JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D21, PAGES 26,767-26,778, NOVEMBER 20, 1999

Intercomparison of total ozone observations at Fairbanks, Alaska, during POLARIS

Steven Lloyd, 1 William H. Swartz, 1,2 Thomas Kusterer, 1 Donald Anderson, 1 C. Thomas McElroy, 3 Clive Midwinter, 3 Robert Hall, 3 Karen Nassim, 3 Daniel Jaffe, 4 William Simpson, 5,6 Jennifer Kelley, 5,6 Dennis Nicks,5, 6 Dale Griffin, 6 Bryan Johnson, 7 Robert Evans, 7 Dorothy Quincy, 7 Samuel Oltmans, 7 Paul Newman, 8 Richard McPeters, 8 Gordon Labow, 9 Leslie Moy, 9 Colin Seftor, 9 Geoffrey Toon, 1ø Bhaswar Sen, 1ø and Jean-Francois Blavier 1ø

Abstract. The pattern of seasonal ozone loss over Fairbanks, Alaska (AK), during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) campaign in the spring and summer of 1997 is defined. Five independent data sets of total ozone observations at Fairbanks are presented, from the Earth Probe and ADEOS Total Ozone Mapping Spectrometer (TOMS) satellite instruments, balloon-borne electrochemical concentration cell ozonesondes, and ground-based (Brewer spectroradiometer, Dobson spectrophotometer, and the Jet Propulsion Laboratory MklV infrared interferometer) instruments. The excellent agreement between different observational techniques lends confidence to the observed rate of summertime loss of total ozone at high latitudes. In addition, the small offsets between the data sets are well understood.

1. Introduction

The 1997 NASA-sponsored Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) campaign based in Fairbanks, Alaska (AK) (65øN, 148øW), was designed to improve our understanding of the seasonal behavior of polar stratospheric ozone at northern high lati- tudes. During the summertime, prolonged periods of continu- ous solar illumination lead to decreases in stratospheric ozone of about 40%. This summertime ozone loss is thought to be largely a naturally occurring phenomenon, in contrast to the rapid springtime ozone loss in the Arctic stratosphere observed only a few weeks before the start of the POLARIS campaign [Newman et al., 1997; Donovan et al., 1997; Fioletov et al., 1997]. In situ observations of stratospheric NOx, HO, and CIO• radicals using NASA's high-altitude ER-2 aircraft enabled the determination of the relative rates

1The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland.

2Department of Chemistry and Biochemistry, University of Maryland, College Park.

3^tmospheric Environment Service, Environment Canada, Downsview, Ontario, Canada.

4Interdisciplinary Arts and Sciences, University of Washington, Bothell.

5Geophysical Institute, University of Alaska, Fairbanks. 6Department of Chemistry, University of Alaska, Fairbanks. 7Climate Monitoring and Diagnostics Laboratory, NOAA,

Boulder, Colorado. 8NASA Goddard Space Flight Center, Greenbelt, Maryland. 9Raytheon Information Technology and Scientific Services,

Lanham, Maryland. 1øJet Propulsion Laboratory, Pasadena, California.

Copyright 1999 by the American Geophysical Union.

Paper number 1999JD900468. 0148-0227/99/1999JD900468509.00

of the photolytic ozone loss cycles, and ultimately the poten- tial impact of anthropogenic emissions of halogen- and nitrogen-containing compounds [Fairlie et al., this issue; Sen et al., this issue; and Toon et al., this issue].

Accurate, traceable ozone column measurements are an

essential cornerstone on which to base studies of photochemi- cal and dynamical changes in total ozone. Before summertime ozone loss at high northern latitudes can be explained, the pattern of ozone loss must first be quantified. The scope of this paper is to empirically define the pattern of seasonal ozone loss over Fairbanks during the 1997 summer season using a variety of instrumental techniques, not to explain the cause(s) of this pattem from any model or a priori informa- tion. However, frequent references are made to the dynamical variability of the ozone fields superimposed on the general downward pattern of summertime ozone loss.

Although POLARIS was designed to investigate the pattern of seasonal ozone loss at northern high latitudes, it was not the original intent of the campaign to include a formal intercomparison of total ozone observations. Only the ozone- sonde measurements based at the Geophysical Institute of the University of Alaska at Fairbanks were specifically funded by POLARIS, primarily to provide ozone profiles as input to the radiation field models used to generate photolysis rate coeffi- cients (j-values) along the flight tracks of the ER-2 aircraft [Swartz et al., this issue]. The Total Ozone Mapping Spec- trometer (TOMS) satellite and Dobson ground-based obser- vations were funded separately as part of other NASA and National Oceanic and Atmospheric Administration (NOAA) ongoing ozone monitoring programs, respectively. The Brewer spectroradiometer measurements were provided by Environment Canada's Atmospheric Environment Service primarily to provide a ground truth calibration for their Composition and Photodissociative Flux Measurement (CPFM) spectroradiometer flown on board the ER-2. The Jet Propulsion Laboratory (JPL) MklV infrared interferometer

26,767

26,768 LLOYD ET AL.: INTERCOMPARISON OF TOTAL OZONE DURING POLARIS

performed ground-based observations in addition to its primary objective of obtaining balloon-based observations as part of the MklV Advanced Earth Observing Satellite (ADEOS) validation campaign, colocated at Fort Wainwright in Fairbanks along with POLARIS and the Observations from the Middle Stratosphere (OMS) balloon campaign. However, both the quality and quantity of total ozone observations over the 6-month period from April through September 1997 are sufficient to allow for a detailed intercomparison of these various techniques for measuring total column ozone.

2. Total Ozone Observations

2.1. TOMS Satellite Instruments

T'ne NASA Total Ozone Mapping Spectrometer (TOMS) instrument aboard the Earth Probe (EP) satellite was launched in July 1996 into a nominal 500-km orbit, and subsequently boosted into a 740-km orbit in December 1997 (after the end of the POLARIS campaign). EP TOMS takes backscatter ultraviolet observations in a cross-track scanning mode, with a nadir field of view (FOV) in the lower orbit of approxi- mately 26x26 km [McPeters et al., 1998].

Level 2 overpass data containing the TOMS FOV nearest to Fairbanks (defined as 64.82øN, 147.87øW) on each date, processed with the TOMS version 7 retrieval algorithm, are presented in Figure l a (and available on the TOMS web site at http://toms.gsfc.nasa. gov/). EP TOMS is the most consis- tent data set presented here, with a single observation reported for each day. All 183 daily observations from April through September 1997 were made within a 2-hour window centered on 20:07 Universal Time (UT).

For comparison, comparable overpass data from a similar TOMS instrument flown aboard the Japanese ADEOS satel- lite are also presented in Figure l a (and available on the TOMS web site at http://toms.gsfc.nasa.gov/) through the end of June 1997, at which time the satellite failed. ADEOS TOMS flew at a nominal 800-km orbit, with a 42x42 km FOV and a nominal observation time centered on 21:48 UT

(almost local noon, 21:52 UT). For the 3 months (90 observa- tions) of overlap shown here, ADEOS TOMS showed slightly higher total ozone over Fairbanks (6.7 Dobson Units (DU) or m-atm cm, about 2%) than EP TOMS.

2.2. Ozonesondes

Electrochemical concentration cell (ECC) ozonesondes [Komhyr, 1969] manufactured by ENSCI coupled to Vaisala, Inc. meteorological radiosondes were launched on weather balloons to altitudes of typically 34_+3 km during the POLARIS campaign by staff at the Geophysical Institute and Department of Chemistry at the University of Alaska at Fair- banks under contract from the NOAA Climate Modeling and Diagnostics Laboratory (CMDL). Sondes were deployed typi- cally at 1-week intervals between the three POLARIS deployments and during most ER-2 flights. Measurements of pressure, temperature, and tropospheric relative humidity were also made simultaneously with the ozone observations. Data and plots of individual soundings as well as summary data are available at http://www.uaf. edu/chem/ozone/ O3_dat.html. The same data in NASA Ames standard format are also available on the POLARIS data CDROM [Gaines, 1998].

Since the balloonsondes do not measure total column

ozone, but rather local ozone concentrations up to the burst altitude, the ozone amount above the burst altitude (residual ozone) must be estimated. The residual ozone is typically about 9_+4% of the total ozone column for the POLARIS

sondes, and depends strongly on the altitude at which the balloon burst. Two methods are readily available to estimate the residual ozone. The standard way to approximate the residual ozone is to assume a constant mixing ratio (CMR) of ozone above the burst point, extrapolating through the remainder of the atmosphere. In practice, the last three valid ozone partial pressure values (in milliPascals, mPa) measured by the ozonesonde before burst are averaged and multiplied by the constant 7.89 to get the residual ozone above that point in DU. The CMR extrapolation is sensitive to changes in the slope of the ozone profile just below the burst altitude, and is an overestimate when the balloon bursts near the maximum of

the mixing ratio profile. The ozonesonde must rise at least 3 or 4 km above the ozone peak to obtain reasonable residual ozone values. Of the 33 successful sonde launches, only two in mid-July burst low enough that the constant mixing ratio approximation was not valid. A second method for estimating the residual ozone is to use climatological tables of ozone residuals from the Solar Backscatter UltraViolet (SBUV) satellite data set [McPeters et al., 1997]. Total ozone esti- mated using each of these two approximations is shown in Figure lb.

The use of nonstandard solutions in the ozonesondes

requires some discussion. The standard ECC sonde solution used in the cathode cell is a 1% KI solution with a sodium

phosphate buffer to maintain a constant pH. In 1996, the sonde manufacturer, ENSCI, recommended switching to a 0.5% KI solution after bench-top laboratory tests using a calibrated source of ozone showed the 1% KI gave 5-15% higher ozone values at and above the ozone peak, reaching the maximum difference near the top end of the profile. Fairbanks was therefore supplied by NOAA/CMDL with a 0.5% KI cathode solution buffered with half of the standard amount of

sodium phosphate buffer. A study at Lauder, New Zealand, directly compared both the 0.5% and 1% KI solution sonde results with simultaneous Dobson and ozone lidar observa-

tions [Boyd et al., 1998], and found that the ozone column derived from the 0.5% KI sondes is consistently -5% lower

with the Dobson and lidar total column ozone.

One of the motivations for switching to the 0.5% KI solu- tion was to minimize the differences between the constant

mixing ratio (CMR) extrapolation for ozone above the burst altitude and the approximation obtained using SBUV clima- tological data. This goal was achieved during the POLARIS campaign, when the average difference between the two approximations using the 0.5% solutions was only 1.7 _+ 4.8 DU (less than 1% of the total column, -+1 standard deviation). Removing the single outlying sounding on September 2, 1997, from this comparison brings the offset down to 1.1 -+ 3.2 DU. This represents a significant improvement over the 11-14 DU (up to 6% of the total column) difference between the CMR and SBUV approximations seen in balloonsonde comparisons made in 1991 and 1992 at Boulder, Colorado, and Hawaii using 1% KI solutions [McPeters et al., 1997].

Recently completed tests at NOAA/CMDL have found that it is primarily the lower concentration of sodium phosphate buffers (rather than the concentration of KI) that is responsi-

LLOYD ET AL.: INTERCOMPARISON OF TOTAL OZONE DURING POLARIS 26,769

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Preliminary studies also show good agreement with the Dobson data when using unbuffered 2% KI solutions.

2.3. Brewer Spectroradiometer

The Brewer spectroradiometer, developed by scientists at Environment Canada and manufactured by Sci-Tec, Inc., provides observations of spectrally resolved irradiance at the Earth's surface throughout the ultraviolet UV-B and into the UV-A (X<360 nm) The instrument deployed throughout the POLARIS campaign on the roof of the aircraft hangar in Fair- banks, AK, is a single-monochromator Brewer spectroradi- ometer (serial number 007, Mark IV model, not to be con- fused with the MkIV infrared interferometer described later). Direct Sun measurements are automated. The Brewer algo- rithm uses ratios of measured irradiances at a series of wave-

lengths in the UV-B to determine the overhead column ozone abundance [Kerr et al., 1984a].

A total of 1888 independent direct Sun ozone observations were made at Fairbanks during POLARIS from late April through late May and again from late June through mid- September with a week-long data gap starting at the end of

Figure 1. Total column ozone observations at Fairbanks, Alaska, from five different observation techniques during the 6-month POLARIS campaign. The units for total ozone are Dobson Units (DU, or m-atm cm). (a) Observations from the Total Ozone Mapping Spectrometer (TOMS) instrument on the Earth Probe satellite, processed with the TOMS version 7 retrieval algorithm. Shown are the daily overpass data for the TOMS observation nearest to Fairbanks, Alaska (64.82øN, 147.87øW). For comparison, daily Fairbanks overpass total ozone data from a comparable TOMS instrument aboard the ADEOS satellite is shown through late June, when the satellite failed. The ADEOS TOMS data are about 6.7 DU

higher on average than the Earth Probe TOMS data. (b) Total column ozone estimates from 33 balloon-borne ozonesondes

launched at the Geophysical Institute of the University of Alaska at Fairbanks. Dates of each sonde launch are indi-

cated. Two approximations are used to estimate the amount of ozone above the balloon's burst altitude: a constant mixing ratio (CMR) of ozone extrapolated from the highest altitude data points, and using SBUV climatological ozone residuals for pressure altitudes above 30 mbar, binned by month and 10 ø latitude bands [McPeters et al., 1997]. The Fairbanks data show excellent agreement between these two approximations using a 0.5% KI cathode solution in the ECC ozonesonde. (c) Brewer spectroradiometer total column ozone data from 1888 independent ozone observations, made atop the ER-2 hangar at Fort Wainwright in Fairbanks, Alaska. The Brewer made multiple measurements on each day, thereby documenting the variability of ozone on an hourly time scale. (d) Dobson spectrophotometer midday total ozone data from Fairbanks, Alaska. Only the 53 direct Sun measurements made between April and September are included. For comparison, 52 direct Sun Dobson observations from Barrow, Alaska, are shown. The divergence of the two data sets throughout April shows the influence of the Arctic winter polar vortex, the edge of which passed over portions of Alaska throughout the late spring. (e) Total column ozone data from the JPL MkIV infra- red interferometer. The MklV balloon instrument was peri- odically wheeled out onto the field in front of the ER-2 hangar at Fort Wainwright to make ground-based total column observations of 24 species. A total of 117 ozone observations were made between April and September.

26,770 LLOYD ET AL.: INTERCOMPARISON OF TOTAL OZONE DURING POLARIS

August. The instrument ran in a fully autonomous mode between the second and third deployments (mid-July through late August). Data are available on the POLARIS data CDROM [Gaines, 1998].

The Brewer data set is presented in Figure l c. A notable contribution of the Brewer is its ability to make multiple (in this campaign, up to as many as 30), sequential measurements of the ozone column throughout the day. This allows differ- ences between the other once-a-day observations to be correlated with changes in the ozone column throughout the day (to be discussed later and shown in Plate 3).

2.4. Dobson Spectrophotometer

Dobson spectrophotometers have been in general use as the standard of total ozone observations across the globe since the 1930s [Dobson et al., 1929]. Ozone is measured by the Dobson instrument using ratios of irradiance at fixed wave- length pairs in the UV-B. While zenith skylight and even moonlight observations may be made with the Dobson, direct solar observations are thought to be the most reliable; there- fore only direct Sun observations are reported here for comparison with other methods.

A total of 53 midday (22:25 + 1:42 UT) direct-Sun Dobson total ozone measurements made at Fairbanks, AK,

over the period April-September 1997 are shown in Figure 1 d. Data gaps include early May through mid-June and when meteorological conditions precluded direct Sun observations. An additional 21 observations made in other viewing modes (mostly zenith cloud observations) are not shown. For com- parison, 52 direct-Sun Dobson observations (out of 113 total observations) from Barrow, AK (about 800 km north of Fair- banks), over this same time period are also presented in Figure ld. Both data sets are available at the NOAA CMDL FTP site, ftp://ftp.cmdl.noaa. gov/dobson/. While the dynami- cally quiescent summer stratosphere results in consistency between the late summer observations at Fairbanks and

Barrow, the rapid uncorrelated oscillations in total ozone during April are evidence of the winter polar vortex moving across Alaska.

2.5. MkIV Interferometer

The MkIV interferometer [Toon, 1991] is a Fourier trans- farm infrared (_ _gTLR•.) spectrometer, which was designed and

built at JPL for remote measurements of atmospheric compo- sition from high altitude research balloons. During POLARIS, MkIV made two balloon flights from Fairbanks in May and July, as well as ground-based observations on 47 days throughout POLARIS. From analyses of these ground-based infrared solar absorption spectra, vertical column abundances of over 20 different species have been determined [Toon et al., this issue], including several of importance to our under- standing of high-latitude summertime ozone loss (e.g., 03, NO, NO2, HNO3, C1ONO2, HC1, and HF).

Figure l e shows the MkIV data for 117 total ozone obser- vations made from April through September. Data for all the MkIV observations are available on the POLARIS data

CDROM [Gaines, 1998] in a single file (data/balloon/ m4970324.bal). The MkIV is the only instrument other than the Brewer that occasionally made multiple observations on a single day. When multiple measurements were made, they were typically within a 2-hour window, and hence they do not show significant daytime variation, as sometimes observed by the Brewer over much longer time intervals.

3. Comparison of Data Sets Table I lists the differences between the EP TOMS total

ozone observations and each of the other instruments over the

6-month period of April through September 1997. In order to define the set of "coincident" measurements to compare, daily EP TOMS overpass data for Fairbanks were interpolated to the exact times of the ozonesonde launches and the ground- based observations. Interpolation, rather than simply comparing those observations nearest in time within a 24- hour window, typically has less than a 1 DU effect on the resulting differences.

The positive offset of EP TOMS relative to the Brewer (9.3 + 9.3 DU, 1 standard deviation) and Dobson (9.6 +_ 9.3 DU, 1 standard deviation) instruments is consistent with compari- sons made by the TOMS retrieval team at NASA Goddard Space Flight Center (GSFC) between EP TOMS and several other high-latitude ground stations (positive offsets of up to 3%) shown in Table 2. For the time period of August 1996 to April 1998, EP TOMS showed a positive bias against the Dobson at Fairbanks of 2.65% with a standard deviation of

3.61%. When ADEOS TOMS was compared to the same l'3nhecm inetinmont fc•r tho riorickel Rontornhor 1QQ/• tc• hmo

Table 1. Differences Between Earth Probe TOMS and Other Instruments

Difference Between Earth Probe Number of Absolute Relative TOMS and... "Coincident" Difference Difference

Measurements +1 s.d., DU +2c•, %

Sondes (CMR approximation) 31 Sondes (SBUV approximation) 33 Dobson spectrophotometer b 53 Brewer spectroradiometer b 1888 MkIV interferometer • 117

16.0 _+ 11.3 4.95 _+ 1.29 14.6 _+ 9.6 4.47 _+ 1.01 9.6 _+ 9.3 2.78 _+ 0.78 9.3 _+ 9.3 2.87 + 0.55 a 6.0 _+ 8.9 1.63 _+ 0.46

In all cases, the EP TOMS values are larger. The relative (percent) difference is relative to the TOMS values, as interpolated to the exact times of the other observations. The stan- dard error (standard deviation of the mean) reported in the last column is +2c• (95% confidence interval).

aAlthough the Brewer made 1888 observations during the POLARIS campaign, the standard error for the TOMS-Brewer comparison has been calculated using 104 rather than 1888 "coincident" measurements, since TOMS provides only 104 independent observations of the ozone to compare with.

•Direct Sun observations

LLOYD ET AL.' INTERCOMPARISON OF TOTAL OZONE DURING POLARIS 26,771

Table 2. Differences Between EP TOMS and Northern Hemisphere High-Latitude Ground Stations

Ground Station Station No. '• Instrument Dates Considered Latitude Offset b _+ l{J

Lerwick, Shetland Is., U.K. 43

Winnepeg, Manitoba, Canada 320 Saskatoon, Saskatchewan, Canada 241

Goose Bay, Newfoundland, Canada 76 Oslo, Norway 165 Yakutsk, Russia 123 Edmonton, Alberta, Canada 21

Reykjavik, Iceland 51 Resolute, N.W. Territories, Canada 24 Regina, Saskatchewan, Canada 338 Churchill, Manitoba, Canada 77 Fairbanks, Alaska, U.S.A. 105 Barrow, Alaska, U.S.A. 199

Dobson July 1996 to Nov. 1998 60.13øN -0.09% + 4.50% Brewer July 1996 to June 1998 49.90øN +0.27% + 4.37% Brewer July 1996 to May 1998 52.11øN +0.62% + 3.55% Brewer July 1996 to May 1998 52.32øN +0.76% + 3.66% Dobson July 1996 to May 1998 59.91øN +1.19% + 4.56% Brewer July 1996 to Dec. 1997 62.08øN +1.31% + 2.93% Brewer July 1996 to Dec. 1997 53.55øN +1.60% + 3.82% Dobson July 1996 to May 1998 64.13øN +1.99% + 4.70% Brewer July 1996 to May 1998 74.72øN +2.04% + 5.23% Brewer July 1996 to April 1998 50.21øN +2.09% + 3.56% Brewer July 1996 to June 1998 58.75øN +2.30% + 3.52% Dobson July 1996 to May 1998 64.82øN +2.65% + 3.61% Dobson July 1996 to May 1998 71.31øN +3.09% + 3.33%

•Vorld Meteorological Organization' s (WMO) World Ozone and Ultraviolet radiation Data Centre (WOUDC) station number. bPositive offsets indicate that the EP TOMS values are larger than the ground-based instrument.

1997, the offset and standard deviation were 3.90% and 2.75%, respectively. While the source of this offset is unknown, it is consistent with differences between Nimbus 7 TOMS (reprocessed with the version 7 TOMS algorithm) and 30 Northern Hemisphere Dobson stations, in which the aver- age difference between TOMS and the Dobson stations increases as a function of total column ozone, up to almost 3% at 500 DU [McPeters and Labow, 1996; see also Fioletov et al., 1999]. Since total ozone is typically higher at high lati- tudes, this may indicate a possible problem with the TOMS retrieval algorithm at high latitudes.

Data from Table 2 are plotted in Figure 2, which illustrates the same phenomenon as observed with Nimbus 7 TOMS with the more recent EP TOMS data. In general, the offset

between EP TOMS and the Brewer and Dobson ground stations increases with increasing latitude in the Northern Hemisphere. However, the magnitude of the offsets is not unambiguous, since there are significant data gaps (most noticeably during the winter) in many of the ground-based observation stations, which may bias the magnitude of the offset, and perhaps mask seasonally induced variations in the offset. Further work on understanding the latitude dependence of the offsets is in progress and will be the subject of a future publication.

Another possible source of disagreement is the fact that the Dobson instruments are routinely intercompared and cali- brated at locations where the total ozone is much lower than

500 DU. The response at high ozone values varies from

Brewer Spectroradiometer Stations [] Dobson Spectrometer Stations A Fairbanks Brewer Data during POLARIS X Fairbanks Dobson Data during POLARIS

4

! EP TOMS Offset Relative to . 3.5' High-Latitude Northern Hemisphere ............................ "• '"B;;"?O';'i"'"AK'i'"'"O'"S'""A ................ 3 i• ii i:: ..:• Fairbanks, A K, USA Brewer and Dobson Ground Stations ............. • ............... ii []

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45 50 55 60 65 70 75

Latitude (Degrees North)

Figure 2. Offset between Earth Probe TOMS and 13 Northern Hemisphere Brewer and Dobson ground stations (listed in Table 2), as a function of latitude. In general, the offset increases with increasing latitude. In addition, the Brewer and Dobson observations made during POLARIS (April-September 1997) are plotted, along with an error bar to show the 2(• standard deviations of the mean (95% confidence interval). The offset between the Fairbanks Dobson and EP TOMS over the longer period July 1996 to May 1998 agrees well with the Dobson- and Brewer-derived offsets observed during POLARIS.

26,772 LLOYD ET AL.: INTERCOMPARISON OF TOTAL OZONE DURING POLARIS

instrument to instrument when compared to TOMS. However, while this uncertainty is on the order of 5-10 DU for the Dobson spectrometer, the equivalent uncertainty for the Brewer spectroradiometer associated with calibrations for high ozone values is only on the order of 1-2 DU (D. Wardle and J. Kerr, personal communication, 1999).

Standard deviations between EP TOMS and all the other

instruments during POLARIS are 9-11 DU (about 3%), which is somewhat smaller than the comparisons between EP TOMS and high-latitude ground-based instruments shown in Table 2. A possible explanation for the smaller standard deviations seen in the POLARIS intercomparison is the fact that the present study is limited to 6 months during spring and summer, when the stratosphere was comparatively quiescent, whereas the comparisons in Table 2 include additional months, during which time dynamical variability in local ozone is greater.

When the local gradient in overhead ozone is large, the noncoincidence in time of the TOMS and ground-based observations of several hours can easily lead to discrepancies of more than 10 DU. The noncoincidence of data during periods of a large gradient in overhead ozone can be seen to produce an enhancement in the standard deviation by artifi- cially shifting the time of the EP TOMS observations by one day. While this does not change the absolute offset between the TOMS and Dobson observations (9.6 DU) for the entire POLARIS campaign, it more than doubles the standard deviation (from 9.3 DU to 20.2 DU).

Figure 3 illustrates the differences between EP TOMS and the ozonesonde total column measurements using both the CMR and SBUV approximations. Since the balloonsonde can be carried by the wind several tens of kilometers, the resulting ozone profile can sometimes be different from the profile at the launch site if the horizontal variability is great. This may

be the case in late spring, when the Arctic winter polar vortex traversed Alaska, resulting in strong gradients in both total ozone (as observed by EP TOMS) and lower stratospheric ozone abundances (as observed by the in situ ozone observa- tions aboard the ER-2 aircraft). Therefore direct comparison of moving ozonesondes with TOMS observations over a fixed location may not be valid when the gradient in ozone is strong. However, if one removes the outlying points (defined as being more than 20 from the mean offset) in Figure 3, it does not appreciably affect the average offsets (from 16.0 and 14.6 DU for CMR and SBUV, respectively, to 16.5 and 14.4 DU), but it does significantly improve the standard deviation (reduced from 11.3 and 9.6 DU for CMR and SBUV, respec- tively, to 4.6 and 4.2 DU).

Since the TOMS measurements are generally the highest and the sondes the lowest during this intercomparison, the offset between these two data sets represents the spread of the data or the precision with which we can know the total column ozone using this range of instrumentation. The offset between EP TOMS and the CMR sonde data is 16 DU, which represents about 3-6% over the typical range of ozone columns encountered during POLARIS (250-450 DU).

Comparisons of the EP TOMS data and the ground-based observations are shown in Figures 4 (Brewer) and 5 (Dobson and MkIV). Comparisons were made by interpolating the TOMS overpass data to the exact times of the Brewer, Dobson, or MkIV observations. The fact that the MklV, which is an infrared instrument, falls between the TOMS and Brewer/Dobson measurements (which all employ UV tech- niques) suggests that there is no apparent bias between the IR and UV cross sections of ozone employed by these instruments.

As with the comparison with the sonde data, the largest deviations are seen in late April and throughout May, when

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'i - -•--Sonde (SBUV) - Sonde (CMR) .... it '"'• ' i .... Sonde (SBUV) - Sonde (CMR) Average Difference (1.7 ñ 4.8 OU)

: .... .,,.,.,..•.., ....... . ........................ .... ',. ,.,.:,.,.,.,.. ........ •.• •. ,: ...... : t : , ,, '• ..... •.•••-• , - ;• - - ••- • - - v..z -...: .•: - :• -•- - - •'- • • - : & • 0 • ' ' '

; • x • Comparison of TOMS Satellite and • .... •; •xtr•p•la[ed.O•on•s•n•e •ot•l •zo•e. : , , , , ' . . ; ß . . '• . . . ; ' . . -15

April May June July August September October Month in 1997

Figure 3. Comparison between Earth Probe TOMS and ozonesonde total column ozone observations. The total ozone for the ozonesondes is the sum of the observed ozone up to the balloon burst altitude and either the constant mixing ratio (CMR) or SBUV climatological estimates for the residual ozone above the burst altitude; both estimates are shown. The solid line shows the average offset between EP TOMS and the CMR sonde values (16.0 + 11.3 DU, one standard deviation), and the short-dashed line shows the average offset between EP TOMS and SBUV sonde values (14.6 _+ 9.6 DU, one standard deviation). Also plotted are the small differences between the CMR and SBUV approximations, and the average value of this difference (1.7 __. 4.8 DU, one standard deviation, long-dashed line).

LLOYD ET AL.: IN•RCOMPARISON OF TOTAL OZONE DURING POLARIS 26,773

c: 30 o

.• 25 o C3 20

o • 10

0

c: E :a 0 o

ß -10 c -15

ß -20 .t

•3 -25

-3O

5O

40

35

-35

' Total Ozone Corn :)arison ' EP TOMS Satellite - Brewer Spectroradiometer

April May June July August September October Month in 1997

Figure 4. Comparison between Earth Probe TOMS and Brewer spectroradiometer total column ozone obser- vations. The solid line represents the average offset between EP TOMS and the Brewer (9.3 DU), and the dashed lines are one standard deviation (_+9.3 DU) around this average offset. Note the larger variability in late April and early May, when dynamical activity induced large variations in total ozone (up to 20 DU) over the course of a single day.

dynamical activity produces strong gradients in total ozone over Alaska, and again in late September, when increased dynamical activity resumes. The day-to-day dynamical activity at high northern latitudes in the upper troposphere and lower stratosphere decreases over the course of the spring and reaches a minimum during midsummer. This dynamical activity has a strong influence on total ozone values as the stratospheric polar vortex moves across Alaska, and as these baroclinic systems advect ozone and lift isentropic surfaces in the Alaskan region.

In the case of the fixed ground instruments, some of the variability may be attributed to the different times at which the "coincident" observations are made. Brewer data for

April/May indicate that total ozone can vary by almost 20 DU or about 5% over the course of a single day. Therefore non- coincident observations on the same day at high latitudes could be off by up to this amount based on the local gradient in total ozone.

Since dynamical variability can be largely averaged out over time, it may not affect the offset between the various

45

4O

35

3O 25

20

15

10

5

0

-5

-10

-15

-25

-3O

............................................................................... I Comparison of EP TOMS Satellite and I ! I Ground-BasedTotal Ozone Observations I ........... /] ..................

- .................. / '•:"•""::"J ?: ....... ß EP TOMS - Dobson' ' ' . .............. E. TOMS- Dobso. Average Di.ere.ce ß DU) .... /.. . • • • ...x-- EP TOMS- Mk•V • • :•: EP TOMS - MklV Average Difference (6 0 ß 8 9 DU) .: ........ : ß . ...................... [.....

•,• , • , , , • , , , • ß ß . • . . . ; . ß . April May June July August September October

Month in 1997

Figure 5. Comparison between Earth Probe TOMS, Dobson ultraviolet spectrophotometer, and MklV infra- red interferometer total column ozone observations. The solid line shows the average offset between EP TOMS and the Dobson (9.6 + 9.3 DU), and the short-dashed line shows the average offset between EP TOMS and MklV (6.0 + 8.9 DU). Errors quoted represent one standard deviation.

26,774 LLOYD ET AL.' INTERCOMPARISON OF TOTAL OZONE DURING POLARIS

instruments by much, but it may significantly increase the standard deviation of the differences. This seems to be the

case when comparing EP TOMS to the Dobson and MklV interferometer, with offsets of 9.6 _+ 9.3 DU and 6.0 _+ 8.9 DU, respectively. Differences of over 20 DU occur between both instruments and EP TOMS throughout April and again in late September. When the outliers (differences greater than two standard deviations, or 18 DU, presumably arising from large gradients in ozone) are excluded, the offsets for the Dobson and MklV are reduced to 8.2 _+ 5.4 DU and 5.7 _+ 7.2 DU, respectively.

All of the ground-based instruments (Brewer, Dobson, and MklV) actually measure the slant column ozone when making direct Sun measurements, which can then be converted to a vertical column abundance using the geometry of the solar observation. At high northern latitudes, this means that the peak of the ozone layer along the line of sight to the Sun (which dominates the total column measurement) can some- times be up to 100 km sunward of the observation site. The resulting vertical column abundance derived from this obser- vation is therefore an effective ozone column, a composite derived from the ozone densities encountered along its line of sight to the Sun. If the gradient in total ozone is large on spatial scales of tens of kilometers, then this effective ozone column can be considerably different from the true vertical column as observed by a sonde ascending vertically in a windless atmosphere.

Table 3 lists the differences between the Brewer spectrora- diometer data and the other instruments over the 6-month

period of April through September 1997. In this comparison, "coincident" measurements are defined by interpolating the Brewer data to the exact times of the sonde launches and

Dobson, MkIV, and EP TOMS observations, provided that comparable observations are made within a 24-hour window. Where multiple observations within a short period of time were made with the MkIV, only those values closest in time to the Brewer observations are considered.

Data gaps in each of the data sets (except EP TOMS) reduce the number of comparisons greatly and also introduce some small sampling bias. For example, since the Brewer did not make any measurements in early April when the dynamic variability in ozone was great, the standard deviation of the comparison of the Brewer and EP TOMS data is perhaps smaller than it would have been had it made early April measurements. Note also that since "coincident" observations

are defined with respect to the Brewer data in Table 3, rather

than with respect to the EP TOMS data as in Table 1, the number of coincident observations is different in the two

tables. However, the absolute and relative (percent) differ- ences between the instruments as shown in Tables 1 and 3

agree to within their stated uncertainties. A comparison between the Brewer and the sonde data is

given in Figure 6. While changing the ECC sonde cathode solutions to a lower concentration may have corrected the observed discrepancy between the CMR and SBUV approxi- mations for ozone above the burst altitude, it may have also introduced another offset at lower altitudes by a slight under- estimation (on the order of 2-5%) of ozone from the surface up to the ozone peak (B. Johnson, unpublished data, 1998). This would explain the sign and magnitude of the offset between the Brewer and the sondes.

A comparison of the Brewer and other ground-based data is given in Figure 7. The Brewer and MklV were colocated at the ER-2 hangar at Fort Wainwright, and the Dobson is located at the Geophysical Institute at the University of Alaska, about 10 km away on the other side of Fairbanks and at a slightly higher altitude. A possible source of differences between the Brewer and Dobson is this difference in location, which could play a role in late spring, when local gradients in total ozone are substantial.

In general, the Brewer and Dobson instruments agree remarkably well, with an offset of less than 1 DU and the smallest standard deviation of differences between the obser-

vations (_+6.0 DU) in this comparison. The remarkable agree- ment between the Brewer and the Dobson instruments should

not be unexpected. Both instruments use similar ozone retrieval techniques (ground-based spectroscopy using a fixed set of UV-B wavelengths) [Kerr et al., 1984b] and made identical assumptions concerning the effective temperature and altitude of the stratospheric ozone (-44øC and 22 km).

The Brewer and MklV also agree well, on the average within about 4%. The standard deviation of the differences is

higher than with the Dobson largely due to observations in the first week of July, when the Brewer, Dobson, and sondes appear noticeably lower than EP TOMS and the MklV (see Plate 3). The analysis of the infrared MklV spectra used actual temperature profiles measured by sondes launched from Fairbanks each day, together with an O3 profile that was based on MklV balloon profiles, but allowed to shift verti- cally from day to day depending on the temperatures and the fits obtained to HF and N20 lines. We therefore believe that a substantial component of the 4% disagreement between the

Table 3. Differences Between the Brewer Spectroradiometer (Direct Sun Observations Only) and Other Instruments

Difference Between Brewer

Spectroradiometer and... Number of Absolute Relative

"Coincident" Difference Difference

Measurements +_1 s.d., DU +_2cL %

Sondes (CMR approximation) 18 Sondes (SBUV approximation) 20 Dobson (direct Sun only) 29 MklV interferometer 37 Earth Probe TOMS satellite 104

10.5 +_ 10.2 3.17 +_ 1.47 7.1 +_ 10.0 2.17 +_ 1.36

0.9 +_ 6.0 0.23 +_ 0.70 -4.1 +_ 9.7 -1.34 +_ 1.00

-7.8 _+ 12.7 -2.63 +_ 0.79

Positive values indicate that the Brewer values are larger than the other instrument. The relative (percent) difference is relative to the Brewer values, as interpolated to the exact times of the other observations. The standard error (standard deviation of the mean) reported in the last column is +_2c• (95% confidence interval).

LLOYD ET AL.: INTERCOMPARISON OF TOTAL OZONE DURING POLARIS 26,775

4O

'•' 35

• 3o o

"' 25 o

ß 20

0 15 E -= 10 o

ß -= 5

ß 0

• -5

Comparison of Brewer :i -.-O--Brewer- Sonde (CMR)

: Spectroradiometer and !:" ' .. Brewer- Sonde (CMR) Average Difference (10.5 ñ 10.2 DU) ' Extrapolated Ozonesonde ' Total Ozone Data ---A-- Brewer- Sondes (SBUV) .... Brewer- Sonde (SBUV)

: i ........ ,•'• ..... Average Difference (7.1 ñ 10.0 DU) ..... .i

-lO

April May June July August September

Month in 1997

October

Figure 6. Comparison between the Brewer UV spectroradiometer and ozonesonde total column ozone obser- vations. Both the CMR and SBUV estimates of residual ozone above the sonde's burst altitude are shown.

The solid line shows the average offset between the Brewer and the CMR sonde values (10.5 _+ 10.2 DU), and the short-dashed line shows the average offset between the Brewer and SBUV sonde values (7.1 +_ 10.0 DU). Errors quoted represent one standard deviation.

MklV data and the Dobson and Brewer instruments (e.g., the first week of July) results from these different assumptions concerning the altitude distribution and temperature of the ozone profile.

4. Composite Total Ozone Data Set

A composite plot of all five data sets is shown in Plate 1, containing 2305 independent observations of total ozone over Fairbanks during the 6-month POLARIS campaign. The

mean ozone column over the entire campaign, using all of the available data weighted equally, was 320 DU. The mean ozone columns for the Fairbanks portions of each of the three POLARIS deployments was 380 DU for the April/May deployment, 318 for June/July, and 275 DU for September. Linear regression of the data reveals an ozone loss of about -30 DU/month between April and August emerges, followed by a roughly constant plateau at about 275 DU.

The most striking feature of this plot is the excellent agreement of the data sets. This consistency is gratifying,

v Brewer- Dobson 15 Brewer - Dobson Average Difference (0.9 ñ 6.0 DU) .......................................

-- -•,-- Brewer - MklV 10-• '-}'•.•. ........................... :iA ..... --- - Brewer - MklV Average Difference (-4.1 ñ 9.7 DU) t....• '

:.. ........ ...... ...... _ .... '5: •, •i ................................................................ ',q ............. .. ' ................................. •i ................................ !"j ........................................

I Comparison of J -15 ', ß Ground-Based ............. -20 Total Ozone Measurements

-25 , i i , i I I I I I I I I

April May June July August September October Month in 1997

Figure 7. Comparison between the Brewer UV spectroradiometer, Dobson UV spectrophotometer, and MklV infrared interferometer total column ozone observations. The solid line shows the average offset between the Brewer and the Dobson values (0.9 _+ 6.0 DU), and the short-dashed line shows the average offset between the Brewer and MklV values (-4.1 _+ 9.7 DU). The overall agreement between the Brewer and MklV would have been much better except for differences in early July (see Plate 3). Errors quoted represent one standard deviation.

z,0,,,nesonde (CMR) ß Ozonesonde (SBUV) & MklV Interferometer IIIIIIII I I I II I IIIl]1111l II I I Ill II Ill II

480

46O

440

420

400

380

360

340 O

= 300 • 280

•-- 240

220

mA ß -mind

Fairbanks, Alaska 1997 Spring/Summer Ozone Loss =

-30 Dobson Units/month

[[[[[[[[,lii[,tii[•[ [[[[[ fl ß a II

April Ma• ,June July August September Month in 1997

Plate ]. Composite plot of all the total ozone observations made at Fairbanks throughout the POL^RIS cam- paign. The data sets agree well in magnitude, forming an envelope with a standard deviation on the order of 10 DU. A seasonal, summertime ozone loss of about 30 DU/month appears between April and August, fol- lowed by a roughly constant plateau at about 275 DU. Dynamically-induced variability in total ozone is evident in April and again in late September, sandwiching a largely quiescent summer period with compara- tively less variability.

Earth Probe TOMS 1997

300 • 30

October

15

Apr May Jun Jul Aug Sep

r• Dobson U•its •'

Plate 2. Longitudinally (zonally) averaged total column ozone observations from the Total Ozone Mapping Spectrometer (TOMS) instrument on the Earth Probe satellite, shown as a function of latitude for the Northern Hemisphere during the POLARIS campaign. Contour lines are labeled in Dobson Units and separated by 10 DU. No observations are available for the white areas at high latitudes in September. The vertical lines represent the time and latitude extent of the ER-2 flight tracks during POLARIS. Note the Arctic winter polar vortex at high latitudes through the end of April, followed by steadily decreasing total ozone at high latitudes.

LLOYD ET AL.' INTERCOMPARISON OF TOTAL OZONE DURING POLARIS 26,777

Brewe r 'S'l•ectroradiometer Ozonesondes (CMR)

i

Earth Probe M• Satellite. Dobson (Direct Sun Only) Ozonesondes (SBUV) ß Mark IV Interferometer

i iii iiii i i I i i i IlllB III millimilli I

37O

360

350 340 330 320 310 300

N

0 290

E 280 = •

• 270= =: o :' • 260 = =,

•-- 250 ß: 240 -; 230

6/17/97

l

I

mm

__

I

I I

Mid-Summer 1997

Total Ozone,,,,c, ,,o,,,,,,m,,,p, arison

iiii

t

6/24197 7/1/97 7•8•97 7/15/97 7122/97 7/29197 8/5/97

Date

Plate 3. Composite plot of all the total ozone observations made at Fairbanks over 7 weeks in midsummer, from mid-June through early August. Each vertical line represents a week, and each tick mark represents a single day' s data. This is a subset of the data shown in Plate 1.

given that the methods of observation are very different. Another general feature is the increased scatter in the data in April and September. In April, the edge of the Arctic winter polar vortex, with comparatively lower column ozone abun- dances, passed over Fairbanks periodically, causing a dynamically induced variability in total ozone. Throughout the late spring and summer, the stratosphere is quiescent, resulting in a much tighter range in the variability, followed by somewhat increased dynamic activity and hence ozone variability in September.

While this summertime ozone loss is thought to be induced by the rapid catalytic loss of stratospheric ozone in the fully illuminated summer, smaller oscillations induced by plane- tary-scale wave activity are discernable in the data set [Hitchman and Buker, this issue]. These oscillations, with a time constant of about 2 weeks and an amplitude of nominally 40 DU, are induced by wave activity in the northern high latitudes. In Plate 2, longitudinally (zonally) averaged total column ozone observations from EP TOMS are shown as a

function of latitude for the Northern Hemisphere during the POLARIS campaign. This plate illustrates both the general photochemical loss of ozone as well as the dynamical wave activity as observed by EP TOMS. During early April, low ozone values over the polar region result from the catalytic losses of ozone inside the stratospheric polar vortex that occurred during the winter of 1997 [Newman et al., 1997]. As the vortex broke up during late April, midlatitude material was carried into the polar region, both dramatically increasing

the polar ozone column and also increasing temperatures. The breakup of the vortex and the typical springtime synoptic activity resulted in large variations of total ozone during this period.

Plate 3 expands a portion of Plate 1 to illustrate the varia- tion of ozone in early summer. During the first week of July, dynamical activity induced short-term increases in total ozone, with considerably more scatter in the data than either the weeks preceding or following. During this time the Brewer spectroradiometer made an average of more than 30 observations each day. Over any given day, total ozone often varied by 15 DU and on occasion by as much as 25 DU. Since the variation of total ozone within a given date is typically greater than the spread between the different methods of observation, this emphasizes the need for simultaneous obser- vations when intercomparing satellite, sonde, and ground- based ozone measurements when ozone gradients are large. At high latitudes, where the gradient and day-to-day variations in total ozone can sometimes be quite large, the differences between TOMS and Dobsons or Brewers may in fact be due more to differences in the times of these

observations than to systematic measurement differences. Therefore coordination of the times of ground- or balloon- based total ozone observations with TOMS overpass times may be key to fine-tuning the absolute "ground truth" calibration of satellite instruments at high latitudes.

Plate 3 illustrates the same general features as Plate 1; that is, EP TOMS is generally a bit higher than the Brewer, the

26,778 LLOYD ET AL.: INTERCOMPARISON OF TOTAL OZONE DURING POLARIS

MklV falls in between EP TOMS and the Brewer, and the

Dobson and balloonsondes generally agree well with the Brewer. On some dates, diurnal patterns within the Brewer's multiple measurements are also apparent in the data from the other instruments.

5. Conclusions

Five data sets of total ozone (TOMS satellites, ozone- sondes, Brewer spectroradiometer, Dobson spectrophoto- meter, and MkIV interferometer) at Fairbanks, AK, are compared over the spring and summer of 1997 during the POLARIS campaign. The data sets agree remarkably well in magnitude, forming an envelope with a standard deviation relative to either EP TOMS or the Brewer on the order of 10

DU. Over the 6 months of data considered, ADEOS TOMS

satellite measurements are about 7 DU higher than EP TOMS, which is higher by about 8 DU than the Brewer, the MklV interferometer lies in the middle between EP TOMS and the

Brewer, the Brewer agrees remarkably well (within 1 DU) with the Dobson, and the sondes using the 0.5% KI cathode solution measure about 7-10 DU less than the Brewer.

Ranking in order of magnitude, ADEOS TOMS > EP TOMS > MklV > Brewer • Dobson > Sondes (SBUV) • Sondes (CMR). In relative terms, the highest (EP TOMS) and lowest (ozonesondes) ozone values measured over the entire 6- month period agree to within 5%. The observed small offsets between the different measurement techniques are consistent with our current understanding. All observations show a sea- sonal, summertime ozone loss of about 30 DU per month between April and August, with minor dynamically induced oscillations in total ozone with a period of about 2 weeks and an amplitude of about 40 DU superimposed on the general decline.

The agreement between the various data sets is best during midsummer, when the ozone fields over central Alaska are largely homogeneous. The agreement is less compact during late April/early May and again after mid-September, when dynamic activity induced significant gradients in total ozone over central Alaska. When the temporal gradient in ozone is large, the variation of ozone within a given day can some- times affect the comparison between noncoincident once-a- day observations. When the spatial gradient in ozone is large, it can impact the compa_n_'son between ground-based and satellite observations.

Acknowledgments. The analysis of the five total ozone data sets was supported by NASA Upper Atmospheric Research Program (UARP) grant NAG5-4780 and the NASA Atmospheric Chemistry Modeling and Analysis Program (ACMAP) grant NAG5-7227. The authors thank the sponsors who enabled the ozone observations, including NASA UARP, NOAA CMDL, NASA's Earth Science Enterprise (ESE), and Environment Canada's Atmospheric Environ- ment Service (AES).

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(Received March 25, 1999; revised June 14, 1999; accepted June 16, 1999.)