8.3C Photochemical smog (HL) Notes

Photochemical Smog: Formation and Components

Primary Pollutants

1. Primary pollutants are directly emitted into the atmosphere through human activities or natural sources.
2. In the case of photochemical smog, the following primary pollutants play a key role:

Nitrogen Oxides ($NO_x$):

1. These are primarily emitted from vehicle exhausts, power plants, industrial processes, and agricultural activities.
2. $NO_x$ plays a crucial role in the formation of tropospheric ozone and PANs in the presence of sunlight.

Volatile Organic Compounds (VOCs):

1. They are emitted from sources such as vehicle emissions, industrial processes, solvents, and the use of gasoline and paints.
2. VOCs, when exposed to sunlight, react with $NO_x$ to form secondary pollutants, contributing to the formation of photochemical smog.

Secondary Pollutants

1. Secondary pollutants are not directly emitted but are formed in the atmosphere through chemical reactions between primary pollutants, often with the help of sunlight.
2. In photochemical smog, the key secondary pollutants are:

Tropospheric Ozone ($O_3$):

1. Ozone at ground level is a harmful secondary pollutant formed when VOCs and $NO_x$ react in the presence of sunlight.
2. Ozone is a major component of photochemical smog and can have harmful effects on human health (e.g., respiratory issues), vegetation, and materials.

Peroxyacyl Nitrates (PANs):

1. PANs are a group of organic nitrates formed by the reaction between VOCs and NOx in the presence of sunlight. They are highly reactive and toxic, often causing irritation to the eyes, lungs, and other respiratory issues.
2. PANs contribute significantly to the harmful effects of photochemical smog and can even cause damage to vegetation.

Chemical Reactions in Photochemical Smog Formation

1. Step 1: Emission of Primary Pollutants
   1. $NO_x$ and VOCs are released into the atmosphere from sources like vehicles, industrial emissions, and the use of solvents and fuels.
2. Step 2: Sunlight-Induced Transformation
   1. Under the influence of sunlight, these primary pollutants undergo a series of photochemical reactions.
   2. For example, $NO_2$ absorbs sunlight, which causes it to dissociate into nitric oxide ($NO$) and a free oxygen atom ($O$).
   3. The free oxygen atom then reacts with molecular oxygen ($O_2$) to form ozone ($O_3$).
3. Step 3: Formation of Secondary Pollutants
   1. The reactions between $NO_x$ and VOCs in the presence of sunlight produce secondary pollutants such as ozone and PANs.
   2. These pollutants accumulate in the atmosphere and contribute to the smog formation

Meteorological and Topographical Factors Intensify Photochemical Smog Formation

1. Photochemical smog is a complex mixture of pollutants, including ozone, nitrogen oxides, and volatile organic compounds (VOCs), formed through chemical reactions driven by sunlight.
2. While human activities like fossil fuel combustion provide the raw materials, meteorological and topographical factors play a critical role in intensifying smog formation.

Key Meteorological Factors

1. Abundant Insolation (Sunlight)

1. Sunlight is the catalyst for the chemical reactions that produce photochemical smog.
2. It breaks down nitrogen dioxide into nitric oxide and a free oxygen atom, which then combines with oxygen to form ozone.

2. Reduced Wind

1. Calm or still air conditions allow pollutants to accumulate rather than disperse.
2. Without wind to carry pollutants away, they remain concentrated near the ground, increasing the likelihood of smog formation.

3. Temperature Inversion

1. Under normal conditions, warm air near the ground rises, carrying pollutants away.
2. However, during a temperature inversion, a layer of warmer air traps cooler, polluted air near the surface, preventing its dispersal.

Topographical Factors

Valleys and Basins

1. Cities located in valleys or basins are more susceptible to smog because the surrounding mountains or hills act as barriers, trapping pollutants.

Urban Canyons

1. Tall buildings in densely populated cities create "urban canyons" that reduce air circulation, allowing pollutants to accumulate.

Diurnal Patterns of Photochemical Smog

Photochemical smog follows a predictable daily pattern:

1. Morning Rush Hour: Emissions of nitrogen oxides ($NO_x$) and VOCs peak due to traffic.
2. Late Morning: Sunlight breaks down $NO_2$, initiating ozone formation.
3. Early Afternoon: Ozone concentrations peak as sunlight-driven reactions reach their maximum.
4. Evening: As sunlight decreases, ozone levels drop, but pollutants like NOx and VOCs may remain.

Analyzing Urban Air Pollution with Graphs and Data

1. Graphs showing diurnal changes in urban air pollutants can help visualize how meteorological factors influence smog formation.
2. For example, a graph might show:
   1. $NO_x$ levels peaking in the morning and evening.
   2. Ozone levels peaking in the early afternoon.
   3. PM2.5 levels remaining high during temperature inversions.