Acids and Bases - Acid deposition

Acid deposition refers to the process by which acidic particles, gases, and precipitation leave the atmosphere. It includes wet deposition (rain, snow, fog) and dry deposition (particulates and gases).

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

Acid rain is specifically defined as rain with a $pH < 5.6$. It is primarily caused by oxides of sulfur ($SO_x$) and nitrogen ($NO_x$).

Sulfur dioxide ($SO_2$) is produced by the combustion of fossil fuels containing sulfur impurities and the smelting of sulfide ores. It oxidizes to $SO_3$ and reacts with water to form $H_2SO_3$ and $H_2SO_4$.

Nitrogen oxides ($NO$ and $NO_2$) are produced in internal combustion engines where high temperatures cause atmospheric $N_2$ and $O_2$ to react. These form $HNO_2$ and $HNO_3$ in the atmosphere.

Environmental impacts include the leaching of nutrients like $Mg^{2+}$ and $Ca^{2+}$ from soil, the mobilization of toxic $Al^{3+}$ ions into water bodies, and the degradation of marble and limestone structures ($CaCO_3$).

Pre-combustion methods to reduce $SO_x$ include hydrodesulfurization, while post-combustion methods include flue-gas desulfurization using lime ($CaO$) or limestone ($CaCO_3$).