The structure of a leaf is highly adapted to maximise the efficiency of photosynthesis. Several features contribute to this:

• Broad, flat shape: This provides a large surface area to capture maximum sunlight.
• Stomata: These are small pores, primarily on the lower surface of the leaf, that allow for the intake of carbon dioxide and the release of oxygen. The stomata are surrounded by guard cells, which regulate their opening and closing to control gas exchange and water loss.
• Mesophyll cells: These are the main photosynthetic cells of the leaf, located between the upper and lower epidermis. They are packed with chloroplasts, which contain chlorophyll and carry out photosynthesis. There are two types: palisade mesophyll (located just below the upper epidermis, with tightly packed, elongated cells optimised for light absorption) and spongy mesophyll (with irregularly shaped cells and air spaces, providing a large surface area for gas exchange).
• Vascular bundles (veins): These contain xylem and phloem. Xylem transports water and minerals from the roots to the leaves, while phloem transports glucose produced during photosynthesis to other parts of the plant. The veins also provide structural support to the leaf.

Oxides of nitrogen, primarily nitrogen monoxide (NO) and nitrogen dioxide (NO₂), play a crucial role in the formation of photochemical smog. The formation process involves a series of reactions initiated by sunlight. Here's a breakdown:

1. Formation of NO₂: NO is formed in the atmosphere through the oxidation of ammonia (NH₃) by nitrogen oxides (NO₂) in the presence of sunlight. This is a complex chain reaction.
2. Photolysis of NO₂: NO₂ absorbs UV radiation from sunlight, leading to its dissociation into NO and a free radical oxygen atom (O).

   
   

   \$NO_2 + h\nu \rightarrow NO + O + \text{radical O}\$

3. Reaction with Oxygen: The free radical oxygen atom reacts with molecular oxygen (O₂) to form ozone (O₃).

   
   

   \$O + O2 \rightarrow O3\$

4. NO₂ as a reactant: NO₂ also acts as a reactant in the formation of ozone.
5. Smog Formation: NO₂ and ozone are key components of photochemical smog. NO₂ contributes to the brownish haze, while ozone is a respiratory irritant. Other pollutants, such as volatile organic compounds (VOCs) react with NO and NO₂ in the presence of sunlight to produce a variety of secondary pollutants, including peroxyacyl nitrates (PANs), which are particularly harmful.

Factors contributing to formation:

• High concentrations of nitrogen oxides from vehicle exhaust, industrial processes, and power plants.
• Abundant sunlight.
• Presence of volatile organic compounds (VOCs) – these react with NO and NO₂.
• Warm, still weather conditions which allow pollutants to accumulate.

Carbon dioxide (CO₂) and methane (CH₄) are crucial components of the Earth's atmosphere and play a significant role in the greenhouse effect. The greenhouse effect is a natural process that warms the Earth's surface, making it habitable. However, the increasing concentrations of these gases due to human activities are enhancing this effect, leading to global warming.

Interaction with Thermal Radiation: Both CO₂ and CH₄ are effective absorbers of infrared radiation. Infrared radiation is emitted by the Earth's surface as it tries to radiate away the heat it has absorbed. The molecules of CO₂ and CH₄ have specific energy levels that correspond to the wavelengths of infrared radiation emitted by the Earth. When infrared radiation strikes these molecules, it causes them to vibrate, absorbing the energy. This absorbed energy is then re-emitted in all directions.

Reduced Thermal Energy Loss: Because greenhouse gases absorb and re-emit infrared radiation, they prevent a significant portion of the thermal energy from escaping directly into space. Instead, the energy is trapped within the atmosphere and radiated back towards the Earth's surface. This process effectively reduces the amount of thermal energy radiated from the Earth into space.

Impact of Increasing Concentrations: As the concentration of CO₂ and CH₄ in the atmosphere increases (primarily due to the burning of fossil fuels, deforestation, and agricultural practices), more infrared radiation is absorbed and re-emitted back towards the Earth. This leads to a greater trapping of heat and a subsequent increase in global temperatures. The more greenhouse gases present, the more heat is retained, and the warmer the planet becomes. This enhanced greenhouse effect is the primary driver of global warming.