Acids and Bases - Acid deposition

Acid deposition refers to any form of precipitation (wet deposition like rain, snow, fog) or dry particles (dry deposition) with a $pH$ lower than $5.6$.

Natural rainwater is slightly acidic with a $pH$ of approximately $5.6$ due to the presence of dissolved carbon dioxide forming carbonic acid: $CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)$.

Sulfur oxides ($SO_x$) are major contributors, primarily from volcanic eruptions and the combustion of sulfur-containing fossil fuels. $SO_2$ oxidizes to $SO_3$, which reacts with water to form sulfuric acid ($H_2SO_4$).

Nitrogen oxides ($NO_x$) are formed during high-temperature combustion in internal combustion engines and by lightning. $NO_2$ reacts with water to form nitric acid ($HNO_3$) and nitrous acid ($HNO_2$).

Environmental impacts include the leaching of essential nutrients like $Mg^{2+}$ and $Ca^{2+}$ from soil and the mobilization of toxic $Al^{3+}$ ions into water systems, which damages fish gills.

Structural damage occurs when acid rain reacts with carbonate-based rocks (like limestone and marble), leading to the formation of soluble salts: $CaCO_3(s) + H_2SO_4(aq) \to CaSO_4(aq) + H_2O(l) + CO_2(g)$.

Methods to reduce $SO_x$ include pre-combustion techniques (hydrodesulfurization) and post-combustion techniques (flue-gas desulfurization using $CaO$ or $CaCO_3$).