Chemistry

What is an Urban Heat Island according to chemistry?

The urban heat island (UHI) effect is a phenomenon where urban areas experience significantly higher temperatures than their surrounding rural regions. This temperature disparity arises due to several chemical and physical factors. Urban materials like asphalt and concrete, which have high thermal capacities, can easily absorb and retain more heat from the sun compared to natural landscapes. At night, these materials slowly release the stored heat, maintaining elevated temperatures.

Additionally, the reduction of vegetation in cities limits the natural cooling process of evapotranspiration, where plants release moisture (H2O) into the air, thereby reducing overall cooling. This absence of greenery, along with the dense concentration of buildings and infrastructure, leads to a marked increase in ambient temperatures.

Chemical pollutants and greenhouse gases also play a crucial role in the UHI effect. Emissions from vehicles, industrial activities, and residential heating and cooling systems release significant amounts of pollutants like nitrogen oxides, sulfur dioxide, volatile organic compounds (VOCs), and greenhouse gases such as carbon dioxide and methane. Not to forget, inorganic molecules that do not derive from living things, like ozone, form when sunlight reacts with chemicals in the air, collecting even more heat.

With less contribution made with energy used in households, like heat from air conditioners, adds to the overall temperature. This combination of factors makes cities much warmer, leading to higher energy use and health issues.

Chemistry - Solution

Materials with high porosity can enhance evaporative cooling. Porous pavements allow water to permeate and evaporate, reducing surface temperatures.

Properties of Porous Materials

• The measure of void spaces (pores) within a material, typically expressed as a percentage of the total volume. High porosity means more empty spaces within the material.
• The ability of a material to allow fluids (liquids or gases) to pass through it. This property is directly related to the size, shape, and connectivity of the pores.
• Porous materials generally have lower density compared to non-porous materials because of the presence of void spaces.
• Porous materials often have lower thermal conductivity because the pores disrupt heat transfer pathways, making them good insulators.
• The mechanical strength of porous materials can vary widely. Some porous materials may be weaker due to the presence of pores, while others, like certain foams, are designed to be lightweight yet strong.
• Porous materials are often effective at absorbing sound, making them useful in acoustic insulation.

Explanation of How It Works

Porous concrete and asphalt are made with larger stones and fewer fine particles, making them full of tiny gaps. When it rains, water can go through these gaps and soak into the ground instead of running off into drains. This helps prevent flooding and recharges groundwater. Additionally, when the water stored in these materials evaporates, it cools the air around them, much like how sweat cools us down. Using porous materials in roads, parking lots, and sidewalks can keep city surfaces cooler and help make urban areas more comfortable during hot weather.

Implementation

Location:

• Choose a suitable location with well-draining soil.
• Clear the area of any existing pavement or debris.
• Dig to the desired depth to create space for the pavement system.

Base Layer Installation:

• Lay down a base layer of gravel or crushed stone.
• Compact the base layer to ensure stability and proper drainage.

Geotextile Fabric:

• Install a geotextile fabric over the base layer to prevent soil erosion and provide support.

Porous Pavement Material Installation:

• Choose and install the porous pavement material (e.g., pervious concrete, permeable asphalt, porous pavers).
• Follow manufacturer's instructions for installation and compaction.

Edge Restraints:

• Install edge restraints along the perimeter of the pavement area to prevent shifting.

Joint Filling (if applicable):

• Fill any joints or gaps between pavers with a porous aggregate or material.

Surface Finishing (if applicable):

• Apply any necessary surface treatments or finishes for durability and aesthetics.

Maintenance:

• Implement a maintenance plan including debris removal, vacuuming, and pressure washing to prevent clogging.
• Inspect the pavement system periodically for any issues and make repairs as needed.

For more information, visit iTrees: Urban Heat Islands.