Airborne Submillimeter Radiometer Research Articles

Constraints for the photolysis rate and the equilibrium constant of ClO‐dimer from airborne and balloon‐borne measurements of chlorine compounds

AbstractWe analyze measurements of ClO across the terminator taken by the Airborne Submillimeter Radiometer (ASUR) in the activated vortices of the Arctic winters of 1995/1996, 1996/1997, and 1999/2000 to evaluate the plausibility of various determinations of the ClO‐dimer photolysis cross section and the rate constant controlling the thermal equilibrium between ClO‐dimer and ClO. We use measured ClO during sunlit conditions to estimate total active chlorine (ClOx). As the measurements suggest nearly full chlorine activation in winter 1999/2000, we compare ClOx estimates based on various photolysis frequencies of ClO‐dimer with total available inorganic chlorine (Cly), estimated from an N2O‐Cly correlation established by a balloon‐borne MkIV interferometer measurement. Only ClO‐dimer cross sections leading to the fastest photolysis frequencies in the literature (including the latest evaluation by the Jet Propulsion Laboratory) give ClOx mixing ratios that overlap with the estimated range of available Cly. Slower photolysis rates lead to ClOx values that are higher than available Cly. We use the ClOx calculated from sunlit ClO measurements to estimate ClO in darkness based on different equilibrium constants, and compare it with ASUR ClO measurements before sunrise at high solar zenith angles. Calculations with equilibrium constants published in recent evaluations of the Jet Propulsion Laboratory give good agreement with observed ClO mixing ratios. Equilibrium constants leading to a higher ClO/ClOx ratio in darkness yield ClO values that tend to exceed observed abundances. Perturbing the rates for the ClO + BrO reaction in a manner that increases OClO formation and decreases BrCl formation leads to lower ClO values calculated for twilight conditions after sunset, resulting in better agreement with ASUR measurements.

Modelling the effect of denitrification on polar ozone depletion for Arctic winter 2004/2005

Abstract. A three-dimensional (3-D) chemical transport model (CTM), SLIMCAT, has been used to quantify the effect of denitrification on ozone loss for the Arctic winter 2004/2005. The simulated HNO3 is found to be highly sensitive to the polar stratospheric cloud (PSC) scheme used in the model. Here the standard SLIMCAT full chemistry model, which uses a thermodynamic equilibrium PSC scheme, overpredicts the ozone loss for Arctic winter 2004/2005 due to the overestimation of denitrification and stronger chlorine activation than observed. A model run with a coupled detailed microphysical denitrification scheme, DLAPSE (Denitrification by Lagrangian Particle Sedimentation), is less denitrified than the standard model run and better reproduces the observed HNO3 as measured by Airborne SUbmillimeter Radiometer (ASUR) and Aura Microwave Limb Sounder (MLS) instruments. Overall, denitrification is responsible for a ~30 % enhancement in O3 depletion compared with simulations without denitrification for Arctic winter 2004/2005, which is slightly larger than the inferred impact of denitrification on Arctic ozone loss for previous winters from different CTMs simulations. The overestimated denitrification from standard SLIMCAT simulation causes ~5–10 % more ozone loss at ~17 km compared with the simulation using the DLAPSE PSC scheme for Arctic winter 2004/2005. The calculated partial column ozone loss from SLIMCAT using the DLAPSE scheme is about 130 DU by mid-March 2005, which compares well with the inferred column ozone loss from ozonesondes and satellite data (127±21 DU).

Aircraft measurements and model simulations of stratospheric ozone and N2O: implications for chemistry and transport processes in the models

Airborne measurements of stratospheric ozone and N2O from the SCIAMACHY (Scanning Imaging Absorption Spectrometer) Validation and Utilization Experiment (SCIA-VALUE) are presented. The campaign was conducted in September 2002 and February–March 2003. The Airborne Submillimeter Radiometer (ASUR) observed stratospheric constituents like O3 and N2O, among others, spanning a latitude from 5°S to 80°N during the survey. The tropical ozone source regions show high ozone volume mixing ratios (VMRs) of around 11 ppmv at 33 km altitude, and the altitude of the maximum VMR increases from the tropics to the Arctic. The N2O VMRs show the largest value of 325 ppbv in the lower stratosphere, indicating their tropospheric origin, and they decrease with increasing altitude and latitude due to photolysis. The sub-tropical and polar mixing barriers are well represented in the N2O measurements. The most striking seasonal difference found in the measurements is the large polar descent in February–March. The observed features are interpreted with the help of SLIMCAT and Bremen Chemical Transport Model (CTMB) simulations. The SLIMCAT simulations are in good agreement with the measured O3 and N2O values, where the differences are within 1 ppmv for O3 and 15 ppbv for N2O. However, the CTMB simulations underestimate the tropical middle stratospheric O3 (1–1.5 ppmv) and the tropical lower stratospheric N2O (15–30 ppbv) measurements. A detailed analysis with various measurements and model simulations suggests that the biases in the CTMB simulations are related to its parameterised chemistry schemes.

Intercomparison of ozone profile measurements from ASUR, SCIAMACHY, MIPAS, OSIRIS, and SMR

The airborne submillimeter radiometer (ASUR) was deployed onboard the Falcon research aircraft during the scanning imaging absorption spectrometer for atmospheric cartography (SCIAMACHY) validation and utilization experiment (SCIAVALUE) and the European polar stratospheric cloud and lee wave experiment (EuPLEx) campaigns. A large number of ozone profile measurements were performed over a latitude band spanning from 5°S to 80°N in September 2002 and February/March 2003 during the SCIAVALUE and around the northern polar latitudes in January/February 2003 during the EuPLEx. Both missions amassed an ample microwave ozone profile data set that is used to make quantitative comparisons with satellite measurements in order to assess the quality of the satellite retrievals. In this paper, the ASUR ozone profile measurements are compared with measurements from SCIAMACHY and Michelson interferometer for passive atmospheric sounding (MIPAS) on Environmental Satellite and optical spectrograph and infrared imager system (OSIRIS) and submillimeter radiometer (SMR) on the Odin satellite. The cross comparisons with the criterion that the ASUR measurements are performed within ±1000 km and ±6 hrs of the satellite observations show a good agreement with all the four satellite sensors. The differences in data values are the following: −4 to +8% for ASUR‐SCIAMACHY (operational product, v2.1), within ±15% for ASUR‐SCIAMACHY (scientific product, v1.62), up to +6% for ASUR‐MIPAS (operational product v4.61) and ASUR‐MIPAS (scientific product v1‐O3‐1), up to 17% for ASUR‐OSIRIS (v012), and −6 to 17% for ASUR‐SMR (v222) between the 20‐ and 40‐km altitude range depending on latitude. Thus, the intercomparisons provide important quantitative information about the quality of the satellite ozone profiles, which has to be considered when using the data for scientific analyses.

Denitrification in the Arctic mid‐winter 2004/2005 observed by airborne submillimeter radiometry

We present measurements of unusually low mixing ratios of HNO3 in the exceptionally cold Arctic vortex of late‐January and early‐February 2005. The measurements were obtained by the airborne submillimeter radiometer ASUR during the polar aura validation experiment (PAVE). The distribution of HNO3 inside the vortex reaches minima below 4 ppbv around 22 km altitude and maxima above 13 ppbv around 16 km altitude, with a considerable spatial variability. We estimate a vortex averaged denitrification of 3.1 ± 0.8 ppbv around 20 km altitude, and slight renitrification below ∼15.5 km altitude. The observed HNO3 deficit is largest (∼6 ppbv) near the center of the vortex, where the air masses had experienced temperatures below the NAT formation threshold for 80–100% of the previous 20 days according to back trajectories. This suggests that the main denitrification mechanism is based on sedimenting nitric acid trihydrate particles.

Expected performance of the Superconducting Submillimeter‐Wave Limb Emission Sounder compared with aircraft data

The simulated retrieval performance of a submillimeter wave limb sounder was compared with that of an up‐looking instrument with identical observation frequency bands and comparable noise temperature. The frequency bands were 624.32–626.32 and 649.12–650.32 GHz, and the retrieval simulations focused on the key trace gas species O3, HCl, and ClO. As expected, the limb geometry leads to a better altitude resolution and larger measurement altitude range. The same retrieval setup was applied to measured spectra, taken by the up‐looking Airborne Submillimeter Radiometer (ASUR) instrument on 4 September 2002 at 19.11°E, 71.90°N and on 19 September 2002 at 44.10°E, 4.10°S. The observed structures in the fit residual near the HCl spectral lines at 625.9 GHz lead to the conclusion that the pressure shift parameter of HCl is likely to be higher than the value in the HITRAN spectroscopic database. Depending on the assumed temperature dependence of the shift, the HCl pressure shift value consistent with the ASUR data is 0.090–0.117 MHz/hPa instead of the 0.030 MHz/hPa reported in HITRAN. This result is in good agreement with very recent independent laboratory work which suggests a value of 0.110 MHz/hPa for the shift.

Rapid meridional transport of tropical airmasses to the Arctic during the major stratospheric warming in January 2003

Abstract. We present observations of unusually high values of ozone and N2O in the middle stratosphere that were observed by the airborne submillimeter radiometer ASUR in the Arctic. The observations took place in the meteorological situation of a major stratospheric warming that occurred in mid-January 2003 and was dominated by a wave 2 event. On 23 January 2003 the observed N2O and O3 mixing ratios around 69° N in the middle stratosphere reached maximum values of ~190 ppb and ~10 ppm, respectively. The similarities of these N2O profiles in a potential temperature range between 800 and 1200 K with N2O observations around 20° N on 1 March 2003 by the same instrument suggest that the observed Arctic airmasses were transported from the tropics quasi-isentropically. This is confirmed by 5-day back trajectory calculations which indicate that the airmasses between about 800 and 1000 K had been located around 20° N 3–5 days prior to the measurement in the Arctic. Calculations with a linearized ozone chemistry model along calculated as well as idealized trajectories, initialized with the low-latitude ASUR ozone measurements, give reasonable agreement with the Arctic ozone measurement by ASUR. PV distributions suggest that these airmasses did not stay confined in the Arctic region which makes it unlikely that this dynamical situation lead to the formation of dynamically caused pockets of low ozone.

Ozone depletion observed by the Airborne Submillimeter Radiometer (ASUR) during the Arctic winter 1999/2000

In the winter 1999/2000 the Airborne Submillimeter Radiometer (ASUR) participated in the Stratospheric Aerosol and Gas Experiment III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone project on board the NASA research aircraft DC‐8. During three deployments in early December 1999, late January, and early March 2000, the ASUR instrument took various measurements of ozone and key species related to stratospheric ozone chemistry. After the sunlight reached the vortex region in January 2000 peak values of about 1.8 ppb ClO were measured by ASUR. There was nearly no ozone destruction observed during the period between mid December 1999 and late January 2000. As expected from ASUR observation of high chlorine activation and continuously low temperatures until mid March, significant ozone depletion was observed between late January and mid March 2000. In order to determine ozone loss it is important to separate dynamical and chemical effects. Since N2O is a good tracer due to its chemical stability in the lower stratosphere for determining ozone changes due to descent of air, ozone loss can be estimated from simultaneous measurements of ozone and N2O by ASUR. Between mid December 1999 and mid March 2000 a chemical ozone loss of about 30% (eq 1.1 ppm) in the altitude range between 19.0 and 22.2 km and of about 40% (eq 1.15 ppm) between 16.0 and 18.1 km was observed. The air masses subsided 2.1–3.2 km in the lower stratosphere due to diabatic descent in the period from mid December 1999 to mid March 2000 as derived from ASUR N2O measurements. Vortex‐averaged ASUR measurements of ozone are systematical greater than results from the Global Ozone Monitoring Experiment (GOME) which has a similar vertical resolution than ASUR. This, however, has little impact on the determination of delta ozone and chemical loss estimates.

Using gas‐phase nitric acid as an indicator of PSC composition

The composition of polar stratospheric cloud particles is investigated using data from several remote sensing instruments: gas‐phase HNO3 measured by the Airborne Submillimeter Radiometer (ASUR), temperature measured by the Airborne Raman Ozone, Temperature and Aerosol Lidar (AROTEL), and aerosol backscatter wavelength dependence and depolarization measured by the UV Differential Absorption Lidar (DIAL). All three instruments have been operated onboard the NASA DC‐8 during the SOLVE winter 1999/2000. A simple thermodynamical model is used to calculate the gas‐phase amount of HNO3 in the presence of polar stratospheric clouds (PSCs) along the flight track of the DC‐8 for one flight into the polar vortex on 7 December 1999. Three types of PSCs are considered in the model: nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), and supercooled ternary solutions (STS). The comparison of the modeled and measured gas‐phase HNO3 in the presence of PSCs shows a very good agreement if a NAT composition of the particles is assumed. However, sensitivity studies show that while the PSCs observed on this flight are not in agreement with a STS composition, the model is very sensitive to temperature, and a NAD composition of the PSC cannot be ruled out.

Vortexwide denitrification of the Arctic polar stratosphere in winter 1999/2000 determined by remote observations

Denitrification has been studied using measurements of stratospheric HNO3 and N2O by the Airborne Submillimeter Radiometer (ASUR), operated on board the NASA DC‐8 during SOLVE/THESEO 2000. Lidar measurements taken on board the same aircraft have been used to distinguish between temporary uptake of HNO3 in polar stratospheric clouds (PSCs) and denitrification events. To derive an NOy budget, ClNO3 data by balloonborne and ground‐based Fourier transform infrared measurements and a model estimate of NOx + 2N2O5 have been considered. The HNO3 profiles of sporadic ASUR measurements without PSC coverage in January suggest that denitrification had started in the vortex core region by then. Vortexwide denitrification was found in mid‐March 2000. Corrected for diabatic descent using the N2O measurements, a vortex‐averaged NOy deficit between 1.2 ± 0.9 ppb at about 16 km altitude and 5.3 ± 2.7 ppb at about 20.5 km altitude was derived compared to December 1999, based on an observed decrease in HNO3 between 2.2 and 3.5 ppb during this time period. A shift in the NOy partitioning from HNO3 toward ClNO3 of about 0.4 to 0.7 ppb was observed in mid‐March compared to December, indicating that chlorine deactivation was occurring. Comparisons with the SLIMCAT three‐dimensional chemical transport model applying denitrification schemes based on ice and nitric acid trihydrate particles in equilibrium, respectively, reveal agreement within the error bars at higher altitudes (∼19 km) but show discrepancies at lower altitudes (∼16 km). It is suggested that more sophisticated denitrification schemes are needed to generally describe denitrification processes.

Remote sensing of ClO and HCl over northern Scandinavia in winter 1992 with an airborne submillimeter radiometer

In February and March 1992 stratospheric ClO and HCl over northern Scandinavia were observed with the submillimeter‐wave atmospheric sounder (SUMAS), as part of the European Arctic Stratospheric Ozone Experiment (EASOE). SUMAS is operated on board the German research aircraft Falcon and observes thermal emission lines in the frequency range 620–650 GHz. In this paper the instrument design and techniques to retrieve volume mixing ratios (VMR) from the measurement will be discussed. Results from the EASOE campaign will be presented including a detailed error analysis. A comparison of the retrieved profiles in the lower stratosphere indicates a strong decrease of ClO abundances from February to March, whereas no corresponding increase in HCl is observed.