This is a question we often get asked. Especially by people intent on safely sterilizing locations or in less built-up areas and are surprised to find they have an ozone pollution problem which they associate with big cities. Ozone formation and dispersion is complex and is influenced by many factors. We wrote this blogpost to do our best to explain.

Ozone is often said to be good and bad. For a quick video explanation, check out this animated video.

Good ozone is found in the upper atmosphere and helps to screen out the sun’s harmful UV rays. A hole in the ozone layer was caused by the release of CFCs and is only now starting to close up. Because it’s way up high in the atmosphere good ozone doesn’t affect us in the way bad ozone does (even though it’s exactly the same gas).

Bad ozone is ozone found at ground level where it gets ‘breathed’ in by people and plants. When we breathe ozone even in quite small quantities it has a negative effect on our respiratory systems. It’s especially bad for the young and old and people with pre-existing respiratory problems e.g. asthma. It’s bad ozone that we refer to as ozone pollution.

Exposure to ground-level ozone can:

Health effects from ozone inhalation often occur without any noticeable signs. So understanding ozone pollution and seeking to control it is essential for maintaining the health of our populations.

Ozone is a secondary pollutant

Ozone isn’t emitted by any particular industry or human activity. Rather it is created through the combination of other pollutants (precursors), such as nitric oxides (NO_X) and volatile organic compounds (VOCs), in the presence of sunlight.

Put simply: VOCs + NO_X + sunlight = ozone

The presence of sunlight is important to the production of ozone. This is why ozone levels are generally higher in the summer compared to the winter.

Try this link here for a more in-depth explanation of the chemical reaction.

Where do VOCs come from?

Volatile organic compounds (VOCs) are a wide range of carbon-based (organic) chemicals (compounds) found in various man-made and naturally occurring solids and liquids. They evaporate easily at ordinary room temperature which is why they are termed volatile.

Typical indoor VOC sources include paint, cleaning supplies, furnishings, glues, permanent markers, and printing equipment. Levels can be particularly high when there is limited ventilation.

Typical outdoor sources include emissions from the oil and gas industry, solvent usage, agricultural burn-offs, and transportation. Although biological VOC emissions tend to be larger overall, man-made sources are a greater concern in urban areas.

Where does NO_X come from?

Nitrogen oxides are produced in combustion processes, partly from nitrogen compounds in the fuel, but mostly by direct combination of atmospheric oxygen and nitrogen in flames. Nitrogen oxides are also produced naturally by lightning, and by microbial processes in soils. Most anthropogenic NOx comes from vehicle emissions and from power stations. The rest comes from a range of industrial processes and from domestic sources.

What happens next?

As you can imagine VOCs and NOx and sunlight are often found in the same place at the same time. When this occurs you get ozone. The extent of ozone pollution depends on a number of factors including:

For that reason, ozone is often associated with cities in summertime. In such cases, you have lots of cars and industrial activity plus you have high levels of sunshine. That leads to higher levels of ozone over the summer months. The problem gets particularly bad when you have large cities in very sunny places surrounded by mountain ranges such as Santiago in Chile or Los Angeles in the USA.

Ozone exposure can be harmful. As discussed above shortness of breath, airway obstruction, and irritation to diseases such as bronchitis are symptoms. It can be harmful at low levels, even if no scent is present. Some countries have set limits to the exposure of 70 ppb (parts per billion) ozone. There are six common pollutants limited by the United States Environmental Protection Agency and other international regulatory bodies, ozone is on that list. If used repeatedly and at unsafe levels, it can lead to increases in air pollution and long term risk to health. This is why monitoring ozone levels is important to individuals, companies, and governments.

We often speak to people from small cities and towns with limited industrial activity who have a pronounced summertime ozone problem. What surprises people is that ozone can form even when you don’t have much NOx or VOCs. A small city or a large town in the right geographical conditions with the right weather (lots of sun and no wind) can still experience relatively high levels of ozone. This is because the rate of ozone formation is related to the relative amounts of VOC and NOx, not their actual levels.

Also, ozone can be transported long distances by wind. It can be generated by photochemical processes in large cities or by wildfires or burnoffs and then be transported by the wind to a rural area or small town hundreds of kilometers away.

This is seen in areas adjacent to large metropolitan areas, for example, the Fraser Valley (near Vancouver) and the Central Valley (near Los Angeles). In such cases, small rural towns can pay the price for the pollution created by their big-city neighbors. In rural areas, ozone affects people and it can also have an impact on horticulture and livestock. Rural areas can also contribute to ozone creation, often by way of VOC emissions from agricultural activities, especially burn-offs.

The first step as always is to get a proper understanding of the dynamics of ozone pollution in the area. This includes an understanding of ozone daily cycles and the meteorology (temperature, humidity, wind speed, prevailing wind direction, solar radiation index). Through measurement of ozone pollution, we can answer questions such as:

Ozone pollution monitoring has become a lot easier in recent years with the introduction of lower-cost instruments such as those made by Aeroqual. Low-cost instruments, with low power requirements and a small footprint, allow the deployment of large numbers of sensors. Having a large network greatly aids the spatial and temporal understanding of ozone pollution. Check out this case study from the USA.

To start dealing with ozone pollution we must look at reducing the pollutants (precursors) that lead to ozone formation (NOx and VOC). This is a long term effort that requires an active air quality policy and enforcement by local air quality managers. Because of the dispersion of ozone pollution, it is also necessary for local government agencies to work together. Measurement of VOCs and NOx can identify sources and emitters in an area and help agencies target their efforts.

Fortunately, there are many examples of places where this has been done successfully.

Our specialty is helping people and governments get better air quality information to enable better decision making.

Our sensors and instruments can help to build an accurate picture of ozone pollution, its precursors and levels harmful to human life on both a daily and application basis. We also talk to governments all over the world and can advise you on and connect you with organizations that are tacking the ozone pollution problem successfully.