Methyl Chloroform Measurements Research Articles

Changes in Global Tropospheric OH Expected as a Result of Climate Change Over the Last Several Decades

AbstractThe oxidizing capacity of the troposphere is controlled primarily by the abundance of hydroxyl radical (OH). The global mean concentration of tropospheric OH, [OH]TROP (the burden of OH in the global troposphere appropriate for calculating the lifetime of methane) inferred from measurements of methyl chloroform has remained relatively constant during the past several decades despite rising levels of methane that should have led to a decline. Here we examine other factors that may have affected [OH]TROP such as the changing values of stratospheric ozone, rising tropospheric H2O, varying burden of NOx (=NO+NO2), rising temperatures, and widening of the climatological tropics due to expansion of the Hadley cell. Our analysis suggests the positive trends in [OH]TROP due to H2O, NOx, and overhead O3, and tropical expansion are large enough (Δ [OH]TROP = +0.95 ± 0.18%/decade) to counter almost all of the expected decrease in [OH]TROP due to rising methane (Δ [OH]TROP = −1.01 ± 0.05%/decade) over the period 1980 to 2015, while variations in temperature contribute almost no trend (Δ [OH]TROP = −0.02 ± 0.02%/decade) in [OH]TROP. The approximated impact of Hadley cell expansion on [OH]TROP is also a small but not insignificant factor partially responsible for the steadiness of tropospheric oxidizing capacity over the past several decades, which free‐running models likely do not capture.

Renewed growth of atmospheric methane

Following almost a decade with little change in global atmospheric methane mole fraction, we present measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) and the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) networks that show renewed growth starting near the beginning of 2007. Remarkably, a similar growth rate is found at all monitoring locations from this time until the latest measurements. We use these data, along with an inverse method applied to a simple model of atmospheric chemistry and transport, to investigate the possible drivers of the rise. Specifically, the relative roles of an increase in emission rate or a decrease in concentration of the hydroxyl radical, the largest methane sink, are examined. We conclude that: 1) if the annual mean hydroxyl radical concentration did not change, a substantial increase in emissions was required simultaneously in both hemispheres between 2006 and 2007; 2) if a small drop in the hydroxyl radical concentration occurred, consistent with AGAGE methyl chloroform measurements, the emission increase is more strongly biased to the Northern Hemisphere.

On the role of hydroxyl radicals in the self-cleansing capacity of the troposphere

Abstract. Thousands of megatons natural and anthropogenic gases are released and subsequently removed from the troposphere each year. Photochemical reactions, initiated by hydroxyl (OH) radicals, oxidise most gases to products which are more easily removed by precipitation and dry deposition at the earth's surface. Since human-induced pollution emissions strongly affect OH formation and loss, large global changes in OH concentrations are possible. Global models and observations of trace gas distributions from global networks have been used to study geographical and temporal changes in tropospheric OH. Here we present a synopsis of recent studies, indicating that global mean OH has changed remarkably little in the past century, even though regional changes have probably been substantial. Globally, depletion of OH by reactive carbon gases has been compensated by increased OH formation by nitrogen oxides, an act of "inadvertent geo-engineering". However, OH analyses for the past 1-2 decades, partly based on methyl chloroform measurements, are inconclusive. Some work, assuming that methyl chloroform emissions have largely ceased, suggests a very strong downward global OH trend in the 1990s, inconsistent with modelling studies. The discrepancy could be much reduced by assuming continued small emissions of methyl chloroform. We recommend the continuation of high precision monitoring of this compound and improved analyses based on detailed meteorological-chemical models.

Recent changes in the air‐sea gas exchange of methyl chloroform

Atmospheric measurements of methyl chloroform provide important constraints on the rate of oxidation of hydrocarbons in Earth's atmosphere. Estimates of the loss of methyl chloroform to the oceans play a small but important role in these calculations. Here, we examine the ocean‐atmosphere interaction of methyl chloroform in a global ocean model. Contrary to previous assumptions, these simulations suggest that the high‐latitude oceans are currently a source of this chemical to the atmosphere. If confirmed, this finding alters estimates of the change in the atmospheric oxidation rate of hydrocarbons. We highlight the potential usefulness of methyl chloroform as a tracer of ocean circulation.

Continuing emissions of methyl chloroform from Europe

The consumption of methyl chloroform (1,1,1-trichloroethane), an industrial solvent, has been banned by the 1987 Montreal Protocol because of its ozone-depleting potential. During the 1990s, global emissions have decreased substantially and, since 1999, near-zero emissions have been estimated for Europe and the United States. Here we present measurements of methyl chloroform that are inconsistent with the assumption of small emissions. Using a tracer transport model, we estimate that European emissions were greater than 20 Gg in 2000. Although these emissions are not significant for stratospheric ozone depletion, they have important implications for estimates of global tropospheric hydroxyl radical (OH) concentrations, deduced from measurements of methyl chloroform. Ongoing emissions therefore cast doubt upon recent reports of a strong and unexpected negative trend in OH during the 1990s and a previously calculated higher OH abundance in the Southern Hemisphere compared to the Northern Hemisphere.

Reply [to “Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.”

Journal of Geophysical Research: AtmospheresVolume 106, Issue D19 p. 23159-23164 CommentariesFree Access Reply [to “Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.”] Maarten Krol, Maarten KrolSearch for more papers by this authorPeter Jan van Leeuwen, Peter Jan van LeeuwenSearch for more papers by this authorJos Lelieveld, Jos LelieveldSearch for more papers by this author Maarten Krol, Maarten KrolSearch for more papers by this authorPeter Jan van Leeuwen, Peter Jan van LeeuwenSearch for more papers by this authorJos Lelieveld, Jos LelieveldSearch for more papers by this author First published: 01 October 2001 https://doi.org/10.1029/2001JD900039Citations: 8AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL References Cunnold, D. M., R. G. Prinn, Comment on “Tropospheric OH in a three-dimensional chemical tracer model: An assessment based on observation of CH3CCl3” by C. M. Spivakovsky et al, J. Geophys. Res., 96, 17,391– 17,393, 1991. Karlsdottir, S., I. S. A. Isaksen, Changing methane lifetime: Possible cause for reduced growth, Geophys. Res. Lett., 27, 93– 97, 2000. Krol, M. C., P. J. Van Leeuwen, J. Lelieveld, Global OH trend inferred from methylchloroform measurements, J. Geophys. Res., 103, 10,697– 10,711, 1998. Prinn, R. G., andJ. Huang, Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.,J. Geophys. Res., 106(D19), 2001. Prinn, R. G., et al., Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978–1990, J. Geophys. Res., 97, 2445– 2461, 1992. Prinn, R. G., R. F. Weiss, B. R. Miller, J. Huang, F. N. Alya, D. M. Cunnold, P. J. Fraser, D. E. Hartley, P. G. Simmonds, Atmospheric trends and lifetime of CH3CCl3 and global OH concentrations, Science, 269, 187– 190, 1995. Spivakovsky, C. M., Reply to comment by D. M. Cunnold and R. G. Prinn on “Tropospheric OH in a three-dimensional chemical tracer model: An assessment based on observations of CH3CCl3”, J. Geophys. Res., 96, 17395– 17398, 1991. Citing Literature Volume106, IssueD1916 October 2001Pages 23159-23164 ReferencesRelatedInformation

Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.

Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.

Global OH trend inferred from methylchloroform measurements

Methylchloroform (MCF) measurements taken at the Atmospheric Lifetime Experiment / Global Atmospheric Gases Experiment (ALE/GAGE) measurement stations are used to deduce the tropospheric OH concentration and its linear trend between 1978 and 1993. Global three‐dimensional fields of OH are calculated with a transport model that includes background photochemistry. Despite the large uncertainties in these OH fields, the simulated MGF concentrations at the five ALE/GAGE stations compare reasonably well to the measurements. As a next step, the OH fields are adjusted to fit the measurements optimally. An ensemble (Monte Carlo) technique is used to optimize the OH scaling factor and to derive the linear trend in OH. The optimized OH fields and trend imply a MCF lifetime in the troposphere of 4.7 years in 1978 and of 4.5 years in 1993. For CH4 these lifetimes (due to OH destruction only) are 9.2 and 8.6 years in 1978 and 1993, respectively. Uncertainties in these estimates are discussed using box‐model calculations. The optimized OH concentration is sensitive to the strength of other MCF sinks in the model and is constrained to 1.00−0.15+0.09 × 106 × molecules cm−3 in 1978 and to 1.07−0.17+0.09 × 106 molecules cm−3 in 1993. The deduced OH trend is sensitive to the trend in the MCF emissions and is confined to the interval between −0.1 and +1.1% yr−1 with a most likely value of 0.46% yr−1. Possible causes of a global increase in OH are discussed. A positive OH trend is calculated due to stratospheric ozone depletion, declining CO concentrations, increased water vapor abundance, and enhanced NOx emissions. Although the changes in the atmospheric composition are to a large extent unknown, it seems that the observed changes are consistent with significant increases in OH over the past decades.

Global monitoring shows ozone treaty is working

Ongoing measurements of methyl chloroform (CH 3 CCl 3 ) in the atmosphere show levels of the ozone-destroying compound have been declining since 1991—solid confirmation that the Montreal Protocol on Substances That Deplete the Ozone Layer is working. In addition, a new calibration technique has led researchers to significantly lower the absolute values for methyl chloroform in the atmosphere. And the scientists have deduced that the average global concentration of hydroxyl radical (OH) is significantly greater than previously estimated. OH is the lower atmosphere's primary cleansing agent, so this new appraisal suggests many pollutants—including chlorofluorocarbon (CFC) substitutes—are removed from the environment more quickly than had been thought. It's exciting to finally see one of the halocarbons actually decreasing, says Ronald G. Prinn, professor of atmospheric chemistry at Massachusetts Institute of Technology. There's no doubt about it: Industry has gone all out to decrease the manufacture and ...

Tropospheric OH and the lifetimes of hydrochlorofluorocarbons

Hydrochlorofluorocarbons (HCFCs) may be used as alternatives for the chlorofluorocarbons (CFCs). Lifetimes for the HCFCs are predicted here in two ways: integrating their loss with a global model, and scaling to another compound with a better known lifetime. Both approaches are shown here to yield similar results. Three‐dimensional fields of modeled tropospheric OH concentrations are used to calculate lifetimes against destruction by OH for the HCFCs and other hydrogenated halocarbons. The OH fields are taken from a three‐dimensional chemical transport model [Spivakovsky et al., 1990a] that accurately simulates the global measurements of methyl chloroform (derived lifetime of 5.5 yr). The lifetimes of various hydro‐halocarbons are shown to be insensitive to possible spatial variations and seasonal cycles. It is possible to scale the HCFC lifetimes to that of methyl chloroform or methane by using a ratio of the rate coefficients for reaction with OH at an appropriate temperature, about 277 K.

Global atmospheric distribution and trend of methylchloroform (CH3CCl3)

Over the last two years atmospheric measurements of methylchloroform (CH3CCl3) have been made at six locations using EC/GC techniques. The concentrations are reported, and it is shown that CH3CCl3 concentrations increased at rates of 3.8%±2.1% per year at Alaska (∼70°N) between 8/79 and 1/81, 6.6%±3.2% per year at Oregon (∼45°N) between 1/79 and 1/81, 4.3%±4% per year at Hawaii (24°N) between 11/79 and 1/81, 4.1%±4.8% per year at Samoa (14°S) between 2/80 and 1/81, 11.5%±1.5% per year at Tasmania (42°S) between 1/79 and 6/80, and at 8.7%±5.7% per year at the south pole between 1/79 and 1/81. These recent yearly increases are smaller than earlier rates of increase but are consistent with a lifetime of ∼7 years (6 years‐10 years) and a decline in the rate of global emissions.

Atmospheric Halocarbons, Hydrocarbons, and Sulfur Hexafluoride: Global Distributions, Sources, and Sinks

The global distribution of fluorocarbon-12 and fluorocarbon-11 is used to establish a relatively fast interhemispheric exchange rate of 1 to 1.2 years. Atmospheric residence times of 65 to 70 years for fluorocarbon-12 and 40 to 45 years for fluorocarbon-l1 best fit the observational data. These residence times rule out the possibility of any significant missing sinks that may prevent these fluorocarbons from entering the stratosphere. Atmospheric measurements of methyl chloroform support an 8-to 10-year residence time and suggest global average hydroxyl radical (HO) concentrations of 3 x 10(5) to 4 x 10(5) molecules per cubic centimeter. These are a factor of 5 lower than predicted by models. Additionally, methyl chloroform global distribution supports Southern Hemispheric HO levels that are a factor of 1.5 or more larger than the Northern Hemispheric values. The long residence time and the rapid growth of methyl chloroform cause it to be a potentially significant depleter of stratospheric ozone. The oceanic sink for atmospheric carbon tetrachloride is about half as important as the stratospheric sink. A major source of methyl chloride (3 x 10(12)grams per year), sufficient to account for nearly all the atmospheric methyl chloride, has been identified in the ocean.