Chemistry of Ozone

Introduction

In the troposphere near the Earth’s surface, ozone forms through the splitting of molecules by sunlight as it does in the stratosphere. However in the troposphere, nitrogen dioxide, not molecular oxygen, provides the primary source of the oxygen atoms required for ozone formation. Sunlight splits nitrogen dioxide into nitric oxide and an oxygen atom. 

NO2 + sunlight ---> NO + O

A single oxygen atom then combines with an oxygen molecule to produce ozone. 

O + O2 --->  O3

Ozone reacts readily with nitric oxide to yield nitrogen dioxide and oxygen, so it is removed quickly from the atmosphere.

NO + O3 --->  NO2 + O2

The process described above results in no net gain in ozone. 

The ozone molecule consists of three oxygen atoms that are bound together (triatomic oxygen, or O3). Unlike the form of oxygen that is a major constituent of air (diatomic oxygen, or O2), ozone is a powerful oxidizing agent. Ozone reacts with some gases, such as nitric oxide or NO, and with some surfaces, such as dust particles, leaves, and biological membranes. These reactions can damage living cells, such as those present in the linings of the human lungs. Exposure has been associated with several adverse health effects, such as aggravation of asthma and decreased lung function.

Ozone was first observed in the Los Angeles area in the 1940s. The ozone that the ARB regulates as an air pollutant is mainly produced close to ground (tropospheric ozone), where people live, exercise, and breathe. A layer of ozone high up in the atmosphere, called stratospheric ozone, reduces the amount of ultraviolet light entering the earth’s atmosphere. Without the protection of the stratospheric ozone layer, plant and animal life would be seriously harmed. In this document, ‘ozone’ refers to tropospheric ozone unless otherwise specified.

Most of the ozone in California’s air results from reactions between substances emitted from vehicles, industrial plants, consumer products, and vegetation. These reactions involve volatile organic compounds (VOCs, which the ARB also refers to as reactive organic gases or ROG) and oxides of nitrogen (NOx) in the presence of sunlight.

Formation of ozone

The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs), such as xylene, react in the atmosphere in the presence of sunlight. NOx, CO, and VOCs are called ozone precursors. Motor vehicle exhaust, industrial emissions, and chemical solvents are the major anthropogenic sources of these chemicals. Another source is windshield washer fluid. Although these precursors often originate in urban areas, winds can carry NOx hundreds of kilometers, causing ozone formation to occur in less populated regions as well. Methane, a VOC whose atmospheric concentration has increased tremendously during the last century, contributes to ozone formation but on a global scale rather than in local or regional photochemical smog episodes. In situations where this exclusion of methane from the VOC group of substances is not obvious, the term Non-Methane VOC (NMVOC) is often used.

The chemical reactions involved in tropospheric ozone formation are a series of complex cycles in which carbon monoxide and VOCs are oxidised to water vapour and carbon dioxide. The reactions involved in this process are illustrated here with CO but similar reactions occur for VOC as well. The oxidation begins with the reaction of CO with the hydroxyl radical (•OH).[1] The radical intermediate formed by this reacts rapidly with oxygen to give a peroxy radical HO2•

Peroxy radicals then go on to react with NO to give NO2 which is photolysed to give atomic oxygen and through reaction with oxygen a molecule of ozone:

HO2• + NO → •OH + NO2

NO2 + hν → NO + O(3P)

O(3P) + O2 → O3

The balance of this sequence of chemical reactions is:

CO + 2O2 + hν → CO2 + O3

The amount of ozone produced through these reactions can be calculated using the Leighton relationship.

This cycle involving HOx and NOx is terminated by the reaction of OH with NO2 to form nitric acid or by the reaction of peroxy radicals with each other to form peroxides. The chemistry involving VOCs is much more complex but the same reaction of peroxy radicals oxidizing NO to NO2 is the critical step leading to ozone formation.

Ozone  & NOx Distribution

Peak ozone concentrations are usually highest downwind from urban centers. Light winds carry ozone from urban centers, and photochemical reactions create ozone from urban emissions of VOC and NOx. Also, away from sources of NOx emissions, less NO is available to destroy ozone. Due to the time needed for transport, these peak ozone concentrations in downwind areas tend to occur later in the day compared to peak ozone concentrations in urban areas.

Due to the lack of ozone-destroying NO, ozone in rural areas tends to persist at night, rather than declining to the low concentrations (<30 ppb) typical in urban areas and areas downwind of major urban areas, that have plenty of fresh NO emissions. Ratios of peak ozone to average ozone concentrations are typically highest in urban areas and lowest in remote areas (ARB 2002).