First observations of surface ozone concentration from the summit region of Mount Everest

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L20818, doi: /2008gl035295, 2008 First observations of surface ozone concentration from the summit region of Mount Everest John L. Semple 1 and G. W. K. Moore 2 Received 10 July 2008; revised 6 September 2008; accepted 15 September 2008; published 25 October [1] The extreme height of Mount Everest is such that its summit region may periodically be in the lower stratosphere. In this regard it provides a unique location for observing the exchange of ozone between the upper troposphere and lower stratosphere. Here we report the first surface ozone measurements from the summit region of Mount Everest. Simultaneous measurements were recorded at different elevations on the north side from base camp (5676 m) to the summit (8848 m) during May The concentrations measured were as high as 70 ppb. Meteorological diagnostics suggest that the stratosphere as well as the long range transport of polluted tropospheric air masses from South East Asia are sources of the observed ozone. There is evidence that the source region for ozone in the vicinity of Mount Everest may vary with the onset of the summer monsoon. Citation: Semple, J. L., and G. W. K. Moore (2008), First observations of surface ozone concentration from the summit region of Mount Everest, Geophys. Res. Lett., 35, L20818, doi: /2008gl Introduction [2] Measurements in the Alps have shown transient elevated concentrations of ozone that are the result of both the downward excursion of ozone rich stratospheric air in the form of tropopause folds as well as the uplifting of polluted air [Stohl et al., 2003]. Extensive measurements in other mountainous regions are not common as a result of the difficulties in accessing these high and remote locations. [3] With respect to the Himalaya, Moore and Semple [2004, 2006] reported that high impact weather events on Mount Everest are often associated with the passage of tropopause folds. They hypothesized that these folds would have resulted in the presence of ozone rich stratospheric air near the summit. Moore and Semple [2005] reported on surface ozone measurements from 5000 meters above sea level, hereafter abbreviated to m, in the Bhutanese Himalaya that indicated an increase in ozone with height that was associated with the passage of a tropopause fold. Observations on the north side of Mount Everest at 5000 m recorded a diurnal pattern in ozone concentration with maxima during the daytime that was hypothesized to be the result of katabatic flow or glacier winds [Zhu et al., 2006]. It was suggested that these downslope winds were responsible for the transport of ozone rich air from the upper troposphere to the observation 1 Department of Surgery, University of Toronto, Toronto, Ontario, Canada. 2 Department of Physics, University of Toronto, Toronto, Ontario, Canada. Copyright 2008 by the American Geophysical Union /08/2008GL site [Cai et al., 2007; Song et al., 2007]. Recently surface ozone measurements made during 2006 at the ABC-Pyramid Observatory, located approximately 20 km south of Mount Everest at an elevation of 5079 m, have been reported [Bonasoni et al., 2008]. They showed that ozone concentrations at the site was higher during the pre-monsoon period than during the monsoon period. In addition, during the premonsoon period there were a number of relatively short, 1 2 day, events in which the ozone concentration was elevated. One such event was found by Bonasoni et al. [2008] to be associated with the passage of an upper-level trough of the sort described by Moore and Semple [2004, 2006]. During the monsoon period, there was an extended event, 10 days in length, that was associated with elevated levels of ozone at the observatory. As discussed by Bonasoni et al. [2008], this event was associated with the transport of a polluted air mass from the Indian subcontinent. [4] During May 2005, surface ozone concentration measurements were made at elevations from 5676 m to 8848 m on the north side of Mount Everest. The measurements were made with OMC-1108 meters that use an electrochemical sensor to determine ozone concentration. The instruments were calibrated by the manufacturer prior to deployment and the calibrations were confirmed by the authors in an ozone chamber at the Gage Occupational and Environmental Health Unit at the University of Toronto after deployment. Measurements were made twice daily, typically in the morning and afternoon. Two instruments were available allowing for simultaneous observations at different elevations and in this paper, we report on two events in which a vertical gradient in ozone was observed. On May 17, one of the authors (JLS) left the Advanced Base Camp (elevation 6400 m) as part of a supply mission to higher camps on the mountain. During this mission, ozone measurements were made at a number of different heights on the north side of Mount Everest including an observation just below Camp V on the North Ridge (elevation 7500 m) on the 19th. During the summit climb, from May 27 to May 30, simultaneous observations of surface ozone were made from the Advanced Base Camp by JLS and by members of the summit party at various camps higher up on the mountain. On May 30, a measurement of surface ozone concentration was made from the summit of Mount Everest. [5] In this paper, we discuss these two events and employ a number of meteorological diagnostics to argue that the stratosphere and the transport of pollution from South East Asia are the sources for the elevated levels of ozone observed. 2. Results [6] The results of these measurements are presented in Figure 1. With regard to the May 19th event, the measure- L of5

2 Figure 1. Surface ozone concentration (ppb) observed on the upper slopes of Mount Everest during May 2005: (a) the May 19th event and (b) the May 30th summit event. The elevations at which the observations were made are indicated. ments show a general increase in ozone concentration with height with the highest values occurring at Camp V, where a value of 70 ppb was observed. The rapid increase in ozone concentration (40 ppb) between the measurements at 7100 m and that at 7500 m over an 18 hour period suggest that the elevated levels were transient in nature. This unfortunately cannot be confirmed because operational constraints were such that the measurements were not simultaneous. With regard to the May 30th summit event, the ozone concentrations measured at 6400 m were between 0 and 10 ppb with no evidence of diurnal variability. In contrast, the measurements made at higher camps on the mountain show evidence of a clear diurnal signal with higher values measured during the afternoon and lower values during the morning. The amplitude of this signal increased with height. On May 30th, an ozone concentration of 50 ppb was recorded at the summit of Mount Everest. [7] To explore the atmospheric circulation patterns associated with these two events, the potential vorticity (PV) and horizontal wind fields on the 330 K potential temperature surface, chosen to be representative of flow near the summit, were calculated from the NCEP Reanalysis [Kalnay et al., 1996]. A comparison with measurements made during a 4 month period at the South Col of Mount Everest, elevation 8000 m, suggests that the NCEP Reanalysis is able to capture the synoptic-scale variability in the region [Moore and Semple, 2004]. PV is a widely used diagnostic that allows one to distinguish between air of tropospheric and stratospheric origin with values in the range of 1 to 2 PV units (1 PVU = 10 6 Ks 2 kg 1 ) usually serving as the cut-off [Hoskins, 1991]. In addition, back-trajectories using the NCEP reanalysis and HYSPLIT [Draxier and Hess, 1998] were computed at 3 different levels (5000 m, 6000 m and 9000 m) on Mount Everest in order to assess the impact that elevation has on the source region. All back-trajectories were 120 hours in length, although for the May 19th case only 96 hours are shown. The back-trajectory analysis was repeated using the higher resolution fields from NCEP s operational Global Data Assimilation System (GDAS). No significant differences were found. It must also be emphasized that there will be variability in the meteorological circulations, such as mountain winds and convection, that are not resolved in any global analysis as a result of its coarse resolution [Moore and Semple, 2004]. These diagnostics should therefore be interpreted as being representative of the synoptic-scale atmospheric circulation associated with these elevated surface ozone concentration events. [8] On May 19, Figure 2a indicates that there was a sharp discontinuity in PV at a latitude of approximately 35 N that represents the location of the tropopause and the subtropical jet [Luo and Yanai, 1983]. In addition, a filamentary region of high PV air known as a tropopause fold along with a jet streak on its southern flank can be seen extending southwards from mid-latitudes towards Mount Everest [Keyser and Shapiro, 1986]. The back-trajectories (Figure 2b) reflect the predominant westerly wind flow. The high wind speeds in the subtropical jet are reflected by the large distance, 10,000 km, covered by the summit back-trajectory that was situated to the west of the Greenwich Meridian four days earlier on the 16th. The 5000 m and 6000 m backtrajectories did not travel as far, 6000 km, as a result of the lower wind speeds at these heights. The height of the backtrajectories all show descent (Figure 2c). [9] The PV and horizontal wind fields on the 30th (Figure 3a) were markedly different from those on the 19th (Figure 2a). In particular the tropopause and the subtropical jet were located approximately 5 degrees northwards of their positions on the 19th and there was an anti-cyclonic circulation over the region. This change in the atmospheric circulation reflects the onset of the summer monsoon in the region [Krishnamurti, 1971]. The back-trajectories on this day (Figure 3b) are reflective of this circulation pattern. The summit back-trajectory was the longest in length with an origin over South East Asia. However its length, 5000 km, was considerably shorter than that on the 19th. The height of the summit back-trajectory also underwent a descent from a height of approximately 12 km. In contrast, the heights of the lower elevation back-trajectories were approximately constant over this period. [10] Figures 2 and 3 indicate that on both days, the summit back-trajectories descended from heights of approximately 12 km. There is however a crucial difference; the back-trajectory on the 19th originated in the mid-latitudes where the tropopause height is lower, while that on the 30th originated in the tropics where the tropopause height is higher. It is therefore possible that summit air parcels on the 19th were in fact of stratospheric origin, while those on the 30th were of tropospheric origin. To test this hypothesis, PV along the 6000 m and 9000 m back-trajectories was calculated for the two events. As discussed by Stohl and Seibert [1998], the conservation of tracers, such as PV, along trajectories is often violated as a result of discretization errors associated with the trajectory calculation as well 2of5

3 Figure 2. (a) Potential vorticity (PVU) and horizontal winds (m/s) on the 330 K potential temperature surface at 12 GMT on May The bold 1PVU contour denotes the approximate location of the tropopause. (b) Position of 96 hour long back-trajectories that end up on Mount Everest at 12 GMT on May (c) Height of the back-trajectories that end up on Mount Everest at 12 GMT on May The location of Mount Everest is indicated by the plus sign in Figures 2a and 2b. as errors in the underlying analysis fields. The PV along the May 19th summit back-trajectory was 2.3 ± 0.5 PVU, while that along the May 30th summit back-trajectory was 0.25 ± 0.25 PVU. Along both the 6000 m back-trajectories, PV was approximately 0.33 ± 0.2 PVU. The magnitude of the errors in this calculation is consistent with the findings of Stohl and Seibert [1998]. It is therefore clear that PV along the May 19th summit back-trajectory assumed stratospheric values while that along all the other back-trajectories, including the summit back-trajectory on the 30th, assumed tropospheric values. 3. Conclusions [11] In this paper, we present the first surface ozone measurements from the summit region of Mount Everest. The availability of two instruments allowed for the possibility of identifying vertical gradients in ozone through 3of5

4 Figure 3. (a) Potential vorticity (PVU) and horizontal winds (m/s) on the 330 K potential temperature surface at 12 GMT on May The bold 1PVU contour denotes the approximate location of the tropopause. (b) Position of 120 hour long back-trajectories that end up on Mount Everest at 12 GMT on May (c) Height of the back-trajectories that end up on Mount Everest at 12 GMT on May The location of Mount Everest is indicated by the plus sign in Figures 3a and 3b. simultaneous measurements at different heights. The measurements are relatively modest compared to efforts in other mountainous regions such as the Alps [Stohl et al., 2000]. However, the extreme height of Mount Everest and the concomitant difficulty in collecting data from its summit region along with its more southerly location, where it experiences both extra-tropical and tropical air masses, resulted in observations that are of interest. The results obtained complement and are consistent with more detailed observations made at lower elevations in the region [Zhu et al., 2006; Bonasoni et al., 2008]. [12] For the May 19th event, the synoptic-scale conditions were typical of those in the pre-monsoon period with the tropopause and subtropical jet just to the north of Mount Everest. As has been observed in previous studies during this period [Moore and Semple, 2004, 2006], a tropopause fold was present in the vicinity of Mount Everest suggesting that the stratosphere was most likely responsible for the elevated ozone levels. This is consistent with the back- 4of5

5 trajectories that indicated that the air parcels in the vicinity of the summit on that day were of stratospheric origin. [13] The synoptic-scale circulation on May 30th was markedly different with the tropopause displaced northwards and anti-cyclonic flow over the Tibetan Plateau. These conditions are typically found during the monsoon period [Krishnamurti, 1971]. As a result of this circulation pattern, the air parcels that were in the vicinity of the summit of Mount Everest on the 30th originated over South East Asia. This is a region known to be a source for Asian Brown Clouds and it is likely that the elevated ozone observed at the summit was associated with this phenomenon [Ramanathan et al., 2007]. The summit backtrajectory showed no evidence of ascent over the source region and this is likely that result of the inability of the reanalysis to resolve the convection responsible for the vertical transport of the pollution [Miyazaki et al., 2002]. [14] The diurnal cycle in ozone concentration that was observed to increase in amplitude with height (Figure 1b) is consistent with the observations made by Zhu [2006] and support their hypothesis that glacier winds are involved in the vertical transport of ozone in the region. No evidence of such a diurnal cycle was however observed at Advanced Base Camp during this period. Indeed the values measured at Advanced Base Camp were significantly lower than background values measured in the surrounding area [Zhu et al., 2006; Bonasoni et al., 2008]. It may be that the steep topography in the vicinity of this camp acted to shelter it from the downward transport of ozone. This hypothesis is supported by the model results of Cai et al. [2007] that show the transport associated with the glacier winds on the north face of Everest separates from the surface at an elevation of approximately 7000 m resulting in a region of low wind just below this elevation. Wind speeds at Advanced Base Camp were indeed observed to be low during the observational period. In such conditions, calibration exercises with the instrument indicated a lack of sensitivity. It is therefore likely that a combination of these two effects resulted in the low ozone concentrations observed at Advanced Base Camp. [15] Our observations and analysis also suggest a difference in temporal evolution of high ozone events on the summit before and during the monsoon period. Prior to the onset of the monsoon, the high ozone events appear to be associated with the passage of tropopause folds and as such are transient in nature. While during the monsoon, they appear to be associated with long-range transport of tropospheric air by a stable anti-cyclone over Tibet and as such may be of longer duration. This change in the nature of the surface ozone variability across the onset of the monsoon is consistent with the observations made at the ABC-Pyramid observatory [Bonasoni et al., 2008]. [16] Acknowledgments. The authors would like to thank members of the Karrimor 2005 Everest Expedition, most notably Dan Short and Ian Wade, for their assistance in making the ozone measurements as well as Bruce Urch for his assistance with the calibration of the instruments. GWKM acknowledges the support of the Natural Sciences and Engineering Research Council of Canada. References Bonasoni, P., et al. (2008), The ABC-Pyramid Atmospheric Research Observatory in Himalaya for aerosol, ozone and halocarbon measurements, Sci. Total Environ., 391, Cai, X., Y. Song, T. Zhu, W. Lin, and L. Kang (2007), Glacier winds in the Rongbuk Valley, north of Mount Everest: 2. Their role in vertical exchange processes, J. Geophys. Res., 112, D11102, doi: / 2006JD Draxier, R. R., and G. D. 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Ogawa (2002), Springtime photochemical ozone production observed in the upper troposphere over east Asia, J. Geophys. Res., 107(D3), 8398, doi: /2001jd000811, [printed 108 (D3), 2003]. Moore, G. W. K., and J. L. Semple (2004), High Himalayan meteorology: Weather at the South Col of Mount Everest, Geophys. Res. Lett., 31, L18109, doi: /2004gl Moore, G. W. K., and J. L. Semple (2005), A Tibetan Taylor Cap and a halo of stratospheric ozone over the Himalaya, Geophys. Res. Lett., 32, L21810, doi: /2005gl Moore, G. W. K., and J. L. Semple (2006), Weather and death on Mount Everest: An analysis of the Into Thin Air storm, Bull. Am. Meteorol. Soc., 87, Ramanathan, V., et al. (2007), Atmospheric brown clouds: Hemispherical and regional variations in long-range transport, absorption, and radiative forcing, J. Geophys. Res., 112, D22S21, doi: /2006jd Song, Y., T. Zhu, X. Cai, W. Lin, and L. Kang (2007), Glacier winds in the Rongbuk Valley, north of Mount Everest: 1. Meteorological modeling with remote sensing data, J. Geophys. Res., 112, D11101, doi: / 2006JD Stohl, A., and P. Seibert (1998), Accuracy of trajectories as determined from the conservation of meteorological tracers, Q.J.R. Meteorol. Soc., 124, Stohl, A., et al. (2000), The influence of stratospheric intrusions on alpine ozone concentrations, Atmos. Environ., 34, Stohl, A., et al. (2003), Stratosphere-troposphere exchange: A review, and what we have learned from STACCATO, J. Geophys. Res., 108(D12), 8516, doi: /2002jd Zhu, T., W. Lin, Y. Song, X. Cai, H. Zou, L. Kang, L. Zhou, and H. Akimoto (2006), Downward transport of ozone-rich air near Mt. Everest, Geophys. Res. Lett., 33, L23809, doi: /2006gl G. W. K. Moore, Department of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada. (gwk.moore@utoronto.ca) J. L. Semple, Department of Surgery, University of Toronto, Toronto, ON M5S 1A7, Canada. 5of5