6.3 - Photochemical Smog

Photochemical Smog

Has significant impacts on societies and living systems. It can reduced by decreasing human reliance on fossil fuel.

It’s mainly nitrogen dioxide and ozone. The biggest contributor to this is motor vehicle exhausts in cities. It is formed when ozone, nitrogen oxides, and gaseous hydrocarbons from vehicle exhausts interact with strong sunlight.

Primary Pollutants

Emitted directly from a process. The process may be natural like volcanic eruptions or anthropogenic (human-made) from moto vehicle exhausts.

Ex:

• Carbon monoxide from incomplete combustion of fossil fuels

• Carbon dioxide

• Unburned hydrocarbons

• other sources of this include: building sites, forest fires

Secondary Pollutants

Formed when primary pollutants undergo a variety of reactions with other chemicals already present in the atmosphere. Sometimes this is a photochemical reaction in the presence of sunlight.

Ex:

• Tropospheric ozone

• peroxyacetyl nitrate (PAN)

What forms nitrogen dioxide?

When NO reacts with O₂ in the atmosphere, which causes the brown haze over many cities like Shanghai and Los Angeles

What forms tropospheric or ground-level ozone (O₃)

When NO₂ reacts with sunlight to release single, reactive oxygen atoms (O₁), which combines with (O₂)

What is the bad ozone?

Tropospheric ozone

What is the good ozone?

Stratospheric Ozone

Why is smog harmful?

• Burning eyes, throat

• Shortness of breath - coughing and wheezing

• Asthma attacks - chest pain

• Increased risk of respiratory diseases

Factors that affect photochemical smog

• Topography

• Climate

• Population Density

• Fossil Fuel Use

Topography as a factor for photochemical smog

Acts as barriers to air movement, trapping pollutants. It increases the likelihood of thermal inversion

Climate as a factor for photochemical smog

Higher temperatures increase frequency and severity since it will occur at a faster rate. Low winds cause less movement of particles

Population density as a factor for photochemical smog

Higher population density increases frequency and severity since more people leads to more cars/more electricity

Fossil fuel use as a factor for photochemical smog

Higher fossil fuel use increases frequency and severity since it produces more primary pollutants which would create more secondary pollutants

Horizontal Winds

These normally disperse pollutants from urban areas (up and out). But thermal inversions trap pollutants at ground level, usually until a storm system moves in to disrupt the inversion layer

Tropospheric Ozone

An example of a secondary pollutant, formed when oxygen molecules react with oxygen atoms that are released from nitrogen dioxide in the presence of sunlight.

It’s highly reactive and damages crops/forests/plants, irritates eyes, creates respiratory illnesses and damages fabrics and rubber materials

Thermal Inversions

Occur due to lack of air movement when a layer of dense, cool air is trapped beneath a layer of less dense, warm air.

This causes concentrations of air pollutants to build up near the ground instead of being dissipated by “normal” air movements

Possible effects of Ozone

• Damage to plants

  • tropospheric ozone is absorbed by plant leaves where the ozone degrades chlorophyll so photosynthesis and productivity are reduced.

• Damage to humans - smog (see other flashcard)

• Damage to materials and products

  • Ozone attacks natural rubber, cellulose, and some plastics

  • It reduces the lifetime of car tyres

  • It bleaches fabrics

How are particulates formed?

Through burning almost any organic material or fossil fuel releases small particles of carbon and other substances referred to as _______________.

Poorly maintained diesel engines release large amounts of ______ in exhaust fumes

Dangers of particulates:

• Our respiratory fillers the nose and hairs lining the passages of the bronchi and lungs, cannot filter them out, so they enter our bodies and stay there

  • causing asthma, lung cancer, respiratory problems, premature death

• They are carcinogenic (cancer-causing)

• In areas close to industrial/dense urban areas, crops become covered with particulates, which reduce their productivity because less sunlight reaches the leaf.

Strategy for reducing urban air pollution
(Altering the human activity producing pollution)

• Consume less, burn less fossil fuel - especially in the internal combustion engine

• Act as informed consumers for the purchase of energy-efficient technologies

• Lobby governments to increase renewable energy use

Strategy for reducing urban air pollution
(Regulating and reducing the pollutants at the point of emission)

• Government regulation/taxation

• Catalytic converters to clean exhaust of primary pollutants from car exhaust

• Fuel quality may be regulated by government

Strategy for reducing urban air pollution
(Clean up and restoration)

• Afforestation to increase carbon sinks and filter air (but this does not reduce emissions)

• Re-greening of cities - more trees, more parks - absorbs more carbon dioxide