4. Ozone and Water Vapor

S. OLTMANS (EDITOR), D. HOFMANN, J. HARRIS, B. JOHNSON, R. EVANS,
G. KOENIG, J. LATHROP, M. O'NEILL, H. VÖMEL, D. QUINCY,
W. KOMHYR, M. CLARK, AND E. HACKATHORN

4.1. CONTINUING PROGRAMS

4.1.1. TOTAL OZONE OBSERVATIONS

Total ozone observations continued throughout 1994 and 1995 at 15 of the 16 stations that comprise the U.S. Dobson spectrophotometer network (Table 4.1). Of the 16 stations, 5 were operated by CMDL personnel, 5 by NWS, 2 are domestic cooperative stations, and 4 are foreign cooperative stations. All stations are either fully or semiautomated. In addition, a Brewer spectrophotometer was operated on a nearly continuous basis at Boulder.

TABLE 4.1. U.S. Dobson Ozone Spectrophotometer Station Network for 1994-1995

Station Period of Record Instrument No. Agency
Bismarck, North Dakota Jan. 1, 1963-present 33 NOAA
Caribou, Maine Jan. 1, 1963-present 34 NOAA
Wallops Is., Virginia July 1, 1967-present 38 NOAA; NASA
SMO Dec. 19, 1975-present 42 NOAA
Tallahassee, Florida May 2, 1964-Nov. 30, 1989;

Nov. 1, 1992-present

58 NOAA; Florida State University
Boulder, Colorado Sept. 1, 1966-present 61 NOAA
Fairbanks, Alaska March 6, 1984-present 63 NOAA; University of Alaska
Lauder, New Zealand Jan. 29, 1987-present 72 NOAA; DSIR
MLO Jan. 2, 1964-present 76 NOAA
Nashville, Tennessee Jan. 2, 1963-present 79 NOAA
Perth, Australia July 30, 1984-present 81 NOAA; Australian Bureau Meteorology
SPO Nov. 17, 1961-present 82 NOAA
Haute Provence, France Sept. 2, 1983-present 85 NOAA; CNRS
Huancayo, Peru Feb. 14, 1964-Dec. 31, 1992 87 NOAA; IGP
BRW June 6, 1986-present 91 NOAA
Fresno, California June 22, 1983-March 13, 1995 94 NOAA
Hanford, California March 15, 1995-present 94 NOAA

The Peruvian station was still out of operation at the end of 1995, although a new baseline monitoring station is under construction. Operations will likely start in late 1996. In May 1995 the Fresno instrument and shelter were moved 30 miles southwest of Hanford, California. The Bismarck instrument and shelter were moved about 150 m in August 1994.

Provisional daily total ozone amounts applicable to local apparent noon for the stations listed in Table 4.1 were archived at the World Ozone Data Center (WODC), 4905 Dufferin Street, Ontario M3H 5T4, Canada, in Ozone Data for the World. Table 4.2 lists the monthly mean total ozone amounts measured at the various stations for 1994 and 1995. (Monthly means are computed for stations where observations were made on at least 10 days each month.).

TABLE 4.2. Provisional 1994 Monthly Mean Total Ozone Amounts (M-Atm-CM)
Station Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.
1994
Bismarck, North Dakota 361 367 372 351 327 322 319 302 286 294 286 300
Caribou, Maine 340 382 390 377 369 338 324 324 316 299 293 304
Wallops Is., Virginia 320 343 347 325 355 327 309 302 292 281 255 270
SMO 241 242 244 237 237 241 240 244 244 248 251 242
Tallahassee, Florida 289 291 306 308 338 321 322 299 - - 241 269
Boulder, Colorado 321 331 333 341 315 295 299 280 277 274 265 276
UAF-GI, Alaska - [371] 420 408 380 347 312 278 - - - -
Lauder, New Zealand 277 267 262 265 289 315 337 361 370 360 341 302
MLO 244 256 - - 279 271 268 265 258 256 238 220
Nashville, Tennessee 321 323 339 328 334 322 313 304 296 277 252 276
Perth, Australia 266 262 265 257 269 291 294 300 309 317 307 278
SPO 266 253 [245] [231] [277] [231] [239] [226] - 132 269 316
Haute Provence, France 326 373 314 382 352 332 320 305 311 295 273 293
Huancayo, Peru Station closed
BRW - - 457 445 379 346 310 293 - - - -
Fresno, California 314 324 325 332 333 303 305 292 - 269 275 280
1995
Bismarck, North Dakota 334 358 341 347 347 321 311 282 286 290 292 297
Caribou, Maine 349 382 357 372 348 323 320 304 302 - 311 325
Wallops Is., Virginia 307 344 316 325 316 323 312 297 288 267 299 284
SMO 242 243 243 244 243 247 245 250 259 262 251 251
Tallahassee, Florida 267 282 284 288 302 313 302 308 294 266 277 263
Boulder, Colorado 296 290 304 328 325 299 281 276 284 270 276 275
UAF-GI, Alaska - [349] 365 367 341 326 300 302 268 326 - -
Lauder, New Zealand 277 267 282 273 280 302 335 336 361 348 328 287
MLO 217 223 254 268 280 274 262 262 261 255 254 239
Nashville, Tennessee 321 323 339 328 334 322 313 304 296 277 252 276
Perth, Australia 272 257 268 276 268 282 295 292 312 311 306 273
SPO 287 270 [231] [256] [255] [245] [246] [223] - 120 192 257
Haute Provence, France 293 319 333 340 340 331 322 329 304 277 283 303
Huancayo, Peru Station closed
BRW - [354] 407 386 339 345 304 300 269 [260] - -
Handford, California 293 292 315 316 338 330 298 292 286 272 264 285

Monthly mean ozone values in square brackets are derived from observations made on fewer than 10 days per month.

Ten Dobson ozone spectrophotometers in the CMDL network as well as 29 others were calibrated during 1994 and 1995. Table 4.3 lists all the instruments calibrated and the resulting calibration difference expressed as a percent ozone difference. This percent difference is between ozone calculated from the test and the standard instrument measurements with the ADDSGQP observation type at a value of 2, and a total ozone value of 300 Dobson Units (DU), before any repair or calibration adjustment is made. The table also lists the place of the calibration and the standard instrument used.

TABLE 4.3. Dobson Ozone Spectrophotometers Calibrated in 1994-1995

Instrument Calibration Calibration Standard
Station Number Date Correction (%) Number Place
1994
Lisbon, Spain D013 Aug. 2, 1990 +0.7% 65 Izaña Observatory
Oslo, Norway D056 Aug. 21, 1986 +0.5% 65 Izaña Observatory
Potsdam, Germany D064 Aug. 2, 1990 +0.6% 65 Izaña Observatory
Huancayo, Peru D087 May 15, 1985 +0.9% 65 Izaña Observatory
Natal, Brazil D093 May 20, 1986 +2.9% 65 Izaña Observatory
Buenos Aires, Argentina D097 July 15, 1992 +0.1% 65 Izaña Observatory
El Arenosillo, Spain D120 Aug. 9, 1990 +1.3% 65 Izaña Observatory
Ushuaia, Argentina D131 None N/A 65 Izaña Observatory
Tallahassee, Florida D058 Sept. 9, 1991 -.3 83 Boulder
Boulder, Colorado D061 Aug. 27, 1992 0.0% 65 Boulder
MLO D076 June 13, 1993 N/A 83 Boulder
SPO D080 May 26, 1988 0.5% 83 Boulder
BRW D091 May 26, 1989 0.0% 83 Boulder
Fresno, California D094 June 26, 1989 1.0% 83 Boulder
Mexico D.F Mexico D098 August 1978 -0.3% 83 Boulder
1995
RA VI Spare 15 None N/A 65 LKO Arosa
Vindeln, Sweden 30 May 10, 1990 +0.7% 65 LKO Arosa
United Kingdom 32 May 1995 N/A 65 LKO Arosa
Uccle, Belgium 40 Aug. 1, 1990 +2.0% 65 LKO Arosa
United Kingdom, Standard 41 Aug. 2, 1990 +1.2% 65 LKO Arosa
Sestola, Italy 48 Nov. 12, 1980 +2.4% 65 LKO Arosa
Bordeaux, France 49 July 10, 1990 +0.3% 65 LKO Arosa
Reykjavik, Iceland 50 Aug. 2, 1990 +1.1% 65 LKO Arosa
Arosa, Switzerland 62 Aug. 7, 1992 +1.7% 65 LKO Arosa
Belsk, Poland 84 Aug. 2, 1990 +0.2% 65 LKO Arosa
l'Obs. Haute Provence, France 85* July 10, 1990 +0.7% 65 LKO Arosa
Denmark 92* Aug. 2, 1990 +1.0% 65 LKO Arosa
Arosa, Switzerland 101 Aug. 2, 1990 +2.1% 65 LKO Arosa
Hohenpeissenberg, Germany 104 Aug. 2, 1990 +1.6% 65 LKO Arosa
Moscow, Russia 107 Aug. 5, 1990 +1.4% 65 LKO Arosa
Budapest, Hungary 110 Aug. 2, 1990 +0.5% 65 LKO Arosa
Tsukuba, Japan, Standard 116* June 29, 1992 +0.6% 65 LKO Arosa
Bucharest, Romania 121 Aug. 5, 1990 Not consistent 65 LKO Arosa
Bismarck, North Dakota D033 Oct. 1, 1993 +1.5 83 Boulder
Caribou, Maine D034 Sept., 9, 1991 +0.3 83 Boulder
Wallops Island, Virginia D038 Sept. 16, 1991 +1.0 83 Boulde
Nashville, Tennessee D079 Aug. 14, 1991 +0.6 83 Boulder
Comodoro Rivadavia, Argentina D133 New Dobson N/A 83 Boulder
Montevideo, Uruguay D134 New Dobson N/A 83 Boulder

The Mauna Loa Observatory, Hawaii (MLO) instrument D076 failed mechanically in February 1994. This automated instrument was repaired and back in service in May 1994. During the repair the left side mirror was replaced due to a deteriorating surface.

CMDL participated in an international Dobson spectrophotometer calibration at Izaña Observatory (Tenerife, Spain) in June 1994 and at Arosa, Switzerland (LKO) in July 1995 as part of its role as the World Center for Dobson Calibrations. Instruments for Mexico City, Mexico; Comodoro Rivadavia, Argentina; and Montevideo, Uruguay, were calibrated in Boulder during this period. A representative from the Czech Hydrological and Meteorological Institute assisted with the latter two instruments. These were new instruments from Ealing Opto-Electronics.

Reevaluation of more than 400 station-years of total ozone data from 25 CMDL (and its predecessor organizations) Dobson spectrophotometer stations was completed in 1995. Corrections were based on field instrument calibrations made throughout the years with World Primary Standard Dobson Instrument no. 83 [Komhyr et al., 1989]. This instrument's long-term ozone measurement precision has been maintained since the early 1960s at 1% by means of Langley calibration observations conducted periodically at MLO and with standard lamps. Procedures used in reevaluating the data are described in detail in Komhyr [1993].

Seasonal and annual downward trends in ozone during 1979-1995, determined from the reevaluated data for five U.S. mainland stations (Caribou, Maine; Bismarck, North Dakota; Boulder, Colorado; Wallops Island, Virginia; and Nashville, Tennessee) and for MLO and Samoa Observatory, American Samoa (SMO), are shown in Table 4.4. Also included in the table are the ozone trends measured at Fresno, California, during 1985-1995. The statistical method used in determining the trends (G. C. Reinsel, University of Wisconsin-Madison) was similar to that employed by the 1988 Ozone Trends Panel [WMO, 1988] whereby solar cycle and ozone QBO effects are removed from the data. Note that the downward trend in ozone, averaged over the five contiguous U.S. stations with the longest records, is largest at -5.45% per decade for spring (March-May) months and smallest at -1.6% per decade for autumn (September to November) months. On an annual basis, the downward ozone trend at these sites averages -3.58% per decade. These trends exhibit a slight recovery from values determined from 1979-1993 data that encompassed record low ozone values over the U.S. during the winter of 1992-1993 [Komhyr et al., 1994]. For the earlier time period, the average five-station downward ozone trend for spring months (not shown) was -5.79% per decade and -3.8% per decade on an annual basis. Downward trends in ozone measured nearer the equator at MLO and SMO are significantly smaller.

TABLE 4.4. Annual and Seasonal Trends January 1979-December 1993

Annual Dec.-Feb. March-May June-Aug. Sept.-Nov.
Station Latitude Trend Std. Error Trend Std. Error Trend Std. Error Trend Std. Error Trend Std. Error
Caribou, Maine 46.9N -4.00 0.71 -4.84 1.73 -5.64 1.12 -3.34 0.79 -1.52 1.16
Bismarck, North Dakota 46.8N -3.30 0.63 -3.25 1.43 -5.69 1.01 -2.00 0.93 -1.62 0.95
Boulder, Colorado 40.0N -3.85 0.61 -3.97 1.23 -6.50 1.22 -2.68 0.75 -1.51 0.98
Wallops Is., Virginia 37.9N -3.67 0.68 -4.79 1.33 -4.77 1.28 -2.66 0.78 -2.10 1.18
Nashville, Tennessee 36.3N -3.09 0.70 -4.08 1.25 -4.62 1.35 -2.03 0.92 -1.23 1.22
Fresno, California* 36.8N -3.81 1.37 -3.18 2.00 -3.84 3.00 -3.52 1.71 -4.74 1.31
MLO 19.5N -0.53 0.77 -1.55 1.15 -0.51 1.44 -0.20 0.94 0.08 0.74
SMO 14.3S -1.66 0.66 -1.48 0.96 -2.13 0.80 -1.42 1.09 -1.61 0.85
Average over first five stations -3.58 -4.19 -5.45 -2.54 -1.60

4.1.2. UMKEHRS

Umkehr observations made with the Automated Dobson Network instruments continued in 1994 and 1995 at Boulder; Haute Provence, France; Lauder, New Zealand; MLO; Perth, Western Australia; and at the University of Alaska's Geophysical Institute. Umkehr processing is set to resume early in 1996. Processing will begin with MLO, followed by Lauder and the other stations in a collaborative effort with the University of Alabama, Huntsville. Since the reprocessing of the total ozone from these stations will have been completed, the updated calibration tables and ozone values will be incorporated into the Umkehr processing. Under conditions of high stratospheric aerosol loading, which was the case following the eruption of Mt. Pinatubo, reliable ozone profiles can be obtained from the Umkehr technique only by correcting for aerosols. Such conditions prevailed through 1992 at most of the sites. The effort with University of Alabama, Huntsville, will include the application of proper aerosol corrections to the profile data.

4.1.3. SURFACE AND TROPOSPHERIC OZONE

At least 20-year records of observation are now available for each of the four CMDL baseline sites. Records at Bermuda, Barbados, and Niwot Ridge are at least 5 years in length. At Westman Islands, Iceland, observations began in 1992. For several years, data were being obtained from Mace Head, Ireland, in a cooperative program as part of the Atmosphere/Ocean Chemistry Experiment (AEROCE). Data continues to be received from Mace Head but CMDL is no longer actively involved in that measurement program. The aging complement of surface ozone monitors, some of which are 20 years old, has experienced a number of breakdowns. Significant blocks of data were lost at Barbados and SMO during 1994 and 1995.

The extent to which tropospheric ozone may have changed since preindustrial times and over the past 20 years is of significant interest. Surface ozone measurements using modern instruments were made only during the past 25 years. Some quantitative measurements using wet chemical techniques were made in Europe in the 1950s [Staehelin et al., 1994], and one set of measurements dates from the turn of the century [Volz and Kley, 1988]. These measurements show that over Europe ozone in the lower troposphere at least doubled from the beginning of the measurements to the early 1980s. Many of the more recent measurements (since 1970) show that at least over Europe, and probably over other areas in the midlatitudes of the northern hemisphere, tropospheric ozone continued to increase through the 1970s and early 1980s [Oltmans et al., 1995]. An analysis of most of the recent data sets from surface stations (some located above the boundary layer) suggests that over at least the past decade there has been a significant slowing in tropospheric ozone growth at midlatitudes of the northern hemisphere [Oltmans et al., 1995]. Most other regions show no evidence for tropospheric ozone increases over the past 20 years and in some cases, such as South Pole Observatory, Antarctica (SPO), significant decreases are evident.

The four CMDL baseline observatory surface ozone data records are among the longest available. The annual averages and long-term trends at each location are shown in Figure 4.1. The numerical trend displayed in the figure is a linear regression of the monthly mean observations. At Barrow Observatory, Barrow, Alaska (BRW) there was a significant upward trend prior to 1990, primarily due to summer increases. Smaller annual averages over the last 5 years have driven the trend downward to show an overall small but insignificant increase. The lower amounts in the 1990s are consistent with the results seen in the ozonesonde record at the Canadian stations [Tarasick et al., 1995]. At MLO the overall record beginning in 1974 shows a small but significant increase. At the two southern hemisphere sites there are long-term decreases. At SMO this decrease is not significant but at SPO a large and significant decline is evident. This is most apparent after 1986. This decline at SPO was discussed in the 1993 Summary Report [Peterson and Rosson, 1994].

Annual average surface ozone mixing ratios for BRW, MLO, SMO, and SPO

Fig. 4.1. Annual average surface ozone mixing ratios in parts per billion (ppbv) for BRW, MLO, SMO, and SPO. The solid line is a linear trend fit to the monthly anomalies. The trend and 95% confidence levels in percent per year are also shown.

The monthly ozone means for each of the four CMDL baseline sites for the period of observation are given in Table 4.5. For MLO the means are for the hours 0000-0800 LST, which falls within the time of downslope flow at the observatory.

TABLE 4.5. Monthly Mean Surface Ozone Mixing Ratios (ppbv)

Year Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.
BRW
1973 - - 22.3 15.8 15.6 19.3 15.1 18.8 23.3 30.8 30.2 34.8
1974 28.7 27.4 - - 23.3 22.0 17.7 18.1 26.1 31.6 32.8 30.9
1975 27.4 29.9 32.7 22.5 23.4 23.7 19.7 19.2 23.0 30.3 28.3 26.3
1976 29.2 31.2 24.4 9.7 11.3 24.1 18.6 18.1 20.3 31.6 30.7 29.1
1977 31.3 32.9 20.8 9.7 20.2 24.5 21.1 20.8 22.4 25.6 33.6 28.0
1978 32.1 32.2 31.0 21.7 22.0 24.1 21.4 22.3 23.6 32.8 29.6 30.3
1979 33.9 31.1 22.6 19.7 25.1 22.5 18.9 18.5 21.1 33.2 32.2 29.3
1980 33.5 33.6 24.5 19.6 18.1 24.7 20.3 19.1 27.9 25.6 34.3 30.0
1981 30.5 27.9 24.5 23.8 26.9 26.1 19.8 23.2 26.2 31.3 34.6 30.9
1982 31.6 31.1 22.5 12.2 15.3 27.1 24.5 23.3 29.7 34.6 36.2 34.5
1983 27.9 34.2 25.9 21.6 21.4 20.4 23.6 22.4 28.2 33.5 30.8 33.0
1984 26.8 19.9 21.5 6.8 16.4 25.5 21.9 22.1 24.3 35.7 38.3 32.2
1985 31.8 28.7 - - 20.2 28.0 21.2 25.0 28.8 31.4 33.0 33.4
1986 35.5 28.0 20.5 13.9 17.4 24.7 20.4 21.6 25.0 30.3 34.6 31.4
1987 31.1 27.2 22.0 15.0 24.0 30.9 23.3 25.6 31.1 32.5 38.8 36.4
1988 36.4 30.9 22.7 23.8 22.2 26.2 21.7 23.8 26.9 31.6 34.2 31.4
1989 30.2 36.8 32.9 24.6 20.2 24.3 19.9 21.0 31.4 34.8 35.0 35.8
1990 32.3 29.0 27.1 19.5 24.6 23.6 21.1 25.5 32.5 33.1 34.3 32.4
1991 31.6 30.2 16.8 14.3 29.4 25.4 24.4 31.6 28.3 27.5 29.8 27.3
1992 29.1 27.8 27.4 15.5 16.9 19.4 18.3 18.8 23.9 27.2 28.6 26.0
1993 26.1 26.5 14.7 17.5 20.6 22.4 16.3 19.1 24.1 29.1 30.3 31.8
1994 31.4 29.7 21.7 10.9 11.3 25.6 21.5 19.4 29.6 31.7 34.7 31.4
1995 33.0 28.5 18.0 18.8 22.4 27.6 20.5 21.3 25.4 35.1 32.2 28.0
MLO
1973 - - - - - - - - - 36.5 33.4 36.0
1974 34.5 42.8 51.8 51.8 49.6 40.8 38.0 31.5 31.2 31.9 29.6 31.9
1975 31.4 38.9 47.5 51.7 48.7 47.3 43.3 43.7 34.1 31.5 29.6 -
1976 37.3 41.6 36.7 41.2 39.3 35.6 32.2 30.0 25.6 30.9 39.3 37.5
1977 - 35.8 48.5 41.6 46.4 41.5 29.2 28.9 - - 39.0 33.2
1978 36.1 39.2 45.0 49.2 37.2 31.8 33.5 29.2 31.9 28.8 27.7 33.6
1979 39.5 36.6 48.5 50.3 48.5 43.7 36.7 29.4 37.4 31.0 38.2 41.6
1980 41.0 42.1 45.0 53.8 47.1 42.0 35.8 38.6 27.4 36.8 38.4 34.5
1981 43.5 42.2 52.1 61.1 60.8 38.6 39.1 37.0 35.9 37.8 38.3 39.9
1982 35.1 40.7 48.8 52.5 54.3 39.3 31.6 30.2 32.4 32.9 36.7 45.7
1983 46.8 53.6 59.1 63.4 56.7 47.6 44.4 32.2 31.0 38.6 34.3 40.2
1984 40.4 40.5 48.4 - 47.1 46.8 37.8 33.8 36.9 31.0 31.8 37.6
1985 43.2 41.6 52.4 50.7 48.4 43.9 40.3 36.9 31.4 33.2 36.2 38.1
1986 39.9 40.0 43.2 46.4 47.2 45.4 31.3 38.7 24.3 38.1 30.2 41.6
1987 38.4 40.5 50.0 - - - - 36.3 31.9 34.4 40.5 33.4
1988 44.8 43.3 50.3 53.1 47.5 32.3 41.2 33.5 34.2 30.0 29.5 36.1
1989 40.7 37.4 40.6 48.4 45.9 35.8 37.1 38.5 38.6 24.8 35.0 40.7
1990 37.8 38.3 49.6 52.0 55.0 43.3 36.7 35.3 30.5 38.1 28.8 39.6
1991 45.3 41.8 48.4 56.9 55.5 41.5 39.2 33.1 31.5 33.9 32.6 33.3
1992 40.2 42.8 53.8 61.0 46.9 49.5 38.8 30.6 26.3 24.9 29.2 32.2
1993 42.2 41.3 50.0 63.3 55.2 50.6 38.0 30.6 26.6 35.3 42.7 38.3
1994 43.3 37.5 48.5 57.9 48.7 35.4 30.1 29.8 33.5 41.4 40.5 45.5
1995 34.4 42.0 51.1 50.5 52.1 44.6 34.6 45.8 42.1 36.7 30.7 35.5


TABLE 4.5. Monthly Mean Surface Ozone Mixing Ratios (ppbv) - Continued

Year Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.
SMO
1976 9.9 9.1 8.8 8.3 11.2 13.5 17.6 21.7 17.9 15.2 12.4 13.2
1977 11.0 8.6 8.8 9.2 12.2 20.7 19.9 20.7 15.9 16.7 15.2 15.0
1978 10.2 9.2 9.2 8.4 13.6 15.9 22.8 15.9 17.7 18.6 13.9 13.3
1979 10.5 8.3 8.1 12.5 15.1 17.2 18.6 19.7 19.3 18.8 13.8 14.8
1980 9.8 7.9 8.5 11.6 15.1 19.3 16.8 21.8 17.7 - 11.4 9.7
1981 9.4 9.4 9.5 9.3 14.0 17.0 20.0 16.4 15.1 15.3 10.9 11.8
1982 8.2 9.1 6.7 7.3 14.0 16.9 16.7 16.1 20.2 13.5 17.0 12.2
1983 8.9 6.3 9.4 11.5 11.2 18.3 19.8 20.8 13.0 15.3 13.0 8.5
1984 7.4 7.1 8.0 5.7 17.3 18.4 18.7 17.6 14.5 17.2 14.7 -
1985 10.3 10.0 7.3 11.2 11.8 17.1 20.2 17.7 17.1 14.0 14.9 10.7
1986 8.4 8.6 8.4 7.2 - 14.0 20.3 21.3 14.5 16.6 12.9 12.7
1987 8.1 6.4 6.5 13.6 16.3 20.1 20.7 24.8 17.1 20.2 10.1 11.9
1988 7.4 8.4 8.1 9.9 11.8 13.9 21.6 17.1 18.3 15.8 13.3 9.1
1989 9.3 8.0 9.0 12.1 10.6 19.6 21.5 20.1 19.4 16.1 18.8 13.8
1990 11.4 11.1 11.3 10.8 16.7 16.1 20.0 19.3 13.4 12.7 13.2 13.2
1991 6.7 8.7 6.6 7.9 13.8 14.4 20.9 20.0 23.2 15.1 12.2 15.4
1992 12.2 - 12.1 - 11.7 18.5 15.2 13.7 12.2 13.9 13.0 9.9
1993 9.4 9.6 9.1 - - - - - - - - -
1994 8.9 8.3 9.4 6.9 12.7 18.0 18.1 22.5 13.6 10.1 13.7 8.0
SPO
1974 - - - - - - - - - - - 28.7
1975 24.6 25.7 24.9 29.4 35.7 34.9 35.3 34.4 35.1 36.2 33.9 31.6
1976 26.2 21.3 21.9 25.2 29.4 31.7 34.4 33.6 26.6 25.2 25.8 24.9
1977 23.5 22.0 21.5 26.7 31.5 33.1 34.2 36.0 33.1 29.9 26.7 30.8
1978 27.4 24.3 - - - 33.1 34.8 34.3 33.5 31.8 33.0 31.3
1979 25.1 22.2 23.6 29.6 33.1 34.7 37.9 34.5 33.1 34.4 - 25.1
1980 24.9 22.5 22.3 26.9 29.3 33.3 35.0 34.4 33.0 29.1 26.8 23.2
1981 21.3 19.6 20.1 24.6 29.0 32.5 35.3 37.7 37.8 37.6 35.3 29.5
1982 - 21.5 24.0 31.5 33.6 35.5 36.4 34.9 33.1 28.5 27.7 24.5
1983 21.3 20.5 21.2 27.3 30.6 33.1 34.3 32.7 33.4 30.6 27.8 25.6
1984 20.5 20.1 21.3 29.0 34.2 36.1 37.5 36.1 35.9 31.5 34.0 28.2
1985 22.7 19.3 22.0 24.4 30.5 36.6 36.7 34.4 32.3 28.3 26.7 24.8
1986 18.3 20.1 21.9 27.1 34.4 36.3 38.7 38.2 36.9 33.0 29.4 25.4
1987 20.0 17.8 19.3 23.5 28.0 30.9 29.7 - - - 24.4 26.0
1988 18.9 21.5 23.7 27.4 31.0 34.1 34.0 33.0 33.9 31.1 29.5 25.3
1989 22.9 19.7 18.7 25.4 35.3 35.4 36.2 36.0 35.5 24.2 28.6 23.4
1990 20.2 20.2 23.1 24.5 27.6 31.2 32.3 30.1 28.8 27.0 27.4 23.7
1991 23.8 19.9 18.9 23.8 26.8 30.9 31.8 34.1 27.7 27.2 22.8 22.1
1992 18.0 17.9 16.9 22.4 29.7 33.5 34.9 34.4 28.5 25.8 26.7 29.3
1993 23.4 20.1 17.5 23.1 26.4 29.7 30.7 30.3 29.1 27.5 29.7 22.9
1994 25.0 19.7 21.1 24.4 27.7 33.1 34.1 33.5 31.6 27.3 29.3 26.8
1995 25.3 18.0 18.0 19.8 24.0 28.3 34.5 33.0 21.8 26.3 29.2 26.4

Monthly means are computed from daily (24-hr) averages.

4.1.4. OZONESONDES

Table 4.6 summarizes the 1994-1995 CMDL ozonesonde project involvement. This includes supplying receiving stations and all ozonesonde supplies, training where needed, personnel launching ozonesondes at several of the sites, and final data processing.

TABLE 4.6. Summary of 1994-1995 Ozonesonde Projects

Ozonesonde
1994
1995
Sites
Totals
Dates
Totals
Dates
Project
Station (weekly)
Boulder 52 Full year 52 Full year NOAA long term
MLO 52 Full year 62 Full Year NOAA long term + MLO3
SPO 69 Full year 69 Full Year NOAA long term
McMurdo 65 Feb. 3-Dec. 25 6 Jan. 1-Feb. 12 NSF and NOAA
Tahiti - 24 July 31-Dec. 29 PEM-Tropics
SMO - 16 Aug. 1-Dec. 14 PEM-Tropics
Intensives (~daily)
Azores 30 May 5-June 3 42 June 2-July 27 AEROCE
Bermuda 10 Jan 21-May 31` 55 April 17-July 27 AEROCE
Maryland - 12 April 13-May 16 AEROCE
Rhode Island - 7 April 18-May 15 AEROCE
Newfoundland - 20 April 12-Aug. 3 AEROCE
Nashville - 14 June 27-July 21 Southern Oxidant Study
Ship Cruises
Indian Ocean - 21 Feb. 12-April 14 NSF R/V Malcom Baldrige
Pacific Ocean - 17 Oct. 17-Dec. 11 ACE R/V Discoverer
Totals 278 418

PEM-Tropics - Pacific Exploratory Mission in the Tropics (a global tropospheric experiment).

The CMDL long-term stations at Boulder, Colorado; Hilo, Hawaii; and SPO, continued operating at one launch per week in 1994 and 1995, with SPO increasing to three per week during the ozone-hole period. The SPO minimum total ozone, measured by ozonesondes, reached 102 and 98 DU in 1994 and 1995, respectively. The minimum profiles and the predepletion profiles are shown in Figure 4.2. Severe depletion was observed in the 14-20 km region (nearly 100%) but did not extend down to the 10-14 km region as it did in 1993 when a record low of 91 DU was measured [Hofmann et al., 1994]. This extended ozone-depletion layer in the lower stratosphere, observed in 1992 and 1993, was due to the effects of the Mt. Pinatubo volcanic aerosol layer [Hofmann and Oltmans, 1993]. By 1994, the Mt. Pinatubo volcanic layer had decayed to background levels over McMurdo Station, Antarctica [Deshler et al., 1996].

Vertical profiles of ozone partial pressure in mPa at SPO during the ozone hole of 1994 and 1995


Fig. 4.2. Vertical profiles of ozone partial pressure in millipascals (mPa) at SPO during the ozone hole of 1994 and 1995. The lighter line represents the predepletion profile while the thicker line is the profile observed at the total ozone minimum.

NOAA was also involved in regular ozonesonde and water vapor measurements at McMurdo Station, Antarctica, from February to August 1994 during a winterover project designed to study the development of polar stratospheric clouds using balloonborne instruments. The University of Denver, University of Wyoming, and NOAA conducted balloon flights to measure ozone, water vapor, nitric acid, and particle concentration profiles during the austral summer, fall, and winter of 1994 prior to and during the development of polar stratospheric clouds and ozone depletion. An early sign of ozone depletion was observed in the June 1994 profile in the 12-20 km layer [Vömel et al., 1995a] (section 4.2.3., this report). CMDL continued measuring ozone profiles on a weekly basis at McMurdo from November 1994 to February 1995. This was done in order to complete the first full year of ozonesonde profiles from McMurdo (February 1994-February 1995). The University of Wyoming launched ozonesondes during the ozone hole period from August to November 1994.

Weekly ozonesondes began at Tahiti and SMO in July 1995 as part of the Global Tropospheric Experiment Pacific Exploratory Mission in the Tropics (PEM-Tropics).

The intensive, short-term ozonesonde projects were all part of the AEROCE II and AEROCE III and the North Atlantic Regional Experiment (NARE). Ozone profiles were measured on a nearly daily basis from several sites (Table 4.6) in the spring and summer of 1994 and 1995 to investigate the sources (anthropogenic and natural) of high ozone layers in the troposphere over the north Atlantic Ocean region.

The 21 ozonesondes flown from the R/V Malcom Baldrige cruise in the Indian Ocean began near South Africa at 30°S, 30°E and ended near Sri Lanka at 7°N, 73°E. This was a preliminary study for the Indian Ocean Experiment (INDOEX) planned for January 1998 to study the chemical and radiative composition of the atmosphere over the Indian Ocean particularly in the region south of the Indian subcontinent. The Aerosol Characterization Experiment (ACE) R/V Discoverer cruise in the Pacific Ocean extended from 31°N, 214°E to 45°S, 145°E. This set of measurements provided the first ozone profiles in a long cross section through the mid-Pacific.

4.1.5. ATMOSPHERIC WATER VAPOR

Monthly water vapor profile measurements continued at Boulder. As was noted earlier [Oltmans and Hofmann, 1995; Ferguson and Rosson, 1992], water vapor in the stratosphere over Boulder has increased significantly. The updated trend information is summarized in Table 4.7. As was reported in the past, the largest trends are seen in the lowest part of the stratosphere over Boulder (16-20 km). This change of about 0.8% yr-2 is somewhat less than reported earlier. This is primarily because the seasonal minimum which occurs in winter and early spring was somewhat lower than in recent years (Figure 4.3). This may be associated with enhanced transport from the tropics during early 1995. Lower stratospheric ozone amounts were also less than normal, indicative of tropical transport. Above 20 km the increase is about 0.5% yr-2, which is consistent with the expected increase resulting from increasing CH4 concentrations in the atmosphere.

TABLE 4.7. 1981-1995 Water Vapor Mixing Ratios Over Boulder, Colorado

Level Mean Standard Deviation Number of Trend* 95% Confidence†
(km) (ppmv) (ppmv) Observations (% yr-1) Interval (% yr-1)
10-12 60.46 40.27 137 1.47 2.60
12-14 12.23 6.97