5.1.3. Radiatively Important Trace Species (RITS) Measurements

Operations Update

The major operational change in this program over the 2-year period of this report was the relocation of equipment to new buildings at three sites. The air line intakes were not moved during this period, but in November 1995 the tower at SPO was moved and the lines extended to accommodate construction of the Atmospheric Research Observatory (ARO). Normal equipment and software maintenance continued as usual based on failures and problems reported by the site personnel.

At the C-1 site on Niwot Ridge, Colorado, a 3 m Ž 5 m Tall Ranch Tuff Shed was placed at the site in early October 1995 east of the existing facility. Over the next 6 months as time and weather permitted, a window was installed and the interior of the structure was wired for electricity. The building was insulated and wall board, flooring, heating equipment, and air conditioning were installed. Finish work was done by NOAH staff. University of Colorado personnel brought power to the building and installed a fiber optic computer network. On April 3, 1996, all of the equipment was moved into the new building. As the air lines were moved, one of the lines was found to have a small hole in it about 1 m from where it entered the old building. The Dekabon tubing was cut at that point and attached to the pump in the new building. A comparison of the previous month’s data shows no significant difference between the two air lines for any of the chemicals measured.

The new building at SMO was completed in early July 1996 and our equipment was the first to be moved from the old EKTO building. The equipment was checked and then turned off on July 20, 1996, moved and tested, and was operational on July 25th. Because the existing air lines were too short to reach into the new building, a union fitting was put in each line and about 9 m of new Dekabon tubing was added to reach the pump board.

On January 21, 1997, the computer and GC equipment were shut down in the Clean Air Facility at SPO, crated for transport, and quickly moved to the new ARO. By January 27 all of the equipment was operational again. The very long air sampling lines were moved and cable tied above the snow with other lines on poles to avoid being crushed by people and equipment. Since the distance from the ARO to the sampling tower was closer than the Clean Air Facility to the tower, the excess air line was coiled up in the crawl space below the first floor where the equipment is located.

Three 4-channel Chromatograph for Atmospheric Trace Species (CATS) (old STEALTH system) type gas chromatographs are currently in operation at HFM, ITN, and LEF. A fourth single-channel version measures N₂O and SF₆ at ALT. The RITS 3-channel GCs will be phased out after a 6-month comparison with the CATS system. This period will be used to ensure comparable results. The first CATS system was shipped in December 1997 and was installed at SPO in January 1998.

Data Analysis

A thorough review of our current calibration scale for CFC-12 was undertaken before an intercomparison meeting with AGAGE staff in May 1997. This resulted in changes on the order of -2% to assigned mixing ratios for all calibration tanks used at the field sites from 1993 to the present. Likewise, by applying the same techniques to other gases, the calibration scales for CFC-11, nitrous oxide, methyl chloroform, and carbon tetrachloride have changed, though not dramatically. All data presented here have had these corrections applied and are our current best estimates of what is happening in the global tropospheric atmosphere.

The revised, globally averaged maximum CFC-11 mixing ratio was 272.5 ppt in late 1993 (Figure 5.9). The mixing ratio was 268.8 ppt at the end of 1997, the growth rate was -1.3 ppt yr^-1, and the interhemispheric difference was 2.6 ppt. The global CFC-12 mixing ratio at the end of 1997 was 531.4 ppt (Figure 5.10). The growth rate slowed until mid-1996 and now appears to be holding steady at 3.6 ppt yr^-1. The average interhemispheric difference continues to decline and was 7.9 ppt in late 1997.

Fig. 5.9. (a) Monthly average CFC-11 mixing ratios in ppt from the in situ GCs, (b) hemispheric and global average mixing ratios, and (c) global average growth rate in ppt yr^-1.

Fig. 5.10. (a) Monthly average CFC-12 mixing ratios in ppt from the in situ GCs, (b) hemispheric and global average mixing ratios, and (c) global average growth rate.

Carbon tetrachloride has been decreasing in the troposphere over the last 6 years at the rate of -0.7 ppt yr^-1. At the end of 1997 the global mixing ratio was 102.0 ppt and the interhemispheric difference about 1.4 ppt (Figure 5.11). The revised calibration scale has increased mixing ratios by approximately 1% from 1993 and the rate of decrease is less than previously reported.

Fig. 5.11. (a) Monthly average carbon tetrachloride mixing ratios in ppt from the in situ GCs, (b) hemispheric and global average mixing ratios smoothed using a LOWESS routine, and (c) global average growth rate in ppt yr^-1.

As noted in section 5.1.2, methyl chloroform mixing ratios continue to decrease in the atmosphere (Figure 5.12). At the end of 1997 the global mixing ratio was 76.9 ppt, and the interhemispheric gradient was near zero. The global distribution in 1997 reflects the distribution of sinks more than sources now that emissions have dropped substantially.

Fig. 5.12. (a) Monthly average methyl chloroform mixing ratios in ppt from the in situ GCs, (b) hemispheric and global average mixing ratios, and (c) global average growth rate.

The atmospheric burden of nitrous oxide continued to increase at an average rate of 0.68 ppb yr^-1 (RITS measurements) over the past 4 years (Figure 5.13). The global mixing ratio at the end of 1997 was 313.1 ppb and the average hemispheric difference over the 1987-1997 period was 1.2 ppb. There is an annual cycle in the southern hemisphere that is in phase with the annual cycle in the northern hemisphere, generally peaking in the first quarter of each year.

Fig. 5.13. Monthly average nitrous oxide mixing ratios in ppt from the in situ GCs, (b) hemispheric to global average mixing ratios, and (c) global average growth rate.

Chromatography and Software

Each in situ gas chromatograph system is custom designed specifically for each site. Table 5.5 lists the compounds to be sampled by the new in situ gas chromatograph at the CMDL observatories.

TABLE 5.5. Peak Characteristics on Four-Channel GC at SPO

*Porapak Q column

†Unibeads 1s column (replacing old Porasil a column)

‡OV101 column

§Capillary column (Poraplot Q)

Chromatography is controlled and data acquired with custom programs developed on the QNX operating system run on personal computers. QNX is a UNIX-based operating system with structured C programming language support. As a multitasking operating system, QNX provides simultaneous programming functions that allow for concurrent control, data acquisition, user interface operation, and data retrieval. The enhanced capabilities of QNX have allowed the addition of features that take advantage of available technology.

The control functions of the in situ gas chromatograph fall into two categories: those controlled by a custom digital interface and those controlled by an RS-485 network. Sample selection, chromatographic valves, cut-off solenoids, and flow controllers are controlled by a custom digital interface. Temperature controllers use an RS-485 network. Two data acquisition circuit boards handle input from the gas chromatograph electrometers, temperature sensors, and pressure sensors. Chromatographic data from the electrometers and engineering data are stored in a file buffer on the hard drive of the computer. Raw chromatograms and a representative subset of the engineering data are extracted from the buffer, compressed, and stored in a retrieval sub-directory, set up as a first-in-first-out (FIFO) on the hard drive and archived on 100 mb Iomega Zip disks at the site.

A computer workstation at CMDL Boulder automatically retrieves the chromatographic and engineering data from the in situ gas chromatograph on a daily basis for processing. Routines for this retrieval were programmed using TCP/IP - Internet links. Additional programs for use in trouble-shooting and routine maintenance have been included in the QNX software. A World Wide Web (WWW) site allows scientists and technicians to determine the operational status of the instrument system over the Internet. The WWW page includes real-time and near real-time engineering and chromatographic data displayed in tabular and graphic formats. This user interface, accessed with widely available WWW browser software, adds considerable flexibility for scientists and technicians to anticipate problems to resolve quickly.

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