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: [email protected] |
