Ozone mixing ratios in the upper stratosphere, observed with a millimeter‐wave radiometer at Rikubetsu (43.46°N, 143.77°E), Japan, from November 1999 to February 2017 showed both interannual and seasonal variation, which was characterized by a winter maximum and a summer minimum. During the study period, the summer minima were nearly constant whereas the winter maxima varied interannually and also displayed short‐term variability. The observed ozone mixing ratios at 1 hPa were anticorrelated with temperature at 1 hPa from MERRA‐2 data. The slope of the relationship between the logarithm of ozone concentration and the reciprocal of temperature differed between winter data and both summer and annual data. Therefore, we inferred that both chemistry and dynamics affect short‐term variation of ozone mixing ratios in winter. We then examined the contribution of the polar vortex to interannual variations in ozone and temperature at 1 hPa. When the polar vortex was strong, wave number‐1 planetary waves at high latitude propagated toward the midlatitudes instead of vertically. The vertical component of the wave number‐1 Eliassen‐Palm flux along 43°N at 1 hPa was strongly correlated with zonal mean zonal wind along 60°N at 50 hPa. When the zonal mean westerly wind was strong in December, upper stratospheric (~1 hPa) temperatures over Rikubetsu and over a point on the opposite side of the globe (by longitude) were significantly lower and higher, respectively, than the climatological temperature. Thus, planetary wave propagation related to zonal mean westerly wind strength induced early winter interannual variation in upper stratospheric ozone in the midlatitudes.

Fluctuations of ozone concentrations with dimensions of a few kilometers (i.e., ozone laminae) are frequently found in ozone‐sounding profiles. We used ozonesonde measurements made at the southern tip of South America to examine the relationship between ozone laminae and atmospheric waves near the edge of the polar vortex and on the leeward side of the Andes Mountains. Laminar structures are formed by vertical and horizontal displacements of isopleths due to gravity waves and by isentropic advection of vortex air filaments with low ozone concentration due to Rossby wave breaking. We extracted components of these ozone fluctuations by applying a high‐pass filter to the observed ozone profiles and normalizing them to background concentrations, which were extracted with a low‐pass filter. Ozone fluctuations due to displacements caused by gravity waves were individually evaluated with experimental data. We assumed that the residuals between the observed and gravity wave‐induced fluctuations were Rossby waves‐induced fluctuations. We found that the gravity wave‐induced variability was larger in the upper troposphere than in the lower stratosphere and was a maximum in winter. Rossby wave‐induced variability showed a distinct seasonal pattern in the lower stratosphere and accounted for a large portion of the observed variability. We also examined the relationship between gravity wave‐induced and Rossby wave‐induced ozone variability and the differences in equivalent latitudes between the sonde positions and the polar vortex edge. We found that variability was larger inside than outside the polar vortex.

We examined long-range transport events of carbon monoxide (CO) and black carbon (BC) from East Asia to the Arctic with satellite measurements and a global chemical transport model GEOS-Chem. The transport patterns and meteorological conditions associated with the events were analyzed. We used Infrared Atmospheric Sounding Interferometer (IASI) data during 2007–2011 to identify transport events of CO reaching the Arctic. Temporal variations of CO column over the Pacific side of the Arctic (160°–200°E, 60°–80°N) showed that episodic increases occurred several times in each season. We identified 11 strong events (6 in spring, 3 in autumn, and 2 in winter) caused by the long-range transport from East Asia using tagged BC and CO simulations. Two transport pathways from East Asia to the Arctic were found over Siberia and the North Pacific. In the pathway over Siberia, pollutants were transported northeastward from China mainly through the Okhotsk Sea and East Siberia. The low pressures passing from the East Siberia to the Okhotsk Sea played important roles on the transport in the lower troposphere and uplifting to the middle troposphere. In the pathway over the North Pacific, pollutants were transported eastward from the Asian continent and subsequent northward transport took place over the North Pacific. The poleward transport occurred west of the high pressure that stayed around the Bering Sea. While the poleward transport occurred mainly in the middle troposphere over Siberia, the enhancement of BC was also observed by the surface measurement at Barrow for the North Pacific route. Arctic, carbon, Long-range

The subpolar regions of the Southern Hemisphere are affected by short periods of low total ozone values directly linked to overpasses of the ozone hole. These events often happen during spring and early summer time and the polar flames of ozone poor air travel northward to low latitudes as much as Buenos Aires ( 34 S). During these episodes, the subpolar regions experience a pronounced ozone reduction with generally an enhance- ment of UV-B radiation, depending on cloud cover conditions. But the level of surface UV radiation not only depends of total ozone column (TOC). Also play an important role the solar zenith angle (SZA), the cloud cover, the aerosols loading and the surface albedo. The combination of low ozone content and high solar elevations at noon during the transports of polar ozone poor airmasses to lower latitudes can induce a relative change of surface irradiance respect to climatological conditions with greater UV indexes at lower than in the higher latitudes. In this paper we evaluate the increase of UV index at surface during this kind of episodes using a parametric UV model that use as input parameters TOC from OMI/AURA and calculated noon solar zenith angle for three selected sites considered in this study: Rio Gallegos ( 51S), Comodoro Rivadavia (45S) and Buenos Aires (34S). The study cases analyzed in this work were selected using potential vorticity maps at isentropic level of 550 K form NCEP reanalysis as tracer of the perturbation of polar vortex. By means of ozone climatological database of Multi Sensor Reanalysis (MRS) (1978-2013) for the selected sites we modeled the climatological UV radiation level at noon time and calculate the relative impact that induces the transported ozone poor airmasses to subpolar regions on surface radiation levels. The results show that the poor ozone airmass transported to middle latitudes increase the UV radiation level and the combination with higher solar elevation at solar noon at lower latitudes induce a high impact in the solar UV radiation.

Profiles of CFC-11 (CCl3F) and CFC-12 (CCl2F2) of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aboard the European satellite Envisat have been retrieved from versions MIPAS/4.61 to MIPAS/4.62 and MIPAS/5.02 to MIPAS/5.06 level-1b data using the scientific level-2 processor run by Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK) and Consejo Superior de Investigaciones Científicas (CSIC), Instituto de Astrofísica de Andalucía (IAA). These profiles have been compared to measurements taken by the balloon-borne cryosampler, Mark IV (MkIV) and MIPAS-Balloon (MIPAS-B), the airborne MIPAS-STRatospheric aircraft (MIPAS-STR), the satellite-borne Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) and the High Resolution Dynamic Limb Sounder (HIRDLS), as well as the ground-based Halocarbon and other Atmospheric Trace Species (HATS) network for the reduced spectral resolution period (RR: January 2005–April 2012) of MIPAS. ACE-FTS, MkIV and HATS also provide measurements during the high spectral resolution period (full resolution, FR: July 2002–March 2004) and were used to validate MIPAS CFC-11 and CFC-12 products during that time, as well as profiles from the Improved Limb Atmospheric Spectrometer, ILAS-II. In general, we find that MIPAS shows slightly higher values for CFC-11 at the lower end of the profiles (below ∼ 15 km) and in a comparison of HATS ground-based data and MIPAS measurements at 3 km below the tropopause. Differences range from approximately 10 to 50 pptv ( ∼ 5–20 %) during the RR period. In general, differences are slightly smaller for the FR period. An indication of a slight high bias at the lower end of the profile exists for CFC-12 as well, but this bias is far less pronounced than for CFC-11 and is not as obvious in the relative differences between MIPAS and any of the comparison instruments. Differences at the lower end of the profile (below ∼ 15 km) and in the comparison of HATS and MIPAS measurements taken at 3 km below the tropopause mainly stay within 10–50 pptv (corresponding to ∼ 2–10 % for CFC-12) for the RR and the FR period. Between ∼ 15 and 30 km, most comparisons agree within 10–20 pptv (10–20 %), apart from ILAS-II, which shows large differences above ∼ 17 km. Overall, relative differences are usually smaller for CFC-12 than for CFC-11. For both species – CFC-11 and CFC-12 – we find that differences at the lower end of the profile tend to be larger at higher latitudes than in tropical and subtropical regions. In addition, MIPAS profiles have a maximum in their mixing ratio around the tropopause, which is most obvious in tropical mean profiles. Comparisons of the standard deviation in a quiescent atmosphere (polar summer) show that only the CFC-12 FR error budget can fully explain the observed variability, while for the other products (CFC-11 FR and RR and CFC-12 RR) only two-thirds to three-quarters can be explained. Investigations regarding the temporal stability show very small negative drifts in MIPAS CFC-11 measurements. These instrument drifts vary between ∼ 1 and 3 % decade−1. For CFC-12, the drifts are also negative and close to zero up to ∼ 30 km. Above that altitude, larger drifts of up to ∼ 50 % decade−1 appear which are negative up to ∼ 35 km and positive, but of a similar magnitude, above.

Atmospheric Chemistry Convener:*Nobuyuki Takegawa(Research Center for Advanced Science and Technology, University of Tokyo), Yousuke Sawa(Geochemical Research Department, Meteorological Research Institute), Yugo Kanaya(Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology), Kenshi Takahashi(Research Institute for Sustainable Humanosphere, Kyoto University), Hiroshi Tanimoto(National Institute for Environmental Studies), Chair:Nobuyuki Takegawa(Research Center for Advanced Science and Technology, University of Tokyo) Thu. May 1, 2014 4:15 PM 6:00 PM 511 (5F) This session provides a forum for the presentation of the broad spectrum of tropospheric and stratospheric chemistry, including various research topics (air quality and climate), approaches (modeling, field measurements, satellite data analysis, and laboratory studies), and species (gas and aerosol). This session also provides an opportunity for discussing possible future collaboration with other research fields relevant to atmospheric chemistry.

We present vertical profiles of hydrogen chloride (HCl) and chlorine monoxide (ClO) as observed by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station (ISS) inside the Antarctic vortex on 19–24 November 2009. The SMILES HCl value reveals 2.8–3.1 ppbv between 450 K and 500 K levels in potential temperature (PT). The high value of HCl is highlighted since it is suggested that HCl is a main component of the total inorganic chlorine (Cly), defined as Cly ≃ HCl + ClO + chlorine nitrate (ClONO2), inside the Antarctic vortex in spring, owing to low ozone values. To confirm the quality of two SMILES level 2 (L2) data products provided by the Japan Aerospace Exploration Agency (JAXA) and Japan's National Institute of Information and Communications Technology (NICT), vis-à-vis the partitioning of Cly, comparisons are made using other satellite data from the Aura Microwave Limb Sounder (MLS) and Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). HCl values from the SMILES NICT L2 product agree to within 10% (0.3 ppbv) with the MLS HCl data between 450 and 575 K levels in PT and with the ACE-FTS HCl data between 425 and 575 K. The SMILES JAXA L2 product is 10 to 20% (0.2–0.5 ppbv) lower than that from MLS between 400 and 700 K and from ACE-FTS between 500 and 700 K. For ClO in daytime, the difference between SMILES (JAXA and NICT) and MLS is less than ±0.05 ppbv (100 %) between 500 K and 650 K with the ClO values less than 0.2 ppbv. ClONO2 values as measured by ACE-FTS also reveal 0.2 ppbv at 475–500 K level, resulting in the HCl / Cly ratios of 0.91–0.95. The HCl / Cly ratios derived from each retrieval agree to within −5 to 8 % with regard to their averages. The high HCl values and HCl / Cly ratios observed by the three instruments in the lower stratospheric Antarctic vortex are consistent with previous observations in late Austral spring.

We observed ozone (O3) in the vertical region between 250 and 0.0005 hPa (~ 12–96 km) using the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the Japanese Experiment Module (JEM) of the International Space Station (ISS) between 12 October 2009 and 21 April 2010. The new 4 K superconducting heterodyne receiver technology of SMILES allowed us to obtain a one order of magnitude better signal-to-noise ratio for the O3 line observation compared to past spaceborne microwave instruments. The non-sun-synchronous orbit of the ISS allowed us to observe O3 at various local times. We assessed the quality of the vertical profiles of O3 in the 100–0.001 hPa (~ 16–90 km) region for the SMILES NICT Level 2 product version 2.1.5. The evaluation is based on four components: error analysis; internal comparisons of observations targeting three different instrumental setups for the same O3 625.371 GHz transition; internal comparisons of two different retrieval algorithms; and external comparisons for various local times with ozonesonde, satellite and balloon observations (ENVISAT/MIPAS, SCISAT/ACE-FTS, Odin/OSIRIS, Odin/SMR, Aura/MLS, TELIS). SMILES O3 data have an estimated absolute accuracy of better than 0.3 ppmv (3%) with a vertical resolution of 3–4 km over the 60 to 8 hPa range. The random error for a single measurement is better than the estimated systematic error, being less than 1, 2, and 7%, in the 40–1, 80–0.1, and 100–0.004 hPa pressure regions, respectively. SMILES O3 abundance was 10–20% lower than all other satellite measurements at 8–0.1 hPa due to an error arising from uncertainties of the tangent point information and the gain calibration for the intensity of the spectrum. SMILES O3 from observation frequency Band-B had better accuracy than that from Band-A. A two month period is required to accumulate measurements covering 24 h in local time of O3 profile. However such a dataset can also contain variation due to dynamical, seasonal, and latitudinal effects.

The International Space Station/Japanese Exposure Module (ISS/JEM) borne instrument, the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES), has successfully measured chemical species in the middle atmosphere between October 2009 and April 2010. We focus on inorganic chlorine species measured inside the late spring Antarctic vortex, when hydrogen chloride (HCl) was a main component of the total inorganic chlorine (Cly). Comparisons with other satellite instruments, the Aura Microwave Limb Sounder (MLS) and Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS), are also presented to show the SMILES HCl and chlorine monoxide (ClO) data quality.

The horizontal structure of processes causing increases in ozone in the Antarctic polar vortex was examined using data measured in 2003 from an ozonesonde observation campaign at Syowa Station (39.6 degrees E, 69.0 degrees S) and from the Improved Limb Atmospheric Spectrometer II (ILAS-II) onboard the Advanced Earth Observing Satellite II. The ILAS-II data are daily and distributed uniformly at 14 points in the zonal direction, mostly at polar latitudes. The Antarctic ozone hole that developed in 2003 was one of the largest recorded. The period of focus in this study is 26 September through 24 October, when a strong polar vortex was situated in the stratosphere. An ozone mixing ratio contour (1.0 ppmv) moved downward near a height of 20 km during the period of focus. This increase in ozone is likely to result from downward transport of ozone-rich air originating from lower latitudes by Brewer-Dobson circulation. First, the descent rate of the mixing ratio contour was estimated by taking the geometric height as the vertical coordinate for the deep vortex interior around 20 km. A significant longitudinal dependence was observed. An analysis using ECMWF operational data shows that this dependence can be approximately explained by longitudinally dependent vertical movements of the isentropes caused by a zonal wavenumber-1 quasi-stationary planetary wave with amplitude and phases varying on a seasonal time scale. Next, the descent rate was calculated around 500 K ( around 20 km) by taking the potential temperature (isentrope) as the vertical coordinate. The longitudinal dependence was still present using this coordinate, meaning that the ozone mixing ratio and its increase are not constant on the isentropic layer even in the interior of the polar vortex. A backward trajectory analysis showed that air parcels with large ozone mixing ratios were mostly transported from the polar vortex boundary region. This result suggests that lateral transport/mixing is important even before the breakup of the polar vortex. Results from a tracer-tracer correlation analysis of O-3 and long-lived constituent N2O were also consistent with this inference. The contribution of lateral mixing to the increase in ozone was estimated at about 17% +/- 4% that of the Brewer-Dobson circulation around 20 km, using the calculated descent rates. The results of this study also imply that Lagrangian downward motions in the vortex interior are not correctly estimated without accounting for lateral mixing, even if the polar vortex is dynamically stable.

Abstract. Vertical profiles of stratospheric water vapour measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) with the full resolution mode between September 2002 and March 2004 and retrieved with the IMK/IAA scientific retrieval processor were compared to a number of independent measurements in order to estimate the bias and to validate the existing precision estimates of the MIPAS data. The estimated precision for MIPAS is 5 to 10% in the stratosphere, depending on altitude, latitude, and season. The independent instruments were: the Halogen Occultation Experiment (HALOE), the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), the Improved Limb Atmospheric Spectrometer-II (ILAS-II), the Polar Ozone and Aerosol Measurement (POAM III) instrument, the Middle Atmospheric Water Vapour Radiometer (MIAWARA), the Michelson Interferometer for Passive Atmospheric Sounding, balloon-borne version (MIPAS-B), the Airborne Microwave Stratospheric Observing System (AMSOS), the Fluorescent Stratospheric Hygrometer for Balloon (FLASH-B), the NOAA frostpoint hygrometer, and the Fast In Situ Hygrometer (FISH). For the in-situ measurements and the ground based, air- and balloon borne remote sensing instruments, the measurements are restricted to central and northern Europe. The comparisons to satellite-borne instruments are predominantly at mid- to high latitudes on both hemispheres. In the stratosphere there is no clear indication of a bias in MIPAS data, because the independent measurements in some cases are drier and in some cases are moister than the MIPAS measurements. Compared to the infrared measurements of MIPAS, measurements in the ultraviolet and visible have a tendency to be high, whereas microwave measurements have a tendency to be low. The results of χ2-based precision validation are somewhat controversial among the comparison estimates. However, for comparison instruments whose error budget also includes errors due to uncertainties in spectrally interfering species and where good coincidences were found, the χ2 values found are in the expected range or even below. This suggests that there is no evidence of systematically underestimated MIPAS random errors.

This paper assesses the mean differences between the two ILAS-II data versions (1.4 and 2) by comparing them with MIPAS measurements made between May and October 2003. For comparison with ILAS-II results, MIPAS data processed at the Institut für Meteorologie und Klimaforschung, Karlsruhe, Germany (IMK) in cooperation with the Instituto de Astrofísica de Andalucía (IAA) in Granada, Spain, were used. The coincidence criteria of ±300 km in space and ±12 h in time for H2O, N2O, and CH4 and the coincidence criteria of ±300 km in space and ±6 h in time for ClONO2, O3, and HNO3 were used. The ILAS-II data were separated into sunrise (= Northern Hemisphere) and sunset (= Southern Hemisphere). For the sunrise data, a clear improvement from version 1.4 to version 2 was observed for H2O, CH4, ClONO2, and O3. In particular, the ILAS-II version 1.4 mixing ratios of H2O and CH4 were unrealistically small, and those of ClONO2 above altitudes of 30 km unrealistically large. For N2O and HNO3, there were no large differences between the two versions. Contrary to the Northern Hemisphere, where some exceptional profiles deviated significantly from known climatology, no such outlying profiles were found in the Southern Hemisphere for both versions. Generally, the ILAS-II version 2 data were in better agreement with the MIPAS data than the version 1.4, and are recommended for quantitative analysis in the stratosphere. For H2O data in the Southern Hemisphere, further data quality evaluation is necessary.

The Improved Limb Atmospheric Spectrometer (ILAS)-II sensor aboard the Japanese ADEOS-II satellite was launched into its sun-synchronous orbit on 14 December 2002 and performed solar occultation measurements of trace species, aerosols, temperature, and pressure in the polar stratosphere until 25 October 2003. Vertical trace gas profiles obtained with the balloon version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) provide one of the sparse data sets for validating ILAS-II version 2 and 1.4 data. The MIPAS-B limb emission spectra were collected on 20 March 2003 over Kiruna (Sweden, 68° N) at virtually the same location that has been sounded by ILAS-II about 5.5 h prior to the sampling of MIPAS-B. The intercomparison of the new ILAS-II version 2 (Northern Hemispheric sunrise) data to MIPAS-B vertical trace gas profiles shows a good to excellent agreement within the combined error limits for the species O3, N2O, CH4, H2O (above 21 km), HNO3, ClONO2, and CFC-11 (CCl3F) in the compared altitude range between 16 and 31 km such that these data appear to be very useful for scientific analysis. With regard to the previous version 1.4 ILAS-II data, significant improvements in the consistency with MIPAS-B are obvious especially for the species CH4 and H2O, but also for O3, HNO3, ClONO2, NO2, and N2O5. However, comparing gases like NO2, N2O5, and CFC-12 (CCl2F2) exhibits only poor agreement with MIPAS-B such that these species cannot be assumed to be validated at the present time.

Nitric acid (HNO3) is one of the key products that are operationally retrieved by the European Space Agency (ESA) from the emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard ENVISAT. The product version 4.61/4.62 for the observation period between July 2002 and March 2004 is validated by comparisons with a number of independent observations from ground-based stations, aircraft/balloon campaigns, and satellites. Individual HNO3 profiles of the ESA MIPAS level-2 product show good agreement with those of MIPAS-B and MIPAS-STR (the balloon and aircraft version of MIPAS, respectively), and the balloon-borne infrared spectrometers MkIV and SPIRALE, mostly matching the reference data within the combined instrument error bars. In most cases differences between the correlative measurement pairs are less than 1 ppbv (5–10%) throughout the entire altitude range up to about 38 km (~6 hPa), and below 0.5 ppbv (15–20% or more) above 30 km (~17 hPa). However, differences up to 4 ppbv compared to MkIV have been found at high latitudes in December 2002 in the presence of polar stratospheric clouds. The degree of consistency is further largely affected by the temporal and spatial coincidence, and differences of 2 ppbv may be observed between 22 and 26 km (~50 and 30 hPa) at high latitudes near the vortex boundary, due to large horizontal inhomogeneity of HNO3. Similar features are also observed in the mean differences of the MIPAS ESA HNO3 VMRs with respect to the ground-based FTIR measurements at five stations, aircraft-based SAFIRE-A and ASUR, and the balloon campaign IBEX. The mean relative differences between the MIPAS and FTIR HNO3 partial columns are within ±2%, comparable to the MIPAS systematic error of ~2%. For the vertical profiles, the biases between the MIPAS and FTIR data are generally below 10% in the altitudes of 10 to 30 km. The MIPAS and SAFIRE HNO3 data generally match within their total error bars for the mid and high latitude flights, despite the larger atmospheric inhomogeneities that characterize the measurement scenario at higher latitudes. The MIPAS and ASUR comparison reveals generally good agreements better than 10–13% at 20–34 km. The MIPAS and IBEX measurements agree reasonably well (mean relative differences within ±15%) between 17 and 32 km. Statistical comparisons of the MIPAS profiles correlated with those of Odin/SMR, ILAS-II, and ACE-FTS generally show good consistency. The mean differences averaged over individual latitude bands or all bands are within the combined instrument errors, and generally within 1, 0.5, and 0.3 ppbv between 10 and 40 km (~260 and 4.5 hPa) for Odin/SMR, ILAS-II, and ACE-FTS, respectively. The standard deviations of the differences are between 1 to 2 ppbv. The standard deviations for the satellite comparisons and for almost all other comparisons are generally larger than the estimated measurement uncertainty. This is associated with the temporal and spatial coincidence error and the horizontal smoothing error which are not taken into account in our error budget. Both errors become large when the spatial variability of the target molecule is high.

To examine the characteristics of polar stratospheric clouds (PSCs) in the Antarctic, we have analyzed short-time (≤ 5 days) changes in nitric acid (HNO3) and aerosol extinction coefficient (AEC) at 780 nm, focusing near 20 km altitude in June 2003 as observed by the Improved Limb Atmospheric Spectrometer (ILAS)-II. The Match technique based on the air parcel trajectory was applied to the ILAS-II data. The several Match pairs have revealed decreased HNO3 values with increased AEC values within short times, indicating “temporary” denitrification. It is also suggested that the observed PSCs could be nitric acid trihydrate (NAT) particles, considering that the temperatures were above existence temperatures for supercooled ternary solution, but below those for NAT. Given appropriate size distributions for NAT particles, it is suggested that the median radius of particles was less than 3 µm.

The temporal evolution of the volume mixing ratio (VMR) of chlorine nitrate (ClONO2) observed with the Improved Limb Atmospheric Spectrometer (ILAS) is described for the Arctic late winter and early spring of 1997. The temporal development of ClONO2 on the 475‐K isentropic surface during winter and spring is characterized by high variability in the VMR with seasonal enhancement to about 2 ppbv. In February, depleted values of ClONO2 were also observed; some of these low values are attributable to denitrification or to occurrence of polar stratospheric clouds. After mid‐March, when ClONO2 reached peak values, ClONO2 decreased and showed much less variability. Comparison of ClONO2 with HCl observed by the Halogen Occultation Experiment/Upper Atmospheric Research Satellite (HALOE/UARS) suggests a conversion of ClONO2 into HCl earlier at high altitudes than at lower altitudes. During the period a marked enhancement in NO2 was observed with a reduction in ClONO2 in the vortex, providing the first evidence from space of the NO2 time evolution in conjunction with ClONO2. Continuous measurements of ClONO2 through winter and spring over the Arctic are limited to date. The ILAS measurements reported in this paper will be useful for reanalyzing the seasonal variation of chlorine activation/deactivation processes in the Arctic lower stratosphere that control the degree of ozone destruction.