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 (CH 3 CCl 3 ) 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 (CH 3 CCl 3 ) 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 (CCl 4 ) accounted for 359 ppt (about 11%) of total tropospheric Cl in 2008.Mixing ratios of CCl 4 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) 1.2Chapter 1ppt.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 (CH 3 Br) 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 0.03-0.04ppt/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 (CH 3 Br) 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 CH 3 Br 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 CH 3 Br had declined by about 70%.Sulfuryl fluoride (SO 2 F 2 ) 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 (GWP 100 ) of 4740.The SO 2 F 2 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 CH 3 Br 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 (COCl 2 ) 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, CH 2 Cl 2 ; chloroform, CHCl 3 ; 1,2 dichloroethane, CH 2 ClCH 2 Cl; perchloroethylene, CCl 2 CCl 2 ), 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 contribution

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