Precipitation made acidic by the presence of acids stronger than CO2(aq) is commonly called acid rain; the term applies to all kinds of acidic aqueous precipitation, including fog, dew, snow, and sleet. In a more general sense, acid deposition refers to the deposition on the earth’s surface of aqueous acids, acid gases (such as SO2), and acidic salts (such as NH4HSO4) . According to this definition, deposition in solution form is acid precipitation, and deposition of dry gases and compounds is dry deposition. Although carbon dioxide is present at higher levels in the atmosphere, sulfur dioxide, SO2, contributes more to the acidity of precipitation for two reasons. The first of these is that sulfur dioxide is significantly more soluble in water than is carbon dioxide, as indicated by its Henry’s law constant (Section 5.3) of 1.2 mol × L-1 × atm-1 compared to 3.38 × 10-2 mol × L-1 × atm-1 for CO2. Secondly, the value of Ka1 for SO2(aq)