This is a question we often get asked. Especially by people who are in less built up areas and are surprised to find they have an ozone 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 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.
Health effects of ozone pollution
Exposure to ground level ozone can:
- irritate your respiratory system
- reduce lung function
- aggravate asthma
- inflame and damage cells that line your lungs
- aggravate chronic lung diseases
- cause permanent lung damage
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 (NOx) and volatile organic compounds (VOCs), in the presence of sunlight.
Put simply: VOCs + NOx + 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 NOx 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:
- How much VOC is present
- How much NOx is present
- The intensity of sunlight
- How much wind there is and from what direction
- Geographical features such as mountains and valleys which trap pollutants
For that reason ozone is often associated with cities in summer time. 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.
Why does ozone also affect small towns?
We often speak to people from small cities and towns with limited industrial activity who have a pronounced summer time 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 kilometres 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 neighbours. 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.
What can be done about it?
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:
- Where ozone pollution occurs (spatial distribution)?
- Is the ozone generated locally or has it travelled from another place?
- Does the ozone disperse to other areas?
- What are the average levels of ozone pollution (long term exposure)?
- When does ozone spike (maximum short term exposure)?
- When does ozone pollution occur (temporal distribution i.e. which hours, days, seasons, years)?
- What are the best ways of reducing ozone for this area?
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.
How can we help?
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 and its precursors. We also talk to governments all over the world and can advise you on and connect you with organisations who are tacking the ozone pollution problem successfully. Feel free to get in touch