The detection and quantification of inorganic halogens in the remote marine boundary layer

Reactive halogens play an important role in the oxidising capacity of the troposphere, particularly in the depletion of tropospheric ozone. Therefore, a good understanding of halogen chemistry is important. The remote marine boundary layer represents one of the largest areas of the troposphere but measurements of reactive halogens in this region have been lacking, particularly for iodine compounds. This has been due to a combination of low mixing ratios, difficult to reach locations, and, in the case of iodine, suitably sensitive instrumentation.
In this thesis, a new high-resolution chemical ionisation mass spectrometer has been used to measure halogen compounds in the remote marine boundary layer, during deployment in field campaigns on the island of Bermuda. The instrument is able to alternate between employing an iodide and bromide reagent ion, allowing for the simultaneous measurement of chlorine, bromine and iodine compounds at a high sensitivity. Here, measurements of I2, HOI, Cl2 and Br2 are reported.
Calibrations for these compounds have been performed, including accounting for variations in sensitivity due to humidity. In the case of HOI, a novel calibration technique has been used, based on finding its relative sensitivity to I2.
The mixing ratios of these compounds from the campaign are compared to model predictions using the global chemistry transport model, Geos-Chem (v14.2), to assess its performance in predicting reactive halogen mixing ratios. The model was shown to overestimate nighttime emissions of I2, HOI, Cl2 and Br2, and underestimate daytime emissions of I2, Cl2 and Br2. For HOI, model predictions vastly overestimate observations by a factor of 10. The potential reasons for these discrepancies are explored to identify areas where the model can be improved.