What is Ground Level Ozone and How It Affects Us
People living in rural or undeveloped areas may live under the assumption that the air they breathe is pure and uncontaminated, which is why they are often surprised to learn that it actually contains unhealthy levels of ozone pollution. Ozone (O3) formation and dispersion are complex processes influenced by many factors, as this article explains.
O3is a strong oxidizing gas that can be good or bad for humanity, depending on where it is found. BeneficialO3is found high up in earth’s stratosphere where it forms a protective layer that shields us from the sun’s harmful ultraviolet rays. This crucial O3layer is sensitive to degradation by manmade chemicals, namely chlorofluorocarbons (CFCs), which are now banned in many countries. This destruction has resulted in the well-known“ hole in the ozone”, which, fortunately for us, has steadily diminished in recent years.
O3 becomes problematic to animal and plant life when it is found at ground level. In haling even relatively small quantities of O3can do damage to our respiratory systems, and it is especially dangerous for young children, seniors, and those with pre-existing respiratory conditions such as asthma. Ground level O3 pollution can also have negative consequences for livestock and agriculture, and is one of the principal components of what we refer to as“smog”.
How and where does ground level ozone occur?
O3 is not directly emitted to the atmosphere by any human activities; rather, it occurs as a secondary pollutant resulting from the emission of other, primary pollutants in the air. Carbon monoxide (CO) and/or volatile organic compounds (VOC) in the atmosphere interact with the surrounding air to create free radicals which convert nitric oxide (NO) to nitrogen dioxide (NO2). This NO2 is then photolyzed by UV-A radiation from the sun to form ground-state atomic oxygen:
NO2 + Sunlight → NO + O
The ground-state oxygen is extremely short-lived, and rapidly reacts with oxygen (O2) to create O3:
The overall process can therefore be thought of as:
If NO emissions are sufficiently high they will act as a limiting factor on O3 formation as these gases react to form additional NO2:
The relationship between levels of nitrogen oxides (NOX) and ozone formation can therefore be visualized as follows:
The above system of reactions is an over-simplification of the actual chemistry, but useful for illustration purposes. The main takeaway is that formation of ground level O3requires three things:
- NO2, which originates as NO emitted by combustion of fuels. Thermal power plants and wildfires contribute to this pollutant, but the major source is transportation. Diesel engines are the main culprit as they favour a fuel-lean mixture which introduces excess nitrogen to the combustion process. NO2 concentrations tend to be highest around busy roadways and dense urban centres.
- VOC, which may also originate from diesel combustion, but can also result from oil or chemical refineries, storage facilities, or spills; and/or CO, which, in the absence of fires, originates mostly from mobile source, especially poorly tuned gasoline engines.
- Sunlight, especially UV-A radiation. Generation of O3 is strongly correlated with the intensity of solar radiation, and therefore tends to be greatest when the sun is at a high angle, i.e., around midday during the summer months.
Although the chemical precursors to O3 are strongly associated with cities and roadways, winds may blow them hundreds of kilometers away into rural and undeveloped areas.O3formed in these remote areas tends to degrade more slowly due to the lack of locally sourced NOX by which it would otherwise be consumed. This is the reason why many rural locations regularly experience poor air quality that conflicts with commonly held beliefs about “clean country air”.
Health effects of ground level ozone pollution
Inhaled O3is harmful to human health, especially on hot sunny days when its concentration can reach hazardous levels. Even relatively low concentrations of O3can cause health effects which include:
- Irritation of the respiratory system, with associated coughing and sore/scratchy throat
- Inflammation of lung tissue leading to reduced lung function and painful breathing
- Aggravation of lung diseases such as asthma, emphysema, and chronic bronchitis
- Increased susceptibility to respiratory infections
- Permanent lung damage
People most at risk from breathing O3-contaminated air include asthmatics, young children, seniors, those with reduced intake of vitamins C and E, and those who are active outdoors, e.g., athletes and outdoor workers. Long-term exposure to O3 is likely to contribute to asthma development, which is particularly worrisome for children as their lungs are not fully mature and they are more likely to be active outdoors when O3levels are high.
How AirSENCE can help?
AirSENCE is a continuous ambient air quality Micro Monitoring Station (caaqMMS) which is the ideal solution for measuring smog-related parameters like NO, NO2, CO, particulates, VOCs, temperature, and, of course, ground level O3.AirSENCEprovides continuos real-time data (1-minute average) that can be analyzed over long periods to predict generation of smog at places where it is known to be a problem.
As described above, O3generation follows a distinctive daily pattern which is more pronounced on hot summer days, especially in urban areas. NO2 levels increase in the morning due to morning rush hour traffic; but, as UV intensity increases through the day, NO2 is depleted and O3 concentrations rise. NO2 then characteristically rises again later in the day due to the evening rush hour, while O3 falls along with UV radiation levels. Every ambient air monitoring system must demonstrate this daily trend as a key indicator of its operational capabilities.
The following chart shows O3, NO2, and temperature profiles over a 3-week period for one AirSENCE Standard device installed in an urban setting in India. It can clearly be seen that O3 concentration (in red) rises in accordance with the ambient temperature (black) each day, as temperature is a useful indicator of light intensity. The temperature and O3 peaks occur in the mid-afternoon each day, coinciding with dips in NO2 concentration (blue), as expected.
In addition to accurately capturing well-established pollutant trends, AirSENCE has several other useful features to help authorities monitor key air pollutants and predict the formation of smog well in advance. Its small footprint and low weight allow for easy installation on light or traffic signal poles to capture vehicular pollutants as accurately as possible before they are dispersed. Various communication alternatives like GPRS, Wi-Fi and LAN allow user to integrate AirSENCE very easily with smart city infrastructure, and data can be viewed and retrieved using any Internet-enabled device, e.g., smart phone. Pre-calibrated sensors, an all-weather enclosure, and carefree maintenance makes it very cost effective to procure, operate and maintain in any environment.
AirSENCE is a groundbreaking caaqMMS that monitors smog-related pollutants reliably and accurately, giving authorities rock-solid data upon which to base timely and critical decisions.
