In our last issue, we looked at how different sectors contribute to pollutants in the air we breathe, of which ozone is one of the major pollutants. Ozone is a gas that is present in Earth’s upper atmosphere (stratosphere), where it protects all life on Earth from the Sun's harmful ultraviolet radiation. At ground level, ozone is extremely harmful to humans, plants, and other animals, and is the main component of smog. Ozone is not directly emitted to the environment from any source, but is produced when nitrogen oxides (NOX) and volatile organic compounds (VOCs) react in the presence of sunlight. NOx emissions are a byproduct of land transportation, thermal power plants and other industries using fossil fuels. VOCs are released from burning of fossil fuels and many consumer products like paints, adhesives, air fresheners, and insect repellents.
Ozone pollution is greatest during summer with long hours of sunlight and warmer temperatures helping speed up the photochemical reactions that produce ozone. As the planet gets hotter due to global warming, the ground level ozone concentrations will continue to rise as well. The National Center for Atmospheric Research in the US has projected that by the year 2050, Americans are likely to suffer a 70 percent increase in unhealthy ozone smog, while no such data is currently available for Indian population.
Ozone, at around 20 kilometres altitude in the atmosphere, acts as a greenhouse gas by trapping heat, and is part of chemicals like methane, black carbon, sulphur dioxide, and organic carbon, collectively called short-lived climate pollutants (SLCPs). These SLCPs are known to have very high radiative forcing effect as compared to carbon dioxide, and ozone has a radiative forcing effect 1,000 times more than that of carbon dioxide. Radiative forcing is the difference in energy flowing into the earth’s atmosphere and that radiating out from earth to space, and this is greatly influenced by the atmospheric gases and other chemicals which have different capacities to absorb this energy. When these atmospheric gases and chemicals later release the stored energy, it causes a rise in temperature on earth. Due to its short lived nature, as a greenhouse gas ozone does not have strong global effect, but does have a strong regional effect. These regions where ground level ozone is high can be viewed on the page maintained by NASA Goddard Space Flight Centre’s Chemistry and Dynamic Branch, which maintains data on ozone concentration in the lower atmosphere (troposphere).
Figure 1: Ozone behaviour at different altitudes in Earth’s atmosphere. (Image courtesy - NASA Space Place
Image source - https://spaceplace.nasa.gov/review/greenhouse/ozone-profile-lrg.en.png)
Short term exposure to ozone can cause chest pain, throat irritation, coughing, and breathing difficulty; while long term exposure can lead to lung damage, asthma, and premature death in people with lung or heart disease. Prenatal or childhood exposure to ozone is known to increase the chance of autism by ten times in at-risk populations. Ozone being highly reactive and unstable could form reactive oxygen species (a type of unstable molecule containing oxygen) upon entering our body, which are known to cause stress to the cells and DNA damage. According to this study titled “The Impact of Individual Anthropogenic Emissions Sectors on the Global Burden of Human Mortality due to Ambient Air Pollution”, published in 2016, energy sector had the greatest impact with respect to ozone linked mortality.
According to the IPCC special report Global Warming of 1.5 °C (table 3.SM.9 ), global ozone related mortality will continue to rise under 1.5°C as well as 2°C projections and will reach its peak in 2050 for the 2°C projection. The Global Environmental Outlook 6 (GEO-6) report by UNEP, shows that India had the highest seasonal concentration of ozone in 2016 (figure 2). According to GEO-6, ground level ozone and particulate matter are the most important air pollutants from a public health perspective.
Figure 2: Seasonal average population-weighted ozone concentration for season with maximum ozone levels by country (Image courtesy: GEO-6, UNEP; Image source: Figure 5.8 on page 119 in GEO-6 report)
Ground level ozone is known to have a detrimental effect on the yield of major food crops like wheat, soybean, maize, and rice, with nearly 40 percent of this damage happening in India and China. Rising ground level ozone concentrations pose a huge threat to food security for a growing population and also dent economic potential of countries. GEO-6 report mentions that ozone exposure can negatively impact plant species and biodiversity by affecting plant growth, flowering, pollination, and susceptibility to pathogens.
In India, coal fired power plants are responsible for an estimated 30 percent of total NOx production. Nitrogen pollution, a result of higher combustion of fossil fuels and from agriculture, in the form of animal waste and higher use of synthetic fertilizers, is resulting in higher ozone in the atmosphere. In December 2015 in India, stringent controls for emissions from coal power plants were introduced by the Ministry of Environment, Forests and Climate Change (MoEFCC), under which coal power plants were given two years time to reduce NOx emissions by almost 60 percent. This 2015 notification was wilfully stalled and diluted with the timeline extended to 2022. Formation of NOx in power plant happens during coal combustion and is dependent on combustion conditions and nitrogen content in coal, and could either be fuel NOx (from nitrogen in the fuel and local flame stoichiometry), thermal NOx (atmospheric nitrogen under high temperatures), and prompt NOx (from atmospheric nitrogen and coal in front of burner flame).
Land transportation emissions, which are the second largest source of NOx emissions in India after coal fired power plants, are likely to be brought under control with the introduction of new Bharat-VI norms across the country from April 2020. Within land transportation, large passenger vehicles (buses) and heavy duty vehicles (different categories of trucks) are the single largest on-road source of NOx emissions. Taking this into consideration, the government plans to transport coal in covered trucks to curb particulate matter emissions but ignores curbing NOx emissions. This plan to transport coal to power plants in covered trucks would do nothing to reduce emissions from trucks and this would react with emissions from the power plant itself to form ozone. It is only through control of emissions of ozone precursors like NOx that we can limit ground level ozone concentrations. This would be made possible either by reduction in use of fossil fuels or by scientifically robust technology and integrated approach to curb emissions across related sectors like land transportation and thermal energy generation, and we shall explore such existing solutions in our next issue.