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 Intensive field measurements were carried out on the southeast coast of the Hudson Bay (55°N, 75°W) in spring 2001. The study focussed on reactive halogen chemistry and ozone/mercury depletion in the Hudson Bay region/Canadian lower Arctic. Several events of enhanced bromine oxide (BrO) coinciding with ozone depletion in the boundary layer (BL) were simultaneously measured by active longpath differential optical absorption spectroscopy (LP-DOAS) and passive multi axis differential optical absorption spectroscopy (MAX-DOAS). Significant differences in intensity and duration of ozone depletion events compared to high latitudes can be explained considering the daily alternation of daylight/nighttime which prevents complete ozone depletion within one day. First simultaneous measurements of active LP-DOAS and passive MAX-DOAS were carried out and compared. While LP-DOAS monitored precise concentration values near the surface, MAX-DOAS also captured BrO layers elevated from the surface which could not be seen by LP-DOAS.
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 During an intensive field campaign in spring 2001 at the south east coast of the Hudson Bay several periods of enhanced BrO and corresponding ozone losses have been monitored. Here we present results from both, LP-DOAS and MAX-DOAS measurements.
2. Field Experiments
 Between April 15 and May 8, 2001, an intensive field campaign was carried out in the framework of the EU project ELCID (Evaluation of the Climatic Impact of DMS). Differential optical absorption spectroscopy (DOAS, [e.g., Platt, 1994]) was employed both as active long path DOAS and as passive multi axis DOAS (MAX-DOAS, [e.g., Hönninger and Platt, 2002; Hönninger et al., 2003]) to study BrO and other halogen oxides as well as other species like ozone, NO2, SO2 and formaldehyde (HCHO).
 Measurement Site: Kuujjuarapik, Nunavik, Quebec, Canada is located at the south east coast of Hudson Bay at 55.31°N, 77.75°W. It is a small settlement of 1200 inhabitants with no local industry but some pollution from the local community power plant and the village. The DOAS instruments were set up in a container about 2 km north of the village on a hillock to allow an unobstructed view towards the Bill of Portland Islands just off the coast where the main retro reflector array was set up. In April north-westerly winds prevailed transporting Arctic airmasses (associated with enhanced BrO) to the site, while the first week of May was characterized by south-easterly winds.
 LP-DOAS: Active LP-DOAS measurements using an artificial light source (Xe lamp) were performed between April 15 and May 8, 2001. The absorption path of the LP-DOAS extended 7.6 km (15.2 km total light path) across the ice-covered Hudson Bay at an average height of 30 m above the sea ice surface. In case of low visibility due to snow, haze or rain a shorter light path (3.9 km total) was used instead to continue measurements albeit at lower sensitivity. Atmospheric absorption spectra were recorded in the UV (295 nm–375 nm), blue (390 nm–470 nm) and green (510 nm–590 nm) spectral ranges where the halogen oxides BrO, OClO, OBrO, IO, OIO as well as O3, SO2, NO2 and HCHO exhibit strongest spectral features. The time resolution was generally <30 min for each wavelength range.
 UV-spectra were analyzed for O3 and BrO absorption in the wavelength interval between 312 nm and 357 nm. This interval contains the 10 strongest absorption bands (4–0 to 13–0) of the A2Π3/2 ← X2Π3/2 transition of the BrO radical. Reference spectra of SO2, NO2, HCHO and HONO were also included in the fit. Sample fit results of the LP-DOAS evaluation for O3 and BrO are shown in Figure 1. The BrO detection limit averaged at 1.5 ppt (2σ).
 Other Halogen Oxides: A spectral fit optimized for chlorine dioxide (OClO) analysis between 331.5 nm and 354 nm yielded a mean detection limit for OClO of 1.7 ppt (at best instrument performance as low as 0.9 ppt). Iodine oxide (IO) was evaluated between 423 nm and 447 nm resulting in a mean detection limit of 1.3 ppt (minimum 0.7 ppt). The halogen dioxides OBrO and OIO were analyzed in the green spectral range between 500 nm and 550 nm with average detection limits of 5.5 ppt (minimum 0.9 ppt) for OBrO and 6.6 ppt (minimum 1.0 ppt) for OIO, respectively, using the OIO cross section by Bloss et al. .
 MAX-DOAS: From April 19 until May 8, 2001 a MAX-DOAS system was operated parallel to the LP-DOAS. The telescope received sunlight scattered in the atmosphere from 3 different elevation angles (α = 5°, 10° and 20°) above the horizon (direction north) as well as from zenith direction in a sequential mode. The azimuth angle between LP-DOAS (long light path) and MAX-DOAS viewing direction was <10°. Continuous measurements of scattered sunlight were performed in an automated measurement loop for solar zenith angles <93°. During daytime the integration time was ∼5 min for individual measurements (10 min at dawn and dusk) and <30 min for a complete MAX-DOAS series (α = 5°, 10°, 20° and 90°).
 The MAX-DOAS spectra recorded in the wavelength range from 319 nm to 381 nm were analyzed for atmospheric absorbers by the DOAS analysis procedure as previously described [e.g., Hönninger and Platt, 2002]. In order to evaluate the bromine oxide absorption reference spectra of BrO, ozone, NO2 and O4 were simultaneously fitted as well as a Fraunhofer reference spectrum (FRS) taken on April 30, 2001 at 16:25 UTC (SZA = 41°, α = 90°), a calculated Ring spectrum and a polynomial of 2nd order to remove broadband absorption and the effects of Rayleigh and Mie scattering. The spectral range for the fit was 346 nm to 359 nm. The time series of resulting BrO slant column densities (SD) was then converted to ‘tropospheric difference’ slant columns ΔS for all elevation angles α ≠ 90° by subtracting the most recent preceding zenith value SD,α=90°.
 The complete time series of ozone and BrO from active LP-DOAS measurements at the Hudson Bay are shown in the top and center part of Figure 2. Until the end of April several events with BrO mixing ratios increasing to 30 ppt were observed, coinciding with O3 decreasing from background levels (40 to 50 ppb) to 30 ppb and in several cases to <20 ppb. Strong winds and snowdrift frequently prevented LP-DOAS measurements on the long light path. During several periods extremely low visibility of <100 m did not even allow measurements on the short light path. From the beginning of May on BrO mixing ratios always remained below 5 ppt. However, still small events of halogen activation with BrO mixing ratios of 2–3 ppt and corresponding small dips in ozone levels were observed around sunrise on May 3 and May 5, 2001. As previously reported by Poissant et al.  for both O3 and mercury, during our measurements ozone depletion periods were short (several hours) and ozone was generally not completely depleted. Even O3 destruction rates of 2 ppb O3/h [Tuckermann et al., 1997] at 30 ppt BrO would take >20h to destroy 40 ppb O3. At Kuujjuarapik in April/May, 14 h of daylight are followed by 10 h darkness, during which O3 can be replenished by vertical mixing with O3 rich airmasses from higher altitudes or by advection. Only in one exceptional case complete ozone depletion was monitored when it started on April 26 after sunset. At a solar zenith angle of 98.5° ozone levels dropped to below the detection limit and at the same time BrO reached 4 ppt. Significant levels of BrO after sunset can be explained by advection of a BrO rich airmass from the north-west, in which O3 had been completely depleted over several days during the long sunshine hours at high latitudes. During this night the visibility was less than 1 km and winds from the north at >15 m/s led to drifting snow, which rendered LP-DOAS measurements impossible. Indeed, back trajectories show that airmasses in this case were directly transported to the site from the high Arctic. Starting at dawn of April 27, 2001 first MAX-DOAS measurements and, when the visibility improved after local noon, LP-DOAS measurements on the short lightpath show the presence of reactive bromine with enhanced BrO mixing ratios of 20 ppt.
 During the measurement period no halogen oxide besides BrO was identified in the measured absorption spectra. However, it should be noted that IO can even at levels of 1 ppt and less strongly accelerate the O3 loss due to combined bromine/iodine catalytic cycles and could therefore play a role in ozone destruction. If both IO and BrO were present simultaneously, OIO could have been formed in the IO self and BrO/IO cross reactions, however, the high OIO detection limit does not allow to further constrain the IO upper limit. OClO can be produced by the ClO self reaction as well as in cross reactions with BrO and IO. The average upper limit derived for the boundary layer at Kuujjuarapik was 1.7 ppt, indicating that reactive chlorine should only play a minor role during ozone depletion.
 The complete time series of MAX-DOAS BrO ΔS is shown in the bottom part of Figure 2. Like in the LP-DOAS data several events of enhanced BrO in the boundary layer were observed. These events are characterized by highest ΔS for 5° values and decreasing ΔS with increasing elevation angle (shown in different colors/symbols in Figure 2) [Hönninger and Platt, 2002; Hönninger et al., 2003].
 For the BrO data measured at the Hudson Bay the first intercomparison of the active LP-DOAS and passive MAX-DOAS techniques was possible. The LP-DOAS measurements yield average concentrations along the respective lightpath (7.6 km or 1.95 km) at an average altitude of 30 m above the Hudson Bay sea ice surface. The MAX-DOAS measurements on the other hand provide differential slant column densities (SD) for elevation angles of α = 5°, 10°, 20° and 90°. From these SD ΔS can be derived as described above. The approximate height of the BrO layer can also be derived as shown by Hönninger and Platt  and Hönninger et al. . In Figure 2 the BrO ΔS derived for the Hudson Bay MAX-DOAS measurements are shown in the bottom part (right y-axis). For the high boundary layer BrO events visible in the LP-DOAS data (center part in Figure 2) the pattern of the BrO ΔS behavior for the low elevation angles (5°, 10° and 20°, shown in different colors/symbols in Figure 2) is very similar to previous findings in the high Arctic [Hönninger and Platt, 2002], indicating a BrO layer of ≈1 km at the surface. In order to better compare the MAX-DOAS results to the LP-DOAS data, the ΔS for 5° were therefore converted to mixing ratios assuming a BrO layer of 1 km at the surface. Figure 3 shows a scatter plot of BrO from LP-DOAS versus MAX-DOAS. The data points which show a MAX-DOAS signature typical for BrO presence above the BL (enhanced ΔS, but no significant difference between 5°, 10° and 20°) [Hönninger and Platt, 2002; Hönninger et al., 2003] are shown as solid symbols and were excluded from the linear fit. For the events of enhanced BrO in the BL associated with surface ozone depletion the LP-DOAS and MAX-DOAS BrO mixing ratios agree well both in time and absolute levels. The slightly higher values measured by MAX-DOAS can be explained by the larger vertical range to which it is sensitive. The good agreement also justifies the assumption of a 1 km BrO layer and shows that ground-based MAX-DOAS can - to some extent - be applied as substitute for LP-DOAS measurements. Additionally, the MAX-DOAS results provide important information on the vertical BrO profile near the surface. On April 19, 20, 23, 25, 26, 28 and 29, 2001, when no or very small surface ozone loss was reported and the LP-DOAS measurements show only small amounts or no BrO, the MAX-DOAS ΔS indicate elevated BrO layers. These are likely to have a strong influence on the ozone budget of the polar free troposphere during spring and also indicate that vertical mixing of reactive bromine from the BL to the free troposphere is a probable mechanism during polar spring. Rapid mixing of BrO to the free troposphere was also assumed by McElroy et al.  to explain their aircraft BrO observations during polar spring. In fact, BrO can be sustained in the free troposphere by heterogeneous recycling on aerosol surfaces [e.g., Platt and Hönninger, 2003 and references therein].
 An important difference between the LP-DOAS and MAX-DOAS methods is that the passive MAX-DOAS relies on the sun as its light source. Therefore measurements are only possible during daylight hours with limited time resolution during sunrise and sunset. In contrast, active LP-DOAS measurements using an artificial light source provide the possibility of nighttime measurements and high time resolution also during dawn and dusk, when the atmospheric photochemistry involved in the ozone destruction cycles changes rapidly. No LP-DOAS measurements are possible during snowdrift and low cloud/fog episodes frequently observed during ozone depletion events in the Arctic, because no light is received from the retro reflectors at low visibility. MAX-DOAS can provide valuable information during these episodes, e.g., during the Hudson Bay measurements on April 21 and before noon on April 27, when major BrO events were observed and LP-DOAS measurements were not possible due to visibility as low as 100 m.
 The measurements presented here are the first ground-based DOAS measurements of bromine oxide at Hudson Bay. The observation of BrO into nighttime was also possible for the first time here. Hudson Bay was discovered as a major source region for tropospheric BrO by Richter et al.  and Wagner et al. , who noted that the region frequently showed enhanced BrO columns on the GOME satellite maps. These measurements were insofar indirect as the observed amounts of BrO could only be explained by the assumption of BrO in the BL at levels comparable to the ones measured by ground-based instruments in polar regions by e.g., Hausmann and Platt . However, disturbing effects of clouds or surface albedo, especially over ice, may lead to a possible bias in this approach. We performed the first direct BrO measurements at Kuujjuarapik on the south east shore of Hudson Bay and proved its abundance at levels comparable to previous measurements at high Arctic sites [Hausmann and Platt, 1994; Tuckermann et al., 1997; Martinez et al., 1999; Hönninger and Platt, 2002]. These measurements also represent the southernmost (55°N) field data for enhanced BrO during Arctic springtime associated to ozone and mercury depletion in the boundary layer [Poissant et al., 2001]. The first intercomparison of active LP-DOAS and passive MAX-DOAS in this paper showed good agreement between both methods during surface ozone depletion events. While MAX-DOAS could also capture BrO layers elevated from the ground, LP-DOAS allowed to measure the BrO decay after sunset. Due to the daily alternation of daylight and nighttime at this latitude, the location is ideal for studying day/night cycles of the key species involved in surface ozone depletion and the rapid changes in the photochemistry. A detailed analysis of the day/night cycles and comparison with results of a chemical model will be subject of a forthcoming paper.
 We are very grateful to the Centre d'Etudes Nordiques (CEN), Université Laval for the excellent support, especially C. Tremblay. We also thank L. Poissant (MSC Montréal) for his great help and support. This work was funded by the European Union under contract EVK2-CT-1999-00033.