Research |
Transport of air pollutants over the Indian Region4. Pollutants and their impact on climate, human and plant health Ozone and fine particulate matter are the two categories of air-borne pollutants of most concern. Ozone produces well-documented effects on human health and can adversely affect crop viability and yields. PM2.5 is a concern for public health as they can penetrate deep into lungs. The World Health Organization (WHO) has suggested a maximum hourly average ozone concentration of 150 to 200 ug/m3 or 80 to 100 ppbv. Plant damage caused by O3, peroxyacyl nitrates (PAN), and NO2 includes leaf injury, reduced growth, reduced flowering of ornamental and fruit-bearing plants, and reduced crop yields from agricultural plants. O3 also causes cracking of rubber, fading of dyes in fabrics, and accelerated erosion of paints. The combination of NO2 and photochemical aerosols is also responsible for the thick, brownish haze that usually accompanies photochemical smog. Ozone in the lower atmosphere is also considered a greenhouse gas responsible for global climate change, while the particulate matter scatter the incoming radiation from the sun but the black carbon contents of the particulate matter is absorbing in nature. Ozone is a secondary pollutant formed by complex chemical reactions between VOC, NOx and oxygen in the presence of sunlight, as first suggested by the biochemist Arie Jan Haagen-Smit. Ozone is formed in the lower atmosphere primarily by nitrogen oxides (NOx) reacting with volatile organic compounds (VOCs) on warm, sunny days. Nitrogen oxides are released into the atmosphere as a by-product of combustion, including the burning of vegetation during a fire. VOCs, or hydrocarbons, also come from man-made sources such as cars, service stations, dry cleaners, and factories and from natural sources such as trees and other vegetation. Figure 1 gives a schematic diagram of ozone formation pathway.
Figure 1: Schematic diagram of ozone formation pathway VOC + NOx + Sunlight --> O3 + HNO3 + Other products Not only the amount of these precursors but also the ratio between them is important. The loci of combinations of VOC and NOx concentrations, which give, rise to the same ozone concentration level are known as isopleths. The isopleths are important to determine optimal ozone control strategies. The environmental persistence of an ozone molecule ranges from few days to few weeks and varies according to the altitude and latitude.
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