004 to 2008, and by 2008 amounted to 0.74 ± 0.05 GtCO₂-eq/yr.
• Evidence is emerging that lifetimes for some important ODSs (e.g., CFC-11) may be somewhat longer than reported in past assessments. In the absence of corroborative studies, however, the CFC-11 lifetime reported in this Assessment remains unchanged at 45 years. Revisions in the CFC-11 lifetime would affect estimates of its global emission derived from atmospheric changes and calculated values for Ozone Depletion Potentials (ODPs) and best-estimate lifetimes for some other halocarbons.

Other Trace Gases That Directly Affect Ozone and Climate

• The methane (CH₄) global growth rate was small, averaging 0.9 ± 3.3 ppb/yr between 1998-2006, but increased to 6.7 ± 0.6 ppb/yr from 2006-2008. Analysis of atmospheric data suggests that this increase is due to wetland sources in both the high northern latitudes and the tropics. The growth rate variability observed during 2006-2008 is similar in magnitude to that observed over the last two decades.
• In 2005-2008 the average growth rate of nitrous oxide (N₂O) was 0.8 ppb/yr, with a global average tropospheric mixing ratio of 322 ppb in 2008. A recent study has suggested that at the present time, Ozone Depletion Potential-weighted anthropogenic emissions of N₂O are the most significant emissions of a substance that depletes ozone.
• Long-term changes in carbonyl sulfide (COS) measured as total columns above the Jungfraujoch (46.5°N) and from surface flasks sampled in the Northern Hemisphere show that atmospheric mixing ratios have increased slightly during recent years concurrently with increases in "bottom-up" inventory-based emissions of global sulfur. Results from surface measurements show a mean global surface mixing ratio of 493 ppt in 2008 and a mean rate of increase of 1.8 ppt/yr during 2000-2008. New laboratory, observational, and modeling studies indicate that vegetative uptake of COS is significantly larger than considered in the past.

Other Trace Gases with an Indirect Influence on Ozone

• The carbon dioxide (CO₂) global average mixing ratio was 385 parts per million (ppm) in 2008 and had increased during 2005-2008 at an average rate of 2.1 ppm/yr. This rate is higher than the average growth rate during the 1990s of 1.5 ppm/yr and corresponds with increased rates of fossil fuel combustion.
• Hydrofluorocarbons (HFCs) used as ODS substitutes continued to increase in the global atmosphere. HFC-134a is the most abundant HFC; its global mixing ratio reached about 48 ppt in 2008 and was increasing at 4.7 ppt/yr. Other HFCs have been identified in the global atmosphere at <10 ppt (e.g., HFC-125, -143a, -32, and -152a) and were increasing at ≤1 ppt/yr in 2008.
• Emissions of HFC-23, a by-product of HCFC-22 production, have increased over the past decade even as efforts at minimizing these emissions were implemented in both developed and developing countries. These emission increases are concurrent with rapidly increasing HCFC-22 production in developing countries and are likely due to increasing production of HCFC-22 in facilities not covered by the Kyoto Protocol's Clean Development Mechanism projects. Globally averaged HFC-23 mixing ratios reached 21.8 ppt in 2008, with a yearly increase of 0.8 ppt/yr (3.9%/yr).
• The sum of emissions (weighted by direct, 100-year GWPs) of HFCs used as ODS replacements has increased by 8-9%/yr from 2004 to 2008, and by 2008 amounted to 0.39 ± 0.03 GtCO₂-eq/yr. Regional studies suggest significant contributions of HFC-134a and -152a emissions during 2005-2006 from Europe, North America, and Asia. Emissions of HFC-23, most of which do not arise from use of this substance as an ODS replacement, added an additional 0.2 Gt CO₂-eq/yr, on average, during 2006-2008.
• Sulfur hexafluoride (SF₆) and nitrogen trifluoride (NF₃): Global averaged mixing ratios of SF6 reached 6.4 ppt in 2008, with a yearly increase of 0.2 ppt/yr. NF3 was detected in the atmosphere for the first time, with a global mean mixing ratio in 2008 of 0.45 ppt and a growth rate of 0.05 ppt/yr, or 11%/yr.

Direct Radiative Forcing

The abundances of ODSs as well as many of their replacements contribute to radiative forcing of the atmosphere. These climate-related forcings have been updated using the current observations of atmospheric abundances and are summarized in Table S1-1. This table also contains the primary Kyoto Protocol gases as reference.

• Over these 5 years, radiative forcing from the sum of ODSs and HFCs has increased but, by 2008, remained small relative to the forcing changes from CO₂ (see Table S1-1).

Table S1-1. Direct radiative forcings of ODSs and other gases, and their recent changes.

Specific Substance or Group of Substances	Direct Radiative Forcing (2008), milliWatts per square meter (mW/m2)	Change in Direct Radiative Forcing (2003.5-2008.5), mW/m2
CFCs *	262	-6
Other ODSs *	15	-2
HCFCs *	45	8
HFCs #,a	12	5
HFC-23 #	4	0.9
CO2 #	1740	139
CH4 #	500	4
N2O #	170	12
PFCs #	5.4	0.5
SF6 #	3.4	0.7
Sum of Montreal Protocol gases *	322	0
Sum of Kyoto Protocol gases #	2434	163

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* Montreal Protocol Gases refers to CFCs, other ODSs (CCl₄, CH₃CCl₃, halons, CH₃Br), and HCFCs.
^# Kyoto Protocol Gases (CO₂, CH₄, N₂O, HFCs, PFCs, and SF₆).
^a Only those HFCs for which emissions arise primarily through use as ODS replacements (i.e., not HFC-23).