A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination: Reactive Chlorine Emissions Inventory

Abstract

As part of the Reactive Chlorine Emissions Inventory, a global model of chemical processes in the marine boundary layer (MBL), Marine Aerosol and Gas Phase Interactions (MAGPI), was developed to calculate direct monthly production of HCl and ClNO2 from sea salt dechlorination on a 2.8 × 2.8 latitude‐longitude grid. Sea salt mass and size distributions and associated surface exchange fluxes were calculated using the Canadian General Circulation Model; integrated annual production of sea salt Cl− was 1785 Tg Cl yr−1. Corresponding distributions of gas‐phase HNO3, SO2, N2O5, H2O2, O3, H2SO4 and NH3 were calculated using different global chemical transport models in which sea salt reactions were not considered. A chemical scheme was developed to estimate the monthly mean steady‐state phase partitioning of product and reactant species at each grid point. Average annual gridded fluxes of HCl and ClNO2 varied spatially from 1 to 300 mg Cl m−2 yr−1 and from 1 to 8 mg Cl m−2 yr−1, respectively. Maxima occurred in polluted coastal regions of the North Atlantic, the western North Pacific and the Mediterranean where up to 20% of the total Cl and 80% of the sub‐micron Cl volatilized. In remote oceanic regions, available acidity was insufficient to titrate all sea salt alkalinity, thus, significant HCl was not produced via acid displacement. However, in these regions virtually all HNO3 was scavenged by sea salt. The integrated annual global fluxes for HCl and ClNO2 were 7.6 Tg Cl yr−1 and 0.06 Tg Cl yr−1, respectively; virtually all in the Northern Hemisphere. Largest HCl and ClNO2 fluxes occur in northern hemisphere winter due to high sea salt loading and elevated HNO3, SO2 and N2O5 concentrations. 70% of the HCl dechlorination occurs on particles between 0.75 μm and 4 μm radius; ClNO2 volatilized from slightly larger particles. The aerosol pH of each particle size bin equilibrates towards the same value once the alkalinity has been titrated.