Grab samples for nonmethane hydrocarbons (NMHCs) were collected from three sites: daily at Alert, Northwest Territories (82.5° N, 62.3° W) from January 21 to April 19, daily at an ice floe 150 km north of Alert from April 9 to 25, and on an aerial survey conducted in April over the Arctic archipelago. Insitu measurements of n‐butane and i‐pentane were also made hourly at Alert from April 2 to April 15. During the dark period (January to March), C2‐C6 hydrocarbon concentrations correlated with those of methane. Concentrations declined gradually from January to April, consistent with removal by HO radicals. On the other hand, during low‐ozone periods in April an additional decrease in NMHC concentrations and change in distribution were observed. Concentration changes of alkanes were correlated to Cl atom reaction rate constants. Acetylene displayed a greater change in concentration than predicted from chlorine kinetics, possibly indicating additional removal by Br atoms. The Br atom concentration derived from the depletion of acetylene can account for the low‐ozone concentrations periodically observed at Alert. The estimated Cl atom concentration is too small to be a significant loss mechanism for ozone. Thus the data from Alert and the ice floe site provide evidence for Cl and Br atom chemistry during the ozone depletion episodes observed at polar sunrise.

result by pulse radiolysis of solid PMMA.16 Consequently, if mechanism 3 is valid, positive charge may transfer in the polymer to react with the polymer anion.

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Results are presented from the 2‐month Polar Sunrise Experiment 1988, which was undertaken to further investigate the cause of ozone destruction during spring in the lower Arctic atmosphere. A strong anticorrelation between the decrease in ozone and concurrent increase in bromine compounds collected on filters observed during a 21‐day period in 1986 was confirmed. It is shown that the reason for this observation is a meteorological modulation which alternately brings to the sampling location lower boundary layer air, depleted in ozone and enriched in filterable bromine, and free tropospheric air with abundant ozone and few bromine compounds. Several other compounds that may have a bearing on the chemical reactions leading to ozone depletion were measured. Bromoform, a potential source for Br atoms, was found to be present at levels between 1 and 10 parts per trillion by volume (pptv); good evidence was obtained that, as with filterable bromine, it had increased in ozone depleted boundary layer air. The mean NO2 level was 85 pptv with an estimated uncertainty of a factor less than 2. However, this measurement of NO2 may have been the sum of NO2+N2O5. The possibility that Br atoms are formed from an N2O5+NaBr interaction is therefore not ruled out. There was a good indication that apparent NO2 was depleted during episodes of low ozone. An upper limit for the mixing ratio of formaldehyde was found to be 39 pptv, while acetaldehyde was observed at mixing ratios of ca. 65 pptv. Ethylene and acetylene were also found to be depleted concurrently with O3. These compounds do react with Br radicals in air (as do the aldehydes) and it is speculated that they may create a link with HOx. chemistry. Data on several other chemical compounds are also presented. Their participation in the O3 depletion chemistry is less clear. The implications of the measurements with respect to the hypothesis that the ozone depletion are due to a BrOx‐O3 destruction cycle are explored.

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588between the involved species. The study also demonstrates the general use of the applied magnetization-transfer NMR technique in unraveling complex exchange schemes.The rate constant for the reaction CI + CH3CH0 -CH3C0 + HCI was determined to be 7.6 X lo-" cm3 molecule-' s-' at 298 -+ 2 K, using the competitive chlorination method with the reaction CI + C2H6 as the reference. An alternative reaction channel leading to the formation of CH2CH0 + HCI contributed less than 1% to the total CI-CH3CH0 reaction. In the C1-atom-initiated oxidation of HCHO-CH3CH0 mixtures, the reaction scheme H 0 2 + CH3C(0)02 -CH3C(0)OOH + O2 (-75%) and CH3C(0)OH + O3 (-25%) could account for the indicated products detected by the FTIR method.

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Abstract. Air samples for nonmethane hydrocarbon (NMHC) analysis were collected at two ground-based sites: Alert, Northwest Territories (82.5øN, 62.3øW) and Narwhal ice camp, an ice floe 140 km northwest of Alert, from Julian days 90 to 117, 1994, and on a 2-day aerial survey conducted on Julian days 89 and 90, 1994 over the Arctic archipelago. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations relative to their background levels were observed at Alert and Narwhal ice camp. At Nap•hal, a long period (>-7 days) of ozone depletion was observed during which a clear decay of alkane concentration occurred. A kinetic analysis led to a calculated C1 atom concentration of 4.5 x 103 cm -3 during this period. Several low-ozone periods concurrent with NMHC concentration decreases were observed over a widespread region of the Arctic region (82ø-85øN, and 51ø-65øW). Hydrocarbon measurements during the aerial survey indicated that the low concentrations of these species occurred only in the boundary layer. In all ozone depletion periods, concentration changes of alkanes and toluene were consistent with C1 atom reactions. The changes in ethyne concentration from its background level were in excess of those expected from C1 atom kinetics alone and are attributed to additional Br atom reactions. A box modeling exercise suggested that the C1 and particularly Br atom concentrations required to explain the hydrocarbon behavior are also sufficient to destroy ozone. IntroductionThe depletion of O3 in the boundary layer at the onset of 24 hour daylight in the spring has been reported for several locations in the high Arctic such as Alert ( Hence the relative rates of removal of hydrocarbons can be used as an indicator for the dominance of particular atmospheric photochemical reactions such as those initiated by HO radicals or those started by C1 or Br atoms. 13,169