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Vol. 13 No. 4 - October 2007

Ground-Level Ozone in The 21st Century:
Submission of Evidence from the Air Pollution Crop Effect Network (APCEN)

 By: Lisa Emberson*

1. Brief introduction to APCEN

The APCEN Network facilitates information exchange between air pollution effect scientists and air pollution stakeholders with the specific aim of developing methods to reduce impacts on crop productivity and quality in developing country regions. APCEN comprises over 60 members from countries predominantly in Asia, Africa, Europe and North America. The methods employed by the network include observation, experimental and modeling techniques, with a focus on standardization of assessment and application of these various tools within and between different global regions. The network is working towards developing methods to make socio-economic impact assessments that can be used to identify appropriate policy interventions at a range of geographical scales.

2. Evidence of Impacts of ozone by region

Over recent years APCEN has been involved in activities to try to collate and synthesise information describing current day impacts of air pollution, with a focus on surface ozone, in developing country regions. These studies have been synthesised in a number of publications.

Both current day evidence of impacts as well as studies that provide some insight into potential future impacts of this pollutant on crop productivity and crop quality are presented here. Where possible this information is provided in national context in terms of food security and pollution control. We also detail what we believe to be some of the urgent research priorities for the future to provide more comprehensive assessments of damage.

2.1 Current day impacts
In Asia ground-level O3 concentrations are alarmingly high in some large metropolitan areas. Severe O3 episodes are now observed in many countries such as China, Japan, Korea, Taiwan and Thailand. Ground level O3 concentrations of these countries typically show peak concentrations in the range of 90 – 200 ppb in the afternoon during ozone episodes. In Japan, frequent observations of visible foliar injury in many crop plants during the summer months (June to August) coincide with conditions when O3 concentrations frequently exceed 100 ppb) with some of the most frequently affected crops being rice, maize, peanut, tomato and aroid.

In northern Taiwan, observations of O3-induced visible injury on plant species such as leafy sweet potato and spinach have been made frequently since 1992. To help assess the geographical extent and frequency of such O3 symptoms researchers in Taiwan have used an active bio-monitoring approach as a cost-effective means of evaluating the pollution situation. This approach has used 2 native (double-fortune tomato and black nightshade) and 2 foreign (Bel-W3 tobacco and morning glory) indicator plants for O3 and has been successful in identifying annual pollution episodes of four urban areas of Taiwan where the bio-monitoring experiments were established.

Potentially damaging high O3 concentrations have also been found in other parts of south east Asia with frequent exceedences of national ambient air quality. In Southeast Asia the levels in big metropolitan regions such as Bangkok, Jakarta, Manila, Ho Chi Minh City are rising which may already cause substantial impacts on health and crop production but no comprehensive assessment has yet been performed. Monitoring data are limited, and if available, are mainly for the city centres where ozone would not be maximum. There is almost no data on ozone in city plume in the suburb areas where crops grow.

In south Asia, particularly India and Pakistan, evidences of high concentrations of O3 have been reported. For example, in India hourly maximum O3 concentrations of between 10 and 273 ppb have been recorded in Delhi.  The limited monitoring that does exist in the region suggests that in general, the northern and western parts of the country experience higher levels of air pollutants compared to south and eastern parts.

In Pakistan, levels of O3 pollution found in the peri-urban areas outside the city centres reach 72 ppb characterized as 6 hr weekly means. Outside Lahore, controlled experimental investigations using OTCs have investigated the impacts of ambient concentrations of O3 and NOx on the growth of local cultivars of wheat, rice, chickpea, mungbean and soybean crops in comparison with those grown in pollution free-air. The damage caused by the exposure of plants to ambient air pollution included reduced numbers of tillers, shoots and leaves; accelerated leaf senescence and yield reductions of between 23 and 47% (Wahid, Pers Comm.). In addition, studies using “EDU” (N-[2-{2-oxo-1-imidazolidinyl}ethyl]-n2phenylurea) as a chemical protectant to O3 have shown that seasonal mean O3 concentrations of 75 ppb for 6 hr per day are sufficient to cause yield reductions of up to 64% to soybean in remote rural areas 30 km from Lahore.

Concern that impacts may be occurring in the Middle East is also emerging. For example, in Iran and in particular Tehran, such concern has resulted in the establishment of more than 15 air quality monitoring stations for a range of pollutants including O3. The proximity of agricultural areas to Tehran and other large urban centres across the country, raises concerns over the impact that air pollutants such as O3 may be having on crop production. 


The increases in air pollution that have occurred around the urban industrial centres of Cairo and Alexandria in Egypt are particularly problematical since these are in the same location as the primary agrarian region, which is limited to the Nile river basin as the primary source of irrigation water. Studies of the effects of air pollution on vegetation have been carried out in the last 20 years in the greater Cairo area and around the main roads within the Nile delta region.

Hourly mean O3 concentrations were also recorded greater than 100 ppb. Visible injuries included necrosis, red spots and chlorosis with 60 % and 54% of clover and Egyptian Mallow leaves injured respectively.

The impact of O3 has been assessed on the growth and yield of local varieties of radish (Raphanus sativus L. cv. Balady) and turnip  (Brassica rapa L. cv. Sultani) at sub-urban and rural sites in Alexandria using EDU to protect control plants from O3 effects. O3 impacts included the formation of chlorotic spots on the upper leaf surface and reductions in plant biomass. These effects were recorded for radish at both sites and for turnip only at the rural site. The study proved that levels of ambient O3 in Egypt are high enough to have significant impacts on the growth and yield of local varieties of vegetable crops, even at a time of year when O3 levels are relatively low. 

Latin America

We are not aware of any studies investigating the damage of surface ozone to crops in Latin America, however, there is evidence of damage to forest trees. In Mexico, the most serious air pollution occurs in the vicinity of Mexico City associated with high O3. O3 damage to several species of Pinus was observed in a southern forested area of Mexico City. Injury included chlorotic mottling and premature senescence. O3 damage was most prevalent at the end of spring and the beginning of the summer season. P. hartwegii has been identified as one of the most sensitive species to O3 exposures.

The sudden decline in sacred fir trees (Abies religiosa) observed in the “Desierto de los Diones” national park located to the south west of the Mexico valley, is considered to be caused by O3 pollution due to the occurrence of O3 visible injury symptoms.


Most studies of the effects of ozone on Australian native species have been short-term studies using high concentrations to assess acute visible injury. In a longer-term study on eight Eucalyptus species there were differences in the responses of the different species.  Some species showed no visible injury or growth changes, but others showed up to 90% leaf injury and 30% growth reductions.  The findings were generally consistent with those found for North American woody perennials and northern European herbaceous species.

The evidence from Australia would suggest that those planning Eucalyptus plantations in regions of the world with high or increasing levels of tropospheric ozone, such as rapidly industrializing nations of Asia and South America, need to consider ozone tolerance when selecting plantation trees.

2.2 Regional risk assessment studies

Based upon dose-response data from US and European studies and a limited amount of Australian data including field pollution gradient studies using ozone-sensitive varieties of plants, current ambient concentrations of ozone in the regions around cities are likely to reduce growth and yield of sensitive crops, and damage some areas of biodiversity importance.

However, regional studies have been conducted in East Asia and China. These suggest that yield losses of staple crops may be currently in the range of 1 to 10% and predict future losses of 4 to 30% in 2020; local yield reductions may be much greater than these regional-scale estimates. Factors that may alter plant response to O3 include those associated with climate, agricultural management practices, crop phenology, genetically based tolerance or resistance and pollutant exposure patterns. However, there is still an urgent need to establish dose-response relationships for locally grown species and cultivars under local environmental conditions and management practice regimes.

The APCEN network identified some of the most important crops for future studies based on existing information describing ozone injury and damage and the importance of the crop as an agricultural commodity. Here we focus on Asia since this is the region where impacts to agriculture are perhaps most severe outside Europe and North America.

3. Conclusions

  • There is substantial evidence of the effects of ozone damage on both crop productivity and forest health across many parts of the globe outside Europe and North America. In particular, Asia would appear to be at particular risk from loss in agricultural productivity at current ozone levels and studies suggest the situation could significantly worsen in the future.

  • There is evidence of variability both within and between species and species cultivars. Further studies are urgently required to further understand which species and strains may be resistant to ozone pollution so that appropriate crop substitution can be employed within high-risk elevated ozone regions.

  • There is also an urgent need to perform regional risk assessments based on dose-response relationships suitable for local species and varieties. These risk assessments may also benefit from the use of flux-based rather than concentration-based methods since these may provide a better opportunity to disentangle the influence of environmental stresses on crop sensitivity to ozone.

  • Additional experimental research should be conducted in the future. In particular, the use of FACE systems should help to provide reliable crop response data free from many experimental artifacts. These, rather expensive, experimental systems should be supported with a range of experimental techniques including enclosed chamber, filtration, transect and bio-monitoring studies. Where possible, these studies should also investigate the influence of climate change stresses on ozone sensitivity.

  • There is an urgent need to conduct co-ordinated and standardized ozone monitoring across the globe in peri-urban and rural areas to complement existing monitoring in developing country regions that tends to be performed in urban areas. This will capture the higher ozone concentrations that occur downwind of urban areas and also provide information that can help crop-based risk assessment studies.

  • Finally, investigation of the potential indirect effects caused by ozone impacts on vegetation should be investigated as a priority, especially where these have direct links with climate change. For example, the effects of ozone reducing transpiration may well alter local and regional climatic systems.

*Stockholm Environment Institute, York, UK. E-mail: l.emberson@york.ac.uk

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

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