Chemistry of the environment - Air quality and climate (3)
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1.
Photochemical smog is a significant air pollutant with detrimental effects on human health and the environment. Describe the role of oxides of nitrogen (NO and NO2) in the formation of photochemical smog. Include a discussion of the chemical reactions involved and the factors that contribute to their formation.
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:
- 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.
- 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}\$
- Reaction with Oxygen: The free radical oxygen atom reacts with molecular oxygen (O2) to form ozone (O3).
\$O + O2 \rightarrow O3\$
- NO2 as a reactant: NO2 also acts as a reactant in the formation of ozone.
- 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.
2.
The composition of clean, dry air is shown below. Describe the gases present and their key properties. Present your answer in a table.
The composition of clean, dry air consists of several gases, each with distinct properties:
| Gas | Key Properties |
| Nitrogen (N2) | Colourless, odourless, and relatively inert. It is a major component of air and helps to dilute oxygen, preventing combustion from being too rapid. |
| Oxygen (O2) | Colourless, odourless, and essential for respiration in most living organisms. It is highly reactive and supports combustion. |
| Argon (Ar) | Colourless, odourless, and inert noble gas. It is used in light bulbs to prevent the filament from oxidizing. |
| Carbon Dioxide (CO2) | Colourless, odourless, and slightly denser than air. It is a product of respiration and is important for photosynthesis in plants. It also contributes to the greenhouse effect. |
3.
Explain, using the concepts of absorption and reflection of thermal energy, how the presence of greenhouse gases like carbon dioxide and methane contributes to global warming. Include a discussion of how this process affects the amount of thermal energy radiated from the Earth.
Global warming is a consequence of the enhanced greenhouse effect caused by an increased concentration of greenhouse gases in the atmosphere. The process can be explained as follows:
- Solar Energy Input: The Earth receives energy from the sun in the form of solar radiation. Some of this radiation is reflected back into space by clouds and the Earth's surface, but a significant portion is absorbed.
- Thermal Radiation Emission from Earth: The Earth, having absorbed solar energy, emits thermal radiation (infrared radiation) back into the atmosphere. The amount and wavelength of this radiation depend on the Earth's temperature.
- Greenhouse Gas Absorption: Greenhouse gases, such as CO2 and CH4, possess molecular structures that allow them to absorb infrared radiation. This absorption occurs because the energy of the infrared radiation matches the vibrational energy levels of the greenhouse gas molecules.
- Reduced Thermal Energy Loss: After absorbing the infrared radiation, the greenhouse gas molecules re-emit the energy in all directions. A portion of this re-emitted radiation is directed back towards the Earth's surface. This effectively traps thermal energy within the atmosphere.
- Increased Atmospheric Temperature: The trapping of thermal radiation leads to an increase in the average temperature of the Earth's atmosphere and surface. This is because more heat is being retained than would otherwise be the case. The more greenhouse gases present, the more infrared radiation is absorbed and re-emitted back towards the Earth, leading to a greater warming effect.
Therefore, greenhouse gases don't prevent solar energy from reaching the Earth; they prevent thermal energy from escaping back into space. This imbalance in the energy balance of the Earth results in a gradual increase in global temperatures.