The discovery of ozone (O3) in the troposphere was first made in the 1840’s. Consequently, an intensive study about its distribution was made in the latter part of the nineteenth century due to its supposed health-giving properties as a germicide (Schönbein, 1840). The first classical explanation of ozone in the troposphere was that it was made in the stratosphere by direct photolysis of oxygen at wavelengths less than 242 nanometer.
Stratosphere is a layer above the earth’s surface, between approximately 15 and 50 km. It was believed that once the ozone is formed, it is transported downward from the stratosphere to the troposphere, to be ultimately destroyed in contact with the earth’s surface, particularly land surfaces. Although it became known that there was a direct source of ozone in the troposphere in photochemical smogs (first discovered in Los Angeles in the 1940’s) it was not understood to be significant on a global scale.
This classical view of the origin of ozone in the troposphere underwent a complete turnaround during the course of time. It is now well documented that ozone is produced in two regions of the earth’s atmosphere. Most ozone—about 85-90 per cent, resides in the stratosphere, commonly known as the ‘ozone layer’. The remaining ozone is in the lower regions of the atmosphere—the troposphere, which extends from the earth’s surface up to about 8 km near the poles and ~18 km near the equator. Ozone from both these regions have the same chemical composition (three oxygen atoms), but their effects on humans and other living things are very different. Based on their location ozone can either protect or harm life on earth. Low lying ozone is a pollutant and a key component of smog, a familiar problem in cities around the world. Higher than usual amounts of surface ozone is now increasingly being observed in rural areas as well.
Primary air pollutants such as carbon monoxide, nitrogen dioxide and sulfur dioxide etc. are harmful to human health. However, the most deleterious air pollutants are not those emitted directly from a source, but those formed in the atmosphere by chemical reactions. When air pollution is analysed it is important that the chemical processes taking place in the atmosphere be understood. Secondary air pollutants, also known as photochemical air pollutants such as bad ozone, organic nitrate, formaldehyde, hydrocarbon and proxy acetylnitrate, etc., are formed in the air by chemical reactions among primary air pollutants that are emitted directly into the urban as well as rural environments. An urban environment consists of thousands of such chemical compounds, very few of which are identified and even fewer monitored at select locations around the world.
Production of Tropospheric Ozone
Ozone is a by product of a nonlinear and complex atmospheric oxidation process. A moderately polluted environment shows more ozone formation efficiency than unpolluted and polluted environments. A non linear process means that increasing or decreasing ozone precursors concentration may or may not increase or decrease ozone concentration. For instance, ozone increases with increasing nitrogen oxides concentration up to certain limit (~10-15 ppb), after that the increase of nitrogen oxides in the environment corresponds to ozone decrease—the titration reaction. In an environment that is NOx rich, a decrease in nitrogen oxides concentration will result in increase of ozone concentration until the above threshold is reached. Due to this nonlinear behaviour of ozone formation, US and European countries are unable to control ozone within the permissible limits despite large spending. Therefore, a considerable populace of industrial cities are regularly exposed to peak air pollution in excess of the current limits advised by the United Nations Economic Commission for Europe and the World Health Organisation . The ozone standard is set at a level of 0.075 ppm averaged over an 8-hour period. The one-hour short-term limit value of 75 to 100 ppb for health protection and the long-term value of 30 ppb over the growing season for protection of vegetation are frequently exceeded in most parts of Europe. This is termed as a ‘hydrocarbon-limited’ environment by atmospheric chemists (Lawrence et. al, 1997), where hydrocarbons mean chemical compounds mainly formed by carbon and hydrogen atoms as opposed to the other type—the ‘nitrogen oxides-limited’ environment. In other words, hydrocarbon-limited environment indicates a polluted air column while a nitrogen oxides-limited environment indicates low pollution. Indian air quality on the other hand suffers primarily due to high particulate matter—ozone remains relatively low in most part of the year. India, therefore, witnesses a nitrogen oxides-limited environment. With the population pressure increasing and the energy demand growing, the Indian region is likely to change to a moderately polluted environment where the ozone forming potential is high. The region may also change to ‘hydrocarbon-limited’ environment in near future, which would make it difficult to control the ozone below the permissible limit as prescribed by Central Pollution Control Board at 50 ppb on eight sunlit hour average.
The tropospheric ozone is slowly showing an increasing trend at the rate of 1 to 2 per cent per year in most urban locations in India (Saraf and Beig, 2004). This means that after 36 years ozone will be double of the present value. Ozone formation takes place in the presence of sunlight and the peak of ozone formation is attained around noon. Thereafter, ozone starts reducing till the next day. Thus, ozone shows maximum concentration around noon and minimum concentration at sunrise and it is maximum in the summer and minimum in the rainy season. Variation in atmospheric dynamic processes, precursors concentration and reaction rate of ozone formation and destruction are reasons for the diurnal and seasonal variation of ozone concentration.
High ozone concentration during noon can affect the developing lungs of children which are more susceptible to tissue damage. This is why in many developed countries windows of schools are shut in noon. Nearly all mega cities of the world are more or less facing an ozone problem. An ozone warning is issued in Japan if its concentration exceeds 120 ppb. In Taipei, Taiwan, ozone pollution is treated as a serious concern with concentrations often exceeding 120 ppb. Mexico City is unique example where topographical influence increases ozone concentration, often reaching up to 500 ppb. Based on clinical research each country finalises their air quality levels of pollutants and 120 ppb is generally accepted by many nations as the optimum level.
Impact of Tropospheric Ozone
Ozone is an oxidizing agent. It reacts with almost all compounds and living species, reducing the life span of reacting substances. Even small amounts ozone of about 40 ppb at ground level can cause chest pain, coughing, nausea, throat irritation and congestion in healthy people. It may also worsen bronchitis, heart disease and asthma. Increase in the rate of asthmatic, heart and cancer patient in urban cities are related to increasing ozone in environment. Ozone also reacts with rubber, oil paint, concrete buildings, art material and monuments. In developed countries in fact, tyres need frequent replacement due to the high ozone concentration (Layer and Lattimer, 1990).
Ozone is also a greenhouse gas—it warms the earth’s surface. The global warming potential of ozone is 2000 times that of carbon dioxide. This means that the enhancement of ozone can lead to severe climatic consequences.
Loss of Food Productivity
Ozone has been recognised as a threat to agricultural production (Deb Roy, 2009). The projected levels to which ozone will increase are alarming and have become a major cause of concern for global food production. High surface ozone and accumulation can harm human health and agricultural productivity over wide areas. Surface ozone not only deteriorates air quality near the region of emission, but is also likely to have far reaching impacts at remote places. Recent studies (Ghude et. al, 2014) have shown that rapid forest decline and decrease in winter crop yields are due to an increase in ozone concentration at the ground level. Ozone is phytotoxic to plant species—it disturbs the photosynthesis process and can result in acute foliar injuries, reducing biomass production.
Wheat is identified to be sensitive to tropospheric ozone, which enters the plant through the stomata. Ozone directly affects the cell membranes of the wheat plant, generating ozone-induced Reactive Oxygen Species (ROS) and up or down regulating ROS, signalling the molecule-associated genes, proteins, and metabolites, which ultimately affects plant growth resulting in a decline in wheat productivity.
Only a few surface ozone measurements for the Indian region are available. However, these studies seldom describe threshold exceedances and cumulative ozone exposure indices. Since emissions of ozone precursors from Asian countries—East Asia, Southeast Asia and the Indian Subcontinent are rising and may continue to rise for several decades, we must carefully evaluate the contribution of each region and India in particular to facilitate mitigative planning and action on the ground.
An advanced way to understand the impact of ozone on agriculture for a larger geographical region is to use high resolution regional chemistry-transport model. This methodology has been used by the Indian Institute of Tropical Meteorology, Pune in their research to study the distribution of exposure-plant response index— Accumulated Exposure Over a Threshold of 40 ppb, (AOT40) expressed as ppb hours—over the Indian region. AOT40 values across India show a lot of temporal and spatial variation. Many areas in India show ozone values far above the AOT40 threshold limit. The highest value recorded is 3000 ppb hours for three months. The highly fertile Indo-Gangetic plains show substantially high simulated AOT40 values throughout the year. This can have damaging effects on the local flora. The observed monthly AOT40 values reported from an Indian station, match reasonably well with model simulated results (Ghude et. al, 2009). The IITM researchers find that the simulated AOT40 target values for vegetation protection is exceeded even in individual months.
Current research suggests that surface ozone is above critical levels, especially in rural areas and hence pose a significant concern for agricultural productivity. Measures to reduce emission of precursors may be implemented to lower the risk of peak ozone concentrations in the short term. The future seems bleak as the emissions of ozone precursors are rising and may actually continue to rise for several more decades in India. With continued economic development in India in general and the Indo-Gangetic Region in particular, surface ozone will have a large impact on both human health and crops. Hence, necessary and effective emission reduction strategies are required to be developed in order to curb the surface level ozone pollution to protect vegetation and India’s agriculture dependent economy.