Modelling of atmospheric chlorine and its effect on volatile organic compounds

This study uses a recently developed halogen scheme (Sherwen et al., 2016) for the GEOSChem chemical transport model to perform and in-depth investigation into tropospheric atomic chlorine. In the study we find a global atomic Cl average concentration of 495 cm-3 with the highest concentrations of 1 – 5.5 x 104 cm-3 being located in the tropical marine boundary layer in tropical regions with a preference towards the north hemisphere. Globally the reaction between HOBr and HCl on sulfate aerosol dominates the production of Cl (77 %) precursors through its production of BrCl but in the boundary layer sea-salt sources through reaction with N2O5 or iodine species can dominate, particularly in polluted coastal regions. HCl is globally the dominant Cly species (80.9 %) but in the boundary layer other compounds (notably ClNO2) can play a significant role. The rate of heterogeneous processing determines the ratio of HCl to more active Cly components. The model agrees with previous studies that Cl is a small sink compared to the OH sink (0.9 %). However, the Cl sink relative to the OH sink for ethane and propane is a more significant 15% and 6.5% respectively. There is a high degree of regional variability in the Cl removal of ethane, with the removal rising over the northern hemisphere oceans to ~50 % of OH removal at sea level and ~30 % averaged over whole column. Between 1998 and 2008 Cl concentrations increase globally due to developing regions experiencing increased Cl production due to increased concentration of pollutants NOx and SO42- and developed regions experiencing reduced loss due to decreases in NMVOC emissions. With the exception being Africa which as seen an increase in loss with minimal increase in production.