Aerosol and ozone observations in the polar troposphere at Spitsbergen in spring 1994

ATMOSPHERIC RESEARCH ELSEVIER

Atmospheric Research 44 (1997) 175- 189

Aerosol and ozone observations in the polar troposphere at Spitsbergen in spring 1994 S. Wessel a, S. Aoki b, R. Weller c, A. Herber ", H. Gernandt a, O. Schrems c a Alfred Wegener Institute Research Department Potsdam, Potsdam, Germany b Center of Atmospheric and Oceanic Studies, Sendai, Japan c Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Abstract The dynamics of the tropospheric ozone distribution during the transition from polar night to polar day has been investigated in Ny Alesund, Spitsbergen (79°N, 12°E) in the period from March to June 1994. Surface ozone, the vertical ozone stratification as well as aerosols were measured. Surface 0 3 mixing ratios were found to be highest in March during prevailing advection from East Europe, while the lowest surface ozone amounts were observed in late June. The transition from winter to spring was characterized by striking surface ozone variations. In this period we observed five distinct 0 3 minima. Such events were typically associated with advection of marine polar air masses. The low burden of aerosols within the accumulation mode coinciding with low ozone mixing ratios suggests that 0 3 destruction occurred during long transport times in the remote marine Arctic, largely in absence of anthropogenic pollutants. It was found that the 0 3 depletion was restricted to the boundary layer only. Typically a capping inversion defined the upper limit of its vertical extension. © 1997 Elsevier Science B.V.

1. Introduction Arctic Haze and dramatic surface ozone variations in the early spring have provoked recent intensive atmospheric investigations in the Arctic (Bottenheim et al., 1990; Li et al., 1990; Barrie et al., 1994a; Heintzenberg and Leck, 1994). Emphasis has been laid on the examination of long term variations with regard to anthropogenic and natural influences. In terms of atmospheric chemistry, the arctic troposphere shifts from dark and heavily polluted during the polar night to permanently sunlit and largely pristine during the polar day (Ottar, 1989). Anthropogenic trace constituents can efficiently be advected into the arctic boundary layer in winter and early spring, because highly industrialized regions are located north of the polar front (Barrie, 1986; Iversen, 1993; Barrie, 1993). Arctic Haze, for instance is essentially caused by anthropogenic sulfate 0169-8095/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-8095(97)00009-4

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aerosols (Bottenheim, 1993; Heintzenberg and Leck, 1994). Emission sources south of the polar front do not contribute much to arctic boundary layer pollution due to a large probability of wet deposition along the trajectory (Iversen, 1993). In summer the northward shift of the polar front prevents advection of pollutants from lower latitudes. This strong seasonality is also reflected in the ozone levels of the boundary layer. Surface ozone mixing ratios are maximum in late winter/early spring mainly due to stratosphere-troposphere exchange processes which are enhanced during the same period (Gruzdev and Sitnov, 1993; Dibb et al., 1994). Furthermore, pollution derived ozone and the prolonged tropospheric lifetime of 0 3 during the polar night can contribute to some extend to higher surface ozone amounts in the springtime. The transition from polar night to polar day is characterized by massive periodic depletions of surface ozone (Barrie et al., 1988; Barrie et al., 1994b; Anlauf et al., 1994). These low ozone events are associated with an increased occurrence of gaseous and particulate bromine compounds (Barrie et al., 1988; Li et al., 1994) and advection of polar marine air masses (Anlauf et al., 1994). The topic of our research activities was to study the dynamics of the tropospheric ozone distribution during the transition from polar night to polar day. In the period from March to June 1994, a spring campaign took place in Ny Alesund (Spitsbergen, 79°N, 12°E). Daily ozone soundings and continuous aerosol and surface ozone observations comprised the actual research program. The data were completed by routine meteorological measurements, which are part of the German-Norwegian BSRN Station, partly supplied by the local meteorological station of the German Koldewey Station.

2. Methods and instrumentation

Throughout the campaign, ozone sondes were launched every other day and every day during periods of strong surface ozone variations. In addition daily radio soundings by the local meteorological station of the German Koldewey Station in Ny Alesund provided vertical temperature, humidity, wind direction and wind velocity profiles. Complementary to the ozone soundings, surface level 0 3 mixing ratios were measured continuously by means of an UV spectrometer. We used electrochemical concentration cells (ECC sondes) together with modified RS 80 radiosondes (Vaisala) on Totex TA 1200 balloons. The ECC sondes were prepared according to the detailed instructions given by Komhyr (1986). The equipment allowed us to measure simultaneously the ECC current which corresponds to the 0 3 partial pressure, the temperature of the inlet air, and from the meteorological RS 80 sonde, atmospheric temperature, relative humidity, wind speed and direction, and the pressure of the ambient air. Taking into account the inherent time constant of the ECC sondes of approximately 20 s and the ascent velocity of around 5 m / s , the effective height resolution was about 100 m. The particle size distribution and the number densities at ground level were measured by means of an laser-optical particle counter (LAS-X-CRT, Particle Measuring System Inc.). Particle counter and ozone UV spectrometer were installed inside the Japanese Station 'Rabben', located approximately 2 km northwest of Ny ,~lesund at sea level. The aerosol sampling line, a 4 m long teflon tube with an inner diameter of 2 mm was

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attached 3 m above the ground on the roof of the station. The sampling flow rate was 2.1 cm3/s. Aerosols could be detected within the particle size range of 0.09-3 /zm diameter. The instrument divides this size range into 15 individual channels. The particle number was integrated for 10 minutes in each channel. For further data evaluation, the signal was transmitted to a PC. The LAS-X sizing accuracy is _ 1 / 4 channel. Counting is accurate to 5000 counts per second, all channels combined. This count frequency is subject to 10% coincidence loss at 5000 counts/second. Before the campaign the instrument had been calibrated with latex particles in the diameter range between 0.173-0.993 /zm. In order to avoid possible damage of the optics, the particle counter was not operating during wet deposition events. Trajectories were calculated for each day of the campaign by the DWD (German Weather Service) and made available for this scientific work. This global model is based on 3D wind fields. The input data are the horizontal wind components and the surface pressure. From the resulting divergences and convergences of the wind field, subsidence and updraft of air masses and thus the vertical wind component can be derived. In all cases, the calculated five-days back-trajectories reached Ny Alesund at 1200 UTC. For our investigations only endpoint heights corresponding to ground level pressure, 950, 850, and 700 hPa were used.

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3. Results 3.1. Tropospheric ozone

The vertical ozone distribution in the troposphere observed from March 3 through June 30 is shown in a contour diagram (Fig. 1). During March, the measured mean boundary layer 03 mixing ratios were in the range of 40-50 ppbv. Those mixing ratios were observed in connection with continental air masses coming from Northern and Eastern Europe. Fig. 2 shows trajectories of March 21 which are typical for this case. In contrast, we measured considerably lower boundary layer ozone values in the range of 20-35 ppbv from March 8 to March 11 (Julian day 67-70). As indicated by trajectory analyses, this period was characterized by air parcels coming from the Western Arctic (Canada Basin), coinciding with significantly lower temperatures (around -20°C) and relative humidity values between 60-70%. Within the free troposphere the ozone mixing ratios increased to about 60 ppbv at 8 km height with an averaged vertical gradient of

Fig. 2. Five-day backward-trajectories (DWD) for March 21, 1994: (circles) 970.65 hPa, (squares) 950 hPa, (triangles) 850 hPa, (diamonds) 700 hPa.

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2.6 ppbv/km. The situation changed in the following months April, May, and June. Concerning the free troposphere, a steeper monthly averaged 03 gradient of 5.6 p p b v / k m in April and 6.3 p p b v / k m in May and June was typical, leading to ozone mixing ratios of 100 ppbv in 8 km height. Most striking, however, was the temporal development of the 03 mixing ratio within the planetary boundary layer (Fig. 3). Occasionally the ozone amounts decreased dramatically to values as low as 1 ppbv. Such events were observed during Julian days 107-108, 121-125, and 148-152. They are only insufficiently represented by daily mean values, marked with arrows in Fig. 3. Fig. 4a-c show in more detail the five ozone minima with hourly averaged mixing ratios. As a characteristic feature a capping inversion at an altitude between 400 and 900 m generally prevented vertical exchange between the ozone depleted layer and levels above (Fig. 5). Furthermore, trajectory analyses point towards a marine origin of such air parcels: With the exception of May 1 (Julian day 121) where the origin of the surface trajectory was Greenland, they originated from the western Central Arctic (Fig. 6). Due to the marine nature of the ozone depleted air the relative humidity reached 70-90%. The surface ozone records (Fig. 4) revealed that the decrease as well as the subsequent increase of the ozone mixing ratios occurred in the time scale of hours, Apart from such events, the averaged surface ozone mixing ratios in April tend to be as high as the mean value of about 45 ppbv in March, compared to 35 ppbv measured during May and June. Contrary to March the prevailing source regions of the advected air masses in May and June were the Barent Sea/Siberian marginal zone and the North Atlantic. Obviously, the marine component was now dominating.

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3.2. Particle distribution

The particle distribution were measured from March 21 (Julian Day 80) to June 29 (Julian Day 180). Although the observation time was limited to sunny days without rain or strong snow drift, a relatively continuous data record could be obtained. The particle concentrations presented here correspond to daily mean values (Fig. 3). The available particle counter detects particles exclusively in the accumulation- and the coarse range. Information on the nucleation mode is not available. The highest aerosol number densities were detected in the accumulation mode, i.e. the size range 0.09-1.0/xm ESD

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(latex-equivalent optical scattering diameter). The cumulative concentrations varied between 4 cm -3 and 75 cm -3 giving a mean value of around 29 cm -3 (ltr-standard deviation: 17 cm-3). Concerning the seasonal dependence of this mode, highest

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concentrations were detected in March, with decreasing tendency towards the begin of June. Later in June, the number densities in the accumulation mode increased again to a significant second maximum (Fig. 3). From trajectory analyses it is obvious that peak concentration occurred when air masses from Europe reached Ny ,~lesund. In contrast, distinct concentration minima observed at Julian Days 107, 130, and 153 correlated with advections from the West Arctic and Greenland. In the coarse mode the detectable size range was confined to 1-3 /zm ESD, due to the instrumental detection cut-off for particles larger than 3 / z m ESD. As a consequence the coarse range is only partly represented by our measurements. With this restriction, the measured particle densities were between 0.003-0.095 cm -3 with a calculated median of 0.018 cm -3 (1 o'-standard deviation: 0.015 cm -3) and thus comparably low. The concentrations tend to increase slightly from March to June. Peak concentrations were observed at Julian Days 95, 121, 170, and 175 in connection with marine air masses coming from the Northwestern Arctic. On the other hand, minima were measured at Julian Days 91, 103, 106 and 134. In these cases the air masses could be traced back to the Siberian coast (Julian Day 103), North Atlantic/Spitsbergen Sea (Julian Day 106) and the East Arctic (Julian Day 134). By comparing the particle number densities and distribution with surface ozone (Fig. 3), we found that ozone depleted air masses coincided generally with extremely low aerosol number densities in the accumulation range (ESD = 0.09-1.0/zm). In particular this is true for the first and last 0 3 minimum (Julian day 107 and 153, respectively), whereas the second and third 0 3 minimum were only associated with a weak decrease in particle number density. It should be emphasized, however, that the occurrence of the ozone minima did not coincide exactly with the decrease particle number densities in the accumulation mode. The latter decreased a few days in advance and increased again one or two days in delay. The correlation coefficient between ozone mixing ratios and aerosol number densities in the accumulation range calculated from the data measured during the whole observation period was determined to be + 0.4.

4. Discussion During winter and early spring the arctic boundary layer is influenced by long range transport of polluted air masses from lower latitudes. The prominent flow direction in winter and early spring is from Eurasia via the Arctic Ocean to North America (Barrie, 1993). The observations of Worthy et al. (1994) indicate that in spring on average 53% of all air parcels can be traced back to Russia. This is supported by our observations: In March, backward trajectories pointed to Eastern Europe as the main source region. Such polluted air parcels are enriched with 0 3 precursors like NO x and hydrocarbons (Taalas et al., 1992) so that photochemical ozone formation is possible in the sunlit regions.

Fig. 6. Five-day backward-trajectories(DWD) showing the origin and the vertical development of the air masses advected to the observation site during the low 03 event at April 17, 1994. (Circles) 971.65 hPa, (squares) 950 hPa, (triangles) 850 hPa, (diamonds) 700 hPa.

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Worthy et al. (1994) found the highest concentrations of anthropogenic pollutants in air parcels advected from Eastern Europe. This is consistent with our observations: during such periods, surface ozone levels were maximum with measured 03 mixing ratios around 50 ppbv. Lower ozone mixing ratios around 20-35 ppbv indicating background conditions were observed in the middle of March when marine air masses from the West Arctic reached the observation site. As reported by Honrath and Jaffe (1992), concentration levels of the most important 03 precursor NO x were found to be only marginal in those air parcels. Along with a seasonal change in the main atmospheric circulation in the Arctic (Barrie, 1993), a seasonal decrease in surface ozone has been observed from March through June. Now, long range transport of polluted air masses from lower latitudes into the arctic boundary layer is less pronounced due to the northward shifted polar front separating the industrial regions from the Arctic (Barrie, 1993). Consequently, the observed 03 mixing ratios in June represent background conditions mainly influenced by marine arctic air, which is also confirmed by trajectory analyses. In April, air parcels occasionally transported from the Siberian marginal zone may contain anthropogenic NO x and hydrocarbons, leading to the observed elevated 03 levels at Julian days 95-105 and 112-117. In contrast, during advection from Greenland or the western Arctic Ocean, measured 03 mixing ratios were generally low. In some cases ozone levels decreased dramatically during such flow conditions as will be described below. The vertical ozone distribution in the free troposphere changed more or less steadily from late winter to early summer. Increasing vertical ozone gradients implied intensified intrusions of ozone rich stratospheric air into the troposphere. It is known that these dynamic processes are most pronounced during arctic spring (Oltmans et al., 1989; Taalas and KyriS, 1992; Gruzdev and Sitnov, 1993; Dibb et al., 1994). Inspection of individual ozone soundings revealed layers with high ozone mixing ratios ( > 70 ppbv) associated with low relative humidities ( < 20%) indicating stratospheric intrusions. As noted by Curry and Radke (1993), even strong surface inversions do not generally prevent large scale subsidence and exchange processes with the ozone rich free troposphere. Therefore, the influence of stratospheric intrusions on the ozone budget of the boundary layer may be significant, especially during springtime. In summer, photochemical ozone formation along trajectories originating from polluted areas could be an alternative source for elevated ozone concentrations in the free troposphere. As mentioned above, boundary layer ozone showed a dramatic variability during the arctic spring. These events are connected with advection of polar marine air. As confirmed by previous measurements, surface ozone depletion could also be observed during marine arctic flow conditions at several other Arctic sites like Barrow and Alert (Anlauf et al., 1994; Bottenheim, 1993; Hopper et al., 1994). Vertical ozone profiles derived from ozone soundings launched at Alert revealed that ozone depletion was limited to the boundary layer (Anlauf et al., 1994). A capping inversion at 300-400 m altitude posed an upper limit to the extension of 03 depleted air. The observations of Anlauf et al. (1994) are in good agreement with our measurements, although in Ny ,~lesund the vertical extension of the ozone depleted boundary layer air reached 500-870 m (Fig. 5) and low ozone events occurred somewhat later than in Alert. Furthermore, only in June 03 mixing ratios below 5 ppbv could be found in Ny Alesund

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during our campaign. Solberg et al. (1994), however, observed 0 3 levels below the detection limit of 0.5 ppbv in the years 1989-1993 at the same site. The results of our aerosol measurements are comparable with previous detailed investigations performed during the Polar Sunrise Experiment 1992 in Alert. The aerosols in the coarse range can be associated with sea spray particles. In agreement with the results of Staebler et al. (1994) the number densities in this size range tend to be very low with short episodes of higher concentrations. In contrast to our observations, however, could exclusively detect peak concentrations during low ozone events. This discrepancy may be due to the fact that our instrument could not detect particles > 3 /zm diameter, while Staebler et al. (1994) referred to the size range 6 - 1 2 ~ m diameter. By means of a laser scattering particle counter, these authors determined aerosol number densities in the accumulation mode to be in the range of 50-600 cm -3, nearly an order of magnitude higher than our results. The most interesting result of our measurements were the observed striking low particle number densities in the accumulation range associated with ozone depletion. Staebler et al. (1994), however, found that surface ozone mixing ratios were anti-correlated with particles larger than 0.35 /zm diameter and positively correlated with particles _< 0.35 /zm diameter. In the first size range our results did not exhibit any significant correlation with boundary layer ozone amounts. A large fraction of particles with diameters < 1.5 /zm diameter consists of sulfuric acid (Trivett et al., 1988; Barrie and Barrie, 1990). The high burden of anthropogenic sulfate aerosols in late winter is associated with long-range-transport to the Arctic (Staebler et al., 1994), caused by increasing SO 2 photooxidation in sunlit areas at lower latitudes. It was found (Staebler et al., 1994) that the particle densities in the accumulation mode were best correlated with pollutants like PAN, CO2, and black carbon. In contrast, the second weaker maximum in June was most probably caused by photooxidation of biogenic dimethyl sulfide (Heintzenberg and Leck, 1994). From trajectory analyses it can be derived that the ozone depleted air masses originated from the central marine Arctic. In concert with the measured low particle concentrations within the accumulation mode, we assume that surface ozone depletions should coincide with low concentrations of anthropogenic pollutants. We emphasize, however, that the observed positive correlation between aerosol and ozone concentration does not prove a link between particle removal and ozone depletion; it rather indicates that the air masses were advected a much longer time across the remote Arctic than air parcels with a correspondingly higher burden of particles in the accumulation mode. It seems that a long residence time in the remote marine Arctic and reduced exchange of boundary layer air with the free troposphere were a necessary condition for ozone depletion. Hausmann and Platt (1994) estimated a characteristic ozone removal time to be in the order of 5 - 1 0 days. This calculation is based on their BrO measurements performed during the Polar Sunrise Experiment 1992 in Alert. Consequently, our results indicate that ozone destruction most probably occurred in relatively clean air masses, largely in absence of anthropogenic pollutants. Further evidence was given by our observation that air masses enriched with anthropogenic aerosols (size range: 0.1-1.0 /~m ESD) which were advected from the eastern part of the Arctic Ocean (Julian Days 95, 113, 119), never showed any significant ozone depletion. On the other hand, the correlation coefficient calculated from the data measured during the whole observation

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period is not much pronounced. This indicates that air masses may rest for quite a long period in the remote marine Arctic where aerosol removal, but not necessarily ozone depletion occurred. Finally, one may speculate about a plausible mechanism to explain the observed springtime 03 depletions. It is well known, that a liquid water layer exists at ice-grain/air interfaces. The volume or thickness of this layer depends on the temperature and the ionic strength of the dissolved compounds (Conklin and Bales, 1993). In the marine Arctic, seasalt should predominantly determine the ionic strength of this liquid layer, which is further believed to be quite acidic due to the uptake of gaseous SO 2 (Conklin and Bales, 1993) and deposited sulfate aerosols. Furthermore, the photooxidant H 2 0 2 will be preferentially dissolved in the liquid layer (Conklin et al., 1993). In acidic aqueous solutions, H 2 0 2 is capable to oxidize seasalt NaBr to Br2: H202 + 2 B r - + 2 H + ~ Br 2 + 2H20 A similar mechanism has been proposed by McConnell et al. (1992). Molecular bromine itself will accumulate in the boundary layer during the polar night, provoking a dramatic 03 depletion in the springtime, which is initiated by photolytic Br-atom liberation. As a consequence, ozone depleted air masses will be enriched with gaseous and particulate bromine (Barrie et al., 1988; Li et al., 1994). Thus bromine may originate indirectly from sea-salt Br- via heterogenous conversion on snow and ice surfaces and is liberated into the gas phase (Curry and Radke, 1993). One part of the gaseous bromine will undergo gas to particle conversation and will be found as f-Br in the fine aerosol fraction ( < 2.5 /zm, Barrie et al., 1994b). It should be noted that the measurements of Barrie et al. (1994b) indicate that the concentration of inorganic gaseous bromine compounds were generally much higher than the particulate fraction. During the polar day, however, the reactions outlined above will only result in a steady surface 03 sink, because Br 2 will instantly photolyse and not accumulate. It should be noted that the suggested heterogeneous reactions on snow and ice surfaces occurs in the marine Arctic and heterogeneous reactions on aerosols are not necessarily involved.

5. Conclusions

The observed seasonal dynamics of tropospheric ozone in Ny ,~lesund, showing highest mixing ratios during March, episodic low ozone events in April/May, and minimum mean concentrations in June, are in general agreement with previous investigations at different sites in the Arctic (Anlauf et al., 1994, Taalas and Kyr/5, 1992, Honrath and Jaffe, 1992). The contribution of pollution derived ozone to the ozone budget of the arctic boundary layer can not be quantified. Trajectory analyses, however, indicate that during the arctic springtime ozone levels were influenced by anthropogenic ozone. The characteristic tropospheric ozone depletion events turned out to be a boundary layer phenomenon. In combination with trajectory analyses and aerosol measurements we conclude that surface ozone depletion seemed to occur in unpolluted air masses, which originated from the Arctic Ocean and showed a low burden of aerosols in the accumulation range. Furthermore, low ozone mixing ratios were found to

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be generally associated with a strong elevated inversion and high ozone gradients across this layer, in agreement with the observations of Mickle et al. (1989) observations. This indicates that the boundary layer was effectively decoupled from the free troposphere aloft during massive ozone depletions. On the basis of our results and previous observations we suggest that heterogeneous reactions on ice and snow surfaces should be considered. Especially in marine environments the chemical composition and the micro-physical properties of the ice/atmosphere interface is rather complex and heterogeneous chemistry on such surfaces is not well understood. Moreover, up to now, the anthropogenic impact and the primary sources of the reactive bromine compounds are not clarified. It would be of great interest to gain information about the chemical transformations occurring in an air parcel when ozone depletion proceeds, similar to an approach performed to give evidence for photochemical ozone destruction in the arctic stratosphere (Von der Gathen et al., 1995). A campaign which combines trajectory analyses with trace gas and aerosol measurements at different sites in the Arctic could give information about the history and chemical transformations in such an air mass and should be very instructive.

Acknowledgements We would like to thank U. Schwartz, the station manager of the Koldewey Station for her assistance in launching the ozone sondes and for efforts to keep the long-term measurement program operating correctly. Furthermore, we thank Dr. V. Dreiling (University of Mainz) for helpful discussions and the German Weather Service for providing trajectories.

References Anlauf, K.G., Mickle, R.E. and Trivett, N.B.A., 1994. Measurement of ozone during the polar sunrise experiment 1992. J. Geophys. Res., 99: 25345-25353. Barrie, L.A., 1986. Arctic air pollution: an overview of current knowledge. Atmos. Environ., 20: 643-663. Barrie, L.A., Bottenheim, J.W., Schnell, R.C., Crutzen, P.J. and Rasmussen, R.A., 1988. Ozone destruction and photochemical reactions at polar sunrise in the lower arctic atmosphere. Nature, 334: 138-141. Bame, L.A. and Barrie, M.J., 1990. Chemical components of lower tropospheric aerosols in the High Arctic: Six years of observations. J. Atmos. Chem., 11: 211-226. Barrie, L.A., 1993. Features of polar regions relevant to tropospheric ozone chemistry. In: eds. H. Niki and K.H. Becker, The Tropospheric Chemistry of Ozone in the Polar Regions. NATO ASI Series, Vol. 17. Springer, Berlin, pp. 3-24. Barrie, L.A., Li, S.-M., Toom, D.L., Landsberger, S., Sturges, W., 1994. Lower tropospheric measurements of halogens, nitrates, and sulphur oxides during the polar sunrise experiment 1992. J. Geophys. Res., 99: 25453-25467. Barrie, L.A., Staebler, R., Toom, D., Georgi, B., den Hartog, G., Landsberger, S. and Wu, D., 1994. Arctic aerosol size-segregated chemical observations in the relation to ozone depletion during the polar sunrise experiment 1992. J. Geophys. Res., 99: 25439-25451. Bottenheim, J.W., Barrie, L.A., Atlas, E., Heidt, L.E., Niki, H., Rasmussen, R.A. and Shepson, P.B., 1990. Depletion of lower tropospheric ozone during the Arctic Spring: The polar sunrise experiment 1988. J. Geophys. Res., 95: 18555-18568.

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