1. Table 1: Ozone Depletion Events During TOPSE 2000 Major ODE (MODE) O 3 < 4 ppbvin-situ Partial ODE (PODE) 4  O 3  20 ppbvremote DIAL DateFltRouteLocation ~Depth (m)Type 3/2321T-TLincoln Sea150-300MODE/PODE 4/426C-TWest Lancaster S.150PODE 4/627T-TKane Basin?400hint? 4/728T-CDial: West & North1000MODE Hudson Bay (750 km) and over land 4/1029C-CMid & W. Hudson B.300-500MODE DIAL mapping first 4/1130C-JHudson B. near & 500PODE east of Churchill DIAL also over land 4/2734T-TLincoln Sea/Alert1200MODE Kane Basin/Smith S.760PODE DIAL also over N. 1000 Baffin Bay 4/2835T-CN. Foxe Basin?1500? and just east of Church.1000? 5/1940T-TLincoln Sea/Alert150-230PODE 5/2241T-WDIAL from Thule1800-MODE/ across N. Baffin Bay2400PODE to Baffin Isl. (600 km) (in-situ on take-off)1800 Fig. 21 N. Baffin Bay Kane Basin Lincoln Sea Lancaster Sound Foxe Basin Hudson Bay Fig. 1 A32A-02 Characterization of Surface Ozone Depletion Events in the Arctic and Sub-Arctic During TOPSE B. Ridley 1, E. Atlas 1, A. Weinheimer 1, F. Flocke 1, L. Emmons 1, L. Cinquini 1, D. Montzka 1, J. Walega 1, C. Cantrell 1, F. Eisele 1, L. Mauldin 1, A. Fried 1, B. Wert 1, B. Henry 1, R. Shetter 1, B. Lefer 1, S. Hall 1, J. Hannigan 1, M. Coffey 1, F. Grahek 1 E. Browell 2, J. Hair 2, C. Butler 2, W. Grant 2, R. DeYoung 2, M. Fenn 2, M. Clayton 2, V. Brackett 2, L. Brasseur 2, D. Harper 2, A. Notari 2, J. Williams 2, G. Alexander 2, G. Insley 2, J. Moody 3, A. Wimmers 3, J. Snow 4, B. Heikes 4, J. Merrill 4 A. Richter 5, R. Talbot 6, J. Dibb 6, E. Scheurer 6, G. Seid 6, D. Blake 7, N. Blake 7, R. Cohen 8, J. Thornton 8, B. Rosen 8, P. Wooldridge 8, R. Weber 9, B. Wang 9 1 NCAR, P. O. 3000, Boulder, CO, 80307, 2 NASA Langley Research Center, 3 U. of Virginia, 4 U. of Rhode Island, 5 U. of Bremen, 6 U. of New Hampshire, 7 U. of California-Irvine, 8 U. of California- Berkeley, 9 Georgia Institute of Technology TOPSE: Tropospheric Ozone Production About the Spring Equinox (http://topse.acd.ucar.edu) Table 2: ODE Encounters/Tries ODEsTriesArgue Indep. In-Situ Dial Hudson Bay 2 16flt 32 2 Foxe Basin 0 2flt 35 1? Lancaster S. 1 5flt 20 N. Baffin Bay 1 4 1 Kane Basin 1 2 1? Arctic Ocean 3 3 Hudson Bay (HB) ODE For the first time, an extensive and persistent ODE was found in the sub-Arctic over HB. It was initially detected from higher altitudes by DIAL on the Apr. 7 flight from Thule to Churchill. Figs. 3 & 4 show the ODE location over both land and west HB for ~600 km below the flight track. The first part of the next flight on Apr. 10 (Fig. 5 shows the flight track) used DIAL to successfully locate and map the ODE (Fig. 6). The remainder of the flight used a race track pattern between altitudes of 30- 260 m to investigate the ODE in-situ for ~2 hours. Fig. 7 shows that the ODE contained high aerosol (size ~0.5-2 µ). The ODE persisted in the area east of and overland west of Churchill at least until the next flight (Apr. 11). The first hour of this return flight to JeffCo examined the ODE again at low altitude. By this time, minimum ozone was elevated to ~10 ppbv but the vertical extent was still near 550 m. The increase in ozone was consistent with mixing of air from above the surface layer inversion on the basis hydrocarbon and other tracer data. Fig. x Fig. 3 ODE Fig. 6 Fig. x Fig. 8 Fig. 9 Introduction: Episodes of ozone depletion from winter levels of 30-40 ppbv to a few ppbv or less during spring in the Arctic surface layer have been well studied especially at Alert (82.5 o N, 62.3 o W) and during the ARCTOC program in N. Europe. These studies have shown that removal can occur in the surface based inversion layer over the ocean snow/ice pack when solar insolation returns and becomes sufficient for activation of reactive halogen constituents [Barrie and Platt, 1997]. Episodes have been observed well after polar sunrise into June. Catalytic destruction of ozone by Br atoms has been identified as the principal cause. The ultimate source of Br is believed to be sea salt, but identification of the processes responsible for triggering and maintaining enhanced levels of BrO x (= Br + BrO) remains ellusive. Enhanced BrO has been measured at the surface during ozone depletion events (ODEs) [Hausmann and Platt, 1994] and detected remotely from high altitude aircraft [McElroy et al., 1999] and by the GOME satellite instrument [Richter et al., 1998]. More recently, the discovery that the sunlit snow surface can be a chemically reactive medium that releases carbonyls [Sumner and Shepson, 1999], reactive nitrogen [Honrath et al., 2000; Jones et al., 2000], and other constituents to the surface layer has added complexity to understanding all of the chemical processes involved in ODEs. The snow/ice surface can also be a sink for chemical constituents especially in the dark winter period or at lower high latitudes at night. A secondary objective of TOPSE was to explore ODEs using the NASA Langley ozone/aerosol aircraft lidar (DIAL) and the suite of in-situ instruments on board the NCAR C130 aircraft. An intensive ground-based study (Alert 2000) was also ongoing during TOPSE. surfaces for periods of 15-30 min (~100-200 km) or longer in regions also identified in Fig. 1. A majority of these low level flight legs were made over Hudson Bay because many flights were made into or out of the secondary base at Churchill, Manitoba. Table 1 (below) summarizes the eight cases where ODEs were found on these 30 m legs and others that were identified by DIAL from higher altitude: In the Arctic Ocean region near and north of Alert, Lancaster Sound, Kane Basin/Smith Sound, North Baffin Bay, and Hudson Bay. Ozone as low as 40 pptv was found over the Arctic Ocean and depletion was observed to as high as 2.4 km altitude over North Baffin Bay. Various snow/ice surfaces were over-flown including nearly solid ice, leads, and polynyas but no coarse surface feature seemed to correlate with ODE presence. Observations: TOPSE con- sisted of 7 deployments (38 flights, each ~6 hr) begin- ning on Feb. 4 and ending on May 23, 2000. The flight track for deployment 6 bet- ween JeffCo and near the north pole is shown as an example in Fig. 1. Because DIAL could not always probe the surface layer due to clouds, the aircraft was flown at ~30 m above ocean MODE conditions near 63000 s. In this flight there is no evidence for significant increases in PAN or alkyl nitrates in ODEs (cf. Fig. 2). HNO 3 at 23 pptv and HOOH at 27 pptv were very small in the MODE and close to detection limits. In contrast, CH 3 OOH was more variable and averaged 112 pptv. Most species remained quite uniform independent of altitude within the MODE. The large decline in propane and ethyne indicate aged conditions consistent with the trajectories. This decline and the strong increase in soluble bromine (Br - ) confirm prior or active bromine and chlorine chemistry [Jobson et al., 1994]. NO x and the NO/NO 2 ratio both increase in the MODE and from the PODE to MODE transition. Both changes illustrate active BrO chemistry and agree with ground-based data [Ridley & Orlando, submitted]. An interesting question is whether the ODE was triggered over HB or was advected to the region. Back trajectories for the Apr. 10 flight (or for several days prior or after) indicate a source 4-10 days earlier from the Arctic Ocean region to the north and west (Fig. 8). It is perhaps surprising that the ozone mixing ratios of ~0.7 ppbv observed over HB could be maintained during low altitude transport over the moderately rough terrain. As well, a source of reactive bromine from sea salt would be expected to be sharply curtailed inland. In spite of all the caveats of low altitude trajectories, transport from the Arctic Ocean is strengthened by the satellite observations of enhanced BrO (Fig. 9). The GOME data shown is the total column, including the stratosphere, and thus has some ambiguity for the content in the lower troposphere. However, the high BrO features usually remain when allowance for the stratospheric content is made [Richter et. al., 1998]. A time sequence of GOME data is consistent with movement of Arctic Ocean air containing high BrO from north of Alaska and Russia beginning near Apr. 2 and persisting over HB region until at least Apr. 12. GOME strongly suggests that chemistry of enhanced BrO remained active during transport and while over HB, a period ≥10 days. These observations also suggest that if the trigger for the "bromine explosion" [Platt and Lehrer., 1996] occurs over the Arctic Ocean, then gaseous and or gas/aerosol interactions can maintain ozone depletion chemistry after the air mass is removed long distances from the polar ocean region. With the large Br + O 3 rate coefficient, ozone at ~0.7 ppbv is sufficient to maintain a significant fraction of BrO x as BrO as observed by GOME. Fig. 10 (below) shows the vertical structure of ozone for various soundings in the ODE. Low ozone, minimum 0.49 ppbv, was sharply capped by the inversion aloft as expected but the vertical extent varied from 300-550 m depending on location. The final ascent to the NE shows multiple sharp gradients and hints of depletion and/or mixing to ~750 m. The ODE was investigated in-situ under clear-sky con- ditions (e.g., J(NO2) = 1.4-1.6 x 10 -2 s -1 ) and Figs. 11-13 show some of the data. Many species, NO y, PAN, HOOH, HNO 3, HCs, (and O 3 ) show a decline with descent into the ODE and a further smaller decline as sampling changed from PODE to Conclusions: In the northern regions investigated during TOPSE, ODEs were relatively rare south of the Arctic Ocean and especially over Hudson Bay (cf. Table 2). This finding suggests that the trigger for active bromine resides in the Arctic Ocean region or that the long "dark" winter is required for accumulation of constituents in the snow or ice pack or to allow formation of active bromine precursors from sea salt. With one possible exception (Lancaster PODE, April 4), the occurrence of ODEs at lower latitudes resulted from surface layer transport from the Arctic Ocean. Jaeschke et al. [1999] concluded the same from their aircraft program. The Hudson Bay case showed that ODEs can be transported large distances (~1800 km) overland intact. Both GOME data and some of the in-situ data show that bromine chemistry can remain active for periods of 4-10 days during transport. With such transport, a change from continuous sunlight to day/night cycles occurs which must have a strong influence on the chemistry that will require a model evaluation. There were cases (not discussed) where transport from the Arctic Ocean to more southern regions occurred but ozone depletion was not observed. Either mixing occurred during transport or the Arctic Ocean source region was not depleted at the time. Is there a preferential region in the Arctic Ocean that triggers ODEs? Acknowledgments: We thank the National Science Foundation for support, the NCAR Research Aviation Facility, and K. Anlauf for the ozone information from the Alert 2000 study. Barrie, L. A., and U. Platt, Arctic tropospheric chemistry: An overview, Tellus 49B, 450-454, 1997. Hausmann, M. and U. Platt, Spectroscopic measurement of bromine oxide in the high Arctic during Polar Sunrise Experiment 1992, J. Geophys. Res. 99, 25399-25413, 1994. Honrath, R. E., M. C. Peterson, M. P. Dziobak, J. E. Dibb, M. Arsenault, and S. A. Green, Release of NO x from sunlight-irridiated midlatitude snow, Geophys. Res. Lett. 27, 2237-2240, 2000. Jobson, B. T., H. Niki, Y. Yokouchi, J. Bottenheim, F. Hopper, and R. Leaitch, Measurements of C2-C6 hydrocarbons during the 1992 Polar Sunrise Experiment: Evidence of Cl-atom and Br-atom chemistry, J. Geophys. Res. 99, 25355-25368, 1994. Jaeschke, W., T. Salkowski, J. P. Dierssen, J. V. Trümach, U. Krischke, and A. Günther, Measurements of trace substances in the Arctic troposphere as potential precursors and constituents of Arctic haze, J. Atmos. Chem. 34, 291-319,1999. Jones, A. E., R. Weller, E. W. Wolff, and H.-W. Jacobi, Speciation and rate of photochemical NO and NO 2 production in Antarctic snow, Geophys. Res. Lett. 27, 345-348, 2000. McElroy, C. T., C. A. McLinden, and J. C. McConnell, Evidence for bromine monoxide in the free troposphere during the Arctic Polar Sunrise, Nature 397, 338-341, 1999. Platt, U., and E. Lehrer, Arctic troposphere halogen chemistry(ARCTOC-EVSV-CT93-0318), Report to the European Community, 1996. Richter, A., F. Wittrock, M. Eisinger, and J. P. Burrows,GOME observations of tropospheric BrO in northern hemispheric spring and summer, Geophys. Res. Lett. 25, 2683-2686, 1998. Ridley, B. A., and J. J. Orlando, Active nitrogen in surface ozone depletion events at Alert during spring 1998, J. Atmos. Chem, sub. Sumner, A. L., and P. B. Shepson, Snowpack production of formaldehyde and its effect on the Arctic troposphere, Nature 398, 230- 233, 1999. The low level flight legs are arbitrarily binned according to the ozone mixing ratio: MODEs (Major depletion, O 3 20 ppbv). Of the 32 low level legs, 24 were NODEs although there are hints of depletion on flights 20 (Lancaster Sound), 32 (Hudson Bay), and 35 (Foxe Basin). In the three cases where low level flights were made near or north of Alert depletion was always found, the earliest on Mar. 23, the latest on May 19. At the Alert ground site PODEs were observed as early as Mar. 8-12 and the first MODE occurred on Mar. 16. During TOPSE ODES were found relatively rarely south of the Arctic Ocean region (Table 2, top left). A summary of some in-situ data is given in Fig. 2 (left) to give an idea of the variability and changes for the three ozone categories. Seasonal changes observed for many species over Feb. to late May have not been removed in compiling Fig. 2. Although considerable differences in mixing ratios could occur from flight to flight, ozone and other constituents (PAN, NO y, CO, HCs…) were often remarkably uniform on individual 30 m flight legs. Fig. 2: means ± s.d., medians Kane B. N. Baffin Bay Kane B. Lincoln Sea/Alert Note: Time gap Flt. 34, April 27/00 ODE: Arctic Ocean To North Baffin Bay ODE Fig. 7 Fig. 10 Soundings Over Hudson Bay Apr. 10/00 (A-ascent D-descent) The aircraft altitude is indicated by the in-situ O 3 data color coded as for DIAL ODE Fig. 4 Fig. 11Fig. 12Fig. 13 Fig. 5 Apr. 10 Track “Race track” The increase in NO x is interpreted as decreased loss via BrONO 2 formation (and subsequent surface deposition or uptake on aerosols) when ozone is small and when NO x and BrO x partitioning favor NO and Br atoms. At HB latitudes decreased loss via N 2 O 5 formation at night when O 3 is very small in MODEs compared to PODEs can also contribute. Decreased formation of HNO 3 due to reduced formation of OH with low O 3 and expected small HCHO (cf. Fig. 2) may be a small factor. Indeed, on the next day's flight, when O 3 was ~10 ppbv, NO x was reduced to below ~3 pptv. The increase in the NO/NO 2 ratio to 3.1 in the MODE is not as large as would be expected from the decline in O 3 from ~50 to 0.7 ppbv. At steady-state, with no HO 2, RO 2, or BrO, the ratio would be ~100. The smaller increase in the ratio requires an HO 2 + RO 2 abundance alone of ~20 pptv, much larger than the 5-8 pptv measured on the aircraft. The observed ratio requires BrO at a mixing ratio of 4-5 pptv. If it is assumed that that the stratospheric contribution to the column is 0.5 x 10 14 cm -2 [Richter et al., 1998], and that the remainder is uniformly distributed below 350-500 m, then GOME would give a much higher estimate of 35-45 pptv in the MODE. In spite of the discrepancy, which depends sensitively on the interpretation of the satellite data, there is good evidence that reactive halogen activity was maintained during transport from the Arctic Ocean. North Baffin Bay ODE, May 22 Fig. 18 Fig. 19Fig. 20 Fig. 22 ODE from Thule to east side of Baffin Island The last example is a late season ODE mapped by DIAL on the May 22 flight from Thule to Winnipeg. Fig. 22 shows that it extended over ~600 km from Thule to the east side of Baffin Island. During take-off minimum in-situ ozone was 1.8 ppbv which increased to 9 ppbv just below the inversion aloft at 1.8 km altitude and to 55 ppbv immediately above. Dial shows depletion to even higher altitudes (~2.4 km) to the west of Thule. On a flight just over two days earlier no ODE was found in the Kane Basin or N. Baffin Bay region. The low altitude back trajectories to Kane Basin and N. Baffin Bay were from the south. By May 22 the trajectories switched to an air mass source again from the Arctic Ocean 3-4 days earlier, and having a residence over the Ocean for at least another 6 days. However, the transit was not down the channels between Ellesmere I. and Greenland as in Flt. 34, but from almost due west or the Beaufort Sea north of western Canada. GOME also showed enhanced column BrO in the source area and extending eastward toward N. Baffin Bay. The enhancement was confined to latitudes north of 70 o and indeed no ODE was found during a 30 m leg over mid-Hudson Bay on this flight. Flight track near and north of Alert (Apr. 27/00) Fig. 14 Fig. 15 Fig. 16 Kane Basin Fig. 17 Another example of an extensive ODE was observed on the Apr. 27 flight from Thule to north of Alert and return. The transit north was at altitudes of 4-8 km until the descent to 30 m north of Alert for 3/4 hr (Fig. 14). The return was first at 5 km altitude above the channel separating Greenland and Ellesmere Isl., both of which have high surface terrain, followed by a ~230 km 30 m leg in the Kane B. region. DIAL ozone data is shown in Fig. 15. Low to mid-level clouds blocked the nadir probe over much of the channel until just before the descent north of Alert but ODEs were observed over Kane B. on the way north, just before the descent over Kane B. on the way south, and over N. Baffin Bay near Thule. The 30 m legs showed a MODE in the Arctic Ocean region (Fig. 14) and a PODE over Kane Basin. A MODE also moved over the Alert ground site on the same day. The northern edge of the ODE was sampled just before and during ascent near 83.8 o N (Fig. 14). Low altitude back trajectories for the flight region north of Alert were from the ocean north of Siberia (5 days), or confined to the ocean (10 days). Those for the Kane Basin leg (Fig. 16) also showed export from the Arctic Ocean confined to the channel, the time to Kane ~1day from the Lincoln Sea area. Those to N. Baffin Bay were a mix of northern and air masses from the south east over Greenland. The observations are consistent with ODE conditions extending from ~84 o N down the channel to N. Baffin Bay or over a distance of ~900 km. Fig. 17 gives the GOME BrO total column data. Fig. 18 shows that the ODE was sharply capped at 1100 m over the Arctic Ocean and at 500 m over Kane B with hints of depletion up to 900 m. Minimum ozone was 0.1 ppbv over the Arctic Ocean and 4.1 ppbv over Kane B. In both cases wind speeds were moderate to high, 5-10 m/s over the Arctic Ocean and surprisingly strong at 10-25 m/s with vertical shear over Kane B. The shear likely promoted mixing from above as is indicated in the Kane B. sounding. Mixing of only 6-16% of air from above the inversion is required to account for the increase in O 3 to 5-12 ppbv observed as sampling continued south over Kane B. Mixing is also consistent with the increase in many con- stituents (PAN, NO y, CO, propane, etc. and the decrease in Br - ) over Kane B. versus the Arctic Ocean (Figs. 19-20). The sensitivity of NO x and the NO/NO 2 ratio to O 3 changes in the Arctic Ocean leg and versus the Kane B. leg is again well illustrated. In this flight HCHO data were available. In the Arctic Ocean leg mixing ratios were very small, the average being less than 10 pptv but with an increase as PODE conditions were encountered near the end of the Arctic Ocean leg. HCHO was larger and positively correlated with O 3 in the Kane B. PODE, the variations being similar to those reported by Sumner and Shepson [1999] from Alert in 1998. A comparison of the Arctic Ocean data from this flight with those of the Hudson Bay MODE discussed earlier shows that many constituents have quite similar mixing ratios (PAN, NO y, NO x, CO, Br - ) but that HNO 3, HOOH, and CH 3 OOH were up to a factor of two larger. Ethyne and propane were 2-4 times smaller although some of this change is due to the seasonal decline observed during spring. The difference does suggest that the Arctic Ocean MODE was either aged longer or that reactive halogen mixing ratios were considerably larger.