BROMINATED ORGANIC-SPECIES IN THE ARCTIC ATMOSPHERE

. Measurements are reported of four gas-phase, brominated organic species found in the Arctic atmosphere during March and April 1983. Volume mixing ratios for CH3Br, CH2BrCH2Br, CHBr3, and CH2Br2 were determined by GC/MS analysis from samples taken Arctic wide, including at the geographic North Pole and during a tropopause folding event over Baffin Bay near Thule, Greenland. Methyl bromide mixing ratios were reasonably constant at 11 + 4 pptv while the other three brominated organics showed a high degree of variability. Bromoform (2 to 46 pptv) was found to be the dominant contributor to gaseous organic bromine to the Arctic troposphere at 38 + 10% followed by CH2Br2 (3 to 60 pptv) at 29 + 6%. Both CH3Br and CH2BrCH2Br (1 to 37 pptv) reservoirs contained less than 20% of the organically bound bromine. Stratospheric samples, taken during a tropopause folding event, showed mixing ratios for all four species at levels high enough to support a stratospheric total volume mixing ratio of 249 pptv Br (888 ngBr/SCM). leading to an efficient catalytic destruction of ozone in the lower stratosphere. At present a major uncertainty in stratospheric bromine chemistry is the absolute concentration of bromine in both the troposphere and the stratosphere. The only measurement reported to date of an individual stratospheric bromine species was of anthropogenic CBrF3 at < 1 pptv (Fabian et al., 1981).


Introduction
The atmospheric chemistry of bromine has received attention recently because of the role that bromine compounds may play in the stratosphere. Watson (1975) first recognized that bromine could perturb stratospheric ozone in an analogous manner to chlorine. Wofsy et al.(1975) suggested that bromine atoms could be more effective than chlorine atoms in the catalytic destruction of stratospheric ozone. Yung et al. (1980) showed that the reaction BrO + C10 --Br + C1 + O2 represents a synergistic effect between bromine and chlorine, leading to an efficient catalytic destruction of ozone in the lower stratosphere. At present a major uncertainty in stratospheric bromine chemistry is the absolute concentration of bromine in both the troposphere and the stratosphere. The only measurement reported to date of an individual stratospheric bromine species was of anthropogenic CBrF3 at < 1 pptv (Fabian et al., 1981).
In the ba•ground troposphere CH3Br was identified by Singh et al. (1977) at mixing ratios in the 3 to 20 pptv range. Later, Singh et al. (1983a,b) found slightly higher concentrations. Two other brominated organic species (CH2BrCH2Br and CBrF3) have since been measured at mixing ratios less than 3 and 1 pptv, respectively (Singh et al., 1983a;Penkett et al., 1981; reviewed by Cicerone, 1981 The importance of the Arctic bromine maximum to both tropospheric and stratospheric chemistry remains unknown since the major Brg compounds that contribute to the annual peak have not been identified. We report here measurements of four g•-phase, br6minated organic species (CH3Br, CH2BrCH2Br, CHBr3, and CH2Br2) during the March/April 1983 peak period across the Arctic from Anchorage, Alaska to Bodfi, Norway, and for samples taken at the geographic North Pole and during a tropopause-folding event over Baffin Bay near Thule, Greenland.

Experimental Procedures
The present results are based on atmospheric bromine measurements from 37 whole-air (grab) samples taken from aircraft and ground sampling platforms in the Arctic. Sampling locations, dates, times, and altitudes are presented in Table 1 Each of the 37 whole-air samples was collected in an electropolished 2.5 l stainless steel cannister with a bellows-sealed valve. All components are welded to eliminate all polymers and lubricants which have been shown to arbitrarily absorb and desorb the halogenated trace gases. Following high-temperature bake-out, each of the cannisters was filled with an air sample, allowed to equilibrate several hours, and then evacuated while being warmed to 80øC. This conditioning process was repeated 6 times before final storage at < 10 -6 torr until use. Further details of cannister preparation and sample handling techniques have been published by Heidt and Ehhalt (1972) and Heidt (1978).
All samples were analyzed with a Hewlett-Packard Model 5985 gas chromatograph/mass spectrometer operated in the single-ion monitoring mode. Masses monitored were CH3Br (94, 96), CH2Br2 (93, 174), CH2BrCH2Br ( 107, 109), and CHBr3 ( 171,173). A fused silica column 30 m long x 0.255 mm OD (J. and W. Scientific) with a bonded, non-polar silicone phase (1.0/tm) accomplished the trace gas separations. The temperature profile for sample analysis was: OøC for 2 min, programmed temperature elevation at 10øC/min up to 150øC, and maintenance of 150 ø for 20 min. Approximately 1000 ml of sample was preconcentrated in the sample injection loops by pumping away the O2 and N2 while holding the glass-bead-packed stainless steel loops at -185.9øC (liquid Ar). To prevent hysteresis effects from carry-over of one sample to the next, the sample loops were heated during evacuation between sample runs. Additionally, backgrounds were run by purging with purified helium using the same preconcentration techniques employed for samples.

Results and Discussion
Four brominated organic species were identified in the Arctic atmosphere. Volume mixing ratios for CH3Br, CHBr3, CH2Br•_, and CH2BrCH,_Br are presented in Table 1 along with ozone mixing ratios. Plots of the halogen data vs. altitude above mean sea-level are presented in Fig. 1. Other brominated species were identified bu t are not reported here since we lack certified calibration standards for these compounds.

The credibility of the Present data is supported by three observations.
First, as seen in Fig. la , 1983a,b). A mean ratio for CH3Br in the Arctic troposphere was found to be 11 ñ 4 pptv, with a range of 7 to 22 pptv. Similar comparisons could not be made, however, for the other three species since we could find no comparable data sets in the literature. Second, contamination did not appear to be the source of the high levels of bromine found in most of the aircraft samples. As seen in Table 1 and Fig. 1, all of the data taken in the boundary layer and in the free troposphere are comparable. This observation, coupled with the fact that the free troposphere samples were all taken by different personnel (on different dates) employing a different sampling geometry not associated with the aircraft, supports the contention that cabin-air contamination of the flight samples was unlikely. Blank cannister runs further support this conclusion. And third, sample degradation during the time lag after collection and before analysis was not a factor as eviden ,ced by storage tests reported above. Table 1  Sluggish meteorological systems in the Arctic Basin together with strong radiative temperature inversions create stagnation almost Arctic-wide.
Rainout, washout, and other normal atmospheric deansing processes are greatly reduced due to the lack of low-level cloud formation and the near absence of precipitation events (low available water vapor due to the nearly complete ice cover and the low air temperatures). Also, there is little energy input from solar radiation during the dark Arctic winter.
As a result, the production of hy.droxy! radicals, which is initiated by the photolysis of ozone at wavelengths shorter than 315 nm in the dean background troposphere, is effectively shut down. This situation, compounded by the lack of water, inhibits the final step in the tropospheric production of hydroxyl radicals by O(•D) + H20--2OH. Similarly, production of OH from H202, HNO2 and CH20 is very limited in the background troposphere. With tropospheric OH sharply reduced to levels less than 1 x 105 cm -3, the normal removal process, OH attack on the brominated organic species, is also shut down in the Arctic atmosphere. Additionally, any photolyric destruction of these compounds is inhibited during the dark Arctic winter. The overall effect of these Arctic processes most likely is a sharp elevation in particulate and gas-phase bromine throughout the Arctic provided sources exist to aqtively feed the region. However, if the Arctic troposphere contains high levels of NOx and hydrocarbons, as in a polluted environment, several other potential OH sources may exist (e.g., oxidation of aldehydes by NOx, a process that requires no photons (Stockwell and Calvert, 1983)).
Stratospheric mixing ratios for several brominated organic compou•nds were also determined. During the 23 March mission a stratosphere-troposphere exchange event over Baffin Bay was penetrated by the aircraft and whole-air samples were taken at three altitudes. The samples at 8.8 km were taken from stratospheric air (ozone levels averaging 260 ppbv and other supporting chemical and meteorological data corroborate the stratospheric nature of this sample). For this sample CHBr3 and CH2Br2 are the major carriers of bromine, with very little organic bromine as CH3Br. If all the CHBr3, CH2Br2 CH3Br and CH2BrCH2Br in our stratospheric sample were converted to monobrominated inorganic gas its concentration would be 249 pptv (888 ngBr/SCM). The sample taken at slightly lower altitude (8411 m), which was predominantly stratospheric air, also showed these high bromine mixing ratios.