Managing Ambient Air Quality Using Ornamental Plants-An Alternative Approach

Air quality management involves the determination of the relative contribution of current or future source emissions at receptor sites to ground level pollutant concentrations coupled with implementation of effective management plan to reduce emission level. From the last few decades, urban air pollution has become an inevitable issue for its possible consequences on public health. In order to manage the ambient air quality, there are several methodologies with each having some specific advantages and constrains. Improvement of ambient air quality involves the basic approaches like emission reduction at source level, conversion of pollutants to a less damaging compounds and sequestration of pollutants. However, in order to manage the air quality, application of ornamental plants may be considered a well alternative approach. Although, it may not be imperative to consider plants as a sole alternative for urban ambient air quality management, but certainly it may useful to manage air quality for a small confined area, especially in case of indoor air. This paper reviews the possible application of ornamental plants, which may be trees, shrubs or herbs for its possible applications to regulate air quality. Some selective plant species has also been recommended based on its air pollution tolerance index.


Introduction
Air pollution had spread around the world like an evil with rapid urbanization and industrialization, it has resulted in various health problems for man such as respiratory, cardiovascular and ophthalmic diseases (Brunekreef and Holgate, 2002;Miller et al., 2007;Nandasena, 2010; Giles et al., 2011;Gudmundsson, 2011;Jamrozik and Musk, 2011). The significant air pollutants are suspended particles, gases, different ionizing radiation and noise. The gases include the oxidized and reduced forms of nitrogen, Carbon, SO 2 , C 6 H 6 , Vapour, O 3, Hg, C l2 , and volatile phenols. The suspended particles include the various forms of PM2.5, PM10 particulate and heavy metals.
In the past few decades India has witnessed a revolution in technological and industrial advancement coupled with an unexpected explosion of population. The first two of these significant events our society improved with many of the material things. The scientific technology has made these developments possible and user friendly. However, during achieving our goals for material things, subsequent side effects of such technological developments on esthetic values, and the quality of our environment have received very little or no attention.
The increase in population, industrialization and commercialization in urban areas has led to a rapid repercussion in the surrounding environment. Ambient air pollution has become a matter of great concern, particularly in mega cities and urban areas and rapid industrial development coupled with emission from transport sector are recognized as the prime sources. The situation is alarming and gradually becoming more severe and it is expected to increase in near future to cope up with the population expansion (Banerjee et al., 2011). Therefore, development of an adequate management plan is one of the most basic requirements for the well beings of human, animals as well as for plants.
Shrubs play a significant role in monitoring and controlling the air pollution by effectively participating in the cycling of gases like N, CO 2 , O 2 and nutrients. Shrubs also provide large leaf area for absorption, impingement and accumulation of pollutants to reduce the pollution level. There are various way and means to mitigate the effect of air pollution. Air pollution has tremendous impact on health of humans as well as on plants. Planting of shrub and tree is an effective and easy way for air pollution control and treatment of environment (Nugrahani et al., 2012). Several plants used for in landscape for beautification act as bio-indicators of urban air quality (Mondal et al., 2011).
India is blessed with various agro-climatic regions and in every part of India almost different type of plants are grown.
Every single plant not only looks different from others but also has enormous features and utilities. As it is estimated that Indian flora comprises of nearly 2000 species belonging to 150 families of flowering plants. The vast diversity ranks the 10 th in the world (Kishwan et al., 2009). In cities and industrial areas certain proportion of land can be managed by growing some ornamentals. These ornamentals either may be trees, shrubs, climbers, ground covers or grasses which occupy a certain proportion of landscaping and become an important and integral part of that area. Scientific management of landscape techniques and selection of plant species has also opened the doors to minimize the deleterious effects of air pollutions in these areas. The awareness of growing ornamental plants for improvement of air quality is lacking in developing countries like India and there is a complete neglect of air quality improvement planning in big cities and near the industrial areas across the country. In view of the above, this paper will find out the possibility of using ornamental plant as an alternative approach for the management of ambient air quality.

Source and Effects of Ambient Air Pollutants
With the gradual advancement of Indian economy, rapid industrialization and commercialization in urban areas have evoked a concern over its probable impacts on surrounding environment. Scientists have identified nearly 3000 different anthropogenic air pollutants and most of them are organic. Combustion sources, especially motor vehicles, emit different compounds. However, only for about 200 of the pollutants have the impacts been investigated. The ambient concentrations are determined for an even smaller number resulting in a major limitation for urban air quality management.
Some natural factor also affects the air quality viz. Volcano which produces chlorine, ash particles and sulphur. Wildfire produces carbon monoxide and smog. Pine plant produce volatile organic compounds. Cattle and other animal produce methane gas.
In urban areas, combustion of fossil fuels to generate electricity, in industrial processes, transportation and space heating is the predominant anthropogenic source of atmospheric air pollutants (CO, NO 2 , SO 2 , TSPM etc.). The burning of hydrocarbons in motor vehicle engines gives rise to CO 2 , CO, SO 2 , NO x in varying proportions and C 2 H 4 , as well as a variety of other hydrocarbons. Additional SO 2 originates from domestic and industrial burning of fossil fuels. Industrial plants, such as chemical and metal-smelting plants, release SO 2 , H 2 S, NO 2 , and HF (hydrogen fluoride) into the atmosphere.
TSPM are diverse in physical and chemical properties depending on their source (stationary, mobile or natural), geography and meteorology of the particular area. The sources, characteristics and potential health effects of PM 10 (particles with aerodynamic diameter less than 10 mm) and PM 2.5 (particles with aerodynamic diameter less than 2.5 mm or fine particles) are very different. Epidemiological studies emphasized that exposure to airborne particles with aerodynamic diameter less than 10 μm (Respirable Particulate Matter, RPM) induce negative health impacts and adverse meteorological factors may aggravate such kind of exposure. In addition to these, particulates are also responsible for causing reduced visibility and changes in the nutrient balance both through wet as well as dry deposition processes.
Sulphur dioxide is one of the atmospheric pollutants that played a vital role in designating the ambient air quality of a place, since its emission occurs from various industrial sources besides fossil fuel thermal power stations. Sulphur dioxide is also produced by smelting of sulphide ores. Atmospheric sulphur is one of the prominent anthropogenic pollutants and SO 2 level rises 4% annually. This trend of rises in SO 2 level is parallel to global energy consumption. SO 2 is also responsible for formation of sulphate aerosols, deposition of sulphate particles and generation of sulphuric acids droplets (H 2 SO 4 ) under the opposite atmospheric conditions. Sulphur dioxide produce a stinging smell and as a result it causes breathing problems in humans. Researches involving asthmatics indicated that significant proportion of population experience changes in pulmonary function and respiratory symptoms after very short periods of exposure to SO 2 (WHO, 2005). Further, the effects of SO 2 on plants are moderately understood. Primarily, the gas is absorbed into the mesophyll layer of the leaves through stomata and incurs toxicity due to its reducing properties which leads to chlorotic or brownish red colour of leaves, owing to chronic toxicity.
Nitrogen oxides are the by-products of fossil fuel combustion processes, therefore, emission from automobiles and power plants are significant contributors of atmospheric NO 2 . NO x emissions in Indian region are growing at an annual rate of 5.5% per year. Irritation in the respiratory tract is the predominant impact associated with NO 2 when it is converted to nitrates and nitric acid. However, protracted exposure may cause adverse impacts on the lung structure, metabolism and resistance against infections. NO x is one of the pre-cursor of ground level ozone and both can affect crops and other plants. Nitrogen oxides are predominantly responsible of both acid precipitation and ground level ozone formation and both are well blamed for causing injury to plants. While nitric acid is responsible for only a smaller part of hydrogen ion (H + ) concentration in wet and dry acid depositions, the contribution of nitrogen oxide emissions to acid deposition could be more significant.
Common Fluorides pollutant compounds are F 2 , SiF 6 , CF 4 , and HF. The fluoride pollutants include CaF 2 , NH 3 F, Ca 3 AlF 6 , AlF 6 , NaF and Na 2 SiF. NaF, AlF 6 and NaAlF 6 act as a source of aerosol. Major sources of fluoride pollution are glassworks, steelworks, aluminium factories, brick kiln, ceramic factories and uranium smelters. Fluoride pollutant causes the adverse effects when absorbed by plants and ingested by herbivorous animals.

Air pollution state in India
Air pollution is one of the main significant causes of the human health problems. According to World Health Organisation, at present urban outdoor air pollution contributes approximately 1.3 million deaths worldwide. Central Pollution Control Board (CPCB) and the State Pollution Control Boards (SPCBs) are responsible for data collection on air quality and policy enforcement. Centre for Science and Environment analysed the data on state of air quality and trend in Indian cities. Main pollutants in India are nitrogen dioxide (NO 2 ), particulate matter (PM), sulphur dioxide (SO 2 ), and carbon monoxide. Particulate level in some Indian cities is 5 time greater then safety limits. The PM10 level is a biggest and main reason of human health problem. During 2007 data published by National Ambient Air Quality Standard (NAAQS) more than 52% cities were at PM10 critical level (exceeding 1.5 times the standard), 36 cities with high levels (1-1.5 times the annual standard) and 19 cities at moderate level. Highest PM10 level was found in Indian cities like Mumbai, Bengaluru, Lucknow, Delhi and Faridabad with continuously increasing levels. (Hosamane et al., 2012)

Ambient Air Pollution Management through Ornamental Plants
Air quality depends on different factors, including the population density, the volume of traffic, the energy demand and the physical characteristics of the territory (i.e. geographical conformation, topography). The development of a coherent picture of national environmental trends and conditions requires the collection of sufficient data, the statistical analysis and integration of the information and the provision of complete, accurate and understandable presentations. An effective air quality assessment and management strategy involves the consideration of a number of complex physical, chemical, socio-economic, environmental (i.e. pollutant emissions) and meteorological factors. These complexities require that effective tools to be employed in developing and establishing the air quality standards and emission regulations. Among the various adopted approaches, use of plants for the removal of ambient air pollutants has considerable potential. The fundamental purpose of air quality management through plants includes:  To improve the air quality around the affected area  To support and implement the developmental works  To create good working conditions  To provide good healthy condition for aged peoples and children  To enrich the aesthetic value of a site  To help to reduce other types of environmental pollutions  To ensure safe and unhazardous journey  To promote green-belt development  To develop awareness in environment management among the peoples Plants remove a significant amount of pollution from the atmosphere as part of their normal functioning. Plants reduce the concentration of greenhouse gases and their consequence on climate change.Apart from air pollution management, ornamental plants have a wide spectrum of uses in reduction of heat build-up, noise pollution and eliminating dust from air (Baiyewu et al., 2005). Additional benefit to grow ornamental plants for air pollution management is it gives aesthetic value. However, selection of ornamental species varied widely according to the location, purpose, adaptability to the particular soil etc. Different approach for choice of species should also be done for outdoor and indoor conditions. Different plant species vary considerably in their susceptibility to air pollutants. The identification and categorization of plants into sensitive and tolerant groups is important because the former can serve as indicators and the latter as sinks. Research conducted by National Aeronautics and Space Administration (NASA), USA revealed that plants can be useful to clean indoor air. These scientists and other vigorous advocates say that plants have been cleaning the earth's atmosphere for millions of years and may be adopted as a most reasonable method for indoor air pollution control. Through photosynthesis, plant uptake carbon dioxide from the atmosphere and release oxygen. A team of NASA researchers tested the effect of fifteen house plants on three pollutants known to be present in spacecrafts. These same three pollutants viz. benzene, formaldehyde and trichloroethylene are present in homes and office buildings with furnishings, office equipment and some building materials as a primary source of emission. Under controlled conditions, certain houseplants were found to remove as much as 87% of indoor air pollutants within 24 hours. Further, it was revealed that more air that is allowed to circulate through the roots of the plants, the more effective they are at cleaning polluted air (Adopted from http://www.sti.nasa.gov/tto/Spinoff2007/ps_3.html)

Mechanism of Air Quality Improvement through Plant System
There are four main ways in which plants improve air quality parameters. These are:

a) Reduction of Temperature:
There is a direct relationship exists between the emission of many pollutant and/or ozone forming chemicals with atmospheric temperature. Ozone forming chemicals are also reported to decrease with reduction in air temperature. Plant has a direct effect on temperature, incident radiation, radiation absorption, surface roughness, wind velocity, relative humidity and surface albedo. Trees contribute towards cooler summer temperature. These changes help to create a microclimate in surrounding areas which has the ability to alter the pollutant concentrations in urban and industrial areas (Nowak et al., 1998).

b) Removal of Air Pollutant:
The principle process by which plants removed gaseous air pollution is through the stomata, though some gases are also removed by other plant organs. Absorbed gases diffuse into intercellular space react with inner surface of leaves, may be absorbed by water to form acid. Suspended air particle intercepts with the leaf surface, adsorbed on the leaf and dropped to the ground during leaf or twig fall or rain fall (Smith, 1990). It is reported that a large healthy tree (> 77cm diameter) remove approximately 70 times more air pollution annually than small healthy trees which having diameter less than 8 cm (Nowak, 1994).

c) Emission of Volatile Organic Compounds (VOC s ):
Each and every plant emits certain amount of volatile organic compounds (VOC s ) in the atmosphere. These VOC s are mainly responsible for the formation of ozone and carbon monoxide. However, in the presence of low nitrogen dioxide VOC s actually remove ozone (Crutzen et al., 1985). VOC s emission rates dependent upon the species and nine genera have been reported to have very high VOC s emission rate; among them Eucalyptus, Salix, Casuarina are important.

d) Energy Effects on Building:
Tress reduces the building energy by lowering temperatures through shading during the summer and blocking winds during winter (Heisler, 1986); however, shading effect may lead to increase energy use during winter while during summer energy use may increase or decrease in the buildings situated near the sea shore. Therefore, proper knowledge of tree placement near the vicinity of the buildings is required to achieve maximum benefit.

Plant Symptoms in Response to Air Pollution Exposure
Leaves absorb and accumulate pollutants from air and help to reduce the pollution level in the atmosphere. Plants provide an enormous leaf area for impingement. Certain changes in morphological, anatomical and physiological characteristics of plants have been reported when plant experienced air pollution exposure. The rate and total amount of pollutant taken up from the air can affect photosynthesis, respiration, growth, yield, leaf conductance and leaf longevity. All of these factors in trees adversely affect the canopy carbon fixation and biosynthesis of chlorophyll. Air pollutants affect the plants even in very low concentration (0.1 to 55 ppm). However, plant species differ in their sensitivity level to air pollutants. Plants symptoms in response to air pollution are of two types chronic and acute depend upon the extent of exposure. Chronic injury result by low level pollutant exposure. Chronic symptoms usually do not kill tissue. Chronic symptoms characterized by stripping, yellowing, dwarfing or growth loss. Acute injury is due to plant exposure to high level of pollutant. Acute symptom characterized by dead tissue in particular area. Sometime whole leaf or even plant death also occurs. Loss of chlorophyll, bronzing and reddening of the leaves is the commonly associated symptoms related with ozone exposure; whereas, upon the long exposure of SO 2 , broad leaved species produced dark brown inter-veinal bifacial necrosis spots on the leaf lamina (Tingey et al., 1973

Selection Criteria of Ornamental Plants for Improvement of Air Quality
The trees should be tolerant to air pollutants present in the area  The

Ornamental Plant Species Ideal for Improvement of Outdoor Air Quality
The effectiveness of a plant suitable for air quality improvement determined with the air pollution tolerance index (APTI) value (Singh et al., 1991). APTI denotes capability of a plant to combat against air pollution. APTI is used by the landscapers for selection of plant that tolerate air pollution. The plants which have higher index value are tolerant to air pollution and can be utilized to mitigate pollution, while plants with low index value show less tolerance and can be used to indicate levels of air pollution (Singh and Rao, 1983). Changes in parameters such as ascorbic acid, total chlorophyll, relative water content and pH of leaf extract are determined to evaluate the degree of tolerance to air pollution by a plant species (Agbaire, 2009

Source of Air Pollutant in Indoor Condition
The major source from which air pollutant generated in indoor conditions are combustion, house hold waste, asbestos-containing insulation, wet and damp carpet, cabinetry and furniture made of certain pressed wood, chemicals, building materials and bioeffluents. Formaldehyde, CO, SO 2 , NO 2 , and tobacco smokes are the byproduct generated from combustion in indoor conditions. Beside these, several chemical compounds such as cosmetics, deodorant, disinfectant, mosquito repellant, central heating and cooling system and humidification device when used by the householders are also hazardous for indoor air quality.

Ornamental Plant Species Ideal for Improvement of Indoor Air Quality
Current researches exhibit that ornamental plants can drastically reduce levels of stress, adverse health impacts as they have the potential to sequester harmful indoor air pollution. International space station (NASA) discovers that several house plants have ability to remove various toxic gases such as ammonia, formaldehyde, toluene and volatile organic compound (VOC s ) from the environment. Researchers have now identified four "super ornamental plants" which every workplace should have to clean up indoor air (Cruz et al., 2014). A recent study revealed that ornamental indoor plants have ability to remove harmful VOC s from indoor air. The process through which indoor plants effectively remove harmful VOC s from the air called phytoremediation. Plants absorb toxic gases from air into their leaves and then translocated into root, whereas different microbes break them. Indoor plants also add biological and aesthetic compound to interior space by increasing the relative humidity and decreasing the dust accumulation. Removing the dust particles, decreasing potential allergy-inducing particles and increasing the relative humidity, induce relaxing effect. Apart from VOC s , Benzene and toluene can also be sequestered by plants effectively. The research team tested different common indoor ornamental plants for their ability to remove indoor pollutants. Of the species tested, purple waffle plant (Hemigraphis alternata), English ivy (Hedera helix), waxy leaved plant (Hoya carnosa) and Asparagus fern (Asparagus densiflorus) were rated best for removing air pollutants. Beside these plants, following ornamental house plants are also found suitable to clean the indoor air.

Needs for air Quality Management
The most essential need is to discover and use of alternate non-polluting source of energy in a strategic way like solar energy, the most abundant and crucial source of energy for country like India, Hydrogen gas and natural gas fuel. Most accurate strategy is to transform the transportation system to natural gas, especially in megacities. Air quality in industrial area can be improved by using clean development mechanism.  In air quality some obvious research needs. Many ecological and basic physiological studies of the climatic condition and effect on change in atmosphere are required. Monitoring programme also required for assessment of current actual condition and rate of change of air quality. Other pressing need is to control the atmospheric pollution and air quality is institutional management in India are:

•
Encourage the private sector to identify need for more investment in air quality assessment and monitoring tool. Training facilities, capacities and calibration equipment are required.
• It is essential to start institutional capacity building in air pollution and quality monitoring for those countries which do not have this facilities and strengthen for those which have already. It involves the following factors: I Regulation and standards II Government support and interest III Stack-holders and awareness IV NGO V Viabilities and monitoring tools

Conclusions
The applicability of ornamental plants as an approach for air quality management and several plant species can be used, based on their APTI value and recommended based on their adaptability for different locations. It is well understood that such ornamental plants are well applicable as a supplementary approach for improvement of air quality and therefore it was felt necessary to adapt such practices in urban environment.