Scientific Assessment of Ozone Depletion: 2014

Chapter 1 Scientific Summary

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Scientific Summary Chapter 1: Update on ODSs and Other Gases of Interest to the Montreal Protocol

Changes in the global atmospheric abundance of a substance are determined by the balance between its emissions and removal. Declines observed for ozone-depleting substances (ODSs) controlled under the Montreal Protocol are due to global emission reductions that have made emissions smaller than removals. Most ODSs are potent greenhouse gases. As the majority of ODSs have been phased out, demand for hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) substitutes for the substances controlled under the Montreal Protocol has increased; these are also greenhouse gases. HCFCs deplete much less ozone per kilogram emitted than chlorofluorocarbons (CFCs), while HFCs essentially deplete no ozone.

The amended and adjusted Montreal Protocol has continued to reduce emissions and atmospheric abundances of most controlled ozone-depleting substances. By 2012, the total combined abundance of anthropogenic ODSs in the troposphere (measured as Equivalent Chlorine) had decreased by nearly 10% from its peak value in 1994.

The contributions to the overall decline in tropospheric chlorine (Cl) and bromine (Br) from substances and groups of substances controlled and not controlled under the Montreal Protocol have changed since the previous Assessment. The observed declines in total tropospheric Cl and Br from controlled substances during the 5-year period 2008-2012 were 13.4 ± 0.9 parts per trillion (ppt) yr-1 and 0.14 ± 0.02 ppt yr-1, respectively[1].

Substances controlled under the Montreal Protocol

Substances not controlled under the Montreal Protocol

Tropospheric Chlorine

Total tropospheric chlorine from ODSs continued to decrease between 2009 and 2012 to 3300 parts per trillion (ppt) in 2012. The observed decline in controlled substances of 13.4 ± 0.9 ppt Cl yr-1 during 2008-2012 was in line with the A1 (baseline) scenario of the 2010 Assessment.

Of total tropospheric Cl in 2012:

Global emissions of HCFCs remain substantial, but relative emissions of individual constituents have changed notably since the last Assessment. Emissions of HCFC-22 have stabilized since 2008 at around 370 gigagrams per year (Gg yr-1). HCFC-142b emissions decreased in the same period. In contrast emissions of HCFC-141b have increased since the last Assessment, in parallel with reported production and consumption in Article 5 Parties.

Estimated sources and sinks of CCl4 remain inconsistent with observations of its abundance. The estimate of the total global lifetime (26 years) combined with the observed CCl4 trend in the atmosphere (−1.1 to −1.4 ppt yr-1 in 2011-2012) implies emissions of 57 (40-74) Gg yr-1, which cannot be reconciled with estimated emissions from net reported production. New evidence indicates that other poorly quantified sources, unrelated to reported production, could contribute to the currently unaccounted emissions.

Three CFCs (CFC-112, -112a, -113a) and one HCFC (HCFC-133a) have recently been detected in the atmosphere. These four chlorine-containing compounds are listed in the Montreal Protocol and contribute about 4 ppt or ~ 0.1% toward current levels of total chlorine, currently adding less than 0.5 ppt Cl yr-1. Abundances of CFC-112 and CFC-112a are declining and those of CFC-113a and HCFC-133a are increasing. The sources of these chemicals are not known.

Stratospheric Inorganic Chlorine and Fluorine

Hydrogen chloride (HCl) is the major reservoir of inorganic chlorine (Cly) in the mid- to upper strat- osphere. Satellite-derived measurements of HCl (50°N-50°S) in the mid- to upper stratosphere show a mean decline of 0.6% ± 0.1% yr-1 between 1997 and 2012. This is consistent with the measured changes in controlled chlorinated source gases. Variability in this decline is observed over shorter time periods based on column measurements above some ground-based sites, likely due to dynamic variability.

Measured abundances of stratospheric fluorine product gases (HF, COF2, COClF) increased by about 1% yr-1 between 2008 and 2012. This is consistent with increases in measured abundances of fluorinated compounds and their degradation products. The increase was smaller than in the beginning of the 1990s, when the concentrations of fluorine-containing ODSs were increasing more rapidly.

Tropospheric Bromine

Total organic bromine from controlled ODSs continued to decrease in the troposphere and by 2012 was 15.2 ± 0.2 ppt, approximately 2 ppt below peak levels observed in 1998. This decrease was close to that expected in the A1 scenario of the 2010 Assessment and was primarily driven by declines in methyl bromide (CH3Br), with some recent contribution from an overall decrease in halons. Total bromine from halons had stopped increasing at the time of the last Assessment, and a decrease is now observable.

CH3Br mole fractions continued to decline during 2008-2012, and by 2012 had decreased to 7.0 ± 0.1 ppt, a reduction of 2.2 ppt from peak levels measured during 1996-1998. These atmospheric declines are driven primarily by continued decreases in total reported consumption of CH3Br from fumigation. As of 2009, reported consumption for quarantine and pre-shipment (QPS) uses, which are exempted uses (not controlled) under the Montreal Protocol, surpassed consumption for controlled (non-QPS) uses. As a result of the decrease in atmospheric CH3Br, the natural oceanic source is now comparable to the oceanic sink.

Stratospheric Inorganic Bromine

Total inorganic stratospheric bromine (Bry), derived from observations of bromine monoxide (BrO), was 20 (16-23) ppt in 2011, and had decreased at ~0.6 ± 0.1% yr-1 between peak levels observed in 2000-2001 and 2012. This decline is consistent with the decrease in total tropospheric organic Br based on measurements of CH3Br and the halons.

Equivalent Effective Stratospheric Chlorine (EESC)

EESC is a sum of chlorine and bromine derived from ODS tropospheric abundances weighted to reflect their expected depletion of stratospheric ozone. The growth and decline in EESC depends on a given tropospheric abundance propagating to the stratosphere with varying time lags (on the order of years) associated with transport. Therefore the EESC abundance, its peak timing, and its rate of decline, are different in different regions of the stratosphere.

By 2012, EESC had declined by about 10% in polar regions and about 15% in midlatitudes from their peak values, with CH3CCl3, CH3Br, and CFCs contributing approximately equally to these declines. This drop is about 40% of the decrease required for EESC in midlatitudes to return to the 1980 benchmark level, and about 20% of the decrease required for EESC in polar regions to return to the 1980 benchmark level.

Very Short-Lived Halogenated Substances (VSLS)

VSLS are defined as trace gases whose local lifetimes are comparable to, or shorter than, interhemispheric transport timescales and that have non-uniform tropospheric abundances. These local lifetimes typically vary substantially over time and space. As in prior Assessments, we consider species with annual mean lifetimes less than approximately 6 months to be VSLS. Of the VSLS identified in the current atmosphere, brominated and iodinated species are predominantly of oceanic origin, while the chlorinated species have significant industrial sources. These compounds will release their halogen atoms nearly immediately once they enter the stratosphere. The current contribution of chlorinated VSLS to Equivalent Chlorine (ECl) is about one-third as large as the contribution of VSLS brominated gases. Iodine from VSLS likely makes a minor contribution to ECl.

Total chlorinated VSLS source gases increased from 84 (70-117) ppt in 2008 to 91 (76-125) ppt in 2012 in the lower troposphere. Dichloromethane (CH2Cl2), a VSLS that has predominantly anthro- pogenic sources, accounted for the majority of this change, with an increase of ~60% over the last decade.

The estimated contribution of chlorinated VSLS to total stratospheric chlorine remains small. A lack of data on their concentrations in the tropical tropopause layer (TTL) limits our ability to quantify their contribution to the inorganic chlorine loading in the lower stratosphere. Current tropospheric concentrations of chlorinated VSLS imply a source gas injection of 72 (50-95) ppt, with 64 ppt from anthropogenic emissions (e.g., CH2Cl2, CHCl3, 1,2 dichloroethane (CH2ClCH2Cl), tetrachloroethene (CCl2CCl2)). The product gases are estimated to contribute 0-50 ppt giving a total of ~ 95 ppt (50-145 ppt) against a total of 3300 ppt of chlorine from long-lived ODSs entering the stratosphere.

There is further evidence that VSLS contribute ~5 (2-8) ppt to a total of ~20 ppt of stratospheric bromine. Estimates of this contribution from two independent approaches are in agreement. New data suggest that previous estimates of stratospheric Bry derived from BrO observations may in some cases have been overestimated, and imply a contribution of ~5 (2-8) ppt of bromine from VSLS. The second approach sums the quantities of observed, very short-lived source gases around the tropical tropopause with improved modeled estimates of VSLS product gas injection into the stratosphere, also giving a total contribution of VSLS to stratospheric bromine of ~5 (2-8) ppt.

Updated Lifetime Estimates

The uncertainties of estimated lifetimes for key long-lived ozone-depleting and related substances are better quantified following the SPARC Lifetimes Assessment (Stratosphere-troposphere Processes And their Role in Climate, 2013). Of note is the change in the estimated lifetime of CFC-11 (revised from 45 yr to 52 yr). The estimate of the total global lifetime of CCl4 (26 yr) remains unchanged from the previous Assessment, although estimates of the relative importance of the multiple loss processes have been revised.

Other Trace Gases That Directly Affect Ozone and Climate

The emissions of CFCs, HCFCs, and HFCs in terms of their influence on climate (as measured by gigatonnes of carbon dioxide (CO2)-equivalent emissions) were roughly equal in 2012. However, the emissions of HFCs are increasing rapidly, while the emissions of CFCs are going down and those of HCFCs are essentially unchanged. The 100-year GWP-weighted emissions for the sum of CFC, HCFC, and HFC emissions was 2.2 Gt CO2-equivalent in 2012. The sum of GWP-weighted emissions of CFCs was 0.73 ± 0.25 Gt CO2-equivalent yr-1 in 2012 and has decreased on average by 11.0 ± 1.2% yr-1 from 2008 to 2012. The sum of HCFC emissions was 0.76 ± 0.12 Gt CO2-equivalent yr-1 in 2012 and has been essentially unchanged between 2008 and 2012. Finally, the sum of HFC emissions was 0.69 ± 0.12 Gt CO2-equivalent yr-1 in 2012 and has increased on average by 6.8 ± 0.9% yr-1 from 2008 to 2012. The HFC increase partially offsets the decrease by CFCs. Current emissions of HFCs are, however, are less than 10% of peak CFC emissions in the early 1990s (>8 Gt CO2-equivalent yr-1).

From 2008 to 2012 the global mean mole fraction of nitrous oxide (N2O), which leads to ozone depletion in the stratosphere, increased by 3.4 parts per billion (ppb), to 325 ppb. With the atmospheric burden of CFC-12 decreasing, N2O is currently the third most important long-lived greenhouse gas contributing to radiative forcing (after CO2 and methane (CH4)).

Methane (CH4) is an important greenhouse gas and influences stratospheric ozone. In 2012 the average background global mole fraction of CH4 was 1808 ppb, with a growth rate of 5-6 ppb yr-1 from 2008 to 2012. This is comparable to the 2006-2008 period when the CH4 growth rate began increasing again after several years of near-zero growth. The renewed increase is thought to result from a combination of increased CH4 emissions from tropical and high-latitude wetlands together with increasing anthropogenic (fossil fuel) emissions, though the relative contribution of the wetlands and fossil fuel sources is uncertain.

Hydrofluorocarbons (HFCs) used as ODS substitutes are increasing in the global atmosphere. The most abundant HFC, HFC-134a, reached a mole fraction of nearly 68 ppt in 2012 with an increase of 5 ppt yr-1 (7.6%) in 2011-2012. HFC-125, -143a, and -32 have similar or even higher relative growth rates than HFC-134a, but their current abundances are considerably lower.

Worldwide emissions of HFC-23, a potent greenhouse gas and by-product of HCFC-22 production, reached a maximum of ~15 Gg in 2006, decreased to ~9 Gg in 2009, and then increased again to reach ~13 Gg yr-1 in 2012. While efforts in non-Article 5 Parties mitigated an increasing portion of HFC-23 emis- sions through 2004, the temporary decrease in emissions after 2006 is consistent with destruction of HFC-23 in Article 5 Parties owing to the Clean Development Mechanism (CDM) of the Kyoto Protocol. The average global mole fraction of HFC-23 reached 25 ppt in 2012, with an increase of nearly 1 ppt yr-1 in recent years.

Mole fractions of sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), and sulfuryl fluoride (SO2F2) increased in recent years. Global averaged mole fractions of SF6 reached 7.6 ppt in 2012, with an annual increase of 0.3 ppt yr-1 (4% yr-1). Global averaged mole fractions of NF3 reached 0.86 ppt in 2011, with an annual increase of 0.1 ppt yr-1 (12% yr-1). Global averaged mole fractions of SO2F2 reached 1.8 ppt in 2012, with an annual increase of 0.1 ppt yr-1 (5% yr-1). The considerable increases for these entirely anthropogenic, long-lived substances are caused by ongoing emissions.

[1] All uncertainties are one standard deviation unless otherwise specified.