Evaluation of aerosol measurements from SAGE II, HALOE, and balloonborne optical particle counters

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D3, 4031, 10.1029/2001JD000703, 2002 Evaluation of aerosol measurements from SAGE II, HALOE, and balloonborne optical particle counters Mark Hervig 1 and Terry Deshler Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA Received 30 March 2001; revised 4 September 2001; accepted 5 September 2001; published 12 February 2002 [1] Stratospheric aerosol measurements from the University of Wyoming balloonborne optical particle counters (OPCs), the Stratospheric Aerosol and Gas Experiment (SAGE) II, and the Halogen Occultation Experiment (HALOE) were compared in the period 1982 2000, when measurements were available. The OPCs measure aerosol size distributions, and HALOE multiwavelength (2.45 5.26 mm) extinction measurements can be used to retrieve aerosol size distributions. Aerosol extinctions at the SAGE II wavelengths (0.386 1.02 mm) were computed from these size distributions and compared to SAGE II measurements. In addition, surface areas derived from all three experiments were compared. While the overall impression from these results is encouraging, the agreement can change with latitude, altitude, time, and parameter. In the broadest sense, these comparisons fall into two categories: high aerosol loading (volcanic periods) and low aerosol loading (background periods and altitudes above 25 km). When the aerosol amount was low, SAGE II and HALOE extinctions were higher than the OPC estimates, while the SAGE II surface areas were lower than HALOE and the OPCs. Under high loading conditions all three instruments mutually agree to within 50%. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: aerosols, stratosphere, HALOE, SAGE II, OPC 1. Introduction [2] Recent years have seen significant efforts aimed at understanding the impact of stratosphere aerosols on global climate and chemistry. An important aspect of these efforts is the obtaining of representative and well-documented data sets of aerosol physical and optical properties. Investigators are challenged to consider a variety of aerosol measurements, as extensive records are accumulating from different remote and in situ sensors. Among these sensors the Stratospheric Aerosol and Gas Experiment (SAGE) II and the University of Wyoming balloonborne optical particle counters (OPCs) are distinguished as offering perhaps the longest records available. The Halogen Occultation Experiment (HALOE) contributes to this record, offering aerosol measurements since late 1991. This work evaluates measurements of stratospheric sulfate aerosols from SAGE II, HALOE, and the OPCs to determine the consistency among these data sets. [3] The OPCs measure aerosol size distributions, HALOE measures aerosol extinction at four infrared (IR) wavelengths, and SAGE II measures extinction at four visible to near-ir wavelengths. Since each instrument measures a fundamentally different quantity, some or all of the measurements must be manipulated to obtain a common ground for comparison. For example, Lu et al. [1997] compared SAGE II and HALOE aerosol extinctions by applying a constant scaling factor to adjust the HALOE 5.26-mm wavelength extinctions to 0.525 mm. In contrast to the current results, Lu et al. show consistently poor agreement between HALOE and SAGE II, and their 1 Now at GATS Inc., Driggs, Idaho, USA. Copyright 2002 by the American Geophysical Union. 0148-0227/02/2001JD000703 unfavorable results are likely due to the inadequacy of a constant scaling factor. In this work, extinctions at the SAGE II wavelengths were computed using aerosol size distributions from HALOE and the OPCs for comparison to the SAGE II measurements. In addition, aerosol surface areas computed from each data set were compared. 2. Aerosol Measurements 2.1. University of Wyoming OPCs [4] The University of Wyoming balloonborne OPCs were developed initially by Rosen [1964] to measure cumulative aerosol size distributions. This instrument illuminates particles in a sample chamber using a white light source and detects scattered light at a fixed angle in the forward direction. In 1990 the scattering angle was changed from 25 to 40, and the solid angle over which scattered light is measured increased from 30 to 34. This technique works for particles with radii greater than 0.15 mm. To obtain a complete size distribution requires a second instrument sensitive to the concentration of particles with radii greater than 0.01 mm. These particles, generally called condensation nuclei, are measured by forcing the particles to grow to an optically detectable size using a growth chamber ahead of an OPC [Rosen and Hofmann, 1977]. The OPCs measure the concentration of aerosols with radii larger than some specified size (i.e., the cumulative concentration) for a range of sizes. The configuration of individual instruments has varied over the years, measuring radii between 0.01 and 10 mm in 3 12 size intervals [see, e.g., Hofmann and Deshler, 1991]. Table 1 summarizes the OPC configurations. The measured cumulative size distributions are fit using lognormal size distributions with one or two modes at height intervals of 0.5 km. This work used profiles measured in stratospheric sulfate aerosols over Laramie, Wyoming (41.2 N, 254.7 E), from 3 to AAC 3-1

AAC 3-2 HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION Table 1. Threshold Radii for Size Intervals Measured by the Optical Particle Counters Over Laramie Threshold Radius of Time Period Each Size Channel, mm Before June 1982 0.01, 0.15, 0.25 June 1982 through Dec. 1987 0.01, 0.15, 0.25, 0.95, 1.20, 1.80 Jan. 1988 through April 1989 0.01, 0.15, 0.25 May 1989 through Nov. 1989 0.01, 0.15, 0.25, 0.5, 1.0, 2.0, 3.0, 5.0, 10.0 Dec. 1989 through Feb. 1990 0.01, 0.15, 0.25 March 1990 through Aug. 1990 0.01, 0.15, 0.25, 0.5, 1.0, 2.0, 3.0, 5.0, 10.0 Sept. 1990 through March 1991 0.01, 0.15, 0.25 March 1991 through April 1992 0.01, 0.15, 0.25, 0.5, 1.0, 2.0, 3.0, 5.0, 10.0 May 1992 through the present 0.01, 0.15, 0.19, 0.25, 0.3, 0.38, 0.49, 0.62, 0.78, 1.08, 1.25, 1.58, 2.0 15 times per year since 1982 [Hofmann and Rosen, 1984; Hofmann and Rosen, 1987; Hofmann, 1990; Deshler et al., 1993]. 2.2. SAGE II [5] SAGE II is on board the Earth Radiation Budget Satellite and has reported science observations since October 1984 [e.g., Chu et al., 1989]. The SAGE II instrument uses the principle of satellite solar occultation to measure profiles of solar attenuation by the atmosphere s limb as the sun rises or sets relative to the spacecraft. Measurements in seven spectral bands (from 0.386 to 1.02 mm) are used to retrieve the profiles of three gas mixing ratios (NO 2,H 2 O, and O 3 ), temperature, and aerosol extinction, b(l), at four wavelengths (l = 0.386, 0.452, 0.525, and 1.02 mm). SAGE II measurements cover two longitude sweeps each day (15 profiles each), one at the latitude of sunsets and one at the latitude of sunrises. The progression of measurement latitude with time provides near-global coverage over periods of 3 4 weeks. This work used SAGE II version 6 (V6) data, which were not formally validated as of this writing. In general, the V6 extinctions have decreased by 2 30% at altitudes between 10 and 30 km when compared to V5.96. At altitudes above 30 km the 0.452- and 0.386-mm extinctions increased by >30% when compared to V5.96. In addition, the V6 retrieval uncertainties have decreased by up to 50%. [6] Previous versions of SAGE II aerosol data have been validated by numerous authors [e.g., Oberbeck et al., 1989], and these efforts indicate extinction uncertainties from 20 to 30%; however, the errors are different for each channel. One of the earliest validation campaigns took place in 1984 1986, when SAGE II extinctions were compared with lidar and with balloonborne and aircraftborne in situ aerosol instruments [Russell and McCormick, 1989]. Comparisons were made by converting the correlative measurements to SAGE II extinctions [Osborn et al., 1989; Oberbeck et al., 1989] and by inverting the SAGE II extinctions to the correlative measurement quantities [Wang et al., 1989]. These results indicated that the 1.02- mm extinction measurements agree within the measurement uncertainty with the correlative instruments from the tropopause to 28 km. Comparisons at the shorter wavelengths were not as favorable. The extinctions at 0.525 mm and lower agreed with the correlative measurements within mutual error bars from the tropopause to 24 km, although the error bars on these measurements were larger than at 1.02 mm. Above 24 km the comparisons are limited by the low aerosol concentrations and small sizes. [7] Aerosol surface area densities S can be computed from the SAGE II aerosol extinctions using principal component analysis (PCA) [Thomason and Poole, 1993; Steele et al., 1999]. For the results presented here, surface areas were determined from SAGE II using the PCA coefficients of Steele et al., which relate S to a linear combination of the SAGE II extinctions: S = 670 b(0.386) + 488 b(0.452) + 333 b(0.525) 152 b(1.02). Moments of the aerosol size distribution calculated from SAGE II agree with correlative data qualitatively, but there is a tendency for SAGE to overestimate Sat altitudes below 25 km, as observed by Yue et al. [1995] in a comparison of SAGE II and aircraft measurements. Conversely, Steele et al. noted that SAGE II can underestimate Swhen the aerosols are small and the concentrations are low (e.g., above 25 km or during background periods). [8] For this work, clouds were filtered from the SAGE II measurements. Because extinction increases rapidly at cloud top, cirrus were identified when the 1.02-mm extinction increased rapidly with decreasing altitude, as suggested by Hervig and McHugh [1999]. Wang et al. [1996] suggested that cirrus are indicated when the ratio b(0.525)/b(1.02) decreases below 2.1, due to the large particle sizes associated with cirrus. While this approach works well under volcanically quiescent conditions, stratospheric aerosols during volcanic periods were large enough to be mistaken for cirrus using the extinction ratio threshold. Thus cirrus were identified from the vertical extinction gradients alone. 2.3. HALOE [9] HALOE is onboard the Upper Atmosphere Research Satellite and has reported science observations since October 1991. HALOE uses the principal of satellite solar occultation, and the measurement geometry is similar to that of SAGE II. Measurements in eight infrared bands (from 2.45 to 10.01 mm) are used to retrieve the profiles of seven gas mixing ratios (HF, HCl, CH 4, NO, NO 2, H 2 O, and O 3 ), temperature, and aerosol extinction at four wavelengths (2.45, 3.40, 3.46, and 5.26 mm). A complete description of the experiment is given by Russell et al. [1993]. The HALOE aerosol retrievals are described in detail by Hervig et al. [1995], and a validation of these measurements suggests uncertainties on the order of 15 20% [Hervig et al., 1996]. [10] The HALOE aerosol data used in this work are version 19, which is nearly identical to version 18. Version 18 HALOE measurements of sulfate aerosols have been used to derive unimodal lognormal size distributions [Hervig et al., 1998]. The unimodal lognormal size distribution describes the concentration versus radius as a function of the number concentration N, distribution width s, and median radius r. Surface area and volume V densities can be calculated directly from N, s, and r: S =4p r 2 N exp (2 ln 2 s) and V = (4/3) pr 3 N exp (4.5 ln 2 s). Sulfate refractive indices are fundamental to the size distribution retrievals, and the results reported by Hervig et al. [1998] were based on room temperature (300 K) indices [Palmer and Williams, 1975] adjusted to stratospheric temperatures according to the Lorentz-Lorenz relationship using sulfate densities from Luo et al. [1996]. Sulfate indices measured at a colder temperature (215 K), more relevant to the stratosphere, were published by Tisdale et al. [1998]. Comparing the Tisdale et al. indices to the room temperature indices adjusted to 215 K shows notable differences. Thus the Tisdale et al. indices were used to recalculate the HALOE aerosol size distributions for this work. Hervig et al. [1998] compared profiles of HALOE unimodal size distributions and surface areas to OPCs over Laramie for 17 coincidences during 1991 1996. That work used HALOE size distributions based on the Palmer and Williams sulfate refractive indices and showed that the HALOE surface areas were from 10 to 30% higher than the OPC values. HALOE size distributions based on cold temperature refractive indices [Tisdale et al., 1998] result in 25% less surface area than the previous results. Please note that HALOE aerosol size distributions currently available (as of September 2001) on the

HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION AAC 3-3 Figure 1. Time series at three altitudes over Laramie of aerosol extinction at four SAGE II wavelengths. SAGE II measurements are compared to extinctions calculated from optical particle counter (OPC) and HALOE size distributions. HALOE and SAGE II measurements between 41 N and 42 N latitude and 245 E and 265 E longitude were used. Vertical bars on the occasional SAGE II measurement indicate ±50%. The SAGE II uncertainties are less than this, and these bars serve only to add perspective. This time series is comprised of 68 SAGE II, 178 OPC, and 31 HALOE measurements.

AAC 3-4 HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION Figure 1. (continued)

HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION AAC 3-5 Figure 2. Time series at three altitudes of aerosol extinction at four SAGE II wavelengths from HALOE and SAGE II. Measurements used were between the equator and 1 S latitude and 180 E and 270 E longitude. Vertical bars on the occasional SAGE II measurement indicate ±50%. The SAGE II uncertainties are less than this, and these bars serve only to add perspective. This time series is comprised of 80 SAGE II and 50 HALOE measurements.

AAC 3-6 HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION Figure 2. (continued)

HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION AAC 3-7 HALOE web site are based on the Palmer and Williams refractive indices. [11] Aerosol size distributions were only retrieved in the absence of clouds. Cloud tops were identified in HALOE profiles using two indicators described by Hervig and McHugh [1999]. In one approach, the rapid increase in extinction Figure 3. Comparison statistics for 281 coincident HALOE and SAGE II profiles between 38 N and 44 N latitude from 1991 through 1998. The average profile separations were 12.1 hours, 0.8 latitude, and 8.0 longitude. Statistics are shown as mean profiles, mean differences (HALOE minus SAGE II), and difference standard deviations (random differences). Error bars on the mean profiles indicate the reported measurement uncertainties, and error bars on the mean differences indicate Figure 4. Comparison statistics for 293 coincident HALOE and uncertainties in the p mean ffiffiffiffi differences (difference standard SAGE II profiles between 44 S and 38 S latitude from 1991 deviation divided by N ). through 1998. The average profile separations were 12.6 hours, 0.8 latitude, and 7.1 longitude. Statistics are as described in Figure 3.

AAC 3-8 HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION associated with cloud tops was found to be a reliable cloud top indicator. In a second approach, uniformity among the four HALOE extinctions provides another reliable cloud indicator, because the large particles associated with cirrus produce relatively flat extinction spectra when compared to aerosol extinction. These two cloud identifiers were not confused by the presence of volcanic aerosols. 3. Extinction Comparisons [12] Since aerosol extinction varies noticeably over wavelength, comparing measurements made at different wavelengths poses a challenge. Some investigators have dealt with this problem by applying a constant scaling factor to adjust extinctions from visible to IR wavelengths [e.g., Massie et al., 1996; Lu et al., 1997]. While this approach can be valid within the IR region, bridging the gap between visible and IR wavelengths is less straightforward. Complications arise because the relationship between visible and IR extinction is highly dependent on the size distribution and aerosol composition (wt % H 2 SO 4 ), which determines the refractive index. Since both quantities vary noticeably with height and time in the stratosphere, a constant scaling factor is typically inadequate. Here aerosol size distributions from the OPCs and from HALOE were used to calculate extinction at the SAGE II wavelengths. For HALOE-SAGE II comparisons this is a robust approach for resolving inherent differences due to measurement wavelength. While the HALOE measurements were manipulated many times (from IR extinctions to size distributions and then to visible extinctions), it is satisfying to have left the SAGE II data set in its original state. [13] Aerosol extinctions at the SAGE II wavelengths were computed from Mie theory using the HALOE and OPC size distributions with refractive indices from Palmer and Williams [1975], which are adequate for visible wavelengths. The refractive index is a function of aerosol composition, and aerosol compositions were determined as a function of temperature, pressure, and water vapor mixing ratio [e.g., Steele and Hamill, 1981]. The balloonborne sampling did not include water vapor, and a constant 5 ppmv was assumed for the OPC profiles, which is consistent with HALOE measurements in the stratosphere at middle latitudes. For HALOE profiles the HALOE water vapor measurements were used to determine composition. Visible sulfate indices are a weak function of composition, and potential errors in calculated extinction due to errors in estimated composition are <1%. Uncertainties in the calculated extinctions are principally due to uncertainties in the size distributions. Test calculations show that the size distribution errors propagate into extinction uncertainties on the order of 30 40% for HALOE and OPC size distributions. Figure 5. Comparison statistics for 22 coincident OPC and SAGE II profiles over Laramie, Wyoming, from 1984 through 1999. The average profile separations were 10.7 hours, 0.8 latitude, and 5.8 longitude. Statistics are as described in Figure 3, except the differences are OPC minus SAGE II. 3.1. Time Series Comparisons [14] Time series were constructed over Laramie from the HALOE, SAGE II, and OPC data sets and were constructed over the equator from the HALOE and SAGE II data sets. The time series comparisons did not require coincidence of the respective measurements, and all measurements near a desired location were used. Each data set samples a chosen latitude frequently enough that the average magnitude and trend of extinction should be well represented. Meteorological variability is a factor; however, for the long duration of these comparisons, small-scale variations become less important than the overall magnitude and trend. An alternative to our approach would be to require time and space coincidence of the measurements. While this approach can work for comparisons of two data sets, three-way coincidences are rare. Additionally, there are no direct coincidences between HALOE and SAGE II at equatorial latitudes. [15] Time series are shown at three representative altitudes over each location for all SAGE II wavelengths. Average tropopause heights are roughly 12 km at middle latitudes and are roughly 16 km in the tropics, and the lowest levels chosen were above these heights, to minimize data loss from cloud filtering. SAGE II, HALOE, and OPC measurements over Laramie are compared in Figure 1. The appearance and decay of volcanic aerosols are well

HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION AAC 3-9 Figure 6. Time series of HALOE, OPC, and SAGE II measurements over Laramie as in Figure 1, except that aerosol surface areas are compared. represented by the three instruments. Aerosol loading due to the eruptions of El Chichon (1982) and Mount Pinatubo (1991) resulted in similar extinction enhancements during the posteruption years. These extinctions were observed to decay by roughly 2 orders of magnitude during a 6-year period following each eruption. The aerosol amount appears to stabilize after 1997, suggesting that a background state now exists. Pre-Pinatubo extinctions are similar to values after 1997, suggesting that a background state also existed in 1991. [16] The Laramie comparisons show generally good agreement among the three data sets at 16 and 22 km, with greater variability and poorer agreement at 28 km. The comparisons can be roughly divided into volcanic and background periods, where some clear differences emerge. During both background periods at 16 and 22 km, the SAGE II extinctions tend to be higher than the OPC values. In addition, HALOE extinctions tend to be higher than the OPC values after 1997. Volcanic periods were characterized by generally good agreement among the three data sets at all altitudes. [17] HALOE and SAGE II measurements were compared over the equator, at longitudes from 180 E to 270 E, where the occurrence of cirrus is a minimum [e.g., Wang et al., 1996]. These comparisons (Figure 2) show less variability at 31 km than over Laramie at 28 km (Figure 1). At 25 km, HALOE and SAGE II are within 25% of each other; however, the results suggest a small systematic difference in the 0.452-, 0.525-, and 1.02-mm comparisons, with SAGE II slightly higher than HALOE. The comparison at 19 km does not suggest a clear bias, and the variability is similar to the Laramie comparison at 16 km. It is interesting to note that after 1997 at 19 km, the time series hint at a seasonal cycle with high extinctions in December and lower values in June. Greater variability at 28 km over Laramie may be a result of lower aerosol concentrations and smaller sizes at these altitudes. Under these conditions the extinction measurements are approaching their noise levels, and greater uncertainties are expected. Low particle concentrations increase the statistical uncertainty of the OPC measurements due to Poisson counting errors and thus create greater variability in calculated extinction. 3.2. Profile Comparisons [18] Vertical profiles for coincident measurements between SAGE II and HALOE and SAGE II and the OPCs were compared over the time periods sampled by the instrument pairs. For this purpose, coincident measurements were identified using maximum separations of 24 hours, 2 latitude, and 20 longitude. This led to nearly 600 HALOE-SAGE II comparisons and 22 OPC-SAGE II comparisons. Coincident profiles were compared for the record durations, and comparison statistics are presented as mean profiles, mean differences, and difference standard deviations (random differences). At each altitude, mean differences in percent between SAGE II (S) and the other (O) measurements were computed as D ¼ 200 N i¼1ð O i S i Þ= N i¼1ð O i þ S i Þ, where N is the number of coincident measurements. Difference qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi standard deviations in percent were computed as s D ¼ N i¼1 i 2=N, where i = 200 (O i S i )/(O i S i ) and is the mean value. Separating each comparison into individual

AAC 3-10 HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION Figure 7. Time series of equatorial SAGE II and HALOE measurements as in Figure 2, except that aerosol surface areas are compared. years can show varying agreement among the three data sets; however, presenting this analysis is difficult. A hint of such comparisons can be obtained by referring to Figures 1 and 2 as the composite profile statistics are presented. [19] Comparison results for coincident HALOE and SAGE II measurements between 38 N and 44 N (±3 latitude of Laramie) are shown in Figure 3. These comparisons show generally good agreement for all wavelengths and most altitudes, although some exceptions are noteworthy. The mean differences indicate that SAGE II extinctions are greater than HALOE near 24 km for all four wavelengths. In contrast to the other wavelengths, SAGE II 1.02-mm extinctions are less than HALOE above 25 km and greater than HALOE below 25 km, and the SAGE II 0.386-mm extinctions are less than HALOE p below 20 km. Errors in the mean differences ( D = ffiffiffiffi N ) are generally <5%, indicating that these biases are statistically significant. Figure 4 shows comparison statistics for coincident SAGE II and HALOE profiles at southern middle latitudes, and these results are very similar to comparisons in the Northern Hemisphere (Figure 3). There are no direct HALOE- SAGE II coincidences in the tropics. [20] Comparison results for coincident SAGE II and OPC measurements over Laramie are shown in Figure 5. These results do not indicate clear systematic differences, as the biases alternate in altitude and the mean differences are often less than the errors in the mean differences, which range from 5 to 20%. Thus many of the suggested biases are not statistically significant. Larger errors in the OPC-SAGE II mean differences are due to the low number of coincidences. The results in Figures 3 and 5are generally consistent with Figure 1, showing greater variability (random differences) at the highest altitudes and showing systematic differences that change with altitude. [21] While the extinction comparisons required manipulation of the HALOE and OPC measurements, the SAGE II measurements were left alone. Thus these comparisons establish agreement among the instruments at perhaps the most fundamental level possible. While the overall agreement is encouraging, it is clear that the differences change with wavelength, latitude, altitude, and time. As these data sets continue to be used to infer various optical Figure 8. Comparison of HALOE and SAGE II measurements over Laramie as in Figure 3, except the results are for surface area.

HERVIG AND DESHLER: AEROSOL MEASUREMENT EVALUATION AAC 3-11 and physical aerosol properties, the extinction comparisons provide a useful standard to assess the validity of the inferred properties. 4. Surface Area Comparisons [22] This section evaluates aerosol surface area densities (S) determined from SAGE II, HALOE, and the OPCs (see section 2). Thomason et al. [1997] compared SAGE II aerosol surface areas (derived from V5.96) to the OPCs over Laramie for 4 years after the Pinatubo eruption. That work showed good agreement for surface areas less than 15 mm 2 cm 3 and showed systematic differences (SAGE II greater than OPC) under higher aerosol loading conditions. Hervig et al. [1998] compared HALOE surface areas based on room temperature refractive indices to OPCs over Laramie. While that work showed that HALOE surface areas were from 10 to 30% higher than the OPC values, the current HALOE surface areas, which are based on cold temperature refractive indices, are 25% lower than the previous results. [23] Time series of surface area from HALOE, SAGE II, and the OPCs are compared over Laramie in Figure 6. The best agreement is found near and below 22 km when aerosol loading is approximately >0.7 mm 2 cm 3. Notable differences exist nearly all the time at 28 km and during background aerosol periods. Steele et al. [1999] noted that surface areas derived from SAGE II using the PCA technique can be biased low at high altitudes or, more specifically, when the aerosols are small. Figure 6supports this conclusion, as SAGE II surface areas tend to be lower than HALOE and the OPCs at 28 km at most times. This discrepancy is also apparent at all altitudes during background periods, although the differences are smaller at 16 and 22 km. After 1996 at all altitudes, the OPCs tend to suggest higher surface areas than both HALOE and SAGE II. Prior to Pinatubo a similar relationship existed between SAGE II and the OPCs. Time series of aerosol surface area over the equator from HALOE and SAGE II are compared in Figure 7. The HALOE-SAGE II surface area comparisons are similar to the equatorial extinction comparisons (Figure 2), suggesting that the variability shown is primarily related to the measurements and to atmospheric variability rather than to errors in the conversion to S. These results resemble the 0.386-mm extinction comparisons, reflecting the higher weighting placed on b(0.386) in determining S from SAGE II. [24] Profile statistics for HALOE-SAGE II surface area comparisons over Laramie are shown in Figure 8. These results show systematic differences that alternate in altitude between 20% and 40% (HALOE minus SAGE II). The mean difference profile for S appears to be dominated by the 0.386-mm extinctions below 20 km and by the 1.02-mm extinction comparison above 20 km (Figure 3), which may indicate the relative contributions from these channels to SAGE II surface areas. HALOE-SAGE II surface area comparisons in the Southern Hemisphere (not shown) are very similar to the Northern Hemisphere comparisons. Profile statistics for OPC-SAGE II surface area comparisons over Laramie are shown in Figure 9. This comparison shows systematic differences (SAGE II less than OPC) at most altitudes, with the largest differences above 25 km and below 15 km. This pattern most resembles the OPC-SAGE II 0.386-mm extinction comparisons (Figure 5), which show large systematic differences above 27 km and below 16 km. 5. Summary Figure 9. Comparison of SAGE II and OPC measurements over Laramie as in Figure 5, except the results are for surface area. [25] Stratospheric aerosol measurements from SAGE II, HALOE, and balloonborne OPCs were compared. These instruments measure principally different quantities, and a common basis was required to compare the data sets. For this purpose, extinctions at the SAGE II wavelengths and aerosol surface areas were computed from the HALOE and OPC aerosol size distributions for comparison to SAGE II. While the overall impression from these comparisons was encouraging, the agreement can change with latitude, altitude, time, and parameter. The best agreement was generally found when the aerosol extinctions and surface areas were large. For example, the three data sets were in excellent agreement for 1.02-mm extinctions greater than roughly 2 10 4 km 1 and for surface areas greater than 0.7 mm 2 cm 3. Notable differences can emerge when the aerosol amounts are lower, such as during background periods at all altitudes, and at altitudes above 25 km during most years. Under these conditions the OPC extinctions tend to be less than HALOE and SAGE II, and the SAGE II surface areas tend to be less than HALOE and the OPCs. [26] The solar occultation method used by HALOE and SAGE II is essentially self-calibrating, and the possibility of instrumental drift over time is greatly reduced. The OPCs are calibrated before each flight; however, the instrument configuration has changed many times over the comparison period. This does not seem to be a concern, however, as the OPC record exhibits repeatable differences with respect to HALOE and SAGE II over the comparison period. For example, the OPC surface areas tend to be greater than SAGE II during both background periods (late 1980s and late 1990s). The aerosol size distribution is highly variable in altitude and time due to the appearance and decay of volcanic aerosols. Variations in agreement over altitude and time appear to be related to these changes in aerosol loading, with better agreement found under conditions of high aerosol loading. SAGE II is sensitive only to aerosol scattering, while HALOE measures the sum of absorption and scattering. As the aerosol radius rdecreases, scattering decreases as roughly r 6, while absorption decreases as roughly r 3. As a result, SAGE II is less sensitive to smaller aerosols, and this may be a factor in some of the comparisons under low aerosol loading conditions. [27] Acknowledgments. We thank the SAGE II Science Team, HALOE Science Team, and the University of Wyoming Balloon Program for providing years of high-quality measurements. This work was supported through funding from the UARS Guest Investigator Program, the SAGE II Science Team, and the National Science Foundation. References Chu, W. P., M. P. McCormick, J. Lenoble, C. Brogniez, and P. Pruvost, SAGE II inversion algorithm, J. Geophys. 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Deshler, Department of Atmospheric Science, University of Wyoming, 16th and Gibbon, Box 3038, Laramie, WY 82071, USA. (deshler@uwyo. edu) M. E. Hervig, GATS Inc., P.O. Box 449, Driggs, ID 83422, USA. (m.e.hervig@gats-inc.com)