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The kinetics of the formation and decomposition of hydroxymethanesulfonate (HMSA) have been examined at pH 4 and 5. The second‐order rate constant for the formation of HMSA at 25°C was measured to be 1.94 ± 0.11 and 12.60 ± 1.04 mol−1 s−1 at pH values of 4 and 5, respectively. The rate constant of HMSA decomposition to form formaldehyde (HCHO) and aqueous sulfur dioxide S(IV) at pH 4 and 5 was 4.8 ± 0.4 × 10−7 and 3.5 ± 0.2 × 10−6 s−1, respectively. In addition, no reaction between hydrogen peroxide (H2O2) and HMSA was observed when a tenfold molar excess of H2O2 was present at pH values of 4 and 5.

Recent studies of the oxidation of sulfur dioxide by hydrogen peroxide in solution indicate that this process may be the dominant mechanism for the conversion of SO2 to H2SO4 in cloud water where the typical pH is less than 5.5. In the interpretation of theoretical calculations and previous hydrogen peroxide measurements it has generally been assumed that the limiting factor in the acidification of precipitation is the amount of gas‐phase H2O2 available to the cloud‐precipitation system. Field observations of H2O2 during Acid Precipitation Experiment (APEX) missions revealed that in addition to vapor phase H2O2, there were present one or more atmospheric constituents which produced H2O2 in the aqueous phase, and there were other species which consumed the collected H2O2 vapor. We report here an investigation of these phenomena undertaken to ensure that the instrumentation was responding to H2O2 alone and to identify the species which interfered in the aqueous collection and measurement of H2O2 vapor. Our results showed that (1) the H2O2 found within the collectors frequently exceeded the amount initially present as vapor, (2) the species which produced H2O2 were relatively insoluble, (3) NO2 did not enhance H2O2 formation, (4) O3 enhanced H2O2 formation, and (5) SO2 depleted aqueous H2O2. The amount of aqueous H2O2 lost when SO2 vapor was added to ambient air was larger than expected. The results of these experiments have important implications for cloud water chemistry. If the processes occurring within the impinger can be extrapolated to droplets, the amount of H2O2 available for SO2 oxidation would not be limited by the amount of H2O2 vapor in precloud air but would also depend upon the concentration of the precursors capable of generating H2O2 in droplets.

A total halogen collection system employing ultra‐pure activated charcoal traps has been developed for use in the stratosphere aboard aircraft and balloon sampling platforms. Neutron activation techniques for low‐level chlorine, bromine, and iodine analysis within the activated charcoal sampling matrix were developed. Initial results from six aircraft flights and one balloon mission in the lower stratosphere are presented for latitudes ranging from 16°N to 67°N. Little variability was observed in twelve total, gaseous and particulate chlorine (Cltot) determinations as a function of latitude at 20 km with values ranging between 2.7 ± .9 ppbv and 3.2 ± .7 ppbv. Five total bromine (Brtot) values showed substantial variability ranging from 7 ± 4 pptv to 40 ± 11 pptv. No iodine was observed in any samples but a calculated Itot upper limit of < 3 pptv was determined.