5. Nitrous Oxide and Halocompounds

5.1.1. INTRODUCTION

The Nitrous Oxide and Halocarbons Group was formed in 1986. In April 1995, the name was changed to Nitrous Oxide and Halocompounds Group (NOAH) to reflect the fact that noncarbon containing, halogenated gases such as sulfur hexafluoride (SF₆) are now an integral part of our program. The general mission of the group is to quantify the distributions as well as the magnitudes of the sources and sinks for atmospheric nitrous oxide (N₂O) and halocarbons that include the chlorofluorocarbons (CFCs), chlorinated solvents (CCl₄, CH₃CCl₃, etc.), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), methyl halides (CH₃Br, CH₃Cl, CH₃I), halons, and numerous other important ozone-depleting and greenhouse gases. Two chromatographic techniques, electron capture detector-gas chromatography (EC-GC) and gas chromatograph-mass spectrometer (GC-MS), are used primarily to detect these trace atmospheric species. NOAH samples air from ground-based stations, towers, ocean vessels, aircraft, and balloons to accomplish its mission. Achieving these goals requires the production and maintenance of reliable gas calibration standards that are supplied to laboratories throughout the world.

New sites for both the flask and in situ programs were added over the past 2 years. The location of these sites in the CMDL N₂O and Halocompounds Network are shown in Figure 5.1. In cooperation with the Carbon Cycle Group (CCG), two tower sites (WITN Tower, North Carolina (ITN) and WLEF Tower, Wisconsin (LEF)) were added to the in situ program to ascertain source strengths of gases near urban and forested areas. With a similar purpose, an in situ gas chromatograph (GC) system was started at Harvard Forest (HFM) in collaboration with Harvard University scientists. At Alert, Northwest Territories, Canada (ALT) and in cooperation with the Canadian Atmospheric Environment Services (AES) program, an in situ GC for measuring N₂O and SF₆ also was installed and operated in 1995. Monthly, and in some cases weekly, flask samples were collected at these new in situ sites. Another site within CMDL, Cape Kumakahi, Hawaii (KUM), was added to obtain flask measurements of air from the remote, tropical, northern hemispheric marine boundary layer. Combined with MLO, this should allow us to estimate vertical gradients of trace atmospheric halocarbons, particularly those that react readily with sunlight or tropospheric OH. Table 5.1 summarizes the geographic location and type of operations of eleven flask and nine in situ sampling sites.

Fig. 5.1. Geographical locations of old and new stations in the NOAH flask (gray circles) and in situ (pluses) networks.

TABLE 5.1. Geographic and Network Information on NOAH Network Sites (In Order of Highest Latitude)

				Elevations	LST-GMT
Code	Station	Latitude	Longitude	(m)	(hr)	Type
ALT	Alert, North West Territories. Canada*	82.45°N	62.52°W	210	-4	F,I
BRW	Pt. Barrow, Alaska	71.32°N	136.60°W	11	-9	F,I
LEF	WLEF tower, Wisconsin†	45.95°N	90.28°W	470	-6	F,I
HFM	Harvard Forest, Massachusetts‡	42.54°N	72.18°W	340	-5	F,I
NWR	Niwot Ridge, Colorado§	40.04°N	105.54°W	3013	-7	F,I
ITN	WITN tower, North Carolina†	35.37°N	77.39°W	9	-5	F,I
MLO	Mauna Loa, Hawaii	19.54°N	155.58°W	3397	-10	F,I
KUM	Cape Kumukahi, Hawaii	19.52°N	154.82°W	3	-10	F
SMO	Tuluila, American Samoa	14.23°S	170.56°W	77	-11	F,I
CGO	Cape Grim, Tasmania, Australia**	40.41°S	144.64°E	94	+10	F
SPO	South Pole, Antarctica	89.98°S	102.00°E	2841	+12	F,I

Cooperative sites (F = flasks, I = in situ) with:

*AES, in situ GC: Only N₂O and SF₆

†CCG. Flasks will be added to WLEF in 1996. The ACATS-II instrument ran as an in situ GC at WLEF for 1995.

‡Harvard University

§University of Colorado

**Commonwealth Scientific and Industrial Research Organization (CSIRO) and Bureau of Meteorology, Australia
 

5.1.2. FLASK SAMPLES

Overview

NOAH's flask sampling and measurement program underwent a number of changes and improvements in 1994 and 1995. Most significant of these was the retirement of the "old" flask GC, a Hewlett-Packard (HP) Model 5710A GC-ECD, at the end of 1995 used for the measurement of CFC-12 (CCl₂F₂), CFC-11 (CCl₃F), and N₂O since inception of the old GMCC halocarbon monitoring program in 1973. This instrument, which operated in an almost entirely manual mode and for which data reduction was a cumbersome process, was replaced by the NOAH automated flask GC (OTTO). The OTTO GC, which began operation in 1992, analyzes samples for seven gases. A detailed comparison of mixing ratios of CFC-11, CFC-12, and N₂O from all stations between the old HP 5710A and OTTO GCs shows that the new OTTO GC gives better precision and close agreement with the older GC (Figure 5.2). The Whitey 300 mL-flasks were also removed from the sampling network. Over the past 10 years, the flask network was augmented with increasingly large flasks (0.85 L and 2.4 L) to accommodate the increased number of measurements and larger amounts of air required. The last Whitey flasks were filled at the end of 1995 at all stations except at the South Pole Observatory, Antarctica (SPO), which will not be able to return them until its reopening in the fall of 1996. Originally selected for sampling of stable CFCs and N₂O, the Whitey flasks, with their smaller volume and coarser internal surfaces, were also increasingly problematic for some of the more reactive gases such as methyl chloroform (CH₃CCl₃), carbon tetrachloride (CCl₄), and methyl bromide (CH₃Br) that were added to the suite of measurements over time. All flasks in the network are now 0.85-L Biospherics or 2.4-L Max Planck Institute (MPI) electropolished, stainless steel containers with Nupro metal-bellows (SS-4H) valves.
 

Fig. 5.2. Continuity of CFC-11 data from the old GC and automated flask GC (OTTO) during 1994-1995. Data are shown for sites in both hemispheres.
 

Flasks are now filled to 376-505 kPa (40-60 psig) at all sites. Although this typically has not been a problem at sea level where the KNF Neuberger, Inc. pumps (Model UN05SV1) are rated to deliver 410 kPa (45 psi), samples collected at higher altitudes (SPO, Mauna Loa Observatory, Hawaii (MLO), Niwot Ridge, Colorado (NWR)) could only reach pressures around 273 kPa (25 psig), which today are barely enough gas for CMDL measurements. To accommodate this change, the inlets were reconfigured to the automated, in situ GCs to allow the Neuberger pumps to draw air from the pressurized portion of the inlet line. Although the inlet lines (Dekabon) and pumps (Air-Cadet, Cole-Palmer) contain plastic, they are kept clean by continuous flushing, 24 hours per day at 5-10 L min^-1. No noticeable difference was observed in the data as a result of making this change. Pumps and lines have been installed at NOAA sites, e.g., KUM, that do not support in situ GCs.

In addition to the routine, weekly sampling of flask pairs at the CMDL observatories and cooperative sampling sites, NOAH scientists also analyzed air in flasks collected during two cruises in 1994 (Bromine Latitudinal Air/Sea Transect (BLAST I and BLAST II); section 5.4 and SF₆ section) to obtain "snapshots" of the interhemispheric gradient of a number of gases and to support measurements made by in situ GCs onboard. These measurements included over 25 gases and involved all of NOAH laboratory instrumentation. Another project (section 5.6) involved the flask program for the analysis of air sampled from South Pole firn (compressed snow). This provided NOAH scientists with a unique opportunity to observe the N₂O and halocarbon content of air dating back to the end of the 19th century. Although the air was collected into glass flasks with Teflon o-rings, which in the past have caused some problems in the analysis of halocarbons, contamination was minimal for most gases in this instance. The success of these measurements prompted NOAH scientists to pursue similar analyses from firn air collected at Vostok and Greenland. Because of the number of investigators requiring air from the SPO flasks, sharing was limited. Consequently, the samples were analyzed only with the OTTO and Low Electron Attachment Potential Species (LEAPS) GCs, which require less sample than the GC-MS systems.

Finally, a number of changes were made in instrumentation, data acquisition, and data management for flask measurements that have improved the precision of some measurements, enhanced detection limits for others, and dramatically streamlined the processing of data. Today, data from samples run on OTTO and LEAPS can be finalized and evaluated alongside all previous data within minutes following analysis.

Measurements by EC-GC

Improvements in Analysis. The precision of flask measurements by EC-GC has improved dramatically during 1994-1995. Modifications in sampling technique and sample introduction, full automation of CFC, N₂O, and chlorocarbon measurements, and installation of 24-bit interfaces for analyses by OTTO and LEAPS combined to yield analytical and sampling precisions about 0.1 ppt or less for LEAPS gases and on the order of tenths of a ppt for gases measured on OTTO.

The old Nelson Analytical, Inc. data acquisition and handling system on OTTO was replaced with entirely new hardware and software to allow more rapid and consistent processing of samples. The HP Model 210 computer was replaced with an IBM PC compatible 486, the 16-bit A/D converters were replaced with two HP 24-bit A/D boards for the PC, and the Rocky Mountain Basic software was replaced with the 1995 version of HP Chemstation software. Programs were written in Microsoft Visual Basic to consolidate the HP Chemstation output from each run of flasks, compute results, generate flags for erroneous or anomalous data, perform additional quality control tests, and load the results into a Microsoft Access data base. Currently, eight flasks can be run at once on OTTO, obtaining precise measurements of CFC-12, CFC-11, CFC-113, CH₃CCl₃, CCl₄, N₂O, and SF₆.

Results and Trends. The years 1994 and 1995 heralded the downturn in total chlorine, equivalent chlorine, and effective equivalent chlorine in the earth's troposphere [Montzka et al., 1995b, 1996b]. Led by a marked drop in CH₃CCl₃, this suggested that the abundance of ozone-depleting halogen in the stratosphere could begin to decline in the near future. Other gases that began decreasing in abundance during this time include CFC-113, CCl₄, and, to a lesser extent CFC-11 (Figure 5.3). CFC-12 continued to increase in the atmosphere, although not in sufficient quantity to offset the losses in organic chlorine represented by the other compounds (Figure 5.3). As expected, the atmospheric abundances of CFC alternative compounds (HCFCs and HFCs) have been increasing at reasonably fast rates, although these gases contain relatively little chlorine and have shorter lifetimes than the CFCs ([Montzka et al., 1993, 1994, 1996a,b]; section 5.1.5).

Fig. 5.3. CFCs and chlorocarbons measured on the old GC and OTTO in ppt versus time since 1977. The transition from old GC data to OTTO data for CFCs -11 and -12 is shown by a vertical, dashed line at the beginning of 1994. Noticeable are the tighter measurements of OTTO and the lack of an offset between the instruments. Also shown in proportion are the recent growth rates of the major, Class I ozone-depleting, chlorinated compounds and their narrowing interhemispheric gradients.

SF₆An Important Tracer and Strong Greenhouse Gas

On a per molecule basis, SF₆ is one of the strongest greenhouse gases known, about 25,000 times greater than CO₂ [Albritton et al., 1995]. It is solely anthropogenic in origin and used primarily for the insulation of high-voltage electrical equipment. With its increasing use and very long lifetime, SF₆ is rapidly accumulating in the atmosphere at ~7% yr^-1. In addition to its importance as a greenhouse gas, SF₆ is a nearly ideal tracer of atmospheric dynamics due to its well understood sources and long atmospheric lifetime of ~3200 years [Ravishankara et al., 1993].

CMDL scientists recently began monitoring atmospheric SF₆ in weekly flask samples from all baseline stations and many CCG network sites as high-resolution latitudinal profiles during the 1994 BLAST ocean cruises [Geller et al., 1994] as in situ stratospheric measurements from the Airborne Chromatograph for Atmospheric Tracers (ACATS) field missions [Elkins et al., 1996], and as in situ measurements at Alert, Harvard Forest, and North Carolina [Hurst et al., 1995]. SF₆ was measured with ECD-GC as described in Elkins et al. [1996]. Even though ambient levels of SF₆ are only ~3.5 ppt, it is possible to measure direct air injections (no sample preconcentration) to a precision of 1-3%. A suite of gravimetric SF₆ standards ranging from 3 to 108 ppt was developed in the NOAH Group. An intercalibration with the University of Heidelberg (Germany) showed CMDL measurements are less than 2% lower that the German calibration scale.

The long-term trend of SF₆ is illustrated in Figure 5.4, which shows NOAA data together with the University of Heidelberg data. These different data sets, collected and analyzed by different techniques, show good agreement. A northern hemispheric trend was fit to the combined Izaña and NWR data, because data from these two midlatitude sites are close to the latitudinally weighted hemispheric mean. Likewise, in the southern hemisphere, data from Antarctica and Cape Grim well represent the true southern hemisphere mean, therefore the southern hemisphere trend was fit to the Neumayer and Cape Grim data from the University of Heidelberg, and the SPO and Cape Grim data from NOAA data. A preliminary estimate for the global trend (the average of the northern and southern hemispheric trends) shows a quadratic increase described 1987 < year <1996). This yields a late 1995 growth rate of by: y = 3.43 - 0.2352 + 0.00487x2 (for x = (year - 1996); 0.23 ppt yr^-1 (6.86% yr^-1). This trend, which is derived from the combined data sets, shows no significant difference from the trend derived from the University of Heidelberg data alone [Maiss et al., 1996]. Figure 5.5 shows the high resolution latitudinal profiles of SF₆ collected over both the Pacific and Atlantic oceans on the BLAST cruises of 1994. Figure 5.5c also shows a mean latitudinal profile of SF₆ for November 1995 obtained from the flask sampling network. These profiles should not be taken as representative of an overall global distribution of SF₆ since they can vary seasonally.

Fig. 5.4. Temporal trends of SF₆. CMDL data shown together with University of Heidelberg data [from Maiss et al., 1996, marked on the figure legend as #]. The Heidelberg data has been adjusted to the CMDL calibration scale and binned into monthly means. The curve fitting is described in the text.
 

Fig. 5.5. Latitudinal profiles of atmospheric SF₆ (dry, ppt by mole fraction). (a) and (b) are in situ data from the marine boundary layer in 1994. (c) shows the monthly mean mixing ratios for November 1995 obtained from flask samples collected at seven sampling stations and from the in situ North Carolina data. Error bars represent 1 standard deviation of the flask pair mixing ratios at each station.
 

5.1.3. RITS CONTINUOUS GAS CHROMATOGRAPH SYSTEMS

Operations Update

A new watchdog timer turns the power off, then resets the computer, printer, and Nelson A to D boxes if a signal is not received in a specified recurring period of time. This restarts data acquisition when station personnel are not present at night or on weekends and the system locks up. In June 1994 the hardware was installed at the Barrow Observatory, Alaska (BRW) in June and at MLO and Samoa Observatory, American Samoa (SMO) in August. At this same time a new function was added to the system software to decrease paper usage by the printer. A set of calibration gas and air chromatograms were printed only once a day prior to the arrival of the station personnel instead of continuous printouts. In August, system software was modified to include a menu-driven log for problems, failures, and changes to be easily documented. In the past such information was written on daily and weekly check lists and then typed into a database.

Original data acquisition and control computers, HP Model 9816s of mid-1980 vintage, were replaced in 1995 by 486 PCs. The software port from HP Basic to TransEra HTBasic running in Microsoft Windows 3.1 required only minor changes. This software upgrade was installed in April at NWR, in May at MLO and SMO, and in June at BRW. An additional feature of the new computers is network access. Data downloading, software upgrading, and determining equipment status is now possible over the Internet.

Alert

A single-channel GC equipped with an ECD and based on the STEALTH GC design (section 5.5) was built in 1995 and installed at ALT as part of a cooperative research agreement between CMDL and AES. Continuous instrument operation began in late August 1995 with two samples of ambient air analyzed each hour for N₂O and SF₆. These measurements augment the weekly flask samples taken at ALT since late 1987 and allowed detection of episodic pollution events. Of particular interest was the monitoring of polluted air that arrives at Alert from northern Asia and from the former Soviet Union.

Data Analysis

In response to observed depletion of stratospheric ozone, the 1987 Montreal Protocol on Substances That Deplete the Ozone Layer mandated a 50% reduction of chlorofluorocarbons and selected chlorinated solvent production over the next 10 years. In 1990 this was strengthened to a 100% phase out by the year 2000. An additional amendment in 1992 required a 75% reduction by 1994 and a complete ban by 1996. The chemical industry responded quickly with substitutes. Emissions have, therefore, generally been reduced in excess of expectations.

The global average CFC-11 tropospheric mixing ratio reached a maximum of 272 ppt in 1993 (Figure 5.6). The growth rate has now started to decline at -1 ppt yr^-1. Recently the interhemispheric difference declined by half, indicative of a long lifetime and a mostly northern hemisphere source that is diminishing quickly. In situ measurements of CFC-11 and CFC-12 are described in Elkins et al. [1993], and the data were recently updated in Montzka et al. [1996b].

CFC-12 growth continues to slow down with the end of 1995 growth rate about 6 ppt yr^-1. Because CFC-12's major use is in domestic, commercial, and industrial refrigeration and air conditioning, release to the atmosphere is slower than foam blowing, propellant, and solvent applications. Assuming a constant deceleration of -1.66 ppt yr^-2, CFC-12 is estimated to peak in the atmosphere in mid-1999 with a global-average tropospheric mixing ratio of 544 ppt (Figure 5.7 and 5.8).

Fig. 5.6. Monthly average CFC-11 mixing ratios in ppt from the in situ GCs.
 
 

Fig. 5.7. Monthly average CFC-12 mixing ratios in ppt from the in situ GCs.

Fig. 5.8. A decrease in the global average growth rate of CFC-12 is projected to become zero in mid-1999.

Southern hemispheric mixing ratios of methyl chloroform peaked in 1992 and northern hemisphere mixing ratios peaked a little more than a year earlier (Figure 5.9). The time lag is similar to the known interhemispheric mixing time. The large north to south gradient before 1993 is indicative of very strong northern hemisphere sources. The rapid decrease in mixing ratios during phaseout shows the chemical has a short lifetime estimated at about 5 years [Prinn et al., 1995].

Fig. 5.9. Monthly average CH₃CCl₃ mixing ratios in ppt from the in situ GCs.
 

The chlorinated solvent CCl₄, was used as the primary source (feed stock) for the chemical synthesis of all the chlorofluorocarbons. With their ban, this role has diminished significantly. Atmospheric mixing ratios were observed to be slowly decreasing at approximately -0.75 ppt yr^-1 since 1991 (Figure 5.10). One unusual feature is the north to south gradient was near constant during this same period.

Fig. 5.10. Monthly average CCl₄ mixing ratios in ppt from the in situ GCs.
 

The methyl chloroform and carbon tetrachloride data were published in Montzka et al. [1996b]. The mixing ratios of both compounds are decreasing with time as a result of the Montreal Protocol.

N₂O continued to increase in the troposphere (Figure 5.11). The average global growth rate for 1995 was 0.61 ppb yr^-1.

Fig. 5.11. Monthly average N₂O mixing ratios in ppb from the in situ GCs.

5.1.4. LEAPS

Although precision of the Low Electron Attachment Potential Species (LEAPS) analyses was improved by an order of magnitude in 1992 with better chromatography (tenths to hundredths of a ppt; Swanson et al. [1993]; Thompson et al. [1994]), the system still operated with the old Nelson Analytical hardware and software. In 1994 this was replaced with a 24-bit A/D board and an IBM PC-compatible 386, and HP Chemstation software. New software was written for processing data and incorporating it into a Microsoft Access data base manager. As with data from OTTO, final LEAPS data are now available immediately following analysis.

Halons have not been produced by industry since January 1, 1994, except for some small exceptions; however, the mixing ratios of the three major halons (H-1211 or CBrClF₂, H-1301 or CBrF₃, and H-2402 or CBr₂F₄) in the troposphere continued to rise (Figure 5.12), because a considerable amount of halon remains stored in fire suppression systems. The growth rates are, however, considerably lower now than during the 1970s and 1980s [Butler et al., 1992].

Fig. 5.12. Growth of halons in the atmosphere since 1991. Growth rates are given for January 1, 1994, thus approximating today's increase of stratospheric bromine from halons. Pre-1995 data for H-2402 are from the NOAA archive; all other measurements are from the flask network.
 

5.1.5. CHLOROFLUOROCARBON ALTERNATIVE MEASUREMENTS PROGRAM

Flask air analysis by GC-MS continued through 1994-1995. Mixing ratios of selected CFCs, HCFCs, HFCs, chlorinated hydrocarbons, brominated hydrocarbons, and halon-1211 were determined from air collected in flasks at the seven remote flask sampling observatories (four CMDL stations and three cooperative flask sampling locations). Toward the end of 1995, flask samples were also collected at three additional sites: KUM, ITN, and HFM.

During this period, analysis methods were developed on a second instrument for precise measurement of halocarbons such as HFCs at mixing ratios of ~0.1 ppt and higher. This was accomplished by using larger volumes of air per injection than in the original GC-MS instrument (up to 1 L of air per injection versus ~0.17 L in Montzka et al. [1993]). Detection of halocarbons in this second instrument is also performed with mass spectrometry. Larger flasks (2.4 L) were incorporated into the sampling network in early 1995 to allow for air analysis on this new instrument in addition to other instruments. With these changes and the development of the second GC-MS instrument, measurements of selected HFCs and additional HCFCs became possible in modern air starting in early 1995. Furthermore in 1995, enhanced sensitivity has allowed for the analysis of HFCs, HCFCs, and other halocarbons within archived air samples that were collected at NWR and other locations since 1987.

HCFC-22 (CHClF₂) Measurements

The most abundant HCFC, HCFC-22, increased in the global troposphere at a rate of 4.5% yr^-1 (mean exponential rate estimated from flask samples collected between 1992 and 1996; Table 5.2, Figure 5.13a, and Montzka et al., [1996b]). This rate represents a slower annual increase on a relative basis when compared to growth rates reported for time periods encompassing the 1980s and early 1990s [Montzka et al., 1993; Zander et al., 1994; Irion et al., 1994; Rinsland et al., 1996].

Informal exchange of flask air samples and standards in 1994-1995 with the National Center for Atmospheric Research (NCAR), the University of Bristol, England, and the Scripps Institution of Oceanography has suggested that consistent results (within 5%) can be obtained by chromatographic analysis of air even when different detectors are used (MS and O₂-doped ECD). These results are also reasonably consistent with surface mixing ratios inferred from long-path absorption studies [Irion et al., 1994].

Emission estimates compiled by industry can be used to infer an atmospheric lifetime for HCFC-22. However, uncertainties associated with this exercise limit its usefulness for providing constraints to the global mean burden of the hydroxyl radical. With simple box-model calculations and emission estimates [AFEAS, 1995] (without adding additional emission to allow for unreported production), an atmospheric lifetime of 12 2 years is estimated for HCFC-22 from CMDL data. This lifetime is consistent with 11.5 0.7 years, which has been estimated for HCFC-22 based upon a comparison between measurements and model calculations of methyl chloroform [Prinn et al., 1995].

HCFC-141b (CH₃CCl₂F) Measurements

Rapid atmospheric growth continues to be observed throughout both tropospheric hemispheres for HCFC-141b (Table 5.2, Figure 5.13c, Montzka et al. [1996b]). Mixing ratios have increased more than tenfold throughout the global troposphere since the beginning of 1993. Fairly good agreement was reported among different laboratories that have published measurements for this compound [Montzka et al., 1994; Schauffler et al., 1995; Oram et al., 1995].

TABLE 5.2. Annual Mean Growth Rate and Mean Tropospheric Burden (Mixing Ratio) for HCFCs and HFC-134a*, Mid-1994 and Mid-1995

	Mid-1994			Mid-1995
	Growth Rate,	Mixing Ratio,		Growth Rate,	Mixing Ratio,
Compound	ppt yr-1	ppt		ppt yr-1	ppt
HCFC-22	5.3	111.1		5.6	116.6
HCFC-142b	1.2	5.6		1.1	6.8
HCFC-141b	1.4	1.9		1.9	3.5
HFC-134a	~0.7†	N.D.‡		1.2	1.6

Global mean growth rates and mixing ratios estimated from polynomial fits to data binned by monthly, bimonthly, odd month, and even month periods, and by station. HCFC-22 growth of 4.5% yr^-1 is estimated from an exponential fit to global mean data between 1992 and 1996.

*See Figure 5.13. Estimates reported here for HCFC-142b were corrected for an error in Table 1 of Montzka et al. [1996b].

†Estimated from mean of two cruises in 1994.

‡Not determined.
 

Fig. 5.13. Atmospheric dry mole fractions (ppt) determined since 1992 for the most abundant substitutes for ozone-depleting substances. Each point represents a mean of two simultaneously filled flasks from one of seven stations: ALT, open circles; BRW, open triangles; NWR, open diamonds; MLO, open squares; SMO, filled triangles; CGO, filled squares; SPO, filled diamonds. These data were obtained from the original GC-MS instrument (see text). Solid lines represent fits to hemispheric monthly means.

Preliminary emissions have been estimated recently for HCFC-141b by industry (P. Midgley, Alternative Fluorocarbon Environmental Acceptability Study (AFEAS), personal communication, 1996). At the beginning of 1993, the global tropospheric abundance estimated from the measurements was ~2.0 times greater than the burden estimated from these emissions. By the end of 1994, this ratio had decreased to between 1.3 and 1.4. The exact cause for this discrepancy is currently unknown; however, the difference (but not its time dependence) could be reconciled if emissions are a larger fraction of production than currently assumed. Some of this difference could also be explained by larger vertical gradients within the troposphere than assumed in the simple box-model calculation.

Whereas the atmospheric lifetime of HCFC-141b also influences ambient mixing ratios and affects the magnitude of the difference discussed here, mixing ratios are fairly insensitive to the lifetime chosen for HCFC-141b during the initial phase of use and emission. For example, if we were to consider a lifetime for HFC-141b of 20 years instead of the more accepted value of ~10 years [WMO, 1995], the ratio calculated for the end of 1994 would be 1.2-1.3 instead of 1.3-1.4.

Analysis of the CMDL air archive reveals fairly constant mixing ratios of 0.08 - 0.10 ppt for HCFC-141b from 1987 to 1990 (Figure 5.14). After 1990, the abundance increases to ~0.6 ppt in 1993, which is consistent with mixing ratios determined for the northern hemisphere from the flask program at that time [Montzka et al., 1994]. These results are also similar to data reported by Oram et al. [1995] where fairly constant mixing ratios of 0.08 0.01 ppt HCFC-141b were found in samples collected at Cape Grim between 1982 and 1991. A dramatic increase was observed at this southern hemispheric site in 1992 and 1993, or 1 to 2 years after that observed at NWR in the CMDL archive.

Fig. 5.14. Atmospheric dry mole fractions for HCFC-142b (filled diamonds) and HCFC-141b (filled triangles) in archived air samples as determined on the newer GC-MS instrument. Analyses of archived air were performed in early 1995. With the exception of samples filled in mid-1987, all samples were obtained from NWR or MLO. Samples collected in mid-1987 were obtained shipboard in both hemispheres. Solid lines represent hemispheric (northern always higher than southern) and global monthly means for HCFC-142b and HCFC-141b as determined from the data in Figures 5.13b and 5.13c.

HCFC-142b (CH₃CClF₂) Measurements

Rapid atmospheric growth was also observed for HCFC-142b during 1994-1995 (Table 5.2; Figure 5.13b; Montzka et al. [1996b]). Published results from ground-based air samples disagree by ~30%, with CMDL data [Montzka et al., 1994] being higher than mixing ratios reported from the UEA [Oram et al., 1995]. Accurate comparison with a few earlier measurements from NCAR [Pollock et al., 1992; Schauffler et al., 1993] is difficult because these earlier measurements were from air collected above 15 km in northern latitudes. However, from informal exchange of air samples and standards between CMDL and NCAR, and between CMDL and the University of Bristol, mixing ratios determined from these three independent laboratories are expected to span a range of approximately 10% (with CMDL results approximating the mean of the three laboratories: University of Bristol, NCAR, and CMDL).

It is also noted that the mixing ratio reported for HCFC-142b in Table 1 of Montzka et al. [1996b] is too high by approximately 6%. The revised growth estimate for 1995 is reduced by a larger percentage (Table 5.2). This error arose in determining the mixing ratio for HCFC-142b in an air sample used for reference in the analysis of flask samples in 1995. This correction does not affect mixing ratios reported or conclusions drawn in Montzka et al. [1994]. This error was corrected in public accessible data files (CMDL World Wide Web site) in July 1996.

Emissions estimated by industry from production figures [AFEAS, 1995] underestimate the atmospheric burden of HCFC-142b [Montzka et al., 1994; Oram et al., 1995] regardless of which measurements are considered accurate. Oram et al. [1995] have suggested that a portion of this discrepancy arises from non-negligible emission of HCFC-142b in the years before 1981, which is the first year for which industry emission estimates are available. Between 1992 and the end of 1995, mixing ratios deduced from these emissions appear to underestimate the atmospheric burden of HCFC-142b by a consistent factor of ~1.9 (CMDL scale).

Analysis of the CMDL NWR air archive in 1995 for HCFC-142b shows mixing ratios of between 0.9 and 1.0 ppt between 1987 and 1989 (Figure 5.14). This is approximately 1.3 times higher than reported by Oram et al, [1995] for this period at Cape Grim, and this difference is consistent with calibration differences as discussed above. After 1989, enhanced growth was observed at NWR. The rate of accumulation is believed to have accelerated at Cape Grim approximately 1 year later [Oram et al., 1995].

HFC-134a (CH₂FCF₃) Measurements

Development of techniques for determining mixing ratios of halocarbons present in the atmosphere at ~1 ppt and higher were refined in 1995 on a second GC-MS instrument. This allowed for analysis of air samples for numerous HCFCs, HFCs, and other halocarbons. Archived samples were also analyzed to determine how the abundance of HFC-134a has changed over the past 10 years. Results from these analyses show that the abundance of HFC-134a in the northern hemisphere has risen from ~50 parts per quadrillion (ppq) in 1990 (the limit of detection for this instrument) to ~2.5 ppt in mid-1995 [Montzka et al., 1996a]. Analysis of flask samples filled onboard ship during cruises in 1987, early 1994, and late 1994 show similar atmospheric increases. The abundance of HFC-134a approximately doubled in the time elapsed between the two 1994 cruises in both hemispheres. Cruise flask samples were filled and stored prior to analysis in early 1995 under dramatically different pressures and humidities than the archived samples filled at NWR. The consistency observed between archived samples from NWR and cruise flask samples suggests that the amount of HFC-134a has not been altered significantly during storage by container-related effects and that the measurements are likely representative of atmospheric abundances at the time of sampling.

Routine measurements of HFC-134a in flask samples filled at the CMDL observatories and cooperative sampling locations began in early 1995 (Figure 5.13d; [Montzka et al., 1996a,b]). Mixing ratios for this HFC are increasing rapidly at all sampling locations. Although it is not possible to accurately estimate the growth rate from such a short data record, the increase observed between 1994 and 1996 is consistent with exponential growth at ~100% yr^-1.

In simultaneously-filled flasks, mixing ratios determined for HFC-134a were not significantly different. The amount of HFC-134a measured in flasks filled in parallel typically agreed to within 30 ppq and was <100 ppq for 95% of the flask pairs analyzed. Similarly, analysis precision (1 s.d.) for replicate injections of air from flasks collected after 1995 from the ground-based stations was typically <30 ppq (<2%) and was <100 ppq for 95% of the flasks analyzed. This consistency is expected for properly-filled flasks and for molecules not adversely affected by storage in flasks. Flasks received from ground-based stations after February 1, 1995, were analyzed an average of 23 days after sampling.

Preliminary emissions for HFC-134a have recently been estimated by industry (P. Midgley, personal communication, 1996). At the end of 1994, these emissions overestimate the observed abundance of HFC-134a by only ~0.1 ppt (measured/calculated = ~0.8-0.9).

CMDL Instrument Comparison from Routine Flask Analyses

Beginning in early 1995, large flasks (2.4 L) were filled and analyzed at the stations on both GC-MS instruments. For the compounds shown in Table 5.3, mixing ratios were assigned to air samples based upon independent calibration of reference air with CMDL gravimetric standards. Comparisons of results obtained from these independent instruments can provide further estimates of measurement uncertainty for halocarbons at these low mixing ratios, especially because different analytical conditions are used in the two instruments. The second instrument incorporates a different analytical column (DB-1 versus DB-5), trapping of compounds at different temperatures on a different substrate (a section of alumina PLOT column at 80^oC versus a length of uncoated fused silica at 140 to 150^oC), and a different valving arrangement. Different mass fragments were monitored during air analysis on the different instruments to determine HCFC-22 mixing ratios (Table 5.3). Because different ions would likely be influenced to different degrees by any coeluting compounds, consistent results obtained with different ions gives additional confidence that these measurements are not affected by such potential chromatographic problems.

TABLE 5.3. Results of Individual Flask Air Analysis on Two Different GC-MS Instruments*

	New Instrument to Old  Instrument Ratio			Mass to Charge Ratio  Ions Monitored
		Standard		New	Old
Compound	Mean	Deviation		Instrument	Instrument
HCFC-141b	0.96	0.04		81	81
HCFC-142b	0.99	0.03		65	65
HCFC-22	1.00	0.03		67	51
HFC-134a	1.00	0.13		83	83

*Comparison based on ~150 flasks analyzed on both instruments in 1995.
 

Good consistency is observed for measurements of HCFC-22, HCFC-142b, and HFC-134a from the two different instruments. These results suggest that potential problems associated with sample analysis (such as coelution or instrument-specific problems) are not influencing the results that are obtained for these halocarbons on either instrument. A small, consistent offset is apparent for HCFC-141b. The cause of this offset is currently unknown. Variability observed between instruments for measurements of HFC-134a is larger than for other compounds because measured mixing ratios in early 1995 were often near the detection limit on the older GC-MS instrument.

Measurements of Additional Chlorinated Compounds with GC-MS Instrumentation

Mixing ratios for numerous other compounds were determined from flasks during 1994-1995. Data for certain chlorinated hydrocarbons with atmospheric lifetimes of <1 year show dramatic seasonal cycles in both hemispheres (Figure 5.15). Minima for these compounds were observed shortly after midsummer in each hemisphere when loss rates were expected to be greater than at other times of the year.

Fig. 5.15. Atmospheric dry mole fractions (ppt) for selected chlorinated compounds. Symbols are identical to those described in Figure 5.13. These data were obtained from flask air analyses on the original GC-MS instrument (see text). Mixing ratios reported are based on a preliminary calibration scale.
 

Atmospheric Trends for Chlorine and Bromine Contained in Long-Lived Halocarbons

Chlorine and bromine catalyze reactions leading to the depletion of stratospheric ozone. Enhanced use of chlorine- and bromine-containing compounds by mankind has led to a steady increase in the abundance of chlorine and bromine in the atmosphere in recent time and to the depletion of stratospheric ozone [WMO, 1995]. Only bromine and chlorine-containing compounds that are relatively insoluble and have atmospheric lifetimes longer than a year can deliver significant amounts of halogen to the stratosphere. In 1994 the atmospheric abundance of Cl contained in these types of halocarbons was approximately five times greater than the burden estimated in the absence of anthropogenic emissions. Similarly, anthropogenic emissions of bromine-containing compounds have resulted in an atmospheric bromine abundance that is approximately twice that estimated for preindustrial times.

Model studies suggest that the tropospheric abundance of Cl will peak in the mid 1990s at 3.5-4.0 ppb if limits outlined in the most recent Copenhagen amendments to the Montreal Protocol are not exceeded. Not all nations have agreed to the restrictions set forth in the Protocol; in addition, evidence suggests that significant amounts of CFCs are currently produced illegally. Furthermore, developing countries are allowed a 10 year grace period on consumption restrictions under the Montreal Protocol. As a result, much uncertainty has remained regarding the timing and magnitude of peak halogen (Cl and Br) loading of the atmosphere.

In the NOAH Group, global tropospheric distributions and abundances are routinely determined for the most abundant, long-lived anthropogenic halocarbons. Measurements of the halogen burden in the troposphere can supply a reasonable estimate for the stratospheric halogen burden 3 to 5 years in the future [WMO, 1995]. Accordingly, the results provide estimates for the burden of ozone-depleting gases in the future stratosphere.

By accounting for the number of Cl atoms contained in the most abundant CFCs, HCFCs, chlorinated solvents, and halon-1211, it is estimated that the tropospheric abundance of Cl contained within these halocarbons peaked in early 1994 and is currently decreasing at a rate of 25 ppt yr-1 (Figure 5.16a; Table 5.4) [Montzka et al., 1996b]. The current decrease is a dramatic turnaround from reported increases of 110 ppt yr-1 in 1989 and 60 ppt yr-1 in 1992 [WMO, 1995]. Most of the current decline in tropospheric Cl can be attributed to a decrease in the atmospheric abundance of CH₃CCl₃ (Figure 5.9) which has a relatively short atmospheric lifetime (~5 yr) [Prinn et al., 1995]. The abundance of the major CFCs and chlorinated solvents were all stable or decreasing in 1995 with the exception of CFC-12. The abundance of CFC-12 continued to increase in mid-1995 at a rate of ~6 ppt yr^-1 or approximately one-third the rate observed in the late 1980s (Figure 5.7 and 5.8). Increases in the abundance of HCFCs (HCFC-22, -142b, and -141b) accounted for growth in tropospheric chlorine of ~11 ppt per year in 1995 [Montzka et al., 1996b]. After accounting for chlorine contributed from CH₃Cl, other chlorinated hydrocarbons (~700 ppt), and less abundant CFCs, it is estimated that the mean global chlorine loading of the troposphere peaked in 1994 at ~3.7 ppb.
 

Fig. 5.16. (a) The amount of total chlorine, (b) effective equivalent chlorine (EECl) and (c) equivalent chlorine (ECl) contained within anthropogenic halocarbons: CFC-11, CFC-12, CFC-113, CH₃CCl₃, CCl₄, HCFC-22, HCFC-142b, HCFC-141b, halon-1211, and halon-1301. Data were binned by month and hemisphere (northern hemisphere, filled triangles; southern hemisphere, filled squares; global mean, plus symbols). Solid lines represent fits to monthly means.

TABLE 5.4. Mean Rate of Change Estimated for Mid-1995 From Measured Halocarbons (ppt yr^-1)*

Compound	Global	NH	SH
Chlorine	-22	-30	-20
EECl	-23	-35	-16
ECl	-15	-22	- 8

*CFC-11, CFC-12, CFC-113, CH₃CCl₃, CCl₄, halon 1211, halon 1301, HCFC-22,

HCFC-141b, HCFC-142b.
 

Stratospheric ozone is destroyed through reactions of inorganic bromine and chlorine molecules. To estimate how the abundance of stratospheric inorganic halogen will change as a result of the observed trends for halocarbons in the troposphere, stratospheric degradation rates of halocarbons must be considered. Halogen release rates vary over altitude and latitude in the stratosphere, as does the efficiency for bromine to catalyze the destruction of stratospheric ozone when compared to chlorine (the alpha factor). Whereas bromine is estimated to be about 40 times more efficient than chlorine for destroying stratospheric ozone in the polar vortex [WMO, 1995], it may be as much as 100 times more efficient in the lower, midlatitude stratosphere [Garcia and Solomon, 1994]. In the following, the future reactive halogen burden is estimated for the lower, midlatitudinal stratosphere (effective equivalent chlorine, EECl) [Daniel et al., 1995] and for the springtime, polar stratosphere (equivalent chlorine, ECl) with halogen release rates and alpha factors appropriate for each region. The abundance of halons increased over this period so that by considering higher estimates for alpha, the decline in EECl or ECl is underestimated.

The current mix and abundance of halocarbons within the troposphere ultimately will release fewer halogen atoms to the lower, midlatitudinal stratosphere than in previous years. The mean global tropospheric burden of halogen that will become inorganic halogen in the stratosphere reached a maximum in early 1994 and was declining in mid-1995 at 21 8 ppt EECl yr^-1. (Figure 5.16b; Table 5.4; Montzka et al. [1996b]).

The actual rate of change for EECl in mid-1995 may be somewhat lower if the atmospheric abundance of methyl bromide has increased since 1992. However, limits to production outlined in the Copenhagen Amendments and production figures from the major global producers for 1991 and 1992 suggest anthropogenic methyl bromide emissions may have stabilized in the early 1990s. It is unlikely that an increase in methyl bromide over this period would have been large enough to offset the decrease reported here for EECl [Montzka et al., 1996b].

For a mean transport time between the troposphere and lower, midlatitude stratosphere of 3-4 years [Hall and Plumb, 1994; Fahey et al., 1995; Boering et al., 1995], maximum levels of inorganic halogen are expected in the lower midlatitudinal stratosphere between 1997 and 1998. Modeling studies suggest that when stratospheric mixing ratios of reactive halogenated compounds begin declining, column-ozone abundance at midlatitudes will begin to recover [WMO, 1995]. However, because stratospheric ozone is influenced by other variables such as aerosol loading and temperature [Solomon et al., 1996], the exact timing will also depend on how these variables change over this period.

To estimate the stratospheric abundance of ozone-depleting gases in the springtime polar vortex, equivalent chlorine (ECl) was calculated based upon CMDL tropospheric halocarbon measurements. The current mix and growth rates of these gases in the troposphere will result in lower ECl in the polar stratosphere in the future. In mid-1995 equivalent chlorine was decreasing at 18 7 ppt ECl yr^-1 (Figure 5.16c; Table 5.4; Montzka et al. [1996b]). It is unlikely that an increase in atmospheric methyl bromide in recent time would have been large enough to offset this decrease. Because transport of air from the lower troposphere to the polar stratosphere below ~25 km occurs in 3-5 years [Prather and Watson, 1990; Pollock et al., 1992; WMO, 1995], or over a slightly longer period than to the lower, midlatitude stratosphere, levels of equivalent chlorine are expected to reach a maximum in the polar stratosphere between 1997-1999 and decline thereafter as long as current growth rates for halons and CFC-12 and the abundance of other CFCs and halocarbons continue to decline.

Although the abundance of reactive halogen in the polar stratosphere above Antarctica will decline when air currently within the troposphere reaches this region, springtime, total-column ozone levels will not increase there immediately. Ozone was nearly completely destroyed in the lower stratosphere above the Antarctic continent in springtime for the past 8 years [WMO, 1995]. Total-column ozone abundance within this region is expected to begin recovering only when mixing ratios of reactive halogenated compounds drop below those present in the late 1980s [Prather and Watson, 1990; WMO, 1995].

5.1.6. GRAVIMETRIC STANDARDS

One of the strengths of NOAH is the ability to generate unique standards for ìhot-topicî molecules with ease. Almost all of the NOAH standards are produced by actually weighing the individual components in air or by gravimetry. With maximum dilutions of 1:20,000 and accuracy's of better than 0.2%, two or four dilutions are sometimes required to produce standards at the ppt level. Not only does NOAH produce standards for internal use, but some of the clients have included other international and national research institutions. Some of this work over the past 2 years is summarized below.

Aluminum compressed gas cylinders are now being used with brass and stainless steel valves that have all-metal valve stems and seats. Materials such as KEL-F have a high absorption/desorption potential for gases such as 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113).

Five compressed gas cylinders containing pure reagent gases were analyzed for impurities using a CEC-103 mass spectrometer located at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. The results of the analyses indicated that the measured purity levels of the pure methane (CH₄), carbon monoxide (CO), carbon dioxide (CO₂), hydrogen (H₂), and nitrous oxide (N₂O) gases are consistent with the stated purity as specified by the gas supplier. These pure mixtures are being used to prepare gravimetric standards for CCG (CH₄, CO, CO₂ , and H₂ ) and for NOAH (N₂O).

A total of 26 gravimetrically prepared CH₄ in air standards now exist for use by CCG. The nominal mixing ratios of the gas mixtures range from 32 ppb to 20 ppm. The standards are currently being studied for stability.

A suite of gravimetrically prepared sulfur hexafluoride (SF₆) in air standards were prepared for the first time this year. The standards were prepared with nominal mixing ratios ranging from 3 ppt to 110 ppt.

A suite of hydrogen (H₂) in air standards were also gravimetrically prepared for the first time this year. The mixing ratios of these standards range from approximately 450 ppb to 600 ppb.

HFC-134a in air standards were prepared in 1995 and were intercompared with existing HFC-134a standards prepared several years ago. The results confirm that the gas is stable over many years.

Several nine-component standards containing various methyl halide compounds were gravimetrically prepared primarily for the ocean and flask programs. The standards contain methyl bromide (CH₃Br), methyl chloride (CH₃Cl), methyl iodide (CH₃I), dibromomethane (CH₂Br₂), tribromomethane (CHBr₃), chlorodibromo-methane (CHBr₂Cl), bromochloromethane (CH₂BrCl), chloroiodomethane (CH₂ICl), and diiodomethane (CH₂I₂). Nine two-component mixtures were initially prepared with mixing ratios at the ppb level. The pure liquids were handled under darkroom conditions with the use of a black-light source, because compounds with iodine are photochemically active and decompose quickly in sunlight and artificial light.

Existing CH₃CCl₃ and CCl₄ primary standards at the ppb level were compared to standards recently prepared to determine the stability of these gases over a number of years and to resolve a difference in sensitivity between several suites of ppt level standards that were prepared from the ppb standards. The mixing ratios range from 120 ppb to 760 ppb for CH₃CCl₃ and from 190 ppb to 950 ppb for CCl₄. The gases were analyzed using GC with a FID. The results of the analyses indicate that eight of nine CH₃CCl₃ standards and nine of nine CCl₄ standards are consistent to within 2%.

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