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Arctic polar stratospheric clouds observed with the ILAS sensor: Inference of their composition and possible relation to denitrification

Sachiko Hayashida, Nara Women’s University, Nara, Japan (sachiko@ics.nara-wu.ac.jp)

Polar stratospheric clouds (PSCs) are recognised as a key factor in ozone destruction over the polar regions as a result of heterogeneous reactions occurring on their surface, which convert inactive reservoir chlorine into active chlorine. Recent stratospheric data show a trend in decreasing temperature [WMO, 1998] and it is of current scientific interest to detect any trend in the frequency of PSCs. PSCs are also closely related to denitrification (irreversible removal of nitric compounds that inactivate chlorine radicals). Waibel et al. [1999] showed that conditions in the Arctic are at the threshold for denitrification. A temperature decrease of one-degree would enhance PSC formation, which might cause significant denitrification and therefore prolonged ozone loss over the Arctic.

Our research group in the Nara Women’s University have been analysing various kind of satellite data focusing on chemical mechanisms related to aerosols/PSCs in the stratosphere. Most of our recent work is concerned with the Improved Limb Atmospheric Spectrometer (ILAS) onboard the Advanced Earth Observing Satellite (ADEOS). ILAS was designed to observe the profiles of minor stratospheric species, such as ozone, nitric acid, and water vapour, and that of the stratospheric aerosol extinction coefficient (780 nm) at 1-km height intervals [e.g., Sasano et al., 1999; Yokota et al., 2002]. ILAS observed fourteen circumpolar points in each hemisphere every day at high latitudes (57.1-72.7N and 64.3-88.2S) from November 1996 until the end of June 1997.

Arctic PSCs observed with ILAS in winter/early spring of 1997

ILAS captured more than 60 PSC profiles in the Northern Hemisphere during the winter and early spring of 1997. Hayashida et al., [2000a] showed the highest probability of sighting PSCs in mid-January at an altitude of approximately 23 km, with subsequent occurrences of PSCs observed intermittently at lower altitudes until mid-March. The 1997 Arctic winter was characterised by the prolonged appearance of PSCs until mid-March, associated with a long-lasting polar vortex. As the ILAS PSC measurements were the only spaceborne measurements made on a regular basis (about 14 times daily) during that period, our study provided an additional data set for the long-term PSC data archive to examine the long-term trend in PSC frequency. Besides, simultaneous measurements of nitric acid and PSCs by ILAS made it possible to infer the composition of PSCs and their effects on denitrification.

PSC composition inferred from satellite data

The ILAS extinction and nitric acid data were compared with theoretically predicted values assuming the existence of Supercooled Ternary Solution (STS) and nitric-acid-containing solid particles (NAD or NAT) [Saitoh et al., 2002]. As the amounts of nitric acid and water vapour in the atmosphere are closely concerned with PSC composition, it is important to estimate those amounts properly in inferring PSC composition. The details of our scheme to determine the values of ambient nitric acid and water vapour that should be in thermodynamic equilibrium with a specific type of PSCs such as STS, NAT, and NAD, are described in Hayashida et al. [2000b] and in Saitoh et al. [2002].

Our analysis indicated that both the extinction coefficient and nitric acid level for some of the PSC events observed in mid- and late-January 1997 over the Arctic agreed with the theoretical values for STS better than with those of NAT or NAD. Figure 1 shows scatter-plots of UKMO temperature vs. the ILAS extinction coefficient (a1, a2), and UKMO temperature vs. the ILAS nitric acid value (b1,b2), at 20 km (a1, b1) and 21 km (a2, b2) in mid-January. Solid circles represent the data identified as PSCs. The theoretical curves for STS (red lines), NAD (green line), and NAT (blue line) are shown in the figure. In Figure 1, STS formation curves are indicated with the thick solid red lines with the proper conversion factors (mass-to-extinction ratio) depending on temperatures: STS droplets are expected to form at cold temperatures, while the background conversion factor should be used at warm temperatures. In the figure, the red circles indicate events whose extinction and nitric acid values are closest to theoretical STS values among the three types. It is found that the enhancement of the volume corresponds to the decrease in the ambient nitric acid, suggesting the uptake of nitric acid into particles as STS forms.
Fig. 1-a1
Fig. 1-a2
Fig. 1-b1
Fig. 1-b2

Figure 1: Scatter-plots of UKMO temperature vs. the ILAS extinction coefficient (a1, a2) and UKMO temperature vs. the ILAS nitric acid (b1, b2) at 20-km (a1, b1) and 21-km (a2, b2) in mid-January. Solid circles represent the data identified as PSCs, red ones are more likely to be STS considering the extension coefficient and nitric acid concentration. The theoretical curves for STS (red lines), NAD (green line), and NAT (blue line) are shown in the figure. Error bars on ILAS data are from data set of version 5.20. Details on the errors are described in Yokota et al., [2002].
(For a better resolution of the images, please click on the plot or contact the SPARC Office)

 

Figure 2 shows the vertical profiles of the extinction coefficient (solid line with a notation ‘E’), nitric acid (solid line with a notation ‘N’), and the collocated UKMO temperature (upper scale: ‘T’) on these two days. The 10-day averaged background profile of nitric acid for mid-January is also shown in the figure (dashed line). The black circles indicate the data that correspond to the STS formation curve. Comparison of the ILAS nitric acid profile with the background profile illustrates that nitric acid decreased significantly in the region where STS-PSCs was observed.
Figure 2: The vertical profiles of the extinction coefficient (E), nitric acid (N), and collocated UKMO temperature (T) observed on January 19 (65.8 N, 21.6 E,). The 10-day averaged background profile of nitric acid for mid-January is also shown in the figure (dashed line). The black circles indicate the data that categorised as STS.
(For a better resolution of the images, please click on the plot or contact the SPARC Office)

 

However, some gaseous nitric acid data corresponding to PSC events indicated less decrease in gaseous nitric acid than predicted for STS, which leaves an uncertainty in identifying PSC type. On the other hand, Pan et al. [2001] reported a corresponding decrease in water vapour up to ~2.5 ppmv in late-January and discussed the possibility of ice cloud formation. They argued that freezing of water would be a much faster process than forming nitric acid containing particles, which could lead to dehydration before denitrification, or in some cases, without substantial denitrification. It would be possible that the STS particle growth was limited in dehydrated air to explain the synchronous decrease in nitric acid and water vapour with less amount of nitric acid decrease than predicted in STS formation. Although the mixture of STS and ice in a large sampling volume of ILAS would be another possible interpretation, those studies brought up a question in PSC formation process at any rate.

According to our analysis for the late PSC season in the same year, many of the characteristic PSC events were categorised as the nitric-acid-containing solid PSC events [Saitoh et al., 2002]. Those PSC events experienced temperatures below NAT saturation temperature for more than several days. They had not passed over typical mountainous area before their measurements, so the formation mechanisms of these solid particles should be explained from their synoptic scale temperature histories, without considering lee waves. They maintained relatively high nitric acid hydrate saturation ratios along their trajectory, which suggests their homogeneous freezing [Tabazadeh et al., 2001]. In our analysis, some of the PSC events indicated low concentration of nitric acid and less volume than predicted from temperature. Kondo et al., [2000] and Irie et al. [2001] derived clear denitrification in February 1997 from ILAS nitric acid data. It seems that the shortage of nitric acid limited particle growth, even if the temperature did fall low enough to generate PSCs in February.

Recently, S. Oshchepkov (NIES) examined IR channel aerosol extinction data of ILAS in aerosol type analyses using a spectral fitting approach [Oshchepkov et al., 2002]. We are now continuing collaboration on this issue with Oshchepkov and the NIES group. Combined use of our approach with infrared spectral analysis will shed light on unknown microphysical processes of PSCs.

Effects of denitrification on ozone destruction - a new approach with assimilated chemical model

Finally, the effect of PSCs and denitrification on ozone loss was examined by means of our newly developed scheme named Chemical Species Mapping on Trajectories (CSMT) [Kagawa and Hayashida, 2002]. The CSMT is a method to construct synoptic maps of chemical species by combining ‘trajectory mapping’ [Morris et al., 1995, 2000] with a photochemical box model. Minor constituents are time-integrated in a photochemical box model along trajectories that start from satellite measurement points until a target time. In this study, we calculated 7-day forward trajectory for each observed data. The initial values of ozone, HNO3, and N2O at the initial trajectory points were taken from ILAS version 5.20 data. All other initial data were taken from the climatological data derived in a similar way to Becker et al. [2000]. Many long and short-lived species in the stratosphere were successfully mapped by the CSMT. Figure 3(a) and 3(b) are the examples among those maps for ozone and nitric acid on February 20, 1997 on 475-K isentropic surface. Though ILAS measurements are geographically limited, the CSMT scheme can be used to expand the area of data coverage, which furthers the potential of data usage.

Figure 3(a): A CSMT-derived ozone map on the 475-K isentropic surface on February 20, 1997. Colors indicate ozone-mixing ratios.
Figure 3(b): Same as Figure 3(a) but for nitric acid.
(For a better resolution of the images, please click on the plot or contact the SPARC Office)

 

The ozone loss rate was determined by the CSMT scheme. Figure 4 shows the time evolution of ozone loss rate on 475-K isentropic surface with the level of gaseous nitric acid (the latter is shown as color codes). Although sedimentation process was not involved in this calculation, denitrification was implicitly included as initial values of gaseous nitric acid that were observed with ILAS. As reported by Kondo et al., [2000] and Irie et al., [2001], significant denitrification was observed after late-February. As shown in Figure 4, the values of ozone loss rates in late-February are distributed in two clusters, with high and low ozone loss rates. The high ozone loss rate was due to low deactivation of chlorine and the low ozone loss was due to less PSC formation. Although the analysis is still preliminary, it suggests that denitrification has a significant impact on ozone loss and recovery as expected. Our analysis shown here is based on the observed nitric acid data that are almost equivalent to NOy, and therefore the partitioning of NOx/NOy would be more realistic than the result from a mechanistic chemical-transport-model.
Possible relation of PSCs with denitrification and ozone loss rate still needs a more detailed analysis.
Figure 4: Calculated ozone loss rate in ppbv/day. Ozone decrease is taken to be positive. Each dot indicates the ozone loss rate calculated for the target day along different trajectories. Ozone loss was estimated from the difference between the first calculated output of the target day and the value 24 hours later. The color indicates the corresponding nitric acid level. The arrows shown in the figure are the PSC events categorised with Saitoh et al., [2002]: red and blue arrows indicate STS and NAT with numbers of the events.
(For a better resolution of the images, please click on the plot or contact the SPARC Office)

Acknowledgements

I wish to thank all of my graduate students, especially Dr. A. Kagawa and Ms. N. Saitoh. I also thank the members of the ILAS science team and their associates.

References

Becker, B. et al., Ozone loss in the Arctic stratosphere in the winter 1994/95: Model underestimate results of the Match analysis, J. Geophys. Res., 105, 15175-15184, 2000.

Hayashida, S., et al., Arctic polar stratospheric clouds observed with the improved limb atmospheric spectrometer during winter 1996/1997, J. Geophys. Res., 105, 24,715-24,730, 2000a.

Hayashida, S., et al., Stratospheric background aerosols and polar stratospheric clouds observed with satellite sensors – Inference of particle composition and sulfate amount, Soc. Photo Opt. Instrum. Eng., 4150, 76-86, 2000b.

Irie, H., et al., Redistribution of nitric acid in the Arctic lower stratosphere during the winter of 1996-1997, J. Geophys. Res., 106, D19, 23, 139-23,150, 2001.

Kagawa, A. and S. Hayashida, Analysis of ozone loss in the Arctic stratosphere during the late winter and spring of 1997, using the Chemical Species Mapping on Trajectories (CSMT) technique, J. Geophys. Res. (submitted).

Kondo, Y. et al., Denitrification and nitrification in the Arctic stratosphere during the winter of 1996-1997, Geophys. Res. Lett., 27, 337-340, 2000.

Morris, G.A., et al., Trajectory mapping and application to data from the Upper Atmosphere Research Satellite, J. Geophys. Res., 100, 16491-16505, 1995.

Morris, G.A., et al., A tool for validation of trace gas observations, J. Geophys. Res., 105, 17825-17894, 2000.

Oshchepkov, S. et al., Retrieval of stratospheric aerosol composition from spectral limb-scanning observations at the infrared gas window channels, J. Geophys. Res. (submitted)

Pan, L.L., et al., Satellite Observation of Dehydration in the Arctic Polar Stratosphere, Geophys. Res. Lett., GL014147, 2001.

Saitoh, S. et al., Characteristics of Arctic Polar Stratospheric Clouds in winter 1996/1997 inferred from ILAS measurements, J. Geophys. Res., 2002 (in press).

Sasano, Y. et al., Improved Limb Atmospheric Spectrometer (ILAS) for stratospheric ozone layer measurements by solar occultation technique, Geophys. Res. Lett., 26, 2; 197-200,.1999.

Tabazadeh, A. E. et al., Role of the stratospheric polar freezing belt in denitrification, Science, 291, 2591-2594, 2001.

Waibel, A.E. et al., Arctic ozone loss due to denitrification, Science, 283, 2064-2069, 1999.

WMO, Scientific Assessment of Ozone Depletion: 1998, Report n° 44.

Yokota, T. et al., Improved Limb Atmospheric Spectrometer (ILAS) data retrieval algorithm for Version 5.20 gas profile products, J. Geophys. Res., 2002 (in press).