Atmospheric Trace Molecule Spectroscopy Research Articles

NOy‐N2O correlation observed inside the Arctic vortex in February 1997: Dynamical and chemical effects

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°N, 21°) on February 10 and 25, 1997. Kiruna was located at the edge or inside of the Arctic vortex at potential temperatures between 475 (∼19 km) and 675 K (∼26 km). Below 500 K (∼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 NOy 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 (∼24 km) the N2O values were <120 ppbv, and the observed NOy 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) diabatic 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.

Polar stratospheric descent of NOy and CO and Arctic denitrification during winter 1992–1993

Observations inside the November 1994 Antarctic stratospheric vortex and inside the April 1993 remnant Arctic stratospheric vortex by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer are reported. In both instances, elevated volume mixing ratios (VMRs) of carbon monoxide (CO) were measured. A peak Antarctic CO VMR of 60 ppbv (where 1 ppbv = 10−9 per unit volume) was measured at a potential temperature (Θ) of 710 K (∼27 km), about 1 km below the altitude of a pocket of elevated NOy (total reactive nitrogen) at a deep minimum in N2O (<5 ppbv). The Arctic observations also show a region of elevated vortex CO with a peak VMR of 90 ppbv at 630–670 K (∼25 km) but no corresponding enhancement in NOy, perhaps because of stronger dynamical activity in the northern hemisphere polar winter and/or interannual variability in the production of mesospheric or lower thermospheric NO. By comparing vortex and extravortex observations of NOy obtained at the same N2O VMR, Arctic vortex denitrification of 5 ± 2 ppbv at 470 K (∼18 km) is inferred. We show that our conclusion of substantial Arctic winter 1992–1993 denitrification is robust by comparing our extravortex observations with previous polar measurements obtained over a wide range of winter conditions. Correlations of NOy with N2O measured at the same Θ by ATMOS in the Arctic vortex and at midlatitudes on board the ER‐2 aircraft several weeks later lie along the same mixing line. The result demonstrates the consistency of the two data sets and confirms that the ER‐2 sampled fragments of the denitrified Arctic vortex following its breakup. An analysis of the ATMOS Arctic measurements of total hydrogen shows no evidence for significant dehydration inside the vortex.

ATMOS/ATLAS 3 INFRARED PROFILE MEASUREMENTS OF CLOUDS IN THE TROPICAL AND SUBTROPICAL UPPER TROPOSPHERE

Vertical profiles of infrared cirrus extinction have been derived from tropical and subtropical upper tropospheric solar occultation spectra. The measurements were recorded by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer during the Atmospheric Laboratory for Applications and Sciences (ATLAS) 3 shuttle flight in November 1994. The presence of large numbers of small ice crystals is inferred from the appearance of broad extinction features in the 8–12 μm region. These features were observed near the tropopause and at lower altitudes. Vertical profiles of the ice extinction (km -1) in microwindows at 831, 957, and 1204 cm -1 have been retrieved from the spectra and analyzed with a model for randomly oriented spheroidal ice crystals. An area-equivalent spherical radius of 6 μm is estimated from the smallest ice crystals observed in the 8–12 μm region. Direct penetration of clouds into the lower stratosphere is inferred from observations of cloud extinction extending from the upper troposphere to 50 mbar (20 km altitude). Cloud extinction between 3 and 5 μm shows very little wavelength dependence, at least for the cases observed by the ATMOS instrument in the tropics and subtropics during ATLAS 3.

ATMOS/ATLAS 3 INFRARED PROFILE MEASUREMENTS OF TRACE GASES IN THE NOVEMBER 1994 TROPICAL AND SUBTROPICAL UPPER TROPOSPHERE

Vertical mixing ratio profiles of four relatively long-lives gases, HCN, C 2H 2, CO, and C 2H 6, have been retrieved from 0.01 cm -1 resolution infrared solar occultation spectra recorded between latitudes of 5.3°N and 31.4°N. The observations were obtained by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer during the Atmospheric Laboratory for Applications and Science (ATLAS) 3 shuttle flight, 3–12 November 1994. Elevated mixing ratios below the tropopause were measured for these gases during several of the occultations. The positive correlations obtained between the simultaneously measured mixing ratios suggest that the enhancements are likely the result of surface emissions, most likely biomass burning and/or urban industrial activities, followed by common injection via deep convective transport of the gases to the upper troposphere. The elevated levels of HCN may account for at least part of the “missing NO y ” in the upper troposphere. Comparisons of the observations with values measured during a recent aircraft campaign are presented.

Correlations of stratospheric abundances of NOy, O3, N2O, and CH4 derived from ATMOS measurements

Correlations are presented for [NOy] relative to [N2O] and [O3] derived from measurements from the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument from a wide range of altitudes and latitudes, including the tropics for which previous analyses have not extended above ∼20 km. Relationships for [O3] versus [N2O] are also given. The results are shown to be in good agreement with aircraft‐ and balloon‐based observations. Distinct correlations are observed for the tropics, the springtime polar vortex, and the extratropics‐extravortex regions. These correlations demonstrate rapid production of NOy and O3 in the tropical middle stratosphere and episodic export of air from this region to higher latitudes. Isolation of air within the developing polar vortices in the fall is also shown. Arctic vortex data from April 1993 appear to indicate denitrification of 25–30%, which is evident as a 3.0–4.5 ppb deficit in [NOy] when the vortex [NOy]:[N2O] correlation is compared with the extravortex correlation. A mixture of air descended from above 40 km with air from lower altitudes can fully account for this deficit in [NOy], in addition to approximately half of an apparent Arctic ozone loss of 50–60%, as inferred by comparison of the vortex and extravortex [O3]:[N2O] correlations. Comparison of Antarctic vortex and extravortex correlations from November 1994 similarly show a 60–80% deficit in [NOy] and 80–100% deficit in [O3]; at least half of this apparent denitrification and ozone loss can be attributed to mixing of air descended from higher altitudes with air from lower altitudes.

Correlations of stratospheric abundances of CH4 and N2O derived from ATMOS measurements

ATMOS measurements made over a wide range of altitudes and latitudes demonstrate compact correlations between mixing ratios of CH4 and N2O. Tight but distinct correlations are observed for the tropics, the springtime Arctic vortex, and the extra‐tropics/extra‐vortex regions, indicating dynamical isolation between these regions. Little variability is apparent in correlations between [CH4] and [N2O] from measurements made in different years (1992, 1993, and 1994), seasons (March/April and November), and hemispheres.

Denitrification observed inside the Arctic vortex in February 1995

Balloon‐borne in situ measurements of total reactive nitrogen (NOy) and nitrous oxide (N2O) were made from Kiruna (68°N, 21°E), Sweden on February 11, 1995. Ten hours later, N2O 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 NOy 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 N2O profiles indicate significant downward transport of air due to diabatic cooling in the winter. To quantify the degree of irreversible removal of NOy (denitrification) between 12 and 28 km, the unperturbed values of NOy (i.e., NOy*) were estimated from the observed N2O values using the NOy ‐ N2O 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 NOy values were lower than the values by ∼10 ppbv, corresponding to a 70% removal of NOy. In spite of the large uncertainty in the NOy values generally agreed well with the values at ∼14 km as well as between 23 and 28 km. The relationship between NOy and N2O 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.

Non local thermodynamic equilibrium (LTE) atmospheric limb emission at 4.6 μm: 2. An analysis of the daytime wideband radiances as measured by UARS improved stratospheric and mesospheric sounder

An analysis of the measurements taken by the improved stratospheric and mesospheric sounder (ISAMS) in its carbon monoxide wideband channel around 4.6 μm at daytime is presented. The radiances show a good signal to noise ratio up to the lower thermosphere (about 120 km) and have been shown to be mainly due to emission from the weak CO2 4.3 μm isotopic and hot bands. They exhibit a very clear dependence with the solar illumination at tangent heights above about 60 km, where they have been found to be almost exclusively determined by the solar elevation. Below about 50 km they are dominated by the variations of the kinetic temperature. The measurements have been analyzed in the 50–100 km range by using a detailed non local thermodynamic equilibrium (LTE) model of the CO2 states emitting in the 4.3 μm spectral region and the GENLN2 line‐by‐line radiance code. A large number (up to 32) of CO2 isotopic and hot bands emit significantly in this spectral region. The N2O(ν3 = 1) and two O3(ν1 + ν3) bands also give contributions in the stratosphere and lower mesosphere. The CO(1→0) band is of relative importance only in the lower thermosphere. The absolute radiances as well as the solar zenith angle dependence are well reproduced by the model. The dependence on the solar zenith angle is due to the absorption of solar radiation in the CO2 near‐infrared bands. A sensitivity study of the radiances was also conducted. The major conclusions are (1) the inclusion of the excitation of N2(1) from the electronic energy of O(1D) was required to explain the radiances in the lower mesosphere; (2) the value for the rate of the vibrational exchange between CO2(ν1,ν2,1) and N2(1) is very similar to the laboratory measurements and to that used in the analysis of the Spectral Infrared Rocket Experiment (SPIRE) 4.3 μm CO2 atmospheric limb radiances; and (3) the CO2 volume mixing ratio (vmr) in the 70–100 km region is significantly smaller than that measured in rocket experiments and similar to that deduced from the atmospheric trace molecule spectroscopy (ATMOS) absorption measurements. A reanalysis of the SAMS 4.3 μm limb radiance measurements has also been conducted with the same non‐LTE model and the GENLN2 line‐by‐line radiance code. The values for the parameters derived from the ISAMS data also explain the SAMS measurements very satisfactorily.

Model overestimates of NOy in the upper stratosphere

Stratospheric NOy profiles from the Garcia‐Solomon two‐dimensional model are compared to observed profiles from the Upper Atmosphere Research Satellite (UARS) and the Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS). The model consistently overestimates NOy in the upper stratosphere beyond the range of uncertainty in the satellite data. The relatively short lifetime of NOy in this region suggests a largely photochemical explanation for the discrepancy. Uncertainties in the photochemical production of NOy from N2O strongly influence stratospheric NOy profiles, but this influence decreases with increasing height in the upper stratosphere. Photochemical loss of NOy is controlled by the NO photolysis rate and the reaction rates of N + NO and N + O2. All these rates are subject to substantial uncertainties that probably contribute to the discrepancy between model and observations.

Heavy ozone enrichments from ATMOS infrared solar spectra

Vertical enrichment profiles of stratospheric 16O16Ol8O and 16O18O16O (hereafter referred to as 668O3 and 686O3 respectively) have been derived from space‐based solar occultation spectra recorded at 0.01 cm−1 resolution by the ATMOS (Atmospheric Trace MOlecule Spectroscopy) Fourier‐transform infrared (FTIR) spectrometer. The observations, made during the Spacelab 3 and ATLAS‐1, ‐2, and ‐3 shuttle missions, cover polar, mid‐latitude and tropical regions between 26 to 2.6 mb inclusive (≈ 25 to 41 km). Average enrichments, weighted by molecular 48O3 density, of (15±6)% were found for 668O3 and (10±7)% for 686O3. Defining the mixing ratio of 50O3 as the sum of those for 668O3 and 686O3, an enrichment of (13±5)% was found for 50O3 (1σ standard deviation). No latitudinal or vertical gradients were found outside this standard deviation. From a series of ground‐based measurements by the ATMOS instrument at Table Mountain, California (34.4°N), an average total column 668O3 enrichment of (17±4)% (1σ standard deviation) was determined, with no significant seasonal variation discernable. Possible biases in the spectral intensities that affect the determination of absolute enrichments are discussed.

Stratospheric NO and NO2 abundances from ATMOS Solar‐Occultation Measurements

Using results from a time‐dependent photochemical model to calculate the diurnal variation of NO and NO2, we have corrected Atmospheric Trace MOlecule Spectroscopy (ATMOS) solar‐occultation retrievals of the NO and NO2 abundances at 90° solar zenith angle. Neglecting to adjust for the rapid variation of these gases across the terminator results in potential errors in retrieved profiles of ∼20% for NO2 and greater than 100% for NO at altitudes below 25 km. Sensitivity analysis indicates that knowledge of the local O3 and temperature profiles, rather than zonal mean or climatological conditions of these quantities, is required to obtain reliable retrievals of NO and NO2 in the lower stratosphere. Extremely inaccurate O3 or temperature values at 20 km can result in 50% errors in retrieved NO or NO2. Mixing ratios of NO in the mid‐latitude, lower stratosphere measured by ATMOS during the November 1994 ATLAS‐3 mission compare favorably with in situ ER‐2 observations, providing strong corroboration of the reliability of the adjusted space‐borne measurements.

The 1994 northern midlatitude budget of stratospheric chlorine derived from ATMOS/ATLAS‐3 observations

Volume mixing ratio (VMR) profiles of the chlorine‐bearing gases HCl, ClONO2, CCl3F, CCl2F2, CHClF2, CCl4, and CH3Cl have been measured between 3 and 49° northern‐ and 65 to 72° southern latitudes with the Atmospheric Trace MOlecule Spectroscopy (ATMOS) instrument during the ATmospheric Laboratory for Applications and Science (ATLAS)‐3 shuttle mission of 3 to 12 November 1994. A subset of these profiles obtained between 20 and 49°N at sunset, combined with ClO profiles measured by the Millimeter‐wave Atmospheric Sounder (MAS) also from aboard ATLAS‐3, measurements by balloon for HOCl, CH3CCl3 and C2Cl3F3, and model calculations for COClF indicates that the mean burden of chlorine, ClTOT, was equal to (3.53±0.10) ppbv (parts per billion by volume), 1‐sigma, throughout the stratosphere at the time of the ATLAS 3 mission. This is some 37% larger than the mean 2.58 ppbv value measured by ATMOS within the same latitude zone during the Spacelab 3 flight of 29 April to 6 May 1985, consitent with an exponential growth rate of the chlorine loading in the stratosphere equal to 3.3%/yr or a linear increase of 0.10 ppbv/yr over the Spring‐1985 to Fall‐1994 time period. These findings are in agreement with both the burden and increase of the main anthropogenic Cl‐bearing source gases at the surface during the 1980s, confirming that the stratospheric chlorine loading is primarily of anthropogenic origin.

ATMOS measurements of H2O+2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: Dehydration and denitrification in the vortex

Simultaneous stratospheric volume mixing ratios (VMRs) measured inside and outside the Antarctic vortex by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument in November 1994 reveal previously unobserved features in the distributions of total reactive nitrogen (NOy) and total hydrogen (H2O+2CH4). Maximum removal of NOydue to sedimentation of polar stratospheric clouds (PSCs) inside the vortex occurred at a potential temperature (Θ) of 500–525 K (∼20 km), where values were 5 times smaller than measurements outside. Maximum loss of H2O+2CH4due to PSCs occurred in the vortex at 425–450 K, ∼3 km lower than the peak NOyloss. At that level, H2O+2CH4 VMRs inside the vortex were ∼70% of corresponding values outside. The Antarctic and April 1993 Arctic measurements by ATMOS show no significant differences in H2O+2CH4 VMRs outside the vortices in the two hemispheres. Elevated NOy VMRs were measured inside the vortex near 700 K. Recent model calculations indicate that this feature results from downward transport of elevated NOy produced in the thermosphere and mesosphere.

The hydrogen budget of the stratosphere inferred from ATMOS measurements of H2O and CH4

The total hydrogen budget of the stratosphere and lower mesosphere has been examined using vertical mixing ratio profiles of H2O and CH4 measured by the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment from four space shuttle missions. The oxidation of CH4 and H2 is investigated by evaluating the quantity H (=H2O + 2 CH4) entering the stratosphere, and examining its conservation with altitude in the upper atmosphere. Data from all four ATMOS missions indicate H to be nearly conserved in the lower stratosphere and to exhibit a broad maximum in the 35‐ to 65‐km range. The observations provide evidence of a secondary source of H2O from H2 oxidation at altitudes from 35 to 55 km, and net production of H2 at altitudes above ∼55 km. ATMOS measurements of H2O and CH4 permit the first evaluation of a sickle‐shaped vertical profile of H2 that is qualitatively consistent with profiles calculated using two‐dimensional models.

NOy correlation with N2O and CH4 in the midlatitude stratosphere

Total reactive nitrogen (NOy), nitrous oxide (N2O), methane (CH4), and ozone (O3) were measured on board a balloon launched from Aire sur l'Adour (44°N, 0°W), France on October 12, 1994. Generally, NOy was highly anti‐correlated with N2O and CH4 at altitudes between 15 and 32 km. The linear NOy‐N2O and NOy‐CH4 relationships obtained by the present observations are very similar to those obtained on board ER‐2 and DC‐8 aircraft previously at altitude below 20 km in the northern hemisphere. They also agree well with the data obtained by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument at 41°N in November 1994. Slight departures from linear correlations occurred around 29 km, where N2O and CH4 mixing ratios were larger than typical midlatitude values, suggesting horizontal transport of tropical airmasses to northern midlatitudes in a confined altitude region.

Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS‐3 measurements of H2O and CH4

Stratospheric measurements of H2O and CH4 by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer on the ATLAS‐3 shuttle flight in November 1994 have been examined to investigate the altitude and geographic variability of H2O and the quantity H = (H2O + 2CH4) in the tropics and at mid‐latitudes (8 to 49°N) in the northern hemisphere. The measurements indicate an average value of 7.24±0.44 ppmv for H between altitudes of about 18 to 35 km, corresponding to an annual average water vapor mixing ratio of 3.85±0.29 ppmv entering the stratosphere. The H2O vertical distribution in the tropics exhibits a wave‐like structure in the 16‐ to 25‐km altitude range, suggestive of seasonal variations in the water vapor transported from the troposphere to the stratosphere. The hygropause appears to be nearly coincident with the tropopause at the time of observations. This is consistent with the phase of the seasonal cycle of H2O in the lower stratosphere, since the ATMOS observations were made in November when the H2O content of air injected into the stratosphere from the troposphere is decreasing from its seasonal peak in July–August.

ATMOS/ATLAS‐3 observations of long‐lived tracers and descent in the Antarctic Vortex in November 1994

Observations of the long‐lived tracers N2O, CH4 and HF obtained by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument in early November 1994 are used to estimate average descent rates during winter in the Antarctic polar vortex of 0.5 to 1.5 km/month in the lower stratosphere, and 2.5 to 3.5 km/month in the middle and upper stratosphere. Descent rates inferred from ATMOS tracer observations agree well with theoretical estimates obtained using radiative heating calculations. Air of mesospheric origin (N2O < 5 ppbV) was observed at altitudes above about 25 km within the vortex. Strong horizontal gradients of tracer mixing ratios, the presence of mesospheric air in the vortex in early spring, and the variation with altitude of inferred descent rates indicate that the Antarctic vortex is highly isolated from midlatitudes throughout the winter from approximately 20 km to the stratopause. The 1994 Antarctic vortex remained well isolated between 20 and 30 km through at least mid‐November.

Trace gas transport in the Arctic Vortex inferred from ATMOS ATLAS‐2 observations during April 1993

Measurements of the long‐lived tracers CH4, N2O, and HF from the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument during the Atmospheric Laboratory for Science and Applications‐2 (ATLAS‐2) Space Shuttle mission in April 1993 are used to infer average winter descent rates ranging from 0.8 km/month at 20 km to 3.2 km/month at 40 km in the Arctic polar vortex during the 1992–93 winter. Descent rates in the mid‐stratosphere are similar to those deduced for the Antarctic vortex using ATMOS/ATLAS‐3 measurements in November 1994, but the shorter time period of descent in the Arctic leads to smaller total distances of descent. Strong horizontal gradients observed along the vortex edge indicate that the Arctic vortex remains a significant barrier to transport at least until mid‐April in the lower to middle stratosphere.

On the assessment and uncertainty of atmospheric trace gas burden measurements with high resolution infrared solar occultation spectra from space by the ATMOS Experiment

The Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument is a high resolution Fourier transform spectrometer that measures atmospheric composition from low Earth orbit with infrared solar occultation sounding in the limb geometry. Following an initial flight in 1985, ATMOS participated in the Atmospheric Laboratory for Applications and Science (ATLAS) 1, 2, and 3 Space Shuttle missions in 1992, 1993, and 1994 yielding a total of 440 occultation measurements over a nine year period. The suite of more than thirty atmospheric trace gases profiled includes CO2, O3, N2O, CH4, H2O, NO, NO2, HNO3, HCl, HF, ClONO2, CCl3F, CCl2F2, CHF2Cl, and N2O5. The analysis method has been revised throughout the mission years culminating in the ‘version 2’ data set. The spectroscopic error analysis is described in the context of supporting the precision estimates reported with the profiles; in addition, systematic uncertainties assessed from the quality of the spectroscopic database are described and tabulated for comparisons with other experiments.

Increase of stratospheric carbon tetrafluoride (CF4) based on ATMOS observations from space

Stratospheric volume mixing ratio profiles of carbon tetrafluoride, CF4, obtained with the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument during the ATLAS (Atmospheric Laboratory for Applications and Science) −3 mission of 1994 are reported. Overall the profiles are nearly constant over the altitude range 20 to 50 km, indicative of the very long lifetime of CF4 in the atmosphere. In comparison to the stratospheric values of CF4 inferred from the ATMOS/Spacelab 3 mission of 1985, the 1994 concentrations are consistent with an exponential increase of (1.6±0.6) %/yr. This increase is discussed with regard to previous results and likely sources of CF4 at the ground. Further, it is shown that simultaneous measurements of N2O and CF4 provide a means of constraining the lower limit of the atmospheric lifetime of CF4 at least 2,300 years, two sigma.