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

Hydrochloric acid, measured as non-sea-salt chloride (nssCl(-)), is a ubiqu itous component of continental and marine "background" rain. with concentra tions ranging between 1.5 and 3.2 mu eq/l. The potential contribution of HC l to the acid-basic equilibrium ranges from similar to 10% to similar to 40 %: showing that this acid plays a significant role in the rain chemistry of remote regions of the world. Considering that the global amount of rainfal l is similar to 5 x 10(17) liters per year, a total deposition of 1.85 Tg/y r of nssHCl is estimated. The most important source of gaseous HCl in the b ackground atmosphere is the degassing of HCl from sea-salt aerosols: howeve r. due the simultaneous scavenging of HCl and basic CI-depleted aerosols. t his HCl does not contribute to the acidity of rain. Due to the short atmosp heric lifetime of HCl. other minor "local" sources (e.g.. volcanoes and bur ning of coal, waste and biomass) do not affect remote sites of the world. i n a significant and/or permanent way. Therefore. an additional. well-distri buted. significant source of HCl should exist in the global background atmo sphere. In one way or another, all chlorocarbons have the potential to prod uce HCl when they are oxidized in the atmosphere. From the amount of haloca rbon (i.e., CH3Cl, CH2Cl2, CHCl3, CH3CCl3, CH2ClCH2Cl, CHClCCl2, CCl2CCl2 a nd CHF2Cl) that is degradated by chemical reactions. the estimated atmosphe ric production of HCl in the gas and liquid phase is 3.4 Tg/yr and 0.78 Tg/ yr, respectively. Assuming that similar to 30% of the HCl produced in the g as phase is removed by dry deposition, it is obtained that similar to 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 p hotochemical degradation of various chlorocarbons in the troposphere. About 50% of this HCl comes from anthropogenic sources of chlorocarbons.