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Vol. 13 No. 2 - April 2007
‘Golden Jubilee Number’

Trend in tropospheric ozone concentration and its
impact on agriculture: Indian perspective

By: Madhoolika Agrawal*

The increase in tropospheric ozone has been identified to be a serious and critical cause of concern world over. Inter-Governmental Panel for climate change (IPCC) and World Meteorological Organization (WMO) assessment reports have predicted large increases in tropospheric ozone (O3) resulting from the emissions of O3 precursors. Ground level concentrations of O3 have been increasing steadily since the industrial revolution. Ozone concentration has risen by 1-2% per year in the industrialized countries of the northern hemisphere. Most of the countries of western Europe, eastern and mid-western region of USA and eastern Asia are found to show the highest background concentrations of O3. Most developing countries are facing increasingly severe air pollution problems due to recent emphasis on economic liberalization leading to rapid increases in industrialization and urbanization. Tropospheric O3 is one of the secondary air pollutants, predominantly formed by photochemical reactions involving precursors like nitrogen oxides (NOx) and volatile organic compounds (VOCs) generated by anthropogenic activities. In polluted air masses, production of Nitrogen dioxide (NO2) takes place by reaction of NO with HO2 and RO2 and then photolysis of NO2 in the presence of strong solar radiation (l< 424nm) releases atomic oxygen, which combines with molecular Oxygen (O2) to form O3. In the free troposphere, O3 formation also depends on photochemical reaction of methane, carbon monoxide and non-methane organic compounds with O2. Tropospheric O3 concentration also depends upon the stratospheric-tropospheric exchange processes.

Air pollution identified as a localized problem restricted to urban and industrial areas has spread even to rural areas. Tropospheric O3 is considered to be the most widespread atmospheric pollutant reaching to elevated concentrations at suburban and rural areas. A long-term change in tropospheric O3 has been implicated as one of the important factors of climate change because O3 acts as a green house gas. The Indian tropical region is expected to experience higher emissions of O3 precursors such as Oxides of nitrogen (NOx), Volatile organic compounds (VOCs), Carbon monoxide (CO) and NMHCs from transport sector and large-scale biomass burning, which enhance the potential of tropospheric O3 production. The annual emission estimates of different pollutants from transport sector in India showed that during 1990-1997, total emissions increased by 80% for CO, 78% for NO and 104% for hydrocarbons.

Most of the work on long-term variations in tropospheric O3 in India is based on photochemical models. Quantitative estimates of ground level O3 variations are limited over the Indian subcontinent. High concentrations of O3 have been reported for some 90 were recorded between 9.4 and 128.3 ppb (parts per billion). Eight hour mean O3 concentration in Delhi exceeded the World Health Organization (WHO) mean standard of 51 to 102 ppb by 10 to 40%. Annual average O3 concentration of 27 ppb was reported at Pune during 1994-95. Surface O3 measured during 1997-2003 at National Physical Laboratory, an urban site in New Delhi showed monthly average maximum concentration in dry summer (April to June) ranging from 62-95 ppb. Another maxima of monthly average were observed during autumn (October – November) ranging from 50-82 ppb. Daily maxima of O3 exceeding 100 ppb was observed on most of the days during summer with values reaching 175 ppb on few occasions. In Varanasi city, annual average O3 concentrations varied between 8 to 25 ppb­ during 1989-1990. The range of two-hour mean O3 concentrations was 11 to 82 ppb. In a study conducted during 1998-2000 average 8 hourly seasonal O3 concentrations at various urban, suburban and rural sites of Varanasi varied from 10 to 45 ppb during winter and 21 to 59 ppb during summer season. O3 concentration was highest at rural sites with no specific sources of pollutants. The summer time maximum O3 concentration can be explained due to meteorological factors such as high solar radiation, longer day light period, warm weather, stagnant wind patterns and low humidity. A detailed monitoring programme conducted at a rural area of Varanasi during 2004-2006 showed average daytime O3 concentrations of 56, 42 and 32 ppb during summer, winter and rainy seasons, respectively. Upon comparison of this data from earlier monitoring records for the same area, it was observed that average O3 concentrations increased by 32, 41 and 36%, respectively during summer, winter and rainy seasons from 1989-1990 recorded data. A diurnal maximum of O3 concentration was found to occur during the early afternoon in winter and during the late afternoon in summer. The night time ground level O3 concentrations were 8 to 13 ppb throughout the year.

Ozone as a toxic air pollutant was identified long back in 1950s, but now it is recognized as the most important rural air pollutant having negative impact on human health and vegetation including crop plants. The predicted increases in tropospheric O3 concentrations in India have been implicated with large-scale impacts on agriculture with many social and economic consequences. Changing O3 concentrations are suggested to be an important component of change in air pollution scenario. Evidence of visible injury due to O3 on potato leaves in Panjab was recorded by Bambawale in 1986, but its possible consequences for agricultural production have scarcely been explored under natural field conditions till late 1990s.

There are different approaches utilized for studying the impact of ambient O3 on crop yield. One such approach evaluates the impact of urban air pollutants on selected crop plants grown under standardized conditions utilizing a transect along a gradient of pollution. Test crop plants i.e. wheat (Triticum aestivum L. cv HD 2329), mustard (Brassica campestris L. cv Pusa Jaikisan), mung bean (Vigna radiata L. cv Malviya Jyoti) and spinach, showed significant decrease in different yield attributes at a rural site experiencing high O3 concentrations with that of low O3 site. Yield (g plant) declined by 14 % in mung bean and 21 % in palak. Yield of both wheat and mustard did not show significant difference between reference and the rural site. Upon comparison of O3 concentrations at two sites, it was found that during winter, mean O3 concentrations were 11 and 32 ppb, respectively at reference and rural sites, whereas during summer, the respective concentrations of O3 were 17 and 56 ppb. Yield of a late sown cultivar of wheat (M234) decreased by 13.5 % at the above seasonal means of O3 concentration. The study has clearly shown that O3 plays a greater role in yield losses during summer when its formation increases due to favourable meteorological conditions. The study has further shown evidence of urban primary pollutants involved in photochemical reactions leading to formation of O3 causing threat to agricultural production in India. In a field study with pea (Pisum sativum L cv Arkel), yield reduction of 13 % was observed at seasonal 8 hourly mean O3 concentration of 42 ppb as compared to a site having mean concentration of 16 ppb. Yield reduction in pea was directly correlated with loss in photosynthetic rate. Ozone accelerated leaf senescence in pea thus reducing the leaf area.

Another approach of O3 impact evaluation on crop plants is through the use of protectant chemicals. An antiozonant, N-2- (2-oxo-1- imidizolidimyl) ethyl-N phenyl urea (EDU), is one of the most successful and widely used protective chemicals for assessing crop loss from O3 under ambient field conditions. EDU is known to suppress acute and chronic O3 injury on a variety of plants. Mung bean plants (Vigna radiata cv Malviya Jyoti) grown at different sites having variable concentrations of SO2, NO2 and O3 showed that maximum protection by EDU treatment occurred at sites showing higher concentrations of O3. Complete protection in crop yield was not observed because SO2 and NO2 were also present. But the results of multiple regression equation between concentrations of pollutant and yield clearly showed significance of O3 under EDU treatment. Experiments with EDU have also indicated that O3 can cause significant effects on the yield of tomato (Lycopersicon esculentum) in and around New Delhi and potato (Solanum tuberosum) in Panjab. Ambient O3 concentrations present in suburban area of Allahabad city caused negative effects on growth, biomass accumulation and allocation and yield of mung bean plants, but EDU treatment induced tolerance as the treated plants maintained higher levels of photosynthetic pigments, protein and ascorbic acid contents as compared to the non EDU treated plants. To assess the impact of ambient O3 on growth and productivity of two wheat cultivars, three concentrations of EDU were used under truly ambient field conditions. The slow growing cultivar M533 with lower yield potential maintained the yield at lower dose of EDU (150 ppm), whereas in M234, a relatively short duration crop with high yield potential, 300 and 450 ppm EDU could only cause significant increments in yield. Harvest index, which denotes the reproductive potential did not vary in M533 due to EDU, but increased significantly in M234 at 300 and 450 ppm EDU. The study further confirmed that M234 is a sensitive variety of wheat for O3 and, therefore, showed better protection only at higher concentration of EDU.

Filtering of ambient air pollution using especially designed open top chambers is another widely used approach of assessing the impact of O3 on crop plants. National Crop Loss Assessment Network (NCLAN) programme in United States used open top chambers to assess the national economic consequences resulting from the reduction in agricultural yield and to define the O3 exposure and crop yield response relationship for the major agricultural crops. Basic NCLAN methodology was used in different countries of Europe under European Open top chambers (EOTC) programme on a variety of crops. Results of experimental studies indicated that yield reductions were highly correlated with cumulative exposure to O3 above a threshold of 40 ppb during daytime. A cumulative concentration of O3 above a 40 ppb threshold (AOT40) was established as 3000 ppb h for the spring planted crops experiencing high O3 concentrations at the time of their maximum growth in summer.

Extensive OTC studies were conducted around Varanasi with field grown crop plants for assessing the yield response at ambient O3 concentrations. Filtration of ambient O3 with charcoal filter from OTCs led to increments of 23, 28, 11, 22 and 29 %, respectively in the yield of winter palak, summer palak, carrot, brinjal and radish. One of the most sensitive cultivars of wheat M234 showed increments of 20.7% in yield and 8.4 % in harvest index due to filtration of O3 (mean concentration of 45 ppb) from germination to maturity. Wheat plants did not show significant impact of O3 during the vegetative phase as concentrations of O3 were below 40 ppb for 90% of the time during December and January. But during reproductive phase daytime O3 concentrations often exceeded 40 ppb causing negative effect on assimilation in flag leaf and consequently translocation of photosynthate to developing ears. Translocation pattern depends upon sink activities as well as source strength and hence the partitioning priorities at the time of exposure modified the impact of O3. The timing of O3 episode is an important factor for determining the sensitivity of plants to O3. Effect of O3 in accelerating the leaf ageing particularly of flag leaf has a direct correlation with yield losses in two important cereal crops viz., wheat and rice. Ozone not only adversely affected the yield of crops, but also negatively influenced the crop quality. Pea seeds collected from a site of Varanasi experiencing mean O3 concentration of 43 ppb showed significant reductions in protein, starch, nitrogen and energy contents as compared to those collected from a site having mean O3 concentration of 10 ppb. Similarly, reductions in protein and starch contents of wheat and rice seeds, beta-carotene content in carrot and iron content in palak were observed under elevated ambient O3 during OTC studies.

Exposure studies were also conducted to evaluate the influence of higher O3 levels on yield attributes of two major crops of India, wheat and soybean using open top chamber facilities. Plants were exposed to 70 and 100 ppb O3 for four hours daily from germination to physiological maturity. The extent of reductions in yield was many times higher in soybean cultivars as compared to both the cultivars of wheat. Soybean cv PK 472 showed yield reductions of 13.9 and 33.5 % and cv Bragg of 10 and 25 %, respectively at 70 and 100 ppb O3 concentrations. In wheat, yield reductions at 70 and 100 ppb concentrations were 4.7 and 15.5 % in cv M234 8 and 17 % in cv HP 1209, respectively. The relationship also indicated that O3 has a more negative impact on soybean compared to wheat.

A comparison of the yield data of exposure experiments at constant concentrations of O3 with those obtained through filteration experiment showed that yield reductions are not matching. The reduction in yield of wheat cv M234 at 4 hourly O3 exposure of 70 ppb from germination to maturity was lower than the reduction in yield of same variety of wheat grown in non-filtered chambers experiencing mean seasonal concentration of 43 ppb O3. This variation in yield reduction is due to the fact that under ambient conditions O3 concentrations vary seasonally as well as diurnally during the growth period. Ozone concentration exceeded 70 ppb for several hours during the anthesis period of wheat from February to March, which probably modified the photosynthate translocation to developing ears thus causing greater yield reductions.

The trends of O3 concentrations recorded at a few stations showed a general tendency of increase over the time, which are capable of causing risk to agricultural productivity in India. However, more systemic and extensive regional work is needed to quantify the crop yield losses in a country like India with wide variations in climatic conditions as well as anthropogenic activities. There is no work available coupling projected increases in tropospheric O3 with impacts on agricultural yield using modeling. The importance of several factors such as inter- and intra-species variations, climate variables particularly temperature, sunshine hours, soil type, soil moisture, timing of O3 episodes, stage of plant growth need to be explored in relation to O3 risk assessment. More detailed studies at the national level are necessary to identify high and low risk zones of O3 in different regions of the country. Air quality standard for O3 needs to be established for vegetation including agriculture. There is an urgent need for identifying monitoring centers for O3 across the rural and semi urban areas of the country.

* Professor, Department of Botany, Banaras Hindu University, Varanasi, India
Professor Madhoolika Agrawal is an Executive Councilor of ISEB - E-mail: madhoo58@yahoo.com

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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