Unit 4 - Tropospheric Ozone and Carbon Monoxide

Is Tropospheric ozone a primary or secondary pollutant? What does this mean?

It is a secondary pollutant - this means that it is produced by chemical reactions between one or more primary pollutants in the atmosphere

How is tropospheric ozone produced?

Tropospheric ozone is produced by photochemical reactions involving oxides of nitrogen. Nitrogen is broken down by UV-A sunlight to form Nitrogen monoxide and monatomic oxygen. The monatomic oxygen then reacts with diatomic oxygen to produce ozone (triatomic oxygen).

What are the main differences between stratospheric and tropospheric ozone?

Stratospheric ozone is naturally occuring while tropospheric ozone is created by chemical reactions between man-made emissions.
Stratospheric ozone protects the earth from the sun’s UV radiation and it is highly important to ecological and human health, filtering out harmful UV-B radiation, preventing DNA mutations. On the other hand, tropospheric ozone is a powerful greenhouse gas and is an air pollutant that harms human health, crop production and ecosystems.

What other air pollutant does tropospheric ozone make a key component of?

Smog - tropospheric ozone is the main component of smog

How can tropospheric ozone impact human health? What groups of people are most vulnerable?

Ozone is a powerful oxidant which can irritate the airways. It can cause muscles in the airways to constrict, trapping air in the alveoli, leading to wheezing and shortness of breath. Ozone can cause coughing and a sore or scratchy throat; make it more difficult to breathe deeply and vigorously and cause pain when taking a deep breath; inflame and damage the airways; make the lungs more susceptible to infection; aggravate lung diseases such as asthma, emphysema and chronic bronchitis; increase the frequency of asthma attacks — long term exposure to ozone is linked to the aggravation of asthma and asthma development

What might be the result of some of these health issues?

Could lead to increased school absences, medication use, visits to doctors and emergency rooms as well as hospital admissions, putting greater pressure on health services

What groups of people might be most at risk from ozone exposure?

People most at risk include: people with asthma, children, older people, outdoor workers, people with certain genetic characteristics and people with reduced intake of nutrients such as Vitamin C and E

Why are children amongs those most at risk from ozone exposure?

Children are at risk from ozone exposure as their lungs are still developing and they are most likely to be active outdoors when ozone levels are high, increasing their exposure. Children are also more likely than adults to have asthma which ozone exposure can aggravate/worsen.

Other groups of people being more vulnerable to ozone exposure, what else might impact the severity of health impacts caused by ozone?

The length of exposure

How many deaths and diseases are caused by by ozone exposure each year?

Ozone exposure causes 1 million pollution related deaths each year and causes millions more chronic diseases

How does tropospheric ozone affect ecosystems?

Tropospheric ozone is toxic to many plants and can affect sensitive vegetation and ecosystems including forests, parks, wildlife refuges and wilderness areas. Ozone can harm sensitive vegetation during the growing season including trees found in many parts of the US e.g. Black cherry, quaking aspen, tulip poplar.

How does tropospheric ozone affect the growth and survival of sensitive vegetation?

When a sufficient amount of ozone enters the leaves of a sensitive plant, it can reduce photosynthesis, which slows the plants growth, increasing its susceptibility to disease, damage from insects, effects of other pollutants, harm from severe weather

How does damage to individual sensitive vegetation have an impact on entire ecosystems?

Effects on individual plants can have negative impacts on whole ecosystens including changes to the specific assortment of plants present in a forest; changes to habitat quality; changes to water and nutrient cycles

How does ozone exposure affect agricultural yield?

Ozone exposure causes up to 15% in annual yield losses of soy, wheat, rice and maize

What is the impact of tropospheric ozone on climate?

• Tropospheric ozone is a greenhouse gas and therefore contributes to global warming

• Damage to plants caused by ozone affects their ability to sequester carbon dioxide and this doubles its climate impact - the world’s land ecosystems capture about 30% of Carbon Dioxide emissions every year

Why are tropospheric ozone levels highest in warmer and industrial regions?

There are higher rates of photochemical reaction due to higher UV radiation levels in equatorial or tropical areas and because industrial manufacturing regions produce higher emissions of VOCs and NOx

Why are concentrations of ozone high in rural and urban areas?

There are high emissions of NOx in rural and agricultural regions as a result of agricultural application, leading to higher concentrations of ground-level ozone. Urban areas also have relatively high levels due to NOx emissions from vehicle exhausts.

Why are there seasonal variations in concentrations of ozone levels?

Because of variations in UV radiation in different seasons. In summer months UV levels are highest, increasing photochemical reaction rates, producing more ground-level ozone

How does topography affect the distribution of tropospheric ozone?

Areas that have temperature inversions will experience higher concentrations of tropospheric ozone - this might be due to the presence of valleys, where cold air often settles at ground-level and therefore prevents air from rising and distributing pollutants and so the ozone accumulates. Topography may also block wind currents, which can prevent pollutant dispersal. Mountainous topography might also encourage the condensation of water vapour in the air to form mist and fog which can block effective ground-level heating by solar energy due to a lack of sunlight - forms a temperature inversion, further preventing pollutant dispersal.

As tropospheric ozone is a secondary pollutant, which pollutants need to be controlled to prevent its formation?

The primary pollutants which react to form tropospheric ozone - strategies are primarily based on methane reductions and managing NOx emissions from vehicles, power plants and other sources.

How do catalytic converters work to reduce NOx emissions? How effective are they?

Catalytic converters are a chamber which is fitted into exhaust pipes of vehicles which contain a catalyst such as platinum or palladium. The catalyst speeds up the reaction between oxygen in the air and NOx gases, chemically reducing NOx back to nitrogen and oxygen gases. They reduce NOx emissions by up to 90%; carbon monoxide by 85% and hydrocarbon emissions by up to 87%.

How do urea sprays work to reduce NOx emissions?

Urea sprays inject a solution of urea into the exhaust system of diesel engines, where it reacts with NOx gases in the presence of a catalyst and oxygen to convert them into nitrogen, carbon dioxide and water vapour.

How does fluidised bed combustion, a lower temperature combustion technique, work to reduce production of NOx?

Fluidised bed combustion creates an increased surface area for combustion, maintaining rapid combustion without the need for high temperatures. Less NOx is produced using lower temperature combustion techniques.

What is another method of reducing NOx production?

Reducing the use of nitrogen fertiliser in agriculture, using alternative processes like organic fertilisers such as manure to encourage plant growth. Nitrogen fertilisers are exposed to the soil and microbial reactions take place which release nitrous oxide.

How is carbon monoxide produced?

Carbon monoxide is produced when carbon-based materials are burned with a shortage of oxygen, such as the incomplete combustion of hydrocarbons, particularly petrol and diesel in vehicle engines.

What are the main sources of carbon monoxide emissions?

The main sources of carbon monoxide emissions include industrial processes; home heating such as for blast furnaces, fireplaces and woodstoves; burning of agricultural wastes - can cause wildfires; shipping. Burning of fossil fuels in power generation as well as emissions from motor vehicle exhausts.

How persistent is carbon monoxide?

Carbon monoxide is highly persistent, remaining in the atmosphere for around two months before it is broken down.

Describe the distribution of carbon monoxide concentrations globally

In the US, Europe and eastern China, the highest Carbon monoxide concentrations occur around urban areas due to vehicle and industrial emissions while rural areas tend to have lower levels. Fires in the Amazon Rainforest and South East Asia, often clearing forested land, also contribute to elevated CO concentrations in those regions. Pollution emitted on one continent can travel across oceans and have a large impact on air quality on other continents. There is also a significant level of CO concentrations in Africa, due to the use of fuels burnt for cooking - often charcoal or coal is burnt. CO levels are generally higher in the Northern hemisphere as there are more landmasses, people and fires.

Describe possible seasonal variations in the presence and concentration of carbon monoxide

Seasonal variations in carbon monoxide concentrations in Africa are tied to widespread agricultural burning which shifts north and south of the equator with the seasons.
There are seasonal peaks in the spring in both hemispheres which are driven by changes in the availability of sunlight and hydroxyl radical, a “detergent” molecule which removes CO from the atmosphere by converting it to carbon dioxide - reduction in light in the winter reduces the amount of hydroxyl in the air, allowing CO levels to increase in the winter and early spring.

Increased emissions may occur in the winter due to increased heating needs, leading to higher emissions from residential heating sources. Additionally, wildfires during dry seasons can significantly elevate CO levels in affected areas.

How have carbon monoxide levels changes since 2000?

Since 2000, average levels of CO have declined by around 15%. However, this rate of decline has slowed, falling from 1% per year from 2000 to 2010 to around 0.5% per year 2012 to 2023.

Emissions have generally declined in many urban areas, particularly in developed countries, due to stricter regulations and cleaner technologies. However, some regions, especially in developing countries, still experience increasing CO levels due to urbanization and industrial growth.

How can carbon monoxide impact health and living organisms?

Carbon monoxide can adversely affect health as it binds to haemoglobin, preventing it from carrying oxygen from the lungs to tissues around the body. If too much of the haemoglobin in the red blood cells is inactivated by carbon monoxide, then symptoms such as headaches and diziness and in severe cases, brain damage and possibly death can occur. Low concentrations of CO can increase the health problems caused by chronic heart disease. Furthermore, in combination with other pollutants and UV, CO takes part in the formation of tropospheric ozone and urban smog, which can cause further respiratory problems. High levels of CO can also impair the function of living organisms by disrupting cellular respiration.

How do catalytic converters work to control CO concentrations? How effective are they?

Catalytic converters reduce carbon monoxide emissions from vehicle exhaust by facilitating chemical reactions that oxidise CO into carbon dioxide. They make use of a catalyst, usually platinum or palladium, to speed up the chemical reactions between oxygen and pollutants in the air to convert them into less harmful byproducts like water vapour, carbon dioxide and nitrogen gas. Catalytic converters have reduced carbon monoxide emissions by 85%.