Precipitation and Acid Rain Study Notes

Precipitation acts as a primary vehicle for heat transport within the hydric cycle.
Latent heat is released during the process of precipitation, which serves to balance the latent heat required for the initial evaporation of water.
Water evaporates and then condenses to form clouds as temperatures cool at higher altitudes. These clouds are transported to different locations or higher latitudes where the water eventually precipitates.
Precipitation is a vital component of water management, specifically regarding the hydric cycle. For example, the volume of snow falling impacts the recharge of reservoirs via spring snowmelt.
In terms of safety and protection, understanding the timing of snowmelt is essential; delays in this process can result in substantial flooding.

Human activity impacts the water cycle through the acidification of precipitation, which includes both rain and snow.
The primary origin of acidification is coal combustion used for electricity generation or metallurgical works.
Byproducts of combustion include carbon dioxide ( $CO_2$ ), sulfur dioxide ( $SO_2$ ), and nitrogen oxides ( $NO_x$ ). These gases combine with atmospheric water vapor, initiating chemical reactions that form sulfuric acid ( $H_2SO_4$ ) and nitric acid ( $HNO_3$ ).
Acidic precipitation is detrimental to human health, aquatic life, and vegetation.
When acids reach the ground, they can cause the leaching of acids into soils and increase the exposure of bodies of water to aluminum, leading to aquatic life die-offs.
Wet deposition refers to acidification occurring through acid rain, acid snow, or acid fog.
Dry deposition occurs when dust produced by coal combustion contains acids. When water condenses on surfaces where this dust has settled, liquid acid forms. For example, acid dew forming on a leaf surface can damage vegetation.

The specific form of precipitation is determined by the temperature profiles of the atmosphere from approximately $3,000\,m$ in altitude down to ground level.
Standard Rain Profile: Temperature starts below the freezing point at high altitudes (near $3,000\,m$ ) and warms significantly as it reaches the ground (e.g., $20^\circ\text{C}$ ).
Snow Profile: The entire temperature profile from $3,000\,m$ to the ground level remains below the freezing point ( $0^\circ\text{C}$ ).
Freezing Rain (Glaze) and Sleet Profiles: These involve temperature inversions. Precipitation may start as snow, enter a warm air mass at approximately $1,500\,m$ where temperatures rise above freezing, and then dip back below the freezing point (e.g., $-10^\circ\text{C}$ ) near the ground level.
Consequences of Freezing Rain:
- Ice coats surfaces such as power lines and tree branches.
- The accumulated mass of the ice often exceeds the weight-bearing capacity of these surfaces, causing them to break.
- Notable events include the December 2013 widespread power outages in Toronto and the 1998 ice storm in eastern Ontario and southern Quebec, where some locations lost power for nearly two months.

For simplicity in this context, acid is defined as protons or hydrogen ions ( $H^+$ ).
Acids can be buffered by substances such as calcium carbonate ( $CaCO_3$ ).
The chemical reaction for buffering involves the dissociation of the ionic bond between calcium and carbonate, where two protons react with an oxygen atom from the carbonate group:
- $CaCO_3 + 2H^+ \rightarrow Ca^{2+} + H_2O + CO_2$
The reaction produces water ( $H_2O$ ), carbon dioxide ( $CO_2$ ), and soluble calcium ( $Ca^{2+}$ ).
Liming is the application of buffers—basic materials with high pH, such as calcium or magnesium-rich substances—to soil or aquatic ecosystems to neutralize acids and maintain natural pH levels.

Industrial scrubbers are used to prevent acidic gases from entering the atmosphere.
In a scrubber system, combustion gases from a boiler ( $CO_2$ , $SO_2$ , $NO_x$ ) pass through a chamber where a water-calcium mixture is sprayed on them.
This results in a reaction where sulfur is captured as calcium sulfite ( $CaSO_3$ ) in a collection tank at the bottom.
The gases that eventually exit the smokestack are primarily water vapor and carbon dioxide ( $CO_2$ ), which significantly reduces the potential for acidic precipitation.

The susceptibility of an environment to acidification depends on the potential of its soils and bedrock to act as natural buffers.
Regions with high buffering potential (natural ability to reduce acidity) include the Prairies, Southern Ontario, the Golden Horseshoe, and Southern Quebec. These regions are rich in materials like limestone, granite, and feldspar.
Regions with low buffering potential include much of the Arctic and certain patches near James Bay, often associated with the Canadian Shield.
In the late 1970s and early 1980s, acidification was a major problem in parts of Ontario and Quebec due to coal combustion in the United States. Wind carried these gases to Canada, where they precipitated and damaged ecosystems with poor buffering capacity, leading to forest die-offs.
International agreements between Canada and the United States, along with the adoption of scrubber technology, have since caused this problem to substantially disappear.

Sudbury is famous for the Superstack, a very tall smokestack designed to dissipate acidic emissions away from the city to reduce local pollution.
Historical mining and smelting operations in Sudbury led to high sulfur emissions that severely acidified the ground and destroyed vegetation for a century.
The landscape was so barren that NASA sent astronauts to Sudbury to practice for lunar environments, as it was one of the closest terrestrial matches to the surface of the Moon.
Rehabilitation efforts included sulfur dioxide ( $SO_2$ ) emission controls and extensive land reclamation.
The primary tool for soil rehabilitation was liming to change chemical conditions so vegetation could grow. By 2008 and 2018, photographs showed that vegetation, including acid-resistant species, had largely rebounded.

Problem Statement: Calculate the amount of calcium carbonate ( $CaCO_3$ ) required to neutralize $400\,mm$ of precipitation with a pH of $2$ over an area of $0.75\,hectares$ of soil.
Required Constants and Conversions:
- Weight of $1\,mole$ of $CaCO_3 = 100\,g$ .
- $1\,hectare = 100\,m \times 100\,m$ (equivalent to $10,000\,m^2$ ).

How can questions regarding the material be addressed?
- Address questions via email or preferably by posting on the Avenue2Learn discussion board. Questions can also be asked during the Thursday meeting.