Abstract :
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 meq/l. The potential contribution of HCl to the acid–basic equilibrium ranges from ~10%
to ~40%; showing that this acid plays a significant roˆ le 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, CH3 CCl3, CH2 ClCH2Cl,
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.
