入江 仁士

The impact of open crop residual burning (OCRB) on O-3, CO, black carbon (BC) and organic carbon (OC) concentrations over Central Eastern China (CEC; 30-40 degrees N, 111-120 degrees E), during the Mount Tai Experiment in 2006 (MTX2006) was evaluated using a regional chemical transport model, the Models-3 Community Multiscale Air Quality Modeling System (CMAQ). To investigate these pollutants during MTX2006 in June 2006, daily gridded OCRB emissions were developed based on a bottom-up methodology using land cover and hotspot information from satellites. This model system involving daily emissions captured monthly-averages of observed concentrations and day-today variations in the patterns of O-3, CO, BC and OC at the summit of Mount Tai (36 degrees N, 117 degrees E, 1534 m a.s.l., Shan-dong Province of the People's Republic of China) with high correlation coefficients between the model and observations ranging from 0.55 to 0.69. These results were significantly improved from those using annual biomass burning emissions. For monthly-averaged O-3, the simulated concentration of 80.8 ppbv was close to the observed concentration (81.3 ppbv). The MTX2006 period was roughly divided into two parts: 1) polluted days with heavy OCRB in the first half of June; and 2) cleaner days with negligible field burning in the latter half of June. Additionally, the first half of June was characterized by two high-pollution episodes during 5-7 and 12-13 June, separated by a relatively cleaner intermediate period during 8-10 June. In the first high-pollution episode, the model captured the high O3, CO, BC and OC concentrations at the summit of Mount Tai, which were associated with OCRB over southern CEC and subsequent northward transport. For this episode, the impacts of OCRB emissions on pollutant concentrations were 26% (O-3), 62% (CO), 79% (BC) and 80% (OC) at the summit of Mount Tai. The daily OCRB emissions were an essential factor in the evaluation of these pollutants during MTX2006. These emissions have a large impact not only on primary pollutants but also on secondary pollutants, such as O-3, in the first half of June over northeastern Asia. The model reproduced reasonably well the variation of these pollutants in MTX2006, but underestimated daily averages of both CO and BC by a factor of 2, when using emission data from almost solely anthropogenic fuel sources in the latter half of the observation period when field burning can be neglected.

In June 2009, 22 spectrometers from 14 institutes measured tropospheric and stratospheric NO2 from the ground for more than 11 days during the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI), at Cabauw, NL (51.97 degrees N, 4.93 degrees E). All visible instruments used a common wavelength range and set of cross sections for the spectral analysis. Most of the instruments were of the multi-axis design with analysis by differential spectroscopy software (MAX-DOAS), whose non-zenith slant columns were compared by examining slopes of their least-squares straight line fits to mean values of a selection of instruments, after taking 30-min averages. Zenith slant columns near twilight were compared by fits to interpolated values of a reference instrument, then normalised by the mean of the slopes of the best instruments. For visible MAX-DOAS instruments, the means of the fitted slopes for NO2 and O-4 of all except one instrument were within 10% of unity at almost all non-zenith elevations, and most were within 5%. Values for UV MAX-DOAS instruments were almost as good, being 12% and 7%, respectively. For visible instruments at zenith near twilight, the means of the fitted slopes of all instruments were within 5% of unity. This level of agreement is as good as that of previous intercomparisons, despite the site not being ideal for zenith twilight measurements. It bodes well for the future of measurements of tropospheric NO2, as previous intercomparisons were only for zenith instruments focussing on stratospheric NO2, with their longer heritage.

Atmospheric aerosol profile observations using Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) have been conducted at Cape Hedo (26.87 degrees N, 128.25 degrees E), the northernmost point of Okinawa Island in Japan, since 30 March 2007. Comparisons of aerosol extinction at 476 nm by MAX-DOAS with ground-based lidar measurements for cloud-free conditions over more than 1 year showed good agreement on both seasonal and intraseasonal time scales, with differences of less than 30% on average for 0-1 km. Agreement between aerosol optical depths retrieved by MAX-DOAS and the sky radiometer was also observed during the same period, with differences of less than 30% in most cases. A cloud-screening method using MAX-DOAS data based on the physical properties of clouds was developed to evaluate aerosol variations, focused primarily on transboundary air pollution below low-level clouds, using relative humidity derived from MAX-DOAS H2O measurements and the MAX-DOAS color index, defined as the ratio of the intensities at 500 and 380 nm. This method consists of two steps: cloud screening in the troposphere using the color index and cloud-base height determination from the relative humidity. The former was consistent with lidar cloud screening at 0-6 km, and for the latter, a strong negative correlation between the lidar cloud-base height and the relative humidity was found. Using this unique cloud-screening method, we investigated aerosol variations at 0-1 km. A clear annual minimum was found in August-September, with low variability in relation to oceanic sources of clean air masses, whereas the maximum was found in November-May, with large variability in relation to continental sources of polluted air masses. This new cloud-screening method can be useful for evaluating aerosols below clouds, particularly during the northern winter at Cape Hedo, when and where transboundary air pollution from the Asian continent below low-level clouds frequently occurs in conjunction with strong westerly winds.

Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) has been utilized in recent years as a means of deriving vertical profiles of aerosol and trace gases; however, this new technique requires further validation because few studies have investigated its capability. In this study, vertical distributions of aerosol extinction coefficients (AECs) in the lower troposphere were retrieved by applying a recently developed aerosol-retrieval algorithm to O-4 slant column densities (SCDs) measured at a UV wavelength (356 nm) using the MAX-DOAS technique. The MAXDOAS measurements were conducted at the Korea Global Atmosphere Watch Observatory (KGAWO) located off the west coast of Korea during a period of seven cloudless days in May and June 2005. The AECs measured by UV MAX-DOAS varied from 0.05 to 0.73 km(-1) in the 0-1 km layer and from 0.01 to 0.20 km(-1) in the 1-2 km layer. The AECs for the 1-2 km layer from UV MAXDOAS are in agreement with lidar data within about 60%. Our results demonstrate the ability of MAX-DOAS as a remote sensing technique for surface aerosol measurements.

We present vertical profiles of the aerosol extinction coefficient retrieved from ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements at Tsukuba, Japan (36.1 degrees N, 140.1 degrees E), from November 2006 to March 2007. Retrievals utilizing absorption by the oxygen collision complex O-4 are first made at two wavelengths, 354 and 476 nm. A robust assessment of the MAX-DOAS aerosol data is then made using coincident lidar measurements throughout the period. Agreement between aerosol extinction coefficients measured by MAX-DOAS and the lidar tends to be better at the longer wavelength and at lower altitudes. At 476 nm, the best agreement, to within 30%, is found at altitudes of 0-1 km, confirming results from a literature assessment for a two-month measurement period. These findings are supported by comparisons between aerosol optical depths derived from MAX-DOAS and sky radiometer measurements and are further explained by differences in the altitude-dependent measurement sensitivity to the aerosol extinction coefficient between 354 and 476 nm. Thus, uncertainty in MAX-DOAS aerosol measurements is well quantified and characterized, providing a basis for quantitative studies using MAX-DOAS measurements.

In the period from June 2006 to December 2008, we measured the tropospheric nitrogen dioxide (NO2) column by ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) at an urban site in China (Tai'an) and three sites in Japan, covering urban (Yokosuka), suburban (Tsukuba), and remote areas (Hedo). This robust dataset is used to characterize Ozone Monitoring Instrument (OMI) tropospheric NO2 column data (the standard product, version 3). Correlations between MAX-DOAS and OMI data, both of which show very low NO2 at Hedo and moderate/high levels at the other sites, have correlation coefficients (R-2) as high as 0.64, indicating that relative changes in OMI NO2 data are reliable. However, OMI data have a negative bias of 31% on average. Assuming that these results are valid for OMI data taken over China, we find an increasing trend in tropospheric column NO2 at about 5% per year on average in the industrial areas of China (30 degrees-40 degrees N and 110 degrees-123 degrees E) over 2005. 2008, but its spatial distribution is highly inhomogeneous.

A recently developed aerosol retrieval algorithm based on O(4) slant column densities (SCDs) measured at a visible wavelength (476 nm) was utilized to derive aerosol information (e.g., aerosol optical depth (AOD) and vertical distributions of aerosol extinction coefficients (AECs)) in the lower troposphere during a severe Asian dust period. The MAX-DOAS measurements were carried out at Gwangju, Korea for nearly three months from February through May 2008. Comparison with AOD and surface PM(10), measured by collocated sunphotometer and beta gauge sampler, were made to validate the retrieved AODs and AECs in the atmospheric layer surface to 1 km height above ground. On the Asian dust days, temporal variations of the AODs retrieved from MAX-DOAS measurements show similar patterns, but with reduced magnitudes, to those measured by sunphotometer whereas similar AOD magnitude and temporal variation was observed between MAX-DOAS and sunphotometer measurements during the non-episodic days. Smaller correlation was observed between the surface PM(10) and AECs at 0.5 km during the Asian dust period compared to the correlation obtained for the non episodic days. This study demonstrates the ability of MAX-DOAS as a remote sensing technique for surface aerosol measurements under conditions of homogeneously distributed pollution in the planetary boundary layer. However, for the measurement of significantly enhanced aerosol loads with heterogeneous vertical distribution (e.g., Asian dust), measured AODs

Formaldehyde (HCHO), the most abundant carbonyl compound in the atmosphere, is generated as an intermediate product in the oxidation of nonmethane hydrocarbons. Proton transfer reaction mass spectrometry (PTR-MS) has the capability to detect HCHO from ion signals at m/z 31 with high time-resolution. However, the detection sensitivity is low compared to other detectable species, and is considerably affected by humidity, due to back reactions between protonated HCHO and water vapor prior to analysis. We performed a laboratory calibration of PTR-MS for HCHO and examined the detection sensitivity and humidity dependence at various field strengths. Subsequently, we deployed the PTR-MS instrument in a field campaign at Mount Tai in China in June 2006 to measure HCHO in various meteorological and photochemical conditions; we also conducted intercomparison measurements by Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). Correction of interference in the m/z 31 signals by fragments from proton transfer reactions with methyl hydroperoxide, methanol, and ethanol greatly improves agreement between the two methods, giving the correlation [HCHO](MAX-DOAS)=(0.99 +/- 0.16) [HCHO](PTR-MS)+(0.02 +/- 0.38), where error limits represent 95% confidence levels.

Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed at Tsukuba, Japan (36.1 degrees N, 140.1 degrees E), in November-December 2006. By analyzing the measured spectra of scattered sunlight with DOAS and optimal estimation methods, we first retrieve the aerosol optical depth (tau) and the vertical profile of the aerosol extinction coefficient (k) at 476 nm in the lower troposphere. These retrieved quantities are characterized through comparisons with coincident lidar and sky radiometer measurements. The retrieved k values for layers of 0-1 and 1-2 km agree with lidar data to within 30% and 60%, respectively, for most cases, including partly cloudy conditions. Results similar to k at 0-1 km are obtained for the retrieved tau values, demonstrating that MAX-DOAS provides a new, unique aerosol dataset in the lower troposphere.

A challenge for the quantitative analysis of tropospheric nitrogen dioxide (NO2) column data from satellite observations is posed partly by the lack of satellite-independent observations for validation. We performed such observations of the tropospheric NO2 column using the ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique in the North China Plain (NCP) from 29 May to 29 June, 2006. Comparisons between tropospheric NO2 columns measured by MAX-DOAS and the Ozone Monitoring Instrument (OMI) onboard the Aura satellite indicate that OMI data (the standard product, version 3) over NCP may have a positive bias of 1.6 x 10(15) molecules cm(-2) (20%), yet within the uncertainty of the OMI data. Combining these results with literature validation results for the US, Europe, and Pacific Ocean suggests that a bias of + 20%/-30% is a reasonable estimate, accounting for different regions.

The Improved Limb Atmospheric Spectrometer-II (ILAS-II) is a satellite-borne solar occultation sensor onboard the Advanced Earth Observing Satellite-II (ADEOS-II). The ILAS-II succeeded the ILAS. The ILAS-II used four grating spectrometers to observe vertical profiles of gas volume mixing ratios of trace constituents and was also equipped with a Sun-edge sensor to determine tangent heights geometrically with high precision. The accuracy of gas volume mixing ratios depends on the accuracy of the tangent height determination. The combination method is a tangent height registration method that was developed to give appropriate tangent heights for the ILAS-II Version 1.4 data retrieval algorithm. This study describes the method used in the ILAS-II Version 1.4 retrieval algorithm to register tangent heights. The root-sum-square total random error is estimated to be 30 m, and the total systematic error is 180 m at an altitude of 30 km. The influence of the tangent height errors on the vertical profiles of gas volume mixing ratios in ILAS-II Version 1.4 is estimated by using the relative difference. The relative difference for each species is within 7% (20%) for an altitude shift of +/-100 m (+/-300 m). (C) 2007 Optical Society of America.

[1] 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.

[1] Measurements of NOY condensation on cirrus particles found in stratospherically influenced air sampled during the SOLVE-I mission are analyzed and compared with data from other field studies of HNO3 or NOY condensation on ice. Each field study exhibits an order of magnitude data spread for constant HNO3 pressures and temperatures. While others assumed this distribution is due to random error, the data spread exceeds instrument precision errors and instead suggests HNO3 removal had not attained equilibrium at the time of sampling. During the SOLVE-I mission, condensation on ice was a significant sink for HNO3 despite submonolayer surface coverages; we therefore propose condensation of HNO3 on lower-stratospheric cirrus particles is controlled by kinetics and will occur at a kinetically limited rate. Furthermore, we suggest the low accommodation coefficient for HNO3 on ice combined with relatively short-lived clouds causes highly scattered, limited HNO3 uptake on cirrus particles. We couple laboratory data on the accommodation coefficient of HNO3 on ice with field surface coverage data in order to generate a "cloud clock'': a calculation to determine the age of a cloud parcel. Data from the aforementioned field studies are compared to theoretical models for equilibrium surface coverage on the basis of laboratory data extrapolated to atmospheric temperatures and HNO3 pressures. This comparison is difficult because most of the atmospheric data are probably not at equilibrium and follow a condensation time curve rather than an equilibrium surface coverage curve. Finally, we develop a simple mathematical solution for the time required for HNO3 condensation on ice.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the ENVISAT satellite provides profiles of temperature and various trace-gases from limb-viewing mid-infrared emission measurements. The stratospheric nitric acid (HNO3) from September 2002 to March 2004 was retrieved from the MIPAS observations using the science-oriented data processor developed at the Institut fur Meteorologie und Klimaforschung (IMK), which is complemented by the component of non-local thermodynamic equilibrium (non-LTE) treatment from the Instituto de Astrofisica de Andalucia (IAA). The IMK-IAA research product, different from the ESA operational product, is validated in this paper by comparison with a number of reference data sets. Individual HNO3 profiles of the IMK-IAA MIPAS show good agreement with those of the balloon-borne version of MIPAS (MIPAS-B) and the infrared spectrometer MkIV, with small differences of less than 0.5 ppbv throughout the entire altitude range up to about 38 km, and below 0.2 ppbv above 30 km. However, the degree of consistency is largely affected by their temporal and spatial coincidence, and differences of 1 to 2 ppbv may be observed between 22 and 26 km at high latitudes near the vortex boundary, due to large horizontal inhomogeneity of HNO3. Statistical comparisons of MIPAS IMK-IAA HNO3 VMRs with respect to those of satellite measurements of Odin/SMR, ILAS-II, ACE-FTS, as well as the MIPAS ESA product show good consistency. The mean differences are generally +/- 0.5 ppbv and standard deviations of the differences are of 0.5 to 1.5 ppbv. The maximum differences are 2.0 ppbv around 20 to 25 km. This gives confidence in the general reliability of MIPAS HNO3 VMR data and the other three satellite data sets.

The results of a comparison exercise of radiative transfer models (RTM) of various international research groups for Multiple AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) viewing geometry are presented. Besides the assessment of the agreement between the different models, a second focus of the comparison was the systematic investigation of the sensitivity of the MAX-DOAS technique under various viewing geometries and aerosol conditions. In contrast to previous comparison exercises, box-air-mass-factors (box-AMFs) for different atmospheric height layers were modelled, which describe the sensitivity of the measurements as a function of altitude. In addition, radiances were calculated allowing the identification of potential errors, which might be overlooked if only AMFs are compared. Accurate modelling of radiances is also a prerequisite for the correct interpretation of satellite observations, for which the received radiance can strongly vary across the large ground pixels, and might be also important for the retrieval of aerosol properties as a future application of MAX-DOAS. The comparison exercises included different wavelengths and atmospheric scenarios ( with and without aerosols). The strong and systematic influence of aerosol scattering indicates that from MAX-DOAS observations also information on atmospheric aerosols can be retrieved. During the various iterations of the exercises, the results from all models showed a substantial convergence, and the final data sets agreed for most cases within about 5%. Larger deviations were found for cases with low atmospheric optical depth, for which the photon path lengths along the line of sight of the instrument can become very large. The differences occurred between models including full spherical geometry and those using only plane parallel approximation indicating that the correct treatment of the Earth's sphericity becomes indispensable. The modelled box-AMFs constitute an universal data base for the calculation of arbitrary ( total) AMFs by simple convolution with a given trace gas concentration profile. Together with the modelled radiances and the specified settings for the various exercises, they can serve as test cases for future RTM developments.

This study assesses polar stratospheric nitrous oxide (N2O) and methane (CH4) data from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II) retrieved by the Version 1.4 retrieval algorithm. The data were measured between January and October 2003. Vertical profiles of ILAS-II volume mixing ratio (VMR) data are compared with data from two balloon-borne instruments, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) and the MkIV instrument, as well as with two satellite sensors, the Odin Sub-Millimetre Radiometer (SMR) for N2O and the Halogen Occultation Experiment (HALOE) for CH4. Relative percentage differences between the ILAS-II and balloon/satellite data and their median values are calculated in 10-ppbv-wide bins for N2O (from 0 to 400 ppbv) and in 0.05-ppmv-wide bins for CH4 (from 0 to 2 ppmv) in order to assess systematic differences between the ILAS-II and balloon/satellite data. According to this study, the characteristics of the ILAS-II Version 1.4 N2O and CH4 data differ between hemispheres. For ILAS-II N2O VMR larger than 250 ppbv, the ILAS-II N2O agrees with the balloon/SMR N2O within +/- 20% in both hemispheres. The ILAS-II N2O in the VMR range from 30-50 to 250 ppbv (corresponding to altitudes of similar to 17-30 km in the Northern Hemisphere (NH, mainly outside the polar vortex) and similar to 13-21 km in the Southern Hemisphere (SH, mainly inside the polar vortex) is smaller by similar to 10-30% than the balloon/SMR N2O. For ILAS-II N2O VMR smaller than 30 ppbv (>similar to 21 km) in the SH, the differences between the ILAS-II and SMR N2O are within +/- 10 ppbv. For ILAS-II CH4 VMR larger than 1 ppmv (<similar to 25 km), the ILAS-II CH4 agrees with the balloon/HALOE CH4 within +/- 5% in the NH. For ILAS-II CH4 VMR larger than 0.3 ppmv in SH, the ILAS-II CH4 is similar to 9% larger than the HALOE CH4; note that this positive systematic difference between the ILAS-II and HALOE CH4 has a seasonal dependence. Also note that the ILAS-II N2O for its VMR smaller than 50 ppbv (>similar to 30 km) and the ILAS-II CH4 for its VMR smaller than 1 ppmv (>similar to 25 km) only in the NH, are abnormally small compared to the balloon/satellite data.

We report the first continuous measurements of chlorine nitrate (ClONO2) in high-latitude regions taken by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) and processed using the latest data retrieval algorithm (version 6.1). Performance of the measurements, validation with three balloon-borne sensors, and seasonal variation of ClONO2 in the Arctic and Antarctic stratosphere are presented, as well as a brief description of the version 6.1 algorithm and data characteristics for both the Arctic and Antarctic. Although the ILAS-measured ClONO2 data show, on average, similar to 30% lower values than the validation data, they agree with validation data within the combined total error (similar to 20-40%) of the ClONO2 measurements at similar to 15- to 32-km altitudes. In the Arctic, enhancement of ClONO2 amounts was observed in spring 1997 after the appearance of polar stratospheric clouds (PSCs) inside the polar vortex. This is the result of preference for ClONO2 formation rather than HCl after the activation of ClOx in this Arctic spring of 1997. In the Antarctic, ClONO2 amounts showed strong local time/latitudinal dependence around the austral fall equinox in 1997.

The Improved Limb Atmospheric Spectrometer (ILAS)-II on board the Advanced Earth Observing Satellite (ADEOS)-II observed stratospheric aerosol in visible/near-infrared/infrared spectra over high latitudes in the Northern and Southern hemispheres, intermittently from January to March and continuously from April through October 2003. This study assesses the data quality of ILAS-II version 1.4 (V1.4) aerosol extinction coefficient at 780 nm. In the Northern Hemisphere (NH), aerosol extinction coefficient (AEC) from ILAS-II agreed with extinctions from SAGE II and SAGE III within +/- 10% and with extinction from POAM III within +/- 15% at heights below 20 km. From 20 to 26 km, ILAS-II AEC was smaller than extinctions from the other three sensors; differences between ILAS-II and SAGE II ranged from 10% at 20 km to 34% at 26 km in the NH. Over the Southern Hemisphere (SH), ILAS-II AEC from 20 to 25 km in February was 12-66% below SAGE II extinction. The difference increased with increasing altitude. Comparisons between ILAS-II and POAM III from January to May in the SH ("non-PSC season'') yielded qualitatively similar results. From June to October ("PSC season''), ILAS-II extinction was also smaller than POAM III extinction above 17 km; however, ILAS-II extinction agreed with POAM III extinction to within +/- 15% from 12 to 17 km during the PSC season. The comparisons indicate that in both hemispheres the ILAS-II V1.4 AEC is comparable to extinctions from other measurements below approximately 20 km and systematically low above approximately 20 km although the mean difference is as small as similar to 2x10(-5) km(-1) during the non-PSC season.

[ 1] Version 1.4 data from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) were validated through comparison with profile data measured by several instruments at Poker Flat (65.1 degrees N, 147.5 degrees W), Alaska. The height profiles of O-3 and HNO3 provided by ILAS-II were compared with those retrieved from spectra measured by a groundbased Fourier transform infrared spectrometer (FTS). The O-3 and HNO3 abundances measured by ILAS-II and FTS in the 17 - 35 km altitude range agreed within the precision of the two measurements. The O-3 volume mixing ratios measured by ILAS-II above ( below) 20 km were 10% lower ( higher) than those from the FTS measurements. The HNO3 values from ILAS-II and FTS agreed to within 10% above 17 km. The O-3 profiles obtained from electrochemical concentration cell (ECC) ozonesondes launched from Fairbanks (64.8 degrees N, 147.9 degrees W) were also compared to ILAS-II data. The ILAS-II ozone abundance agreed with the ozonesonde values. Comparison of mesospheric and upper stratospheric temperature data obtained by ILAS-II and Rayleigh lidar indicated that the temperature derived from ILAS-II was significantly higher ( lower) than that from lidar at about 60 (40) km.

[1] Uptake of nitric acid (HNO3) in Arctic cirrus ice crystals was observed on 11 February 2003 by in-situ instruments onboard the M55 Geophysica aircraft. The cirrus cloud with a mean ice water content of 5.4 mg m(-3) covered northern Scandinavia for several hours and extended up to the thermal tropopause at 12.3 km. Within the cirrus region, on average 9% of the total HNO3 measured as reactive nitrogen (NOy) is present in ice particles, increasing to 19% at temperatures below 205 K. In contrast to previous studies, we discuss the HNO3 uptake in ice in terms of HNO3/H2O molar ratios in ice crystals. The HNO3 content of the ice increases with increasing gas phase HNO3 concentrations and decreasing temperatures. Enhanced uptake of HNO3 in ice and heterogeneous chemistry on cold cirrus clouds may disturb the upper tropospheric ozone budget.

The Improved Limb Atmospheric Spectrometer-II (ILAS-II) was launched aboard the Advanced Earth Observing Satellite-II (ADEOS-II) in December 2002. Stratospheric vertical profiles of nitric acid (HNO3) concentration observed by ILAS-II (version 1.4) are validated using coincident HNO3 measurements by balloon-borne instruments (MIPAS-B2 and MkIV) in March and April 2003. Further validation is performed by making climatological comparisons of lower stratospheric HNO3-ozone (O-3) correlations obtained by ILAS-II and ILAS ( the predecessor of ILAS-II) for specific potential vorticity-based equivalent latitudes and seasons where and when ILAS data showed very compact correlations in 1997. The reduced scatter of ILAS-II HNO3 values around the reference HNO3, which is derived from ILAS-II O-3 using the ILAS HNO3-O-3 correlation, shows that the precision of the ILAS-II HNO3 data is better than 13-14%, 5%, and 1% at 15, 20, and 25 km, respectively. Combining all of the comparisons made in the present study, the accuracy of the ILAS-II HNO3 profiles at 15-25 km is estimated to be better than -13%/+26%.

[1] A solar occultation sensor, the Improved Limb Atmospheric Spectrometer (ILAS)-II, measured 5890 vertical profiles of ozone concentrations in the stratosphere and lower mesosphere and of other species from January to October 2003. The measurement latitude coverage was 54 - 71 degrees N and 64 - 88 degrees S, which is similar to the coverage of ILAS ( November 1996 to June 1997). One purpose of the ILAS-II measurements was to continue such high-latitude measurements of ozone and its related chemical species in order to help accurately determine their trends. The present paper assesses the quality of ozone data in the version 1.4 retrieval algorithm, through comparisons with results obtained from comprehensive ozonesonde measurements and four satellite-borne solar occultation sensors. In the Northern Hemisphere (NH), the ILAS-II ozone data agree with the other data within +/- 10% ( in terms of the absolute difference divided by its mean value) at altitudes between 11 and 40 km, with the median coincident ILAS-II profiles being systematically up to 10% higher below 20 km and up to 10% lower between 21 and 40 km after screening possible suspicious retrievals. Above 41 km, the negative bias between the NH ILAS-II ozone data and the other data increases with increasing altitude and reaches 30% at 61 - 65 km. In the Southern Hemisphere, the ILAS-II ozone data agree with the other data within +/- 10% in the altitude range of 11 - 60 km, with the median coincident profiles being on average up to 10% higher below 20 km and up to 10% lower above 20 km. Considering the accuracy of the other data used for this comparative study, the version 1.4 ozone data are suitably used for quantitative analyses in the high-latitude stratosphere in both the Northern and Southern Hemisphere and in the lower mesosphere in the Southern Hemisphere.

Long-term tropospheric nitrogen dioxide (NO2) column data obtained by the Global Ozone Monitoring Experiment (GOME) (G-NO2) are evaluated to confirm the trends found in tropospheric NO2 abundances over East Asia between 1996 and 2002. For three locations in Central and East Asia, the G-NO2 values are compared with tropospheric columns estimated from coincident observations of total NO2 by ground-based UV/visible spectrometers and stratospheric NO2 by satellite solar occultation sensors (E-NO2). The comparisons show a slight linear drift in G-NO2 data from 1996 to 2002. However, it is much smaller than the standard deviation of the differences between G-NO2 and E-NO2 and much smaller than the increasing trends in NO2 seen by GOME over the industrial areas of China, demonstrating the validity of the trends estimated using the GOME data.

Satellite observations of denitrification and ice clouds in the Arctic lower stratosphere in February 1997 are used with Lagrangian microphysical box model calculations to evaluate nucleation mechanisms of solid polar stratospheric cloud (PSC) particles. The occurrences of ice clouds are not correlated in time and space with the locations of back trajectories of denitrified air masses, indicating that ice particle surfaces are not always a prerequisite for the formation of solid PSCs that lead to denitrification. In contrast, the model calculations incorporating a pseudo-heterogeneous freezing process occurring at the vapor-liquid interface can quantitatively explain most of the observed denitrification when the nucleation activation free energy for nitric acid dihydrate formation is raised by only similar to10% relative to the current published values. Once nucleated, the conversion of nitric acid dihydrate to the stable trihydrate phase brings the computed levels of denitrification closer to the measurements.

[1] Measurements of CFC-12 were made by the Improved Limb Atmospheric Spectrometer (ILAS) between 57degreesN and 72degreesN in the Northern Hemisphere and between 64degreesS and 89degreesS in the Southern Hemisphere. ILAS was launched on 17 August 1996 on board the Advanced Earth Observing Satellite (ADEOS). The ILAS validation balloon campaigns were carried out from Kiruna, Sweden (68degreesN, 21degreesE), in February and March 1997 and from Fairbanks, Alaska (65degreesN, 148degreesW), in April and May 1997. During these validation balloon campaigns, CFC-12 was measured with the in situ instruments ASTRID, BONBON, and SAKURA and the remote sensing spectrometers MIPAS-B, FIRS-2, and MkIV. ILAS version 6.0 CFC-12 profiles obtained at the nearest location to the validation balloon measurement are compared with these validation balloon measurements. The quality of ILAS CFC-12 data processed with the version 6.0 algorithm improved significantly compared to previous versions. Low relative differences between ILAS CFC-12 and the correlative measurements of about 10% were found between 13 and 20 km. The comparison of vertical profiles shows that ILAS CFC-12 data are useful below about 20-22 km inside the vortex and below about 25 km outside the vortex. However, at greater altitudes the relative percentage difference increases very strongly with increasing altitude. Further, correlations of CFC-12 with N2O show a good agreement with the correlative measurements for N2O values of N2O > 150 ppbv. In summary, ILAS CFC-12 data are now suitable for scientific studies in the lower stratosphere.

Measurements of total reactive nitrogen (NOy) concentrations in the gas and aerosol phases were made on board the NASA DC-8 aircraft within 1 km above the Arctic tropopause in February 2000. As temperatures decreased, NOy was taken up into the background sulfate aerosols. The observed temperature-dependent NOy uptake agrees well with that calculated from a model of nitric acid (HNO3) uptake to form liquid HNO3/H2O/H2SO4 ternary droplets. The observations reported here provide the first direct evidence for the formation of ternary droplets in the Arctic tropopause region.

Measurements of HNO3 mixing ratios from the chemical ionization mass spectrometer have been critically compared with simultaneous measurements of total gas phase NOy from the NO chemiluminescence detector aboard the NASA DC-8 aircraft during the SAGE 3 Ozone Loss and Validation Experiment (SOLVE). The data were obtained in the arctic upper troposphere and lower stratosphere in the winter of 1999-2000. A brief comparison to the NOy instrument aboard the NASA ER-2 is also presented. The time responses, detection limits, relative precision, and stability of relative calibrations for the instruments were in excellent agreement throughout the mission. However, the average slope of the HNO3 to NOy correlation was 1.13 ± 0.03 overall and 1.06 ± 0.03 in stratospheric air, indicating that the two measurements had a systematic calibration offset. Possible sources for the offset error are presented, and methods to reduce the calibration error in future flights are suggested.

[1] Measurements of HNO3 mixing ratios from the chemical ionization mass spectrometer have been critically compared with simultaneous measurements of total gas phase NOy from the NO chemiluminescence detector aboard the NASA DC-8 aircraft during the SAGE 3 Ozone Loss and Validation Experiment (SOLVE). The data were obtained in the arctic upper troposphere and lower stratosphere in the winter of 1999-2000. A brief comparison to the NOy instrument aboard the NASA ER-2 is also presented. The time responses, detection limits, relative precision, and stability of relative calibrations for the instruments were in excellent agreement throughout the mission. However, the average slope of the HNO3 to NOy correlation was 1.13 +/- 0.03 overall and 1.06 +/- 0.03 in stratospheric air, indicating that the two measurements had a systematic calibration offset. Possible sources for the offset error are presented, and methods to reduce the calibration error in future flights are suggested.

In situ aircraft measurements Of O3, CO, total reactive nitrogen (NOy), NO, and nonmethane hydrocarbons (NMHCs) were made over the western Pacific Ocean and Australia during the Biomass Burning and Lightning Experiment (BIBLE) A and B conducted in August-October 1998 and 1999. Generally, similar features were seen in the BIBLE A and B data in the latitudinal variations of these species in the troposphere from 35°N to 28°S at longitudes of 120°-150°E. The focus of this paper is to describe the characteristics of air masses sampled at 15°N-10°S (tropical Pacific) and 10°S-28°S (over Australia). With the exception of occasional enhancements in reactive nitrogen seen over New Guinea associated with lightning activities, the tropical Pacific region is distinguished from the rest of the region by smaller concentrations of these trace species. This can be explained in terms of the absence of surface sources over the ocean, lack of stratospheric intrusion, and the preclusion of midlatitude air and air from the west due to active convection throughout the troposphere. The median O3, CO, NOy, and NO mixing ratios in tropical air above 4 km were about 15-20 parts per billion by volume (ppbv), 60-75 ppbv, 20-100 parts per trillion by volume (pptv), and 5-40 pptv, respectively. Data obtained from PEM-West A and B conducted in 1991 and 1994 showed similar latitudinal features, although the PEM-West A values were somewhat elevated due to dominating westerly winds in the lower troposphere associated with El Niño. Over Australia, the levels Of O3, CO, NOy, NO, and NMHCs were elevated throughout the troposphere over those observed in the tropical Pacific both in 1998 and 1999. The effect from biomass burning that occurred in northern Australia was limited to within the boundary layer because of strong subsidence in the period. Analyses based on 14-day back trajectories identified free tropospheric air over Australia that originated from Indonesia, the Indian Ocean, Africa, and southern midlatitudes. The levels Of O3, CO, NOy, and NMHCs in these air masses were much higher than those from the tropical Pacific due to their stronger sources from biomass burning and lightning. These values are compared with those obtained in the South Pacific during PEM-Tropics A. Effects of biomass burning and lightning are discussed as possible sources of O3 and its precursors in these air masses.

[1] The Improved Limb Atmospheric Spectrometer on board the Advanced Earth Observing Satellite first detected the onset of denitrification at 19 and 20 km in the 1996/1997 Arctic winter stratosphere. Box model calculations along back trajectories were used to estimate the degree of denitrification caused by the formation of nitric acid trihydrate (NAT) particles inside liquid aerosols. The loss of reactive nitrogen calculated assuming conversion of nitric acid dihydrate to NAT and direct NAT formation explains well the observed loss, demonstrating that such NAT particle formation mechanisms play a critical role in Arctic denitrification.

NOy ( total reactive nitrogen) contained in ice particles was measured on board the NASA DC-8 aircraft in the Arctic in January and March 2000. During some of the flights, the DC-8 encountered widespread cirrus clouds. Large quantities of ice particles were observed at 8-12 km and particulate NOy showed large increases. The data indicate that the amount of NOy covering the cirrus ice particles strongly depended on temperature. Similar measurements were made in the upper troposphere over the tropical Pacific Ocean in August-September 1998 and 1999. The data obtained in the Arctic and tropics show very limited uptake of NOy on ice at temperatures above 215 K.

[1] Tropospheric column amounts and mixing ratios of CO, C2H6, C2H2, and HCN were retrieved from ground-based infrared solar spectra using a vertical profile retrieval algorithm (SFIT2). The spectra were recorded with high spectral resolution Fourier transform infrared (FTIR) spectrometers at Moshiri (44.4degreesN) and Rikubetsu (43.5degreesN) in northern Japan from May 1995 to June 2000. The retrievals show significant seasonal variations in the tropospheric content of the four molecules over northern Japan with maxima in winter-spring (February-April) for CO, C2H6, and C2H2 and in summer (May-July) for HCN. Good correlations between CO, C2H6, and C2H2 indicated that they had similar sources and underwent similar dilution processes. Deviation of HCN relative to its seasonal mean value (DeltaHCN) is correlated with the similar deviation of CO (DeltaCO), indicating that enhancements of CO and HCN above the mean levels were probably due to the same sources. Linear trends in tropospheric CO, C2H6, and C2H2 from May 1995 to June 2000 (excluding 1998) were (-2.10 +/- 0. 30), (-2.53 +/- 0.30), and (-3.99 +/- 0.57)%/yr, respectively, while the trend of (-0.93 +/- 0. 49)%/yr in HCN was relatively small. Abnormally high tropospheric amounts of the four molecules were recorded in 1998. HCN amounts were found to be much higher than its seasonal mean value throughout 1998 with a 65% maximum increase in August 1998. Significant increases of CO, C2H6, and C2H2 took place in August-October 1998. Trajectory calculations, global fire maps, and satellite smoke images revealed that biomass burning in eastern Siberia from mid-July to early October 1998 was the major cause of the elevated levels in tropospheric CO, C2H6, C2H2, and HCN observed in northern Japan in 1998.

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 (similar to20-22 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.

[1] The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) measured nitrogen dioxide (NO2) and nitric acid (HNO3) profiles from November 1996 to June 1997 at high latitudes in both hemispheres. The ILAS NO2 profiles (version 5.20) are compared with those obtained by balloon-borne and satellite measurements to validate ILAS NO2 data. Comparisons with balloon-borne measurements indicate that ILAS NO2 at 25-30 km has a positive bias of 0.3-0.4 ppbv (6-11%). The random difference in NO2 at 25-30 km is 0.2-0.3 ppbv (3-9%). The random error in the ILAS NO2 measurements is larger than 100% below 20 km and above 45 km, where the NO2 mixing ratios were less than 1.0 ppbv. It is possible that ILAS NO2 values were lowered by optically thick aerosols with aerosol extinction coefficients at 780 nm of greater than 0.001 km(-1). The lack of diurnal correction along the line of sight contributes to the positive bias in the ILAS NO2 values below 25 km. Agreement of the ILAS NO2 values with those by the Polar Ozone and Aerosol Measurement (POAM) II instrument is within 10-30% at 25-35 km. The agreement with the Halogen Occultation Experiment (HALOE) is as good as +/-10% at 25-40 km. ILAS HNO3 (version 5.20) agrees with balloon-borne HNO3 to within 0.1 ppbv (0-1%), and the random difference is within 10% at 25-30 km.

The goal of this study is to show that trajectory hunting is an effective technique for comparison of multiplatform measurements. In order to achieve this goal, we (1) describe in detail the trajectory hunting technique (THT), (2) perform several consistency tests for THT (self-hunting and reversibility), (3) estimate uncertainties of this technique, and (4) validate THT results against those obtained by the traditional correlative analysis (TCA). THT launches backward and forward trajectories from the locations of measurements and finds air parcels sampled at least twice within a prescribed match criterion during the course of several days. TCA finds matched profiles for a chosen match criterion, averages them for each instrument separately, and compares the averaged profiles. As an example, we consider the 22 October to 30 November 1996 period in the Southern Hemisphere and compare the latest versions of relevant measurements made by the following five instruments: Microwave Limb Sounder (MLS, version 5 (v. 5)), Halogen Occultation Experiment (HALOE, v. 19), Polar Ozone and Aerosol Measurement II (POAM-II, v. 6), Stratospheric Aerosol and Gas Experiment II (SAGE-II, v. 6.1), and Improved Limb Atmospheric Spectrometer (ILAS, v. 5.20). We present results for O-3, H2O, CH4, HNO3, and NO2, which show that (1) ozone measurements from all five instruments agree to better than 0.4 (0.2) ppmv below (above) 30 km; (2) water vapor measurements agree within +/-5-10% above 22 km; (3) methane measurements by HALOE and ILAS agree to better than 10% above 30 km with a possible positive offset of up to 10-15% by ILAS in the lower stratosphere; (4) MLS HNO3 data corrected to account for some excited vibrational lines omitted in the v. 5 HNO3 retrieval agree with ILAS HNO3 measurements to within similar to0.5 ppbv (similar to10-20%) over the range similar to450-750 K; (5) ILAS sunset NO2 measurements are larger than both POAM-II and SAGE-II values by up to 10-15% below 30 km. The self-hunting tests show that the THT RMS noise is of the order of 1-2% for O-3, CH4, and H2O and 4% for NO2 and HNO3 measurements in the stratosphere. Total THT-related uncertainties may be 3-5% for O-3 measurements when photochemical effects and sensitivities of the results to duration of trajectories and match criteria are taken into account. Good agreement is found between the THT and TCA results for each of these products and for each possible pair of instruments, with considerably better statistics (typically by at least an order of magnitude) in the THT case. This agreement validates the THT results.

Vertical profiles of HNO3, N2O, O-3, and the aerosol extinction coefficient at 780 nm were observed by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) during the Arctic winter of 1996-1997. Irreversible redistribution of HNO3 is evaluated using HNO3-N2O and HNO3-O-3 correlations. Denitrification and nitrification started to be observed just after the Arctic vortex cooled to below the ice frost point (T-ICE) on February 10. Trajectory analyses show that denitrification occurred only in air masses, which were once cooled to near T-ICE and were kept at temperatures below the nitric acid trihydrate saturation threshold continuously for more than 4 days. Such a temperature history provides the necessary conditions for nucleation and growth of particles causing denitrification. The average extent of denitrification at 19 km reached 43% at the center of the vortex, suggesting that stratospheric ozone could be affected by denitrification. deep inside the vortex. Denitrification (>2 ppbv) and nitrification (>1 ppbv) covered 40 +/- 10% and 35 +/- 10% of the vortex area, respectively. Redistributed numbers of HNO3 molecules at each altitude were calculated by integrating the area-weighted changes in the HNO3 concentration. The decreases in total HNO3 concentration at 17-21 kin in late February and early March agreed with the increases at 12-15 kin to within 25%, confirming conservation of HNO3 during sedimentation and evaporation of HNO3-containing polar stratospheric cloud particles.

Ground-based infrared solar spectra were recorded at Rikubetsu (43.5 degrees N) and Moshiri (44.4 degrees N) in Japan using Fourier transform infrared (FTIR) spectrometers from 1995 to 1997. Total column amounts and tropospheric mixing ratios of HCN were derived from these spectra in the 3287.05-3287.40 cm(-1) micro-window using a nonlinear least squares spectral fitting method. The HCN values at these two locations showed significant seasonal variations. The HCN total column reached a maximum value of 6.57+/-0.84x10(15) molecules cm(-2) in summer (June-August) and a minimum value of 3.97+/-0.30x10(15) molecules cm-2 in winter (December-February). The maximum and minimum tropospheric HCN mixing ratios were 333+/-44 (summer) and 195+/-116 (winter) parts per trillion by volume (pptv), respectively. These seasonal variations suggest that the lifetime of HCN is shorter than one year. The enhancement of HCN above its seasonal mean (Delta HCN) was correlated with the enhancement of CO (Delta CO). A constant Delta HCN/Delta CO ratio suggests biomass burning as a source of HCN.

The Improved Limb Atmospheric Spectrometer (ILAS), a solar occultation infrared satellite sensor, was launched in August 1996. The ILAS validation balloon campaigns were carried out from Kiruna, Sweden (68 degrees N, 21 degrees E), in February and March 1997 and Fairbanks, Alaska (65 degrees N, 148 degrees W), in April and May 1997. During these campaigns, measurements of nitric acid (HNO3) were made using infrared emission spectrometers (Cold Atmospheric Emission Spectral Radiometer, Michelson Interferometer for Passive Atmospheric Sounding-Balloon-Borne version 2, and far-infrared spectrometer) and infrared solar occultation spectrometers (Limb Profile Monitor of the Atmosphere and Mark IV interferometer). An in situ experiment (Chemiluminescence Detector) measured total reactive nitrogen (NOy), from which HNO3 mixing ratios in the lower stratosphere were calculated. In addition, an in situ NO, measurement was also made at 12 km altitude from the Deutsche Luft-und Raumfahrt Falcon aircraft in January 1997. The ILAS version 3.10 HNO3 mixing ratios obtained at the nearest location and averaged ILAS mixing ratios obtained within certain criteria were compared with the balloon data. The precision of the ILAS measurements was estimated from the random differences to be 0.8 parts per billion by volume (ppbv), corresponding to about 35% at 15 km and 10-15% at 20-35 km. While the absolute accuracy estimated from the systematic differences was as good as 0.5 ppbv (5%) at 20 km, the ILAS HNO3 mixing ratios were systematically lower than the balloon values by 1 ppbv (15-20%) at 25-30 km. The error in the altitude registration in the ILAS retrieval algorithm is a possible cause for the negative bias at higher altitudes.

The concentrations of HNO(3), N(2)O, ozone, and aerosol in the lower stratosphere inside the Arctic vortex were observed by the Improved Limb Atmospheric Spectrometer (ILAS) in the winter of 1996-1997. These data demonstrate that irreversible loss of reactive nitrogen by sedimentation of HNO(3) containing particles (denitrification) at 18-23 km occurred in mid-late February soon after the Arctic vortex cooled below ice saturation temperature (T(ICE)) Denitrification exceeding 40% was observed only in air masses which experienced temperature below T(ICE) It occurred within 2 days in some of these air masses. Increases in HNO(3) by evaporation of the pal-rides (nitrification) at 13-15 km occurred 0-3 days after denitrification was observed, indicating particle radii of 5-10 mu m or larger. It is likely that these particles were composed of nitric acid trihydrate (NAT) or NAT-coated ice particles, given that the temperatures below 16 km were higher than T(ICE) Continued exposure of air masses below NAT saturation temperature for 1-4 days did not lead to any significant denitrification as long as the temperature did nor fall below T(ICE), indicating that possible nucleation of NAT at these temperatures within 4 days did not play a significant role in causing denitrification. There was little change in the average HNO(3) column from February 11 to 28 since HNO(3) decreases at 18-23 km were almost completely offset by increases at 12-17 km.

Simultaneous balloon-borne in situ measurements of total reactive nitrogen (NOy) and nitrous oxide (N2O) were made up to 29 km over Kiruna, Sweden (68 degrees N, 21 degrees E) on February 10 and 25, 1997. Kiruna was located at the edge or inside of the Arctic vortex at potential temperatures between 475 (similar to 19 km) and 675 K (similar to 26 km). Below 500 K (similar to 21 km) the N2O values were >120 ppbv on both days, and the observed NOy mixing ratios agreed well with those calculated using the NOy-N2O correlation previously obtained at northern midlatitudes. An exception was a sharp dip in NO at 445 K (18.4 km) observed on February 25. Back trajectory analyses indicate that this layer had experienced cold temperatures close to ice saturation, i.e., favorable conditions for denitrification. Between 500 and 600 K (similar to 24 km) the N2O values were <120 ppbv, and the observed NO values were some 4-6 ppbv lower than those calculated using the midlatitude NOy-N2O correlation, which includes the NOy reduction due to the N + NO reaction. The temperatures in the Arctic winter above 550 K were too high to cause extensive denitrification. The combined processes of (1) diabetic descent and (2) quasi-horizontal mixing of vortex air are likely causes of the anomalous NOy-N2O correlation. The CH4-N2O correlation obtained inside the Arctic vortex in February 1997 also supports this hypothesis. A similar anomalous NOy-N2O correlation was observed from the ER-2 measurements and from the atmospheric trace molecule spectroscopy ATLAS 2 measurements made inside the vortex in the winters of 1991-1992 and 1992-1993.