Ground-based FTIR Measurements of Stratospheric Trace Species from Aberdeen During Winter and Spring 1993/94 and 1994/95 and Comparison with a 3D Model

Vertical column abundances of HCl, ClONO2, HF and HNO3 have been obtained from infrared solar absorption measurements made at Aberdeen, UK (57°N, 2°W) during the periods January 13 1994 - May 8 1994 and November 23 1994 - April 19 1995. The measurements reveal the partitioning of inorganic chlorine (Cly) inside and outside the polar vortex during these two winter and spring periods. Stratospheric temperatures within the northern polar vortex during 1993/94 were not cold throughout January and most of February. The measurements reported here suggest that following a brief period of chlorine activation in late February and early March, the active chlorine within the vortex recovered rapidly to form ClONO2 resulting in in-vortex ClONO2 columns of 7 × 1015 molecules cm-2. In contrast, measurements during January 1995 suggest extensive invortex activation with in-vortex HCl + ClONO2 as low as 3.6×1015 molecules cm-2. High day-to-day variability in the ClONO2 columns observed during February is evidence for the transport of ClONO2 rich air from high to mid latitudes during the late winter. The implications for mid latitude O3 loss are discussed. A preliminary comparison of the HCl, ClONO2, and HNO3 column data from winter 94/95 with a three-dimensional chemical transport model shows that the model generally reproduces well the day-to-day variability and absolute magnitude of the observed columns, especially for HNO3 outside of the vortex.     

Ground Based FTIR Measurements of Stratospheric Species from Harestua, Norway During Sesame and Comparison with Models

Vertical columns of HF, HCl, HNO3, ClONO2, N2O, ClO and COF2 were measured at Harestua, Norway (60.22° N, 10.75° E, Elevation 600 a.s.l.) beginning on 24 November 1994 and concluding on 1 May 1995 during Phase-III of the SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment) measurement campaign. The vertical columns of HCl, HNO3 and ClONO2 measured on 81 days were compared with columns calculated by the 3-D Cambridge model SLIMCAT. In addition the results were also interpreted by comparison with a photochemical trajectory model. Good agreement was seen for HCl while the nitrogen compounds showed larger discrepancies, especially for ClONO2. Evidence for chlorine activation was seen with 65% reduction of the chlorine reservoirs (HCl + ClONO2) while the levels of ClO were greatly enhanced. Interpretation of the loss with the trajectory model indicated condensation of chlorine on PSCs. The vertical column ratio of COF2 and HF was measured to 0.21 outside the vortex and a factor of two lower inside. The recovery of ClONO2 was seen to be much faster than that of HCl in the early spring.     

Partitioning Between Chlorine Reservoir Species Deduced from Observations in the Arctic Winter Stratosphere

Simultaneous observations of several chlorine source gases, as well asHCl and ClO, have been performed in the Arctic stratosphere on 1 and 9February 1994, using balloon-borne instrumentation as a contribution toSESAME (Second European Stratospheric Arctic and Mid latitude Experiment).The observed mixing ratios of HCl and N₂O show a clearanticorrelation. No severe loss of HCl was observed inside the vortex duringour measurement. These measurements showed that during this period at 20 kmand above, HCl was either in excess, or at least as abundant, asClONO₂ and comprised between 50 and 70% of theavailable chlorine, Cl_y. On 1 February, measurements were madeinside the polar vortex. The air mass sampled on this day showed a clearsignature of diabatic descent, and also enhanced levels of ClO with amaximum of 230 pptv at 22.5 km. A 10 day backward trajectory analysis showedthat these air masses had passed a large region of low temperatures a fewhours prior to the measurement. Temperatures along the back trajectory atthe 475 K and 550 K levels (20.1 and 23.7 km respectively) were cold enoughfor heterogeneous chlorine activation to occur, in agreement with theobserved elevated ClO mixing ratios.     

HNO3 and PSC Measurements from the Transall: Sequestering of HNO3 in the Winter of 1994/95

In the winter of 1994/95 the TRANSALL research aircraft performed several flights in the region of the Arctic vortex during the period of low stratospheric temperatures. The results of simultaneous measurements of HNO3 column amounts by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and of aerosol backscatter profiles by the Ozone Lidar EXperiment (OLEX) are presented for two typical flight scenarios across the polar vortex boundary on December 17, 1994 and January 11/12, 1995. On December 17 and January 12, the column amounts of gaseous HNO3 decreased significantly in regions with low stratospheric temperatures. This decrease was correlated with the extent of the polar stratospheric clouds. Depolarisation measurements showed that type Ib PSCs were observed primarily, but equilibrium calculations for H2SO4/HNO3/H2O aerosols seem to underestimate the observed HNO3 sequestering.     

Column Amounts of ClONO2, HCl, HNO3, and HF from Ground-Based FTIR Measurements Made Near Kiruna, Sweden, in Late Winter 1994

During SESAME phase I ground-based FTIR measurements were performed atEsrange near Kiruna, Sweden, from 28 January to 26 March 1994. Zenith columnamounts of ClONO₂, HCl, HF, HNO₃,O₃, N₂O, CH₄, and CFC-12 werederived from solar absorption spectra. Time series of ClONO₂and HCl indicate a chlorine activation at the end of January and around 1March. On 1 March a very low amount of HCl of 2.09times; 10¹⁵molec. cm^-2 was detected, probably caused by a second chlorineactivation phase starting from an already decreased amount of HCl. The ratioof column amounts of HCl to ClONO₂ decreased inside the vortexfrom about 1 in January to 0.4 in late March compared to values of about 2outside the vortex. Although the Arctic stratosphere was rather warm in winter1993/94 and PSCs occurred seldom, chlorine partitioning into its reservoirspecies HCl and ClONO₂ changed during that winter andClONO₂ is the major chlorine reservoir at the end of thewinter as in cold winters like 1991/92 and 1994/95.     

Observation of the Stratospheric NO2 Latitudinal Distribution in the Northern Winter Hemisphere

During a series of flights in the winters 1991/92 to 1994/95 total stratospheric NO2 was measured by means of the DOAS (Differential Optical Absorption Spectroscopy) technique on board a C160 (Transall) aircraft. In an area covering 60°W to 60°E, and 16°N to 86°N, the total stratospheric NO2 was observed to vary markedly with latitude and season (winter and spring). In the mid-winter Arctic vortex extremely low total stratospheric NO2 (< 3.1014/cm2) was always found, generally larger amounts of NO2 occurred outside the vortex in winter and towards the spring both inside and outside the vortex. This behaviour of stratospheric NO2 can be explained by the denoxification of the wintertime polar stratosphere. Ambient to the vortex in mid-winter however, sudden increases of total stratospheric NO2 by about a factor of 3 were observed. These sudden increases in stratospheric NO2 coincide with a change in the wavenumber 2 of the geopotential height at 60°N, which indicates that most likely the events are caused by planetary waves efficiently transporting air masses rich in NOx from lower to higher latitudes. The monitoring of stratospheric NO2, during latitudinal traverses ranging from the Arctic (80°N) to the Subtropics (18°N) in spring also unexpectedly showed a large variability in total stratospheric NO2 at mid-latitudes. Since photochemistry almost certainly can be excluded, it is proposed that the observed variability may be due to the planetary wave activity of the stratospheric surf zone, known to dynamically connect the tropical and the polar stratosphere.     

Stratospheric aerosol measurements in the Arctic winter of 1996/1997 with the M‐55 Geophysika high‐altitude research aircraft

In‐situ aerosol measurements were performed in the northern hemispheric stratosphere up to altitudes of 21 km between 13 November 1996 and 14 January 1997, inside and outside of the polar vortex during the Airborne Polar Experiment (APE) field campaign. These are measurements of particle size distributions with a laser optical particle counter of the FSSP‐300 type operated during 9 flights on the Russian M‐55 high‐altitude research aircraft Geophysika. For specific flights, the FSSP‐300 measurements are compared with balloon‐borne data (launched from Kiruna, Sweden). It was found that the stratospheric aerosol content reached levels well below the background concentrations measured by the NASA operated ER‐2 in 1988/89 in the northern hemisphere. During the APE campaign, no PSC particle formation was observed at flight altitudes although the temperatures were below the NAT condensation point during one flight. The measured correlations between ozone and aerosol give an indication of the subsidence inside the 1996/97 polar vortex. Despite the lower aerosol content in the winter 1996/97 compared to the 1989 background, the heterogeneous reactivity of the aerosol (as calculated from the measured data with additional model input) is comparable. This is due to the dependency of the reactive uptake coefficients on the atmospheric water vapor content. Under the described assumptions the reaction rates on the background aerosol are significantly smaller than for competing gas phase chlorine activation, as can be expected for stratospheric background conditions especially inside the polar vortex.     

Ground-based FTIR Measurements with High Temporal Resolution

We outline how ground-based Fourier transform infrared (FTIR) measurements of stratospheric trace species, obtained with high temporal resolution, could be used to detect filaments of polar vortex air at mid-latitudes and therefore test high spatial resolution chemical transport models (CTMs). Vertical column abundances of HCl, ClONO2, HNO3, N2O and HF have been obtained from FTIR solar absorption measurements made throughout the day from Aberdeen, UK (57°N, 2°W) on several days during winter/spring 1993/94 and 1994/95. The short-timescale ( 2 hours) variability observed in the columns is attributed to real atmospheric variations and is often associated with the passage of high latitude air over Aberdeen. This is confirmed by 3D modelling studies which qualitatively reproduce and rationalise the observed changes in the column data on January 19 1994, January 20 1995 and February 26 1995. We describe the viewing geometry of ground-based FTIR measurements and we suggest a measurement strategy which should maximise the information retrieved on horizontal gradients in stratospheric trace species columns from FTIR measurements.     

The 1985 chlorine and fluorine inventories in the stratosphere based on ATMOS observations at 30° north latitude

The set of high-resolution infrared solar observations made with the Atmospheric Trace Molecule Spectroscopy (ATMOS)-Fourier transform spectrometer from onboard Spacelab 3 (30 April-1 May 1985) has been used to evaluate the total budgets of the odd chlorine and fluorine chemical families in the stratosphere. These budgets are based on volume mixing ratio profiles measured for HCl, HF, CH₃Cl, ClONO₂, CCl₄, CCl₂F₂, CCl₃F, CHClF₂, CF₄, COF₂, and SF₆ near 30° north latitude. When including realistic concentrations for species not measured by ATMOS, i.e., the source gases CH₃CCl₃ and C₂F₃Cl₃ below 25 km, and the reservoirs ClO, HOCl and COFCl between 15 and 40 km (five gases actually measured by other techniques), the 30° N zonal 1985 mean total mixing ratio of chlorine, Cl, was found to be equal to (2.58±0.10) ppbv (parts per billion by volume) throughout the stratosphere, with no significant decrease near the stratopause. The results for total fluorine indicate a slight, but steady, decrease of its volume mixing ratio with increasing altitude, around a mean stratospheric value of (1.15±0.12) ppbv. Both uncertainties correspond to one standard deviation. These mean springtime 1985 stratospheric budgets are commensurate with values reported for the tropospheric Cl and F concentrations in the early 1980s, when allowance is made for the growth rates of their source gases at the ground and the time required for tropospheric air to be transported into the stratosphere. The results are discussed with emphasis on conservation of fluorine and chlorine and the partitioning among source, sink, and reservoir gases throughout the stratosphere.     

Groundbased Doas UV/visible measurements at Kiruna (Sweden) during the sesame winters 1993/94 and 1994/95.

Zenith sky observations of O3, NO2, OClO and BrO are reported, which were performed at Kiruna (67.9°N, 21.1°E) within the SESAME winters 1993/1994 and 1994/95. For both winters large total amounts of OClO were observed inside the polar vortex at twilight, indicating the degree and the temporal variation of the halogen activation of the polar stratosphere. Occasionally OClO could also be observed outside the polar vortex, most likely due to export of halogen activated vortex air masses into the ambient stratosphere. BrO could also be detected in winter 1994/95, with the largest slant column amounts (5·1014/cm2) occuring in the polar vortex in mid-winter. Similar abundances of stratospheric BrO were observed at dusk and dawn, for both, air masses inside and outside the vortex. This observation is in reasonable agreement with previous studies on stratospheric BrO (observations and models) of Wahner et al. (1992), Arpag et al. (1994), Krug et al. (1996), and Lary et al. (1996a,b), but partly in disagreement with those of Solomon et al. (1989), Fish et al. (1995), and Sessler et al. (1996).     

Middle Stratospheric Polar Vortex Ozone Budget during the Warming Arctic Winter, 2002--2003

The ozone budget inside the middle stratospheric polar vortex(24-36 km) during the 2002-2003 Arctic winter is studied by analyzing Michelson Interferometer for Passive Atmospheric Sounding(MIPAS) satellite data.A comprehensive global chemical transport model(Model for Ozone and Related Chemical Tracers,MOZART-3) is used to analyze the observed variation in polar vortex ozone during the stratospheric sudden warming(SSW) events.Both MIPAS measurement and MOZART-3 calculation show that a pronounced increase(26-28 DU) in the polar vortex ozone due to the SSW events.Due to the weakening of the polar vortex,the exchange of ozone mass across the edge of the polar vortex increases substantially and amounts to about 3.0× 107 kg according to MOZART-3 calculation.The enhanced downward transport offsets about 80% of polar vortex ozone mass increase by horizontal transport.A "passive ozone" experiment shows that only ～55% of the vertical ozone mass flux in February and March can be attributed to the variation in vertical transport.It is also shown that the enhanced downward ozone above ～32 km should be attributed to the springtime photochemical ozone production.Due to the increase of air temperature,the NOx reaction rate increases by 40%-80% during the SSW events.As a result,NOx catalytic cycle causes another 44% decrease in polar vortex ozone compared to the net ozone changes due to dynamical transport.It is also shown that the largest change in polar vortex ozone is due to horizontal advection by planetary waves in January 2003.     

Ground-Based UV Measurements of BrO and OClO over Ny-Ålesund during Winter 1996 and 1997 and Andøya during Winter 1998/99

Presented here are measurements of BrO and OClO performed by ground-based UV-visible zenith-sky viewing spectrometers developed by the Norwegian Institute for Air Research (NILU). Measurements were taken at Ny-Ålesund, Spitsbergen (79° N, 11° E), in winter and spring1996 and 1997 and at Andøya (69.3° N, 16° E) from summer 1998 until summer 1999. AM and PM differential slant column densities (DSCDs) at 90°SZA of BrO and OClO reached their maxima during polar vortex conditions in the winter months and were anti-correlated to temperature andNO₂. Comparison of BrO with a 3-D chemical transport model showed good agreement for seasonal trends and non-vortex conditions. BrO AM/PM ratios were underestimated by the model for vortex conditions, indicating the need for better quantification of BrO source and sink reaction rates. The detection of OClO above 200 K at the 475 K isentropic level indicates the possible activation of chlorine on sulphate particles. Several episodes of boundary layer ozone depletion due to marine-derived BrO were evident in our zenith-skyspectra during April 1997 in Ny-Ålesund.     

Nitrosyl and Nitryl Chloride Formation in H2SO4/HNO3/H2O/HCl Solutions at 200 K

Infrared spectroscopy has been used to measure the vapor pressure of chlorine containing species generated from H2SO4/HNO3/H2O/HCl solutions at 200 K. The vapor pressure was observed to be a function of solution composition. Two solution compositions were investigated. One solution remained a liquid whereas the second solution was a mixed liquid and solid phase (an ice slurry). The liquid solution had a composition of 64.6 wt.% H2SO4/4.8 wt.% HNO3/30.1 wt.% H2O/0.5 wt.% HCl and produced only vapor phase HCl. The ice slurry solution had a composition of 76.6 wt.% H2SO4/3.0 wt.% HNO3/20.1 wt.% H2O/0.3 wt.% HCl and produced HCl, ClNO, and ClNO2 vapor phase components. The sulfuric acid, nitric acid, and water content of these solutions are representative of those present in polar stratospheric clouds (PSCs), however the HCl concentrations are much higher than present within these clouds. The partitioning of chlorine between vapor phase HCl (50%) and ClNO/ClNO2 (50%) for the ice slurry solution suggests a possible mechanism of halogen activation within PSCs. A reaction mechanism to model the observed chemistry is proposed.     

Polar night vortex breakdown and large-scale stirring in the southern stratosphere

The present paper examines the vortex breakdown and large-scale stirring during the final warming of the Southern Hemisphere stratosphere during the spring of 2005. A unique set of in situ observations collected by 27 superpressure balloons (SPBs) is used. The balloons, which were launched from McMurdo, Antarctica, by the Stratéole/VORCORE project, drifted for several weeks on two different isopycnic levels in the lower stratosphere. We describe balloon trajectories and compare them with simulations obtained on the basis of the velocity field from the GEOS-5 and NCEP/NCAR reanalyses performed with and without VORCORE data. To gain insight on the mechanisms responsible for the horizontal transport of air inside and outside the well-isolated vortex we examine the balloon trajectories in the framework of the Lagrangian properties of the stratospheric flow. Coherent structures of the flow are visualized by computing finite-time Lyapunov exponents (FTLE). A combination of isentropic analysis and FTLE distributions reveals that air is stripped away from the vortex’s interior as stable manifolds eventually cross the vortex’s edge. It is shown that two SPBs escaped from the vortex within high potential vorticity tongues that developed in association with wave breaking at locations along the vortex’s edge where forward and backward FTLE maxima approximately intersect. The trajectories of three SPBs flying as a group at the same isopycnic level are examined and their behavior is interpreted in reference to the FTLE field. These results support the concept of stable and unstable manifolds governing transport of air masses across the periphery of the stratospheric polar vortex.     

Enhanced production of stratospheric OH from methane oxidation at elevated reactive chlorine levels in northern midlatitudes

Examination of odd-hydrogen production and loss processes in the lower stratosphere reveals that the large abundances of midlatitude ClO recently measured requires a reassessment of the OH yield from methane oxidation due to a more rapid initiation of the methane oxidation sequence by atomic chlorine. Using a steady state calculation for OH and an iterative procedure for evaluating odd-hydrogen production from the methane oxidation sequence, we demonstrate that both OH and reactive chlorine are amplified below 25 km. The amplified values of OH and ClO are found to be consistent with recent measurements of ClO concentrations and of OH column abundances.     

Measurements of stratospheric chlorine monoxide (ClO) from groudbased FTIR observations

Infrared absorption features due to ClO in the lower stratosphere have been identified from groundbased solar absorption spectra taken from Aberdeen, U.K. (57° N, 2° W) on 20 January 1995. A vertical column abundance of 3.42 (±0.47)×10¹⁵ molec cm^-2 has been derived from 13 independent absorption features in the P and R branches of the (0–1) vibration-rotation band of ³⁵ClO, spanning the spectral region 817–855 cm^-1. The observed absorption features are consistent with very high levels of ClO (approximately 2.6 parts per billion by volume (ppbv)) in the altitude range 16–22 km. A comparison of this profile with a 3D chemical transport model profile indicates the observation was made inside the polar vortex and shows good qualitative agreement but the model underestimates the concentrations of ClO. Simultaneous measurements of other species were made including HCl, HF and ClONO₂. These columns yield a value for HCl+ClONO₂+ClO of 7.02±0.65×10¹⁵ molec cm^-2. This is lower than the total inorganic chlorine (ClO _y ) column of 10.7±1.6×10¹⁵ molec cm^-2 estimated from mean measured (HCl+ClONO₂)/HF ratios together with in-vortex HF measurements. The discrepancy is probably due to significant amounts of the ClO dimer (Cl₂O₂) in the lower part of the stratosphere. The measurements of highly elevated levels of ClO are used to estimate O₃ loss rates at the 400, 475 and 550 K levels making assumptions about the probable distribution of ClO and Cl₂O₂. These are compared with loss rates derived from ozone sonde data.     

Vertical profiles of Volcanic Aerosol and Polar Stratospheric Clouds Above Kiruna, Sweden: Winters 1993 and 1995

Vertical profiles of aerosol were measured in February 1993, and January - March 1995 using balloon-borne particle counters released from Kiruna, Sweden. Condensation nuclei (CN) and aerosol with radii 0.15 - 10.0 µm were measured in 8-12 size classes. The three flights in 1993 were within the polar vortex. Temperatures were below polar stratospheric cloud (PSC) threshold temperatures on one flight and a thin PSC was observed. The volcanic aerosol in the 1993 vortex was similar to that in 1992. In 1993, surface areas were 10 - 20 µm2 cm-3 and volumes 1 - 3 µm3 cm-3. In 1995 three of five flights were within the polar vortex. The volcanic aerosol had decreased to 3 - 7 µm2 cm-3 and 0.1 - 0.4 µm3 cm-3. The top of the volcanic aerosol layer in both years was near 500 K potential temperature (~20 km). A thick nitric acid and water PSC was observed in January 1995. In the thickest region of this PSC nearly all CN were observed to be activated, and surface areas of 5 - 10 µm2 cm-3 were calculated. The volumes observed in this PSC were closer to what would be expected for particles composed of nitric acid trihydrate than for ternary solution droplets. In 1993 the opposite was observed, the volumes in the thin PSC were closer to what would be expected for ternary solution droplets.     

Observational evidence of the delayed response of stratospheric polar vortex variability to ENSO SST anomalies

The temporal and spatial relationship between ENSO and the extratropical stratospheric variability in the Northern Hemisphere is examined. In general, there exists a negative correlation between ENSO and the strength of the polar vortex, but the maximum correlation is found in the next winter season after the mature phase of ENSO event, rather than in the concurrent winter. Specifically, the stratospheric polar vortex tends to be anomalously warmer and weaker in both the concurrent and the next winter season following a warm ENSO event, and vice versa. However, the polar anomalies in the next winter are much stronger and with a deeper vertical structure than that in the concurrent winter. Our analysis also shows that, the delayed stratospheric response to ENSO is characterized with poleward and downward propagation of temperature anomalies, suggesting an ENSO-induced interannual variability of the global mass circulation in the stratosphere. Particularly, in response to the growing of a warm ENSO event, there exist warm temperature and positive isentropic mass anomalies in the midlatitude stratosphere since the preceding summer. The presence of an anomalous wavenumber-1 in the concurrent winter, associated with an anomalous Aleutian high, results in a poleward extension of warm anomalies into the polar region, and thus a weaker stratospheric polar vortex. However, the midlatitude warm temperature and positive isentropic mass anomalies persist throughout the concurrent winter till the end of the next summer. In comparison with the concurrent winter, the strengthening of poleward heat transport by an anomalous wavenumber-2 in the next winter results in a much warmer and weaker polar vortex accompanied with a colder midlatitude stratosphere.     

A numerical study on the effect of an extratropical cyclone on the evolution of a midlatitude front

The extratropical transition (ET) of tropical cyclone (TC) Haima (2004) was simulated to understand the impact of TC on midlatitude frontal systems. Two experiments were conducted using the Advanced Research version of the Weather Research and Forecast (WRF) model. In the control run (CTL), a vortex was extracted from the 24-hour pre-run output and then inserted into the National Centers for Environmental Prediction (NCEP) global final (FNL) analysis as an initial condition, while TC circulation was removed from the initial conditions in the sensitivity run (NOTC). Comparisons of the experiments demonstrate that the midlatitude front has a wider meridional extent in the NOTC run than that in the CTL run. Furthermore, the CTL run produces convection suppression to the southern side of the front due to strong cold advection related to the TC circulation. The easterly flow north of the TC not only decelerates the eastward displacement of the front and contracts its zonal scale but also transports more moisture westward and lifts the air along equivalent potential temperature surfaces ahead of the front. As a result, the ascending motion and diabatic heating are enhanced in the northeastern edge of the front, and the anticyclonic outflow in the upper-level is intensified. The increased pressure gradient and divergent flow aloft strengthen the upper-level jet and distort the trough axis in a northwest-southeast orientation. The thermal contrast between the two systems and the dynamic contribution related to the TC circulation can facilitate scalar and rotational frontogenesis to modulate the frontal structure.