Page 34 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

1.3 Effects of Air Pollution and GHGs 7

• Unhealthy for sensitive groups indicates that the air quality is not acceptable
for those who are particularly sensitive to certain air pollutants. It is likely to
affect at levels lower than that of the general public: for example, children and
adults who are active outdoors and people with respiratory disease are at greater
risk from exposure to ozone. However, the general public is fine with the air.
• Unhealthy AQI values correspond to the air that the majority of the public may
begin to experience an adverse effect on health.
• Very unhealthy air triggers a health alert, meaning everyone may experience
serious health effects.
• Hazardous AQI values trigger health warnings of emergency conditions.

1.3.2 Environmental Impact

Air pollution affects not only air quality, but also indirectly the quality of
groundwater, land, vegetation, forest, and climate. Air pollutants contaminate the
clouds first and then return to Earth together with precipitation. These precipitations
carry air pollutants back to Earth and to any subjects with which they come in
contact, such as land, water, and vegetation. As introduced shortly, acid rain is an
excellent example of this case. Air pollutants, such as SO 2 and NO x , cause direct
damage to leaves of plants and trees when they enter leaves’ stomata. Chronic
exposure of leaves and needles to air pollutants can also break down the waxy
coating that helps prevent excessive water loss and damage from diseases, pests,
drought, and frost. “In the Midwestern United States crop losses of wheat, corn,
soybeans, and peanuts from damage by ozone and acid deposition amount to about
$5 billion a year” [17].

1.3.2.1 Low Visibility
Low visibility is a direct and practical perception of poor air quality. Low visibility
occurs as a result of the scattering and absorption of light by air pollutants. Both
primary and secondary particulate air pollutants contribute to visibility impairment.
Humidity can worsen the low visibility by changing the fate and sizes of the
particulate matter: for example, sulfates accumulate water and grow in size,
becoming more effective in lowering the visibility.
Visibility is also reduced by ground-level ozone, which is a secondary air pol-
lutant formed during the reaction between NO x and VOCs. Sulfate and nitrate fine
particles can suspend in air for a long period of time; and, more importantly, they
give smog its yellowish-brown color and reduce the visibility. Particles falling on
public buildings and vegetation also affect their appearance.