Hydrochloric acid from chlorocarbons: a significant global source of background rain acidity

Hydrochloric acid, measured as non-sea-salt chloride (nssCl⁻), is a ubiquitous component of continental and marine “background” rain, with concentrations ranging between 1.5 and 3.2 μeq/l. The potential contribution of HCl to the acid—basic equilibrium ranges from ~10% to ~40% showing that this acid plays a significant rôle in the rain chemistry of remote regions of the world. Considering that the global amount of rainfall is ~5×10¹⁷ liters per year, a total deposition of 1.8–5 Tg/yr of nssHCl is estimated. The most important source of gaseous HCl in the background atmosphere is the degassing of HCl from sea-salt aerosols; however, due the simultaneous scavenging of HCl and basic Cl-depleted aerosols, this HCl does not contribute to the acidity of rain. Due to the short atmospheric lifetime of HCl, other minor “local” sources (e.g., volcanoes and burning of coal, waste and biomass) do not affect remote sites of the world, in a significant and/or permanent way. Therefore, an additional, well-distributed, significant source of HCl should exist in the global background atmosphere. In one way or another, all chlorocarbons have the potential to produce HCl when they are oxidized in the atmosphere. From the amount of halocarbon (i.e., CH₃Cl, CH₂Cl₂, CHCl₃, CH₃CCl₃, CH₂ClCH₂Cl, CHClCCl₂, CCl₂CCl₂ and CHF₂Cl) that is degradated by chemical reactions, the estimated atmospheric production of HCl in the gas and liquid phase is 3.4 Tg/yr and 0.78 Tg/yr, respectively. Assuming that ~30% of the HCl produced in the gas phase is removed by dry deposition, it is obtained that ~3 Tg of HCl should be annually deposited in rainfall. This estimate agrees well with the “measured” amount of nssCl⁻(1.8-5 Tg/yr) deposited globally in rainfall. Therefore, this analysis suggests that a significant fraction of the HCl found in rainfall at remote sites is most likely produced in the photochemical degradation of various chlorocarbons in the troposphere. About 50% of this HCl comes from anthropogenic sources of chlorocarbons.