SAGE III Ozone Loss And Validation Experiment Research Articles

Sub-micrometer refractory carbonaceous particles in the polar stratosphere

Abstract. Eleven particle samples collected in the polar stratosphere during SOLVE (SAGE III Ozone loss and validation experiment) from January until March 2000 were characterized in detail by high-resolution transmission and scanning electron microscopy (TEM/SEM) combined with energy-dispersive X-ray microanalysis. A total of 4202 particles (TEM = 3872; SEM = 330) were analyzed from these samples, which were collected mostly inside the polar vortex in the altitude range between 17.3 and 19.9 km. Particles that were volatile in the microscope beams contained ammonium sulfates and hydrogen sulfates and dominated the samples. Some particles with diameters ranging from 20 to 830 nm were refractory in the electron beams. Carbonaceous particles containing additional elements to C and O comprised from 72 to 100 % of the refractory particles. The rest were internal mixtures of these materials with sulfates. The median number mixing ratio of the refractory particles, expressed in units of particles per milligram of air, was 1.1 (mg air)−1 and varied between 0.65 and 2.3 (mg air)−1. Most of the refractory carbonaceous particles are completely amorphous, a few of the particles are partly ordered with a graphene sheet separation distance of 0.37 ± 0.06 nm (mean value ± standard deviation). Carbon and oxygen are the only detected major elements with an atomic O∕C ratio of 0.11 ± 0.07. Minor elements observed include Si, S, Fe, Cr and Ni with the following atomic ratios relative to C: Si∕C: 0.010 ± 0.011; S∕C: 0.0007 ± 0.0015; Fe∕C: 0.0052 ± 0.0074; Cr∕C: 0.0012 ± 0.0017; Ni∕C: 0.0006 ± 0.0011 (all mean values ± standard deviation).High-resolution element distribution images reveal that the minor elements are distributed within the carbonaceous matrix; i.e., heterogeneous inclusions are not observed. No difference in size, nanostructure and elemental composition was found between particles collected inside and outside the polar vortex. Based on chemistry and nanostructure, aircraft exhaust, volcanic emissions and biomass burning can certainly be excluded as sources. The same is true for the less probable but globally important sources: wood burning, coal burning, diesel engines and ship emissions. Recondensed organic matter and extraterrestrial particles, potentially originating from ablation and fragmentation, remain as possible sources of the refractory carbonaceous particles studied. However, additional work is required in order to identify the sources unequivocally.

Mother-of-pearl cloud particle size and composition from aircraft-based photography of coloration and lidar measurements.

During a Stratospheric Aerosol and Gas (SAGE)-III Ozone Loss and Validation Experiment (SOLVE)-II science flight on 4 February 2003, a mother-of-pearl cloud over Iceland was underflown by the NASA DC-8 and measured with the lidars onboard. In addition, color photos were taken during the approach. Aided by extensive modeling of cloud coloration, the main results of the analysis of this unique data set are: (1)the polar stratospheric cloud was mountain wave-induced and of type II; (2)the spectacular color display was caused by ice particles with sizes around 2μm.

Mountain Wave–Induced Polar Stratospheric Cloud Forecasts for Aircraft Science Flights during SOLVE/THESEO 2000

Abstract The results of a multimodel forecasting effort to predict mountain wave–induced polar stratospheric clouds (PSCs) for airborne science during the third Stratospheric Aerosol and Gas Experiment (SAGE III) Ozone Loss and Validation Experiment (SOLVE)/Third European Stratospheric Experiment on Ozone (THESEO 2000) Arctic ozone campaign are assessed. The focus is on forecasts for five flights of NASA's instrumented DC-8 research aircraft in which PSCs observed by onboard aerosol lidars were identified as wave related. Aircraft PSC measurements over northern Scandinavia on 25–27 January 2000 were accurately forecast by the mountain wave models several days in advance, permitting coordinated quasi-Lagrangian flights that measured their composition and structure in unprecedented detail. On 23 January 2000 mountain wave ice PSCs were forecast over eastern Greenland. Thick layers of wave-induced ice PSC were measured by DC-8 aerosol lidars in regions along the flight track where the forecasts predicted enhanced stratospheric mountain wave amplitudes. The data from these flights, which were planned using this forecast guidance, have substantially improved the overall understanding of PSC microphysics within mountain waves. Observations of PSCs south of the DC-8 flight track on 30 November 1999 are consistent with forecasts of mountain wave–induced ice clouds over southern Scandinavia, and are validated locally using radiosonde data. On the remaining two flights wavelike PSCs were reported in regions where no mountain wave PSCs were forecast. For 10 December 1999, it is shown that locally generated mountain waves could not have propagated into the stratosphere where the PSCs were observed, confirming conclusions of other recent studies. For the PSC observed on 14 January 2000 over northern Greenland, recent work indicates that nonorographic gravity waves radiated from the jet stream produced this PSC, confirming the original forecast of no mountain wave influence. This forecast is validated further by comparing with a nearby ER-2 flight segment to the south of the DC-8, which intercepted and measured local stratospheric mountain waves with properties similar to those predicted. In total, the original forecast guidance proves to be consistent with PSC data acquired from all five of these DC-8 flights. The work discussed herein highlights areas where improvements can be made in future wave PSC forecasting campaigns, such as use of anelastic rather than Boussinesq linearized gridpoint models and a need to forecast stratospheric gravity waves from sources other than mountains.

Measurements of ice water content in tropopause region Arctic cirrus during the SAGE III Ozone Loss and Validation Experiment (SOLVE)

A new instrument, the closed‐path laser hygrometer (CLH), was flown on the NASA DC‐8 aircraft during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) campaign in 1999–2000 to measure condensed‐phase water. The ice water content (IWC) of Arctic cirrus was determined from in situ measurements of condensed‐ and gas‐phase water. The IWC values obtained from the CLH observations are compared to those determined by integrating particle size distributions measured by a Forward Scattering Spectrometer Probe (FSSP) also flown on the DC‐8. The considerably greater IWC seen by the CLH implies the presence of particles with diameters greater than the FSSP's upper limit of 20 μm. The evidence for and implications of the presence of large ice crystals in Arctic cirrus is discussed.

Intercomparison of MSX/UVISI‐derived ozone and temperature profiles with ground‐based, SAGE II, HALOE, and POAM III data

Recently, we demonstrated a combined extinctive and refractive stellar occultation technique on a proof‐of‐concept basis using data from the Ultraviolet and Visible Imagers and Spectrographic Imagers (UVISI) on the Midcourse Space Experiment (MSX) satellite. Beyond this initial presentation of the technique, we also used MSX/UVISI data obtained during the Stratospheric Aerosol and Gas Experiment III (SAGE III) Ozone Loss and Validation Experiment (SOLVE) campaign in 2000 to demonstrate the utility of stellar occultation data in scientific analyses. In the present work, we compare the retrieved ozone and temperature profiles from numerous MSX/UVISI stellar occultations to ground‐based, SAGE II, Halogen Occultation Experiment (HALOE), and Polar Ozone and Aerosol Measurement III (POAM III) data. The MSX/UVISI results derived using the combined occultation method are shown to compare favorably with all these data. Small biases in the comparisons hint at interesting differences in the nature of the various ozone measurement techniques, but the overall agreement in the comparisons clearly illustrates the utility of the combined technique as a complementary means of long‐term ozone monitoring.

Chlorine budget and partitioning during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE)

The amount of chlorine in the stratosphere has a direct influence on the magnitude of chlorine‐catalyzed ozone loss. A comprehensive suite of organic source gases of chlorine in the stratosphere was measured during the NASA Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) campaign in the arctic winter of 2000. Measurements included chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halon 1211, solvents, methyl chloride, N2O, and CH4. Inorganic chlorine contributions from each compound were calculated using the organic chlorine measurements, mean age of air, tropospheric trends, and a method to account for mixing in the stratosphere. Total organic chlorine measured at tropospheric levels of N2O was on the order of 3500 ppt. Total calculated inorganic chlorine at a N2O mixing ratio of 50 ppb (corresponding to a mean age of 5.5 years) was on the order of 3400 ppt. CFCs were the largest contributors to total organic chlorine (55–70%) over the measured N2O range (50–315 ppb), followed by CH3Cl (15%), solvents (5–20%), and HCFCs (5–25%). CH3Cl contribution was consistently about 15% across the organic chlorine range. Contributions to total calculated inorganic chlorine at 50 ppb N2O were 58% from CFCs, 24% from solvents, 16% from CH3Cl, and 2% from HCFCs. Updates to fractional chlorine release values for each compound relative to CFC 11 were calculated from the SOLVE measurements. An average value of 0.58 was calculated for the fractional chlorine release of CFC 11 over the 3–4 year mean age range, which was lower than the previous value of 0.80. The fractional chlorine release values for HCFCs 141b and 142b relative to CFC 11 were significantly lower than previous calculations.

Tracer Equivalent Latitude: A Diagnostic Tool for Isentropic Transport Studies

Abstract Area equivalent latitude based on potential vorticity (PV) is a widely used diagnostic for isentropic transport in the stratosphere and upper troposphere. Here, an alternate method for calculating equivalent latitude is explored, namely, a numerical synthesis of a PV-like tracer from a long-term integration of the advection–diffusion equation on isentropic surfaces. It is found that the tracer equivalent latitude (TrEL) behaves much like the traditional PV equivalent latitude (PVEL) despite the simplified governing physics; this is evidenced by examining the kinematics of the Arctic lower stratospheric vortex. Yet in some cases TrEL performs markedly better as a coordinate for long-lived trace species such as ozone. These instances include analysis of lower stratospheric ozone during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) campaign and three-dimensional reconstruction of total column ozone during November–December 1999 from fitted ozone...

Mixing events revealed by anomalous tracer relationships in the Arctic vortex during winter 1999/2000

During the 1999/2000 Arctic winter SAGE III–Ozone Loss and Validation Experiment (SOLVE) campaign, high‐resolution, in situ tracer data measured aboard the NASA ER‐2 high‐altitude aircraft revealed anomalous mixing events within the polar vortex. From January to March 2000 in the 350–500 K potential temperature range, we found mixing events during 15% of the flight time on average with significant maxima at potential temperatures of 450, 410, and 380 K. The events were spread throughout the vortex but showed a distinct minimum at 73° N and a peak at 85°N equivalent latitude. About 60% of the observed mixing events were less than 13 km wide. Based on a case study of tracer‐tracer relationships, an objective simple method is introduced to detect such events using the linear nitrous oxide (N2O):potential temperature relationship observed deep in the vortex. Rigorous analysis and supporting evidence from total water data corroborated the validity of the method. These results suggest mixing across the polar vortex edge occurred preferentially in layers at select altitudes in the Arctic winter 1999/2000.

Reconstruction of three‐dimensional ozone fields using POAM III during SOLVE

In this paper we demonstrate the utility of the Polar Ozone and Aerosol Measurement (POAM) III data for providing semiglobal three‐dimensional ozone fields during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) winter. As a solar occultation instrument, POAM III measurements were limited to latitudes of 63°N to 68°N during the SOLVE campaign but covered a wide range of potential vorticity. Using established mapping techniques, we have used the relation between potential vorticity and ozone measured by POAM III to calculate three‐dimensional ozone mixing ratio fields throughout the Northern Hemisphere on a daily basis during the 1999/2000 winter. To validate the results, we have extensively compared profiles obtained from ozonesondes and the Halogen Occultation Experiment to the proxy O3 interpolated horizontally and vertically to the correlative measurement locations. On average, the proxy O3 agrees with the correlative observations to better than ∼5%, at potential temperatures below about 900 K and latitudes above about 30°N, demonstrating the reliability of the reconstructed O3 fields in these regions. We discuss the application of the POAM proxy ozone profiles for calculating photolysis rates along the ER‐2 and DC‐8 flight tracks during the SOLVE campaign, and we present a qualitative picture of the evolution of polar stratospheric ozone throughout the winter.

An overview of the SOLVE/THESEO 2000 campaign

Between November 1999 and April 2000, two major field experiments, the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) and the Third European Stratospheric Experiment on Ozone (THESEO 2000), collaborated to form the largest field campaign yet mounted to study Arctic ozone loss. This international campaign involved more than 500 scientists from over 20 countries. These scientists made measurements across the high and middle latitudes of the Northern Hemisphere. The main scientific aims of SOLVE/THESEO 2000 were to study (1) the processes leading to ozone loss in the Arctic vortex and (2) the effect on ozone amounts over northern midlatitudes. The campaign included satellites, research balloons, six aircraft, ground stations, and scores of ozonesondes. Campaign activities were principally conducted in three intensive measurement phases centered on early December 1999, late January 2000, and early March 2000. Observations made during the campaign showed that temperatures were below normal in the polar lower stratosphere over the course of the 1999–2000 winter. Because of these low temperatures, extensive polar stratospheric clouds (PSC) formed across the Arctic. Large particles containing nitric acid trihydrate were observed for the first time, showing that denitrification can occur without the formation of ice particles. Heterogeneous chemical reactions on the surfaces of the PSC particles produced high levels of reactive chlorine within the polar vortex by early January. This reactive chlorine catalytically destroyed about 60% of the ozone in a layer near 20 km between late January and mid‐March 2000, with good agreement being found between a number of empirical and modeling studies. The measurements made during SOLVE/THESEO 2000 have improved our understanding of key photochemical parameters and the evolution of ozone‐destroying forms of chlorine.

Lidar temperature measurements during the SOLVE campaign and the absence of polar stratospheric clouds from regions of very cold air

NASA Goddard Space Flight Center's Airborne Raman Ozone, Temperature, and Aerosol Lidar (AROTEL) measured extremely cold temperatures during all three deployments (1–16 December 1999, 14–29 January 2000, and 27 February to 15 March 2000) of the SAGE III Ozone Loss and Validation Experiment (SOLVE). Temperatures were significantly below values observed in previous years with large regions regularly below 191 K and frequent temperature retrievals yielding values at or below 187 K. Temperatures well below the saturation point of type I polar stratospheric clouds (PSCs) were regularly encountered, but their presence was not well correlated with PSC observations made by NASA Langley Research Center's aerosol lidar colocated with AROTEL. Temperature measurements by meteorological sondes launched within areas traversed by the DC‐8 showed minimum temperatures consistent in time and vertical extent with those derived from AROTEL data. Calculations to establish whether PSCs could exist at measured AROTEL temperatures and observed mixing ratios of nitric acid and water vapor showed large areas favorable to PSC formation but that were lacking PSCs. The flight on 12 December 1999 encountered large regions having temperatures up to 10 K below the NAT saturation temperature but only small, localized regions that might be identified as PSCs.

Descent and mixing in the 1999–2000 northern polar vortex inferred from in situ tracer measurements

In situ measurements from the Lightweight Airborne Chromatograph Experiment (LACE) and the Airborne Chromatograph for Atmospheric Trace Species (ACATS‐IV) taken during the SAGE III Ozone Loss and Validation Experiment (SOLVE) are used to examine the descent and mixing in the 1999–2000 northern stratospheric polar vortex. LACE was flown twice on an in situ balloon platform, in November 1999 just after the vortex formed and in March 2000 near the end of the vortex lifetime. The aim of this paper is to use simple models of vortex transport to try to explain the changes seen in the long‐lived tracers measured by LACE between the two vortex flights. It is the high precision and long‐term accuracy of the observations that allow small differences in the data from a number of species obtained on different flights to be used to infer transport processes. Changes in tracer profiles as a function of height or potential temperature can be attributed, to first order, to descent in the vortex. A calculation which used six tracers shows that the total descent was a strong function of potential temperature, from 200 K in the middle stratosphere to 50 K in the lower stratosphere over the nearly 4‐month period between flights. Observed changes in long‐lived tracer‐tracer correlations require a mixing process to have occurred since descent alone cannot change the correlations. Two simple models of mixing are used to examine the tracer correlation changes. One model simulates entrainment of midlatitude air across the vortex edge with relatively efficient mixing within the vortex. A second model simulates the effect of differential descent and subsequent efficient mixing within a vortex that is entirely isolated from the midlatitudes. It is shown that the differential descent and mixing calculation produces results which are much more consistent among all the tracer correlations compared to the results from midlatitude entrainment. The unique sensitivity of SF6 as a tracer of mesospheric air makes it an outlier in both the descent and mixing calculations. The inclusion of a small fraction of air from the mesosphere is necessary to bring the SF6 mixing and descent calculations into agreement with the other tracers. The conclusions regarding descent and descent rates in the vortex are consistent with other modeling studies. The results indicate that mixing of midlatitude air into the winter vortex is not a significant contributor to the observed ozone changes in the 1999/2000 season.

In situ observations of HO2 and OH obtained on the NASA ER‐2 in the high‐ClO conditions of the 1999/2000 Arctic polar vortex

Extensive observations of OH and HO2 obtained aboard the NASA ER‐2 inside the Arctic polar vortex during the SAGE III Ozone loss and Validation Experiment (SOLVE) provide the opportunity to identify and test interferences during the measurement of HO2 in the presence of high concentrations of ClO. In‐flight calibrations are consistent with small interferences from CH3O2 (8%) and from ClO (3%) to the total signal on average inside the vortex. Measurements of HO2, OH, ClO, and O3 are used with rate constants derived from laboratory measurements to test the consistency of the reactions that control HO2/OH. Inside the vortex the calculated HO2/OH under predicts the observed HO2/OH by 7% with the JPL‐00 rate constants and over‐predicts the observations by 23% when recent laboratory rates are used. Measurements of HO2 and ClO are used to calculate ozone loss rates and concentrations of HOCl with uncertainties determined from the analysis of HO2/OH. The calculated concentration of HOCl is 52−28+50 pptv, or 0.07 × ClO on average in the vortex. The ozone loss rate from the two cycles controlled by HO2 + ClO is ∼1.5−0.5+1.2 ppbv d−1 in early March.

Redistribution of reactive nitrogen in the Arctic lower stratosphere in the 1999/2000 winter

Total reactive nitrogen (NOy) in the Arctic lower stratosphere was measured from the NASA DC‐8 aircraft during the SAGE III Ozone Loss and Validation Experiment (SOLVE) in the winter of 1999/2000. NOy‐N2O correlations obtained at altitudes of 10–12.5 km in December 1999 and January 2000 are comparable to the reported reference correlation established using the MkIV balloon measurements made during SOLVE prior to the onset of denitrification. Between late February and mid‐March, NOy values obtained from the DC‐8 were systematically higher than those observed in December and January by up to 1 part per billion by volume, although a compact correlation between NOy and N2O was maintained. Greater increases in NOy were generally observed in air masses with lower N2O values. The daily minimum temperatures at 450–500 K potential temperature (∼20–22 km) in the Arctic fell below the ice saturation temperature between late December and mid‐January. Correspondingly, intense denitrification and nitrified air masses were observed from the ER‐2 at 17–21 km and below 18 km, respectively, in January and March. The increases in NOy observed from the DC‐8 in late February/March indicate that influence from nitrification extended as low as 10–12.5 km over a wide area by that time. We show in this paper that the vertical structure of the temperature field during the winter was a critical factor in determining the vertical extent of the NOy redistribution. Results from the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) three‐dimensional chemistry transport model, which reproduced the observed general features only when the NOy redistribution process is included, are also presented.

Construction of a unified, high‐resolution nitrous oxide data set for ER‐2 flights during SOLVE

Four nitrous oxide (N2O) instruments were part of the NASA ER‐2 aircraft payload during the 2000 SAGE‐III Ozone Loss and Validation Experiment (SOLVE). Coincident data from the three in situ instruments and a whole air sampler are compared. Agreement between these instruments was typically good; however, there are several types of important differences between the data sets. These differences prompted a collaborative effort to combine data from the three in situ instruments, using an objective method, to produce a self‐consistent, high‐resolution, unified N2O data set for each SOLVE flight. The construction method developed by the four N2O instrument teams is described in detail. An important step in this method is the evaluation and reduction of bias in each of the in situ data sets before they are combined. The quality of unified N2O data is examined through its agreement with high‐accuracy and high‐precision N2O data from whole air samples collected from the ER‐2 during SOLVE flights. Typical agreement between these two data sets is 2.9 ppb (1.5%), better than the typical agreement between any pair of N2O instruments.

POAM III observations of arctic ozone loss for the 1999/2000 winter

During the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE)/Third European Stratospheric Experiment on Ozone (THESEO) campaign, Polar Ozone and Aerosol Measurement (POAM) III sampled in the vortex core, on the vortex edge, and outside the vortex on a near‐daily basis from December 1999 through mid‐March 2000. During this period, POAM observed a substantial amount of ozone decline. For example, ozone mixing ratios in the core of the vortex dropped from about 3.5 ppmv in mid‐January to about 2 ppmv by mid‐March at 500 K. The ozone chemical loss indicated by these measurements is assessed using two methodologies. First, the POAM data is used to construct vortex‐averaged ozone profiles, which are advected downward using vortex average descent rates. The maximum ozone loss (1 January to 15 March) is found to be about 1.8 ppmv. In a second approach, the REPROBUS 3‐D CTM is used to specify the passive ozone distribution throughout the winter. The chemical loss in the vortex is estimated by performing a point‐by‐point subtraction of the POAM measurements inside the vortex from the model passive ozone evaluated at the time and location of the POAM measurements. Both ozone loss estimates are in general agreement and they agree well with published loss estimates from ER2 and ozonesonde measurements.

Comparison of POAM III ozone measurements with correlative aircraft and balloon data during SOLVE

The Polar Ozone and Aerosol Measurement (POAM) III instrument operated continuously during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) mission, making approximately 1400 ozone profile measurements at high latitudes both inside and outside the Arctic polar vortex. The wealth of ozone measurements obtained from a variety of instruments and platforms during SOLVE provided a unique opportunity to compare correlative measurements with the POAM III data set. In this paper, we validate the POAM III version 3.0 ozone against measurements from seven different instruments that operated as part of the combined SOLVE/THESEO 2000 campaign. These include the airborne UV Differential Absorption Lidar (UV DIAL) and the Airborne Raman Ozone and Temperature Lidar (AROTEL) instruments on the DC‐8, the dual‐beam UV‐Absorption Ozone Photometer on the ER‐2, the MkIV Interferometer balloon instrument, the Laboratoire de Physique Molèculaire et Applications and Differential Optical Absorption Spectroscopy (LPMA/DOAS) balloon gondola, the JPL in situ ozone instrument on the Observations of the Middle Stratosphere (OMS) balloon platform, and the Système D'Analyze par Observations Zénithales (SAOZ) balloon sonde. The resulting comparisons show a remarkable degree of consistency despite the very different measurement techniques inherent in the data sets and thus provide a strong validation of the POAM III version 3.0 ozone. This is particularly true in the primary 14–30 km region, where there are significant overlaps with all seven instruments. At these altitudes, POAM III agrees with all the data sets to within 7–10% with no detectable bias. The observed differences are within the combined errors of POAM III and the correlative measurements. Above 30 km, only a handful of SOLVE correlative measurements exist and the comparisons are highly variable. Therefore, the results are inconclusive. Below 14 km, the SOLVE comparisons also show a large amount of scatter and it is difficult to evaluate their consistency, although the number of correlative measurements is large. The UV DIAL, DOAS, and JPL/OMS comparisons show differences of up to 15% but no consistent bias. The ER‐2, MkIV, and SAOZ comparisons, on the other hand, indicate a high POAM bias of 10–20% at the lower altitudes. In general, the SOLVE validation results presented here are consistent with the validation of the POAM III version 3.0 ozone using SAGE II and Halogen Occultation Experiment (HALOE) satellite data and in situ electrochemical cell (ECC) ozonesonde data.

Airborne spectroscopic observations of chlorine activation and denitrification of the 1999/2000 winter Arctic stratosphere during SOLVE

We have used Fourier transform spectrometers aboard the NASA DC‐8 and on balloons as part of the SAGE III Ozone Loss and Validation Experiment (SOLVE) to record infrared absorption spectra of the polar stratosphere. From the high‐resolution aircraft spectra we derived vertical column amounts of HCl, HF, NO2, HNO3, and a number of other gases for 11 flights in the region of the northern polar vortex. Vertical mixing ratio profiles are derived for a number of gases from two balloon flights. Within the vortex, where low values of total ozone are observed during the latter part of the observation period, we observed markedly reduced columns of HCl and NO2 and elevated columns of HF and HNO3 relative to columns outside the vortex. The low value for the ratio of HCl to HF indicates that HCl had been chemically or physically removed, probably providing a source for active chlorine. High values of HNO3 inside the vortex indicate that nitrogen oxides, which might otherwise mitigate chlorine‐catalyzed ozone destruction, have been converted into more stable reservoirs. Because of incorporation into particles, some nitric acid may be removed from the stratosphere altogether.

Evidence for the widespread presence of liquid‐phase particles during the 1999–2000 Arctic winter

In situ Multiangle Spectrometer Probe (MASP) particle measurements have been analyzed to determine the typical behavior of sulfate particles during the SAGE III Ozone Loss and Validation Experiment (SOLVE) campaign. The study has explored variations in the total particle concentration measured by MASP. A new analysis method has been developed in which increases of the MASP concentration can be interpreted as growth of small particles (those which are smaller than 0.2 μm in radius at midlatitudes). The method also allows all of the MASP measurements made during the SOLVE campaign to be incorporated in a single analysis. At all levels of the stratosphere, the total MASP concentration (and therefore aerosol growth) varies continuously with temperature. This behavior is well‐reproduced by assuming that the sulfate aerosols are liquid solutions, but cannot be reproduced if the aerosol is assumed to be frozen. At sufficiently cold temperatures, larger increases in the MASP concentration are consistently seen; the observed onset temperature for this growth is in good agreement with model expectations for liquid ternary solutions. Liquid‐like behavior is apparent for all measurements made in the Arctic during SOLVE, both inside and outside the vortex, and even at the coldest temperatures sampled during the campaign. At the levels with the coldest measured temperatures, which cause maximum particle sizes and thus the greatest total MASP concentrations, 90% of the particles grow as liquids. Therefore, the freezing that occurred during the 1999–2000 Arctic winter was selective, with most of the particles remaining liquid even in the presence of a small number of frozen particles.

Comparison of ER‐2 aircraft and POAM III, MLS, and SAGE II satellite measurements during SOLVE using traditional correlative analysis and trajectory hunting technique

We compared the version 5 Microwave Limb Sounder (MLS), version 3 Polar Ozone and Aerosol Measurement III (POAM III), version 6.0 Stratospheric Aerosol and Gas Experiment II (SAGE II), and NASA ER‐2 aircraft measurements made in the Northern Hemisphere in January–February 2000 during the SAGE III Ozone Loss and Validation Experiment (SOLVE). This study addresses one of the key scientific objectives of the SOLVE campaign, namely, to validate multiplatform satellite measurements made in the polar stratosphere during winter. This intercomparison was performed by using a traditional correlative analysis (TCA) and a trajectory hunting technique (THT). TCA compares profiles colocated within a chosen spatial‐temporal vicinity. Launching backward and forward trajectories from the points of measurement, the THT identifies air parcels sampled at least twice within a prescribed match criterion during the course of 5 days. We found that the ozone measurements made by these four instruments agree most of the time within ±10% in the stratosphere up to 1400 K (∼35 km). The water vapor measurements from POAM III and the ER‐2 Harvard Lyman α hygrometer and Jet Propulsion Laboratory laser hygrometer agree to within ±0.5 ppmv (or about ±10%) in the lower stratosphere above 380 K. The MLS and ER‐2 ClO measurements agree within their error bars for the TCA. The MLS and ER‐2 nitric acid measurements near 17‐ to 20‐km altitude agree within their uncertainties most of the time with a hint of a positive offset by MLS according to the TCA. We also applied the Atmospheric and Environmental Research, Inc. box model constrained by the ER‐2 measurements for analysis of the ClO and HNO3 measurements using the THT. We found that: (1) the model values of ClO are smaller by about 0.3–0.4 (0.2) ppbv below (above) 400 K than those by MLS and (2) the HNO3 comparison shows a positive offset of MLS values by ∼1 and 1–2 ppbv below 400 K and near 450 K, respectively. Our study shows that, with some limitations (like HNO3 comparison under polar stratospheric cloud conditions), the THT is a more powerful tool for validation studies than the TCA, making conclusions of the comparison statistically more robust.