7.1.1 Acid Deposition

• The process begins with the combustion of fossil fuels, which releases $SO_2$ (from sulfur-containing coal/oil) and $NO_x$ (from high-temperature air reactions in engines).

• These primary pollutants undergo oxidation and hydration in the atmosphere to form secondary pollutants: sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$).

• The general reaction for sulfur involves: $2SO_2 + O_2 \rightarrow 2SO_3$, followed by $SO_3 + H_2O \rightarrow H_2SO_4$.

• These acids are highly soluble in water, allowing them to be incorporated into cloud droplets and eventually fall as acidic precipitation.

• Wet Deposition refers to acidic chemicals that fall to the ground in liquid or solid form, such as rain, snow, sleet, or fog.

• This method is particularly effective at transporting acids over long distances, often hundreds of kilometers from the original source of pollution.

• Dry Deposition occurs when acidic gases and particles (like sulfate and nitrate dust) settle out of the atmosphere directly onto surfaces like leaves, soil, or buildings.

• Dry deposition is often more prevalent in areas close to the emission source and can be converted into liquid acid when it later reacts with surface moisture or dew.

Aquatic Ecosystems

• Acidification lowers the pH of lakes and streams, which can be lethal to fish, amphibians, and invertebrates that have narrow tolerance ranges.
• Low pH increases the solubility of aluminum ions ($Al^{3+}$) from soil; these ions are toxic to fish as they damage gills and cause respiratory failure.

Terrestrial Ecosystems

• Acid rain causes leaching, where essential nutrients like calcium ($Ca^{2+}$), magnesium ($Mg^{2+}$), and potassium ($K^+$) are washed out of the soil, leading to nutrient deficiencies in plants.
• Direct damage to foliage occurs when acid strips the protective waxy cuticle from leaves, and dry particles can block stomata, hindering gas exchange and photosynthesis.

Built Environment

• Acidic compounds react with building materials containing calcium carbonate ($CaCO_3$), such as limestone and marble, causing them to dissolve and crumble.
• Metals, particularly iron and steel, experience accelerated corrosion and rusting when exposed to acidic deposition.

• Transboundary Nature: Always remember that acid deposition is a global issue; pollutants emitted in one country often cause damage in another due to prevailing winds.

• pH Nuance: Be careful with the pH scale. A drop from pH 6 to pH 5 represents a ten-fold increase in acidity because the scale is logarithmic.

• Source Identification: Distinguish between $SO_2$ (mostly from power plants/coal) and $NO_x$ (mostly from vehicle exhausts).

• Chemical Formulas: Ensure you can identify the difference between primary pollutants ($SO_2, NO$) and the resulting secondary acids ($H_2SO_4, HNO_3$).