**2.2. Tropospheric or ground-level ozone (O3)**

Ozone (O3) is a secondary pollutant formed in the troposphere from complex photochemical reactions following emissions of precursor gases such as NOX and non-methane volatile organic compounds (NMVOCs), deriving from paint application, road transport, drycleaning, and other solvent uses. At the continental scale, methane (CH4) and carbon monoxide (CO) also play a role in O3 formation. O3 is a powerful and aggressive oxidizing agent, causing respiratory and cardiovascular problems and leading to premature mortality. High levels of O3 can also damage plants, leading to reduced agricultural crop yields and decreased forest growth.

O3 is found not only in the troposphere but also in the upper regions of the atmosphere. Three oxygen atoms constitute O3, and it is a very reactive molecule. O3 in the upper atmosphere protects the earth from the sun's harmful rays, whereas the ground level O3 is considered as the main component of harmful smog.

In fact, the tropospheric O3 is created by chemical reactions between oxides of nitrogen (NOX) and volatile organic compounds (VOC) after its emission in the air from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapours, and chemical solvents. O3 is likely to reach unhealthy levels on hot, sunny days in urban environments and can also be transported through long distances by wind so that even rural areas can experience high O3 levels.

O3 contributes to what we typically experience as "smog" or haze, which still occurs most frequently in the summertime but can occur throughout the year in some southern and mountain regions. Ground-level O3 that we breathe can harm human health, especially affecting people with lung disease, children, older adults, and people who are active outdoors.

Children are at greatest risk from exposure to O3 because their lungs are still developing and they are more likely to be active outdoors when O3 levels are high, which increases their exposure. O3 also damages vegetation, in particular, trees and plants during the growing season, and ecosystems, including forests, parks, wildlife refuges, and wilderness areas.

### **2.3. Carbon monoxide (CO)**

The main pollutants measured to characterize and monitor the quality of the environment are

PM is one of the most important pollutants in terms of potential to harm human health, as it penetrates into low regions of the respiratory system. PM is a complex heterogeneous mixture of solid particles and liquid droplets found in the air, whose size and chemical composition change in time and space, depending on different emission sources and atmospheric and weather conditions. These particles come in many sizes and shapes and can be made up of hundreds of different chemicals. PM includes both primary and secondary PM; primary PM is the fraction of PM that is emitted directly into the atmosphere from construction sites, unpaved roads, fields, smokestacks, or fires, whereas secondary PM forms in the atmosphere following the oxidation and transformation of precursor gases (mainly SO2, NOX, NH3) and some volatile organic compounds (VOCs) that are emitted from power plants, industries and

Particle pollution includes "inhalable coarse particles," with diameters larger than 2.5 µm and smaller than 10 µm, and "fine particles," with diameters that are 2.5 µm and smaller. Smaller sizes of PM such as PM2.5, with a diameter up to 2.5 µm, are considered particularly harmful

Ozone (O3) is a secondary pollutant formed in the troposphere from complex photochemical reactions following emissions of precursor gases such as NOX and non-methane volatile organic compounds (NMVOCs), deriving from paint application, road transport, drycleaning, and other solvent uses. At the continental scale, methane (CH4) and carbon monoxide (CO) also play a role in O3 formation. O3 is a powerful and aggressive oxidizing agent, causing respiratory and cardiovascular problems and leading to premature mortality. High levels of O3 can also damage plants, leading to reduced agricultural crop yields and decreased forest

O3 is found not only in the troposphere but also in the upper regions of the atmosphere. Three oxygen atoms constitute O3, and it is a very reactive molecule. O3 in the upper atmosphere protects the earth from the sun's harmful rays, whereas the ground level O3 is considered as

In fact, the tropospheric O3 is created by chemical reactions between oxides of nitrogen (NOX) and volatile organic compounds (VOC) after its emission in the air from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapours, and chemical solvents. O3 is likely to reach unhealthy levels on hot, sunny days in urban environments and can also be transported through long distances by wind so that even rural areas can experience high O3

O3 contributes to what we typically experience as "smog" or haze, which still occurs most frequently in the summertime but can occur throughout the year in some southern and

due to their greater ability to penetrate deep into the lungs.

**2.2. Tropospheric or ground-level ozone (O3)**

the main component of harmful smog.

shown below.

422 Current Air Quality Issues

automobiles.

growth.

levels.

**2.1. Particulate Matter (PM)**

Carbon monoxide (CO) is a very dangerous gas because it is odorless and it is emitted generally from all combustion processes both natural and anthropogenic sources. Urban air pollution is characterized by high CO emissions to ambient air from vehicles movement. CO can cause several harmful health effects, all related to reduced oxygen delivery to the body's organs (especially the heart and brain) and peripheral tissues causing death at very high doses of exposure.
