G. Wetzel

Denitrification observed inside the Arctic vortex in February 1995

Balloon-borne in situ measurements of total reactive nitrogen (NO,,) and nitrous oxide (N 2 O) were made from Kiruna (68°N, 21°E), Sweden on February 11, 1995. Ten hours later, N 2 O was again measured by an infrared spectrometer flown on another balloon launched from Kiruna. Both observations were made inside the polar vortex between 380 K (∼14 km) and at least 675 K (∼26 km). In the winter of 1994-1995, temperatures at 475 K (∼19 km) inside the vortex were extremely low, sometimes lower than ice frost point, especially from mid-December to mid-January. The NO y profiles obtained during both the ascent and descent revealed layered structures between 15 and 20 km with mixing ratios ranging from 2.7 to 9.3 parts per billion by volume (ppbv). The observed N 2 O profiles indicate significant downward transport of air due to diabatic cooling in the winter. To quantify the degree of irreversible removal of NO y (denitrification) between 12 and 28 km, the unperturbed values of NO y (i.e., NO * y ) were estimated from the observed N 2 O values using the NO y - N 2 O relationship obtained at midlatitudes by the atmospheric trace molecule spectroscopy ATLAS mission and in situ aircraft and balloon-borne measurements. The largest denitrification was observed at 19± 0.5 km, where the NO y values were lower than the NO * y values by ∼10 ppbv, corresponding to a 70% removal of NO y . In spite of the large uncertainty in NO * y the NO y values generally agreed well with the NO * y values at ∼14 km as well as between 3 and 28 km. The relationship between NO,, and N 2 O measured between 23 and 28 km agreed with that measured above 40 km at northern midlatitudes in fall, indicating that the air masses sampled at 23-28 km over Kiruna were transported from the midlatitude upper stratosphere followed by the descent inside the vortex.

NOy partitioning and budget and its correlation with N2O in the Arctic vortex and in summer midlatitudes in 1997

Vertical profiles of the most important species of nocturnal total reactive nitrogen (NO y = NO 2 + HNO 3 + CIONO 2 + 2 N 2 O 5 + HO 2 NO 2 ) together with its source gas N 2 O were retrieved from infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version (MIPAS-B) instrument inside the late winter arctic vortex from Kiruna (Sweden, 68°N) on 24 March 1997 and in summer midlatitudes from Gap (France, 44°N) on 2 July 1997. The measured data were compared to calculations performed with the three-dimensional chemistry transport model (CTM) Karlsruhe Simulation model of the Middle Atmosphere (KASIMA). The results show that in the late winter arctic vortex most of the available nitrogen and chlorine is in the form of HNO 3 and CIONO 2 , respectively. An anomalous N 2 O-NO y correlation observed in March 1997 appears to be caused to a large extent by quasi-horizontal mixing of air masses across the vortex edge. However, near 20 km some denitrification of ∼1.5 to 2 ppbv NO y could be observed. The N 2 O profile measured in July 1997 indicates remnants of polar vortex air and is not reproduced by the CTM at the same location. However, the profile shapes of the individual compounds of the NO y family as well as the NO x /NO y ratio are reproduced fairly well by the model.

Simultaneous measurements of HDO, H2O, and CH4 with MIPAS‐B: Hydrogen budget and indication of dehydration inside the polar vortex

For the first time, vertical profiles of HDO inside the Arctic vortex along with CH4 and H2O were retrieved from nighttime infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloonborne instrument (MIPAS-B) from Kiruna (Sweden, 68°N) on February 11, 1995 and March 24, 1997. The deuterium to hydrogen ratio (D/H) of water vapor shows a strong depletion in comparison to that of standard mean ocean water (SMOW), particularly in the lower stratosphere for the February 1995 flight. This extraordinarily strong depletion indicates an additional isotopic effect due to dehydration by polar stratospheric cloud particles. The maximum dehydration occurs at a lower altitude than that of the denitrification measured on the same flight. A dehydration of up to 0.7(±0.4) ppmv is seen in the compact correlation between CH4 and H2O. For the March 1997 results the exceptionally low D/H ratios and a deviation from the linear H2O-CH4 correlation could not be found. The H2O results of the February 1995 flight show a peak mixing ratio of 7.1 ppmv at 17.1 hPa and a minimum of 3.6 ppmv at 137.5 hPa. The analysis of the March flight shows a similar profile, but the vertical gradient is less pronounced. The total hydrogen budget of the stratosphere was examined by evaluating the quantity [H] = [H2O] + 2[CH4], revealing values of around 7.25 ppmv on average for both flights. All profiles reflect the subsidence of polar vortex air.

A comparison of Arctic HNO3 profiles measured by the Improved Limb Atmospheric Spectrometer and balloon‐borne sensors

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°N, 21°E), in February and March 1997 and Fairbanks, Alaska (65°N, 148°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 farinfrared 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 NOy, 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.

ClONO2 vertical profile and estimated mixing ratios of ClO and HOCl in winter Arctic stratosphere from Michelson interferometer for passive atmospheric sounding limb emission spectra

Nighttime limb emission spectra recorded by the balloon-borne Michelson interferometer for passive atmospheric sounding (MIPAS) on February 11, 1995, near Kiruna were used to infer a vertical profile of ClONO2 as well as estimates of ClO and HOCl volume mixing ratios. The highest ClONO2 mixing ratio (2.6 parts per billion by volume (ppbv)) was found at 22.69 km altitude and is explained by an early recovery of this chlorine reservoir in the upper part of the formerly chlorine-activated height range. Inferred nighttime ClO mixing ratios appear to be rather high for the lower stratosphere (0.38 ppbv at 16.43 km altitude) and indicate chlorine activation at this altitude region. The HOCl mixing ratio is estimated as 0.03 ppbv at 28.04 km altitude, while for lower altitudes the HOCl concentrations are clearly below the detection limit of MIPAS. The measurements are compared with three-dimensional chemical transport model calculations. Results agree reasonably well but show differences in detail.

Vertical profiles of N2O5, HO2NO2, and NO2 inside the Arctic vortex, retrieved from nocturnal MIPAS-B2 infrared limb emission measurements in February 1995

Vertical profiles of N 2 O 5 , HO 2 NO 2 , and NO 2 inside the arctic vortex were retrieved from nighttime infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version 2 (MIPAS-B2) instrument from Kiruna (Sweden, 68°N) on February 11, 1995, as part of the Second European Stratospheric Arctic and Midlatitude Experiment (SESAME). Spectra were analyzed by a multiparameter nonlinear least squares fitting procedure in combination with an onion-peeling retrieval algorithm. The N 2 O 5 , HO 2 NO 2 , and NO 2 results were derived from spectral features within the bands near 8.0 μm, 12.5 μm, and 6.2 μm, respectively. Peak mixing ratios of 1.14 parts per billion by volume (ppbv) N 2 O 5 and 80 parts per trillion by volume (pptv) HO 2 NO 2 at 17.1 hPa as well as 2.79 ppbv NO 2 at 12.0 hPa corresponding to 25.8 km and 28.0 km altitude were inferred from the spectra. NO 2 mixing ratios measured by MIPAS fit well to the data observed by concurrent flights. A comparison with calculations performed with a three-dimensional chemistry transport model for the time and location of the measurements shows that the best agreement of measured and calculated profiles is reached between 17 and 28 hPa corresponding to 25.8 and 22.7 km altitude, while below and above this altitude region there are some discrepancies between the modeled and observed data.

Validation of GOMOS‐Envisat vertical profiles of O3, NO2, NO3, and aerosol extinction using balloon‐borne instruments and analysis of the retrievals

The UV-visible Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument onboard Envisat performs nighttime measurements of ozone, NO 2 , NO 3 and of the aerosol extinction, using the stellar occultation method. We have conducted a validation exercise using various balloon-borne instruments in different geophysical conditions from 2002 to 2006, using GOMOS measurements performed with stars of different magnitudes. GOMOS and balloon-borne vertical columns in the middle stratosphere are in excellent agreement for ozone and NO 2 . Some discrepancies can appear between GOMOS and balloon-borne vertical profiles for the altitude and the amplitude of the concentration maximum. These discrepancies are randomly distributed, and no bias is detected. The accuracy of individual profiles in the middle stratosphere is 10 % for ozone and 25 % for NO 2 . On the other hand, the GOMOS NO 3 retrieval is difficult and no direct validation can be conducted. The GOMOS aerosol content is also well estimated, but the wavelength dependence can be better estimated if the aerosol retrieval is performed only in the visible domain. We can conclude that the GOMOS operational retrieval algorithm works well and that GOMOS has fully respected its primary objective for the study of the trends of species in the middle stratosphere, using the profiles in a statistical manner. Some individual profiles can be partly inaccurate, in particular in the lower stratosphere. Improvements could be obtained by reprocessing some GOMOS transmissions in case of specific studies in the middle and lower stratosphere when using the individual profiles.

Vertical profiles of N2O5 along with CH4, N2O, and H2O in the late Arctic winter retrieved from MIPAS-B infrared limb emission measurements

Vertical profiles of N2O5, CH4, N2O, and H2O inside the arctic vortex were retrieved from nighttime infrared limb emission spectra obtained during a flight of the Michelson interferometer for passive atmospheric sounding, balloonborne version (MIPAS-B) Fourier spectrometer from Kiruna (Sweden, 68°N) on March 14/15, 1992, as part of the European Arctic Stratospheric Ozone Experiment. Spectra were analyzed by a nonlinear multiparameter least squares fitting procedure in combination with an onion-peeling retrieval algorithm. The N2O5 results were derived from the intensity of the v12 band near 8 μm. These data represent the first ever reported N2O5 profile inside the polar vortex. Between 21.5 and 31.7 km altitude, N2O5 mixing ratios from 0.38 to 0.74 parts per billion by volume (ppbv) were inferred. Below 21.5 km there is a steep decrease in the mixing ratio toward values lower than 0.07 ppbv at 18.9 and 16.1 km. This discontinuity in the vertical profile correlates in altitude with the bulk of the Pinatubo aerosol layer inside the arctic vortex. N2O5 concentrations are calculated as a function of time since local sunset by using initial NO2 concentrations, O3 concentrations, aerosol surface area densities, and reaction rate coefficients, as found in the literature; calculated N2O5 concentrations are consistent with the MIPAS results. These suggest efficient heterogeneous hydrolysis of N2O5 having taken place on sulphate aerosol particles. Retrieved CH4 and N2O profiles reflect the subsided polar vortex air.

Validation of the Improved Limb Atmospheric Spectrometer‐II (ILAS‐II) Version 1.4 nitrous oxide and methane profiles

This study assesses polar stratospheric nitrous oxide (N(2)O) and methane (CH(4)) 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 N(2)O and the Halogen Occultation Experiment (HALOE) for CH(4). Relative percentage differences between the ILAS-II and balloon/satellite data and their median values are calculated in 10-ppbv-wide bins for N(2)O (from 0 to 400 ppbv) and in 0.05-ppmv-wide bins for CH(4) (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 N(2)O and CH(4) data differ between hemispheres. For ILAS-II N(2)O VMR larger than 250 ppbv, the ILAS-II N(2)O agrees with the balloon/SMR N(2)O within +/- 20% in both hemispheres. The ILAS-II N(2)O 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 N(2)O. For ILAS-II N(2)O VMR smaller than 30 ppbv (>similar to 21 km) in the SH, the differences between the ILAS-II and SMR N(2)O are within +/- 10 ppbv. For ILAS-II CH(4) VMR larger than 1 ppmv ( similar to 30 km) and the ILAS-II CH(4) for its VMR smaller than 1 ppmv (>similar to 25 km) only in the NH, are abnormally small compared to the balloon/satellite data.

REMOTE SENSING OF TRACE GASES IN THE MIDINFRARED SPECTRAL REGION FROM A NADIR VIEW

High-resolution IR remote-sensing measurements from space by means of a nadir-viewing geometry are particularly suited to the detection of trace gases and yield high temporal and horizontal resolutions on a global scale. To identify the potential of such a technique, an extensive feasibility study has been performed. The column amount of some trace gases, namely H(2)O, CH(4), N(2)O, CO, and O(3), may be determined with accuracies of approximately 10%. In addition, some information on the vertical distribution of these species is also possible. Concerning CFC-12, an accuracy of 10%-20% may be expected. Furthermore, it is believed that column amounts can be derived with an accuracy of 20% for HNO(3), and 50% for species like NO(2), OCS, and CFC-11.