Ground based millimeter-wave observations of Arctic ozone depletion during winter and spring of 1996/97

Björn-Martin Sinnhuber, Jens Langer, Ulf Klein, Uwe Raffalski, ¹ and Klaus Künzi 
Institute of Environmental Physics, University of Bremen, Bremen, Germany
Otto Schrems
Alfred Wegener Institute for Polar Research, Bremerhaven, Germany
 

Intruduction |  Measurements  |  Computation  |  Discussion  |  Reference  |  Figures

Abstract.Ground based millimeter wave measurements of Arctic stratospheric ozone in the winter 1996/97 are presented. The measurements have been performed at one of the primary Arctic stations of the Network for the Detection of Stratospheric Change (NDSC) in Ny-Ålesund, Spitsbergen ( 78.9°N, 11.9°E). Over the period 11 February to 26 April the measurements show in the lower stratosphere an ozone mixing ratio decrease of 1.3 ppm or 44 %. Correspondingly, stratospheric ozone column densities decreased by more than 50 DU. Taking into account the transport of ozone due to diabatic decent, we estimated chemical ozone loss rates of 22 ppb/day in February decreasing to 15 ppb/day in late April 1997.
 

Introduction

Chemical depletion of lower stratospheric ozone in the Arctic has been observed during the last years in winter and spring [WMO,1994; European Commission,1997]. Although the extent of the Arctic ozone depletion is still less than in the Antarctic Ozone Hole, the processes leading to the ozone depletion are believed to be the same in both hemispheres. Polar stratospheric clouds (PSC) form at very low temperatures and enable heterogeneous reactions to activate chlorine which, in the presence of sunlight, can efficiently destroy ozone. However, dynamical processes usually lead to a high ozone variability in the Arctic, that makes it difficult to seperate chemical ozone depletion.
In late winter and spring 1997 again substantial ozone loss has been observed in the Arctic [Manny et al.,1997;Newman et al.,1997;Müller et al.,1997.] The meteorology of the winter 1996/97 stratosphere was unusual [Coy et al.,1997]. The polar vortex formed unusually late and was stable until the first days in May. Temperatures stayed low enough to allow the formation of PSCs until the end of March.
In this paper we derive chemical ozone loss rates from ground based millimeter-wave ozone measurements. The measurements have been performed at one of the primary Arctic stations of the Network for the Detection of Stratospheric Change (NDSC) in Ny-Ålesund, Spitsbergen (78.9°N, 11.9°E). These continuous measurements of ozone profiles inside the polar vortex from February to the end of April allowed the investigation of the chemical ozone loss evolution during spring 1997.

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Ozone measurements

The Radiometer for Atmospheric Measurements (RAM) is a ground based millimeter-wave radiometer operated continuously at Ny-&Aringlesund, Spitsbergen. An ozone emission line is measured at 142 GHz over a spectral band-width of 1.6 GHz. The high spectral resolution allows the retrieval of ozone profiles between 12 and 55 km from the pressure broaded line shape. The vertical resolution is about 6-8 km. One to four ozone profiles per hour are obtained all year round, largely independent of tropospheric weather conditions. Temperature profiles required for the ozone profile retrieval are taken from analyses of the National Center for Environmental Prediction (NCEP, former NMC). The precision of the measured ozone volume mixing ratio (VMR) profiles is about 0.1 ppm. The RAM measures alternately ozone at 142 GHz and chlorine monoxide (ClO) at 204 GHz. The ClO measurements of late winter-spring 1997 are presented in a companion paper by Raffalski et al. [1998]. For a detailed description of the RAM see Langer et al. [1997].
Fig. 1 shows the RAM measurements of the ozone VMR at the 475 K level (approximately 19 km altitude) between mid of February and begin of May 1997. A continuous decrease from nearly 3 ppm in February to about 1.6 ppm at the end of April can be seen, corresponding to an overall reduction of 44 %. Diabatic decent is expected to increase the lower stratospheric ozone VMR over this time period. The observed ozone decrease therefore has to be attributed to chemical ozone depletion. Potential vorticity (PV), derived from analyses of the European Centre for Medium Range Weather Forecast (ECMWF), indicate that the ozone measurements during this period have been performed well inside the polar vortex, with the exception of only a few days in mid April, when Ny-&Aringlesund was at the vortex edge as seen in the data by a slight increase of the ozone mixing ratio. The vortex edge was determined using the PV gradient with respect to equivalent latitude [Nash et al.,1996]. We defined the inner vortex as the region of relatively low PV gradients, compared to the high PV gradients at the vortex edge. We performed domain filling trajectory calculations [Sinnhuber et al.,1996] to show that the inner vortex at 475 K was well isolated from February to the end of April 1997. No intrusion of outer-vortex air into the inner vortex occurred. After April 26 the inner vortex moved away from Spitsbergen. Higher ozone VMR of up to 2.5 ppm have been measured in the vortex edge region between April 14 and 17 and after April 26. After May 10 measurements have been made clearly outside the vortex, with typical mid-latitude ozone mixing ratios of 2 ppm at 475 K. Stratospheric ozone column densities above 12 km have been computed from the RAM measurements. They show a constant decrease between mid of February and the begin of April, see Fig. 2 (dots).
Fig. 2 also shows UV/visible total ozone measurements performed at Ny-&Aringlesund (asterisks, data provided by F. Wittrock). The RAM column densities above 12 km are in excellent agreement with the UV/visible total ozone measurements, when a mean offset of 66 DU accounting for the remaining ozone column below 12 km is added. Between the beginning of February and beginning of April the ozone column densities decreased bymore than 50 DU. The decrease of the ozone column densities inside the polar vortex was also observed by the two satellite-borne instruments Total Ozone Mapping Spectrometer (TOMS) [Newman et al.,1997] and Global Ozone Monitoring Experiment (GOME) [Bramstedt et al.,1997].

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Computation of ozone loss rates

From the RAM ozone measurements at 475 K loss rates have been determined by fitting a linear trend over periods of 20 days. A 20 day period was chosen because the measurements show some variability on time-scales of a few days, indicating that ozone is not horizontally homogeneously distributed inside the vortex. Fitting a trend over 20 days averages out this horizontal variability. Measurements not made within the inner vortex were excluded from the trend calculations. The observed ozone trends are shown in
Fig. 3 (dots with errorbars). The errorbars indicate two standard deviations of the fit. To derive chemical ozone loss rates from the observed ozone trends, we have to take into account how much of the chemical loss is masked by the diabatic decent of air inside the polar vortex. The ozone change on a given isentropic surface due to diabatic decent is given as


Where O3 is the ozone VMR, Q the diabatic heating rate, p and p0 are pressure and reference pressure respectively, kappathe ratio of of the dry air gas constant to the specific heat at constant pressure (2/7), and --is the ozone gradient with respect to the potential temperature [Braathen et al.,1994]. The heating rates include solar heating through the absorption of shortwave radiation and cooling through longwave emission into space. The longwave cooling rate calculations have been performed with a narrow-band model with a resolution of 10 cm-1 [Shine,1991]. The computation included ozone, water vapor and carbon dioxide. Daily profiles of the RAM ozone measurements at Spitsbergen have been used for the heating rate calculations. Since no routine measurements of stratospheric water vapor were available, a typical late winter vortex profile was assumed with a minimum of 3.5 ppm at 400 K increasing to 6 ppm above 675 K [Ovarlez and Ovarlez,1994. However, water vapor has only a small impact on both, longwave and shortwave stratospheric heating rates. The carbon dioxide VMR was assumed to be 360 ppm. Temperature profiles averaged over the inner vortex, have been taken from the ECMWF analyses. The error through the use of vortex averaged temperatures for the heating rate calculations, Q(overline{T}), instead of averaging the heating rates after the computations, $overline{Q(T)}$, is estimated to be less than 0.05 K/day. The integration of the solar heating rates over the vortex has been performed by computing the heating rates for every 10 degrees of latitude. All calculations have been done for clear sky conditions only. Rosenfield et al.[1994] found that tropospheric clouds increaselower stratospheric cooling rates only by a few percent, while the effect of PSCs on stratospheric cooling rates is negligible. The resulting vortex averaged net heating rates at 475 K decrease from about 0.3 K per day in February to nearly zero at the end of March. There is slightly less cooling during February and March 1997 than found by Rosenfield et al.[1994] for February and March 1992. Our calculations showed no significant diabatic heating during the considered period in 1997. This is a result of the unusually low ozone mixing ratios in April. Since the solar heating in the lower stratosphere depends almost exclusively on ozone absorption, higher ozone VMR would have caused diabatic heating in April. From the diabatic heating rates and observed vertical ozone gradients, the diabatic transportrates are computed using Eq. (1) Fig. 3, solid line). They show that in the absence of any chemical ozone depletion, the diabatic decent would have increased ozone by about 5 to 15 ppb/day during February and March. Estimates on the chemical ozone depletion rates, given as the difference between the observed ozone decrease and the computed increase due to the diabatic decent are indicated in Fig. 3 (open diamonds). Significant chemical ozone loss rates are observed throughout the whole period. The chemical loss rates stayed nearly constant at about 20 ppb/day throughout February and March and decreased to 15 ppb/day in the second half of April. They are in good agreement with results of the 1997 MATCH campaign, which are available until the beginning of April (P. von der Gathen, personal communication, see also von der Gathen et al. [1995]).

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Discussion and Conclusion

The analysis of the millimeter-wave ozone measurements shows substantial chemical ozone loss at the 475 K isentropic level until the begin of April 1997. During this period we also measured high levels of ClO [Raffalski et al.1998]. Ozone loss rates computed from the ClO measurements agree well with the observed ozone loss. The still high ozone loss rates during March and the begin of April are partly due to the high amount of sunlight available inside the vortex at that time. Fig. 4 shows the estimated ozone loss rates per sunlit hour (open diamonds). They have been derived from the loss rates shown in Fig. 3 divided by the average hours of sunlight per day available inside the vortex during the corresponding periods. Between February and March, the ozone loss rates per sunlit hour at 475 K decreased by more than a factor two. For comparison the area of temperatures below 195 K at the 475 K level, where PSCs may exist, is also shown in Fig. 4 . It shows that the reduction of the possible PSC existance area between February and March is correlated to the vortex averaged chemical ozone loss rates per sunlit hours. The ozone loss rates of 15 ppb/day in the second half of April 1997 - three weeks after temperatures were low enough to allow the existance of PSCs - are about a factor of two or three higher than we would expect under this condition from usual gas-phase chemistry without chlorine activation, i.e. due to the NOx- and HOx-cycles alone [Lary1997]. Catalytic chlorine cycles on the other hand would require a vortex averaged chlorine activation of about 0.5 ppm ClO to explain the observed ozone loss in the second half of April, as calculations with the Bremen Atmospheric Photochemical Model indicate [Raffalski et al.,1998]. Under this situation the ClO/BrO- and ClO/O-cycles become dominant compared to the ClO dimer cycle, which means that the ozone loss would be proportional to the ClO concentration. It is presently unclear, what is the cause for the unexpected high ozone loss in late April. The possibility that there may be chlorine activation with ClO mixing ratios as high as 0.5 ppm during this period is disscussed by Raffalski et al. [1998]. The millimeter-wave observations performed at Ny-Ålesund showed that substantial ozone loss occurred again in winter and spring 1997, in agreement with other observations. The late break-up of the polar vortex and record low temperatures during March and early April led to chemical ozone loss that lasted longer than in previous years. However, the overall ozone loss for the winter 1996/97 was lower than that reported for previous years [European Commission,1997; Rex et al.,1997].

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acknowledgments

We thank the staff at Ny-&Aringlesund for their support. Keith P. Shine is gratefully acknowledged for providing the radiation model for cooling rate calculations. Meteorological data were provided by the European Centre for Medium-Range Weather Forecast and by the National Center for Environmental Prediction. We thank Folkard Wittrock for the use of the UV/visible data prior to publication. Part of this work was supported by the Commission of the European Community, by the German Bundesministerium für Bildung und Wissenschaft, and the Alfred Wegener Institute for Polar Research, Germany.
 

Reference

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 B.-M. Sinnhuber, J. Langer, U. Klein, and K. Künzi, Institute of Environmental Physics, University of Bremen, PO Box 33 04 40, D-28334 Bremen, Germany. (e-mail: bms@schalk.physik.uni-bremen.de)
 U. Raffalski, Institute for Space Physics, PO Box 812, S-98128 Kiruna, Sweden. (e-mail: uwe@irf.se)
 O. Schrems, Alfred Wegener Insitute for Polar and Marine Research, PO Box 12 01 61, D-27515 Bremerhaven, Germany.

¹ Now at Swedish Institude for Space Physiks, Kiruana, Sweden.

figures

Figure 1. Ozone volume mixing ratio at the 475 K isentropic level as measured by the ground based millimeter-wave radiometer at NY-Ålesund in late winter 1997. A continuing decrease of the ozone mixing ratio between mid of February and end of April is evident.

Figure 2. Ozone column densities above 12 km as measured by the ground based millimeter-wave radiometer (dots) and total ozone from UV/visible measurements (asterisks) at NY-Ålesund in late winter 1997.}

Figure 3. observed ozone change at 475 K derived from millimeter wave measurements at NY-Ålesund (dots with errorbars). A linear trend was computed over a period of 20 days. The errorbars represent a 2 sigma error of the fit. Compared to computed ozone change due to vortex averaged diabatic decent of ozone (solid line). Estimates on the chemical ozone loss rates indicated by open diamonds.}

Figure 4. Estimated chemical ozone loss rates per sunlit hour at 475 K (open diamonds). For comparison the possible area of PSC existance at 475~K is displayed, approximated by the area of temperatures within the vortex lower than 195 K.}

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