School of Environmental Sciences, University of East Anglia, Norwich, United ... Department, University of Manchester Institute of Science and Technology,.
GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L03111, doi:10.1029/2003GL018956, 2004
Bromine oxide in the mid-latitude marine boundary layer A. Saiz-Lopez and J. M. C. Plane School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
J. A. Shillito Atmospheric Physics Group, Physics Department, University of Manchester Institute of Science and Technology, Manchester, United Kingdom Received 30 October 2003; revised 1 December 2003; accepted 9 January 2004; published 13 February 2004.
[1] We report direct observations of bromine oxide (BrO) in the mid-latitude marine boundary layer (MBL), using long-path Differential Optical Absorption Spectroscopy (DOAS). The measurements were made at the Mace Head observatory on the west coast of Ireland. Over six days of observations, the BrO concentration varied from below the detection limit (0.8 parts per trillion (ppt)) at night, to a maximum daytime concentration of 6.5 ppt. At the average daytime concentration of 2.3 ppt, BrO causes significant O3 depletion in the MBL through catalytic cycles involving the iodine oxide and hydroperoxy radicals, and also oxidises dimethyl sulfide much more rapidly than the hydroxyl radical. A post-sunrise pulse of BrO was observed, consistent with the build up of photolabile precursors produced by heterogeneous reactions on sea-salt aerosol during the previous night. This indicates that significant INDEX bromine activation occurs over the open ocean.
[3] The major source of gas-phase bromine in the remote MBL is almost certainly the release of species such as IBr, Br2 and BrCl from sea-salt aerosol, following the uptake from the gas phase, and subsequent aqueous-phase reactions, of hypohalous acids (HOX, where X = Br, Cl, I) [Vogt et al., 1996]. The uptake of di-nitrogen pentoxide (N2O5), formed at night by the recombination of the nitrate radical (NO3) with NO2, also leads to the release of BrNO2 [Behnke et al., 1994]. These heterogeneous mechanisms are supported by the observation that the bromide ions in sea-salt aerosols are substantially depleted [Ayers et al., 1999; Gabriel et al., 2002]. Once released into the gas phase, these bromine-containing compounds (BrX) will photolyse rapidly during the day, and the resulting atomic Br will react rapidly with O3 to form BrO:
TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0330 Atmospheric Composition and Structure: Geochemical cycles; 0365 Atmospheric Composition and Structure: Troposphere— composition and chemistry. Citation: Saiz-Lopez, A., J. M. C. Plane, and J. A. Shillito (2004) Bromine oxide in the mid-latitude marine boundary layer, Geophys. Res. Lett., 31, L03111, doi:10.1029/2003GL018956.
BrX þ hv ! Br þ X
ð1Þ
Br þ O3 ! BrO þ O2
ð2Þ
1. Introduction [2] The bromine oxide radical (BrO) is likely to play an important role in a number of processes in the lower troposphere. Attention has focused on the bromine-catalysed destruction of O3, especially in the polar boundary layer during springtime [Le Bras and Platt, 1995; Tang and McConnell, 1996]. The BrO column density has been measured by Differential Optical Absorption Spectroscopy (DOAS) using the BrO absorption bands in the near UV, both by ground-based instruments in the Arctic and the Dead Sea [Hausmann and Platt, 1994; Hebestreit et al., 1999], and by satellites [Wagner and Platt, 1998]. By contrast, the direct detection of BrO in the mid-latitude marine boundary layer (MBL) has so far been inconclusive, although spectroscopic measurements using scattered sunlight have shown that BrO may be present in the mid-latitude MBL at a level of about 1 part per trillion (ppt) [Leser et al., 2003]. Copyright 2004 by the American Geophysical Union. 0094-8276/04/2003GL018956$05.00
As we will show below, two important catalytic cycles leading to ozone depletion are reaction 2, followed either by: BrO þ IO ! Br þ I þ O2
ð3Þ
I þ O3 ! IO þ O2
ð4Þ
net :
2O3 ! 3O2
or BrO þ HO2 ! HOBr þ O2
ð5Þ
HOBr þ hv ! Br þ OH
ð6Þ
OH þ CO ! H þ CO2 ðþO2 Þ ! HO2
ð7Þ
net :
O3 þ CO þ hv ! CO2 þ O2
BrO is thus a prime candidate for explaining the substantial O3 depletion rates that have been reported in the remote MBL immediately after sunrise [Galbally et al., 2000; Nagao et al., 1999]. [4] Finally, the presence of BrO would enhance the daytime oxidation of dimethyl sulphide (DMS) in the
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MBL [Boucher et al., 2003; James et al., 2000], because the reaction CH3 SCH3 þ BrO ! CH3 SðOÞCH3 þ Br
ð8Þ
is relatively fast [Bedjanian et al., 1996].
2. BrO Measurements [5] Observations of BrO were made by DOAS at the Mace Head Atmospheric Research Station (MHARS) on the west coast of Ireland (53200N, 9540W), as part of the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) in August 2002. For a map showing the location of the MHARS and the DOAS optical path, see Figure 2 in [Allan et al., 2000]. The site is exposed to prevailing southwestwesterly winds from the Atlantic Ocean, and is situated 150 km from the main Atlantic shipping routes, and more than 80 km from trans-Atlantic air corridors [Jennings et al., 1991]. The MHARS is therefore a relatively clean site, except when impacted by semi-polluted air masses from the European continent (usually during high latitude anticyclonic fronts). [6] The DOAS instrument, which consists of a 31.5 cm diameter f/6 Newtonian telescope housing both the transmitting and receiving optics, was located at the MHARS. The broadband light beam, powered by a 450 W ozone-free Xe lamp, was directed to a small island 4.2 km to the west, and folded back to the MHARS by an array of 14 solid Suprasil quartz corner-cubes (each with a diameter of 6 cm and a precision of 20%) in the ultraviolet. It is front-illuminated, which largely eliminates the effects of e´taloning commonly found in back-illuminated cameras. The detector was cooled to 70C in order to minimise the dark current. [7] BrO was detected from the identification of eight absorption bands between 317 and 358 nm in its A23/2 X23/2 electronic transition, with a spectral resolution of 0.25 nm. For the purposes of subsequent calibration and spectral deconvolution, reference spectra of NO2 and O3 were recorded daily across the same spectral range by inserting an optical absorption cell into the optical path of the instrument. For each measurement of BrO, spectra of the transmitted light beam, scattered sunlight and the Xe lamp were accumulated for 30 min, and converted into a differential optical density (OD) spectrum using a procedure we have described in detail elsewhere [Allan et al., 2000]. The contributions of the individual absorbing species to the OD spectrum were then determined by simultaneously fitting, using singular value decomposition, a library of high resolution reference absorption cross-sections that were first coarsened to the instrument function of the DOAS spectrometer. These fitted species and the sources of their
Figure 1. Spectral fits of BrO. Atmospheric differential optical density spectra after all known absorbers in the 317– 358 nm region, except for BrO, have been subtracted (thin black line). Overlapped is the fitted BrO reference spectrum (thick black line), corresponding to [BrO] of 6.5 ± 0.7 ppt and 1.0 ± 0.9 ppt in the upper and lower panels, respectively. The lower panel shows a spectral fit where the retrieved [BrO] is close to the detection limit (0.8 ppt) obtained from the root mean square of the residual structure.
reference spectra were BrO [Wahner et al., 1988], O3 [Daumont et al., 1992], NO2 [Harder et al., 1997], HONO [Bongartz et al., 1991] and CH2O [Cantrell et al., 1990]. [8] Applying this conventional deconvolution procedure [Allan et al., 2000] revealed that whereas BrO was present in the daytime spectra, at night the BrO concentration was always below the detection limit (1.6 ppt). After the molecular absorbers had been subtracted out, the residual spectra contained small persistent structures that were identical in the daytime and night-time spectra. The source of these structures was probably zero, first- or possibly higher-order diffracted light, a tiny fraction of which was scattered off the inside walls of the spectrometer onto the CCD. The scattered light was shown, using a cut-on filter placed in front of the CCD, to be at wavelengths greater than 400 nm. Although these persistent structures did not significantly overlap the BrO absorption bands in the 320– 360 nm region (and therefore affect the retrieved BrO concentrations), they influenced the BrO detection limit because the minimum detectable optical density is given by the root-mean-square of the residual spectrum. [9] In order to remove the residual structures, an ‘‘instrumental reference’’ spectrum was constructed by averaging 16 night-time spectra. By analogy with the established procedure for measuring NO3 by DOAS [Allan et al., 1999], the daytime spectra were then divided by this night-time reference spectrum to cancel out the residual structures. This reduced the daytime BrO detection limit to 0.8 ppt, an improvement by a factor of 2. A conventional DOAS fit to the instrumental reference spectrum yielded an apparent BrO concentration of 0.006 ppt, so that 0.006 ppt was subtracted from all the retrieved daytime concentrations. Figure 1 shows two examples of BrO fits. In the top panel, BrO is clearly identified with good signal-to-noise,
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Table 1. Rates of O3 Depletion for Various Catalytic Cycles Involving BrO and IO (The Rate-Determining Step is Listed in the First Column) Rate-determining reaction
O3 removal rate/ppb hr1
BrO + IO IO + HO2 IO + IO BrO + HO2 BrO + BrO
0.13 0.08 0.06 0.03 0.01
The O3 depletion rates in the second column are calculated at 285 K, using concentrations of BrO, IO and HO2 of 3, 2, and 3 ppt, typical of Mace Head during the first two hours after sunrise in summer.
Figure 2. Observed BrO concentrations (averaged over 30 minutes): (a) 3rd August; (b) 4th August; (c) 1st August; (d) 10th August; (e) 31st August; and (f ) 1st September 2003. Black dots and white squares indicate [BrO] measured when the wind direction was from the Atlantic Ocean and from the Irish landmass, respectively. The error bars (2s standard deviations) arise from the spectral deconvolution. The average detection limit is around 0.8 ppt. Hatched and white backgrounds indicate night-time and daytime periods, respectively, and a black vertical bar separates non-consecutive days. whereas the bottom panel shows an example where the BrO optical density is close to the instrumental detection limit.
3. Results and Discussion [10] The concentration of BrO, estimated by assuming a homogeneous distribution along the DOAS light path, varied from below the instrumental detection limit (0.8 ppt) to a maximum of 6.5 ppt. An average concentration of 2.3 ppt was observed in the daytime. At night, BrO was not measured above the detection limit. These levels are in good agreement with model predictions of daytime BrO concentrations up to about 4 ppt in the remote MBL [Vogt et al., 1996]. [11] Figure 2 shows the observed BrO concentration profiles during the six days on which the radical was measured. The BrO concentrations indicated by solid black points were measured when the wind was from the oceanic sector (SW-NW). The BrO concentrations indicated by open squares were obtained when the wind was easterly, i.e. from the Irish landmass. It is interesting to note that the BrO concentrations are either small ( 1.5 ppt) or below the detection limit in easterly conditions. During the first four days of measurements (Figures 2a– 2d) there was variable
cloud cover, light winds (
