Rain droplets, acidification

Emission of hydrogen chloride is the third most important contribution to the global acidification from human activities. The two first are SO2 and NO, The HCI is a local pollutant, contrary to the other two. It is soluble in water and easily dissolved in rain droplets and, therefore, usually falls down near the emission source. The hydrogen chlorine emissions from combustion and gasification processes has been calculated to 3,5 Mt./year, The major part of the estimated global contribution of HCI to the atmosphere is evaporation from the oceans. Even with a redeponation of 90-% HCI to the oceans, the estimated emission will reach approximately 120 Mt./year, The majority of the emitted chlorine from a combustion process will leave as HCI in the gas phase which may cause problems like corrosion and formation of dioxins. 

The other removal process, wet deposition, removes sulfur from the atmosphere as sulfates in rain. This would be the fate of sulfuric acid produced via the homogeneous oxidation of SO2, but oxidation also proceeds within droplets. Aqueous sulfur dioxide is oxidized only slowly by dissolved oxygen, but the production of sulfuric acid, which is much stronger, leads to acidification... 

It is important to note that, even in the absence of S02, atmospheric droplets will be acidic through the dissolution of C02 (Box 3.7). This has implications for the geochemistry of weathering (see Section 4.4). The S02, however, does make a substantial contribution to the acidity of droplets in the atmosphere. It can, so to speak, acidify rain (Box 3.7). However, let us consider the possibility of subsequent reactions that can cause even more severe acidification ... 

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