This article foregrounds the challenges India is currently facing in reducing air pollution and bringing the level of air quality to a certain standard. It also discusses solutions that could be adopted to combat the national crisis.
Rising urbanisation, booming industrialisation, and associated anthropogenic activities are the prime reasons that lead to air pollutant emissions and poor air quality. It is expected that by 2030, around 50% of the global population will be residing in urban areas (Gurjar, Butler, Lawrence, et al. 2008). More than 80% of population in urban areas is exposed to emissions that exceed the standards set by World Health Organization (WHO 2016). Air pollution is one of the key global health and environmental concerns (Nagpure, Gurjar, Kumar, et al. 2016) and has been ranked among the top five global risk factors of mortality by the Health Effects Institute (HEI 2019). According to HEI's report, particulate matter (PM) pollution was considered the third important cause of death in 2017 and this rate was found to be highest in India. Air pollution was considered to cause over 1.1 million premature deaths in 2017 in India (HEI 2019), of which 56% was due to exposure to outdoor PM2.5 concentration and 44% was attributed to household air pollution. As per WHO (2016), one death out of nine in 2012 was attributed to air pollution, of which around three million deaths were solely due to outdoor air pollution.
The rising trends in population growth and the consequent effects on air quality are evident in the Indian scenario. For example, the megacities of Delhi, Mumbai, and Kolkata combined holds a population exceeding 46 million (Gurjar, Ravindra, and Nagpure 2016). Over the years, there has been a massive-scale expansion in industries, population density, anthropogenic activities, and the increased use of automobiles has degraded the air quality in India (Gurjar and Lelieveld 2005). In the last few decades, the greenhouse gas (GHG) emissions and other emissions resulting from anthropogenic activities have increased drastically (Gurjar and Nagpure 2016).
As per WHO (2016) estimates, 10 out of the 20 most populated cities in the world are in India. Based on the concentrations of PM2.5 emissions, India was ranked the fifth most polluted country by WHO (2019), in which 21 among the top 30 polluted cities were in India. The Indian cities, on average, exceeded the WHO threshold by an alarming 500%.
The consistent population growth has led to an excessive strain on the energy consumption, thereby affecting the environment and the air quality of the megacities (Gurjar and Nagpure 2016). Kumar, Khare, Harrison, et al. (2015) calculated the increase in the total energy demand for both mobile and point sources and inferred that in Delhi, the energy demand had grown by 57.16% from 2001 to 230,222 TJ in 2011. A subsequent rise in energy consumption can be expected in the coming years, with no reliable sources available for monitoring the rate of energy consumption.
The continuous degradation of ambient air quality in the urban centres of India demands effective measures to curb air pollution. Though various policy measures are being introduced by the Government of India (GoI) to reduce the vehicular and industrial emissions, the extent to which these measures are implemented is questionable. The lack of infrastructural facilities, inadequacy of financial resources to implement advanced infrastructural innovations, difficulty in relocation of the industries from the urban centres even after mandatory court decisions, and above all, the behavioural patterns among people in accepting the green solutions are some of the crucial impediments on the road to environmental protection that our country seems to be struggling to overcome today.
Background
There have been various efforts to study the air quality in Indian cities. The potential of the atmospheric carcinogenic emissions to put human health at risk has been studied by Gurjar, Mohan, and Sidhu (1996). Gurjar, Aardenne, Lelieveld, et al. (2004) framed a comprehensive emission inventory model to understand the emission trends in Delhi, India's capital, for a period from 1990 to 2000. A multi-pollutant index (MPI) rating scale was used by Gurjar, Butler, Lawrence, et al. (2008) to rank the megacities with respect to their ambient air quality. According to this study, out of 18 megacities considered worldwide, the Indian cities, namely, Delhi, Kolkata, and Mumbai were ranked 7, 9, and 11, respectively. Gurjar, Nagpure, Kumar, et al. (2010) evaluated the vehicular emissions in Kolkata between 2000 and 2010 and inferred that the older vehicles in the city contributed more to the pollution load and should be phased out. A Vehicular Air Pollution Inventory (VAPI) model was developed by Nagpure and Gurjar (2012) that could estimate the vehicular emissions from road traffic in Indian cities. Later, Gurjar, Nagpure, and Kumar (2015) evaluated the potential gaseous emissions from the agricultural wetlands of Delhi and inferred that man-made wetlands were responsible for 48–49% of the total GHG emissions in the capital city. The study intended to develop an emission inventory for agricultural activities to evaluate their contribution to pollution in Delhi.
Several policy measures have been taken by the Ministry of Environment, Forest and Climate Change (MoEFCC), GoI to tackle the adverse effects of air emissions in short and long terms. The government's decision to adopt compressed natural gas (CNG) as an alternative fuel to petrol and diesel, the odd-even measures introduced in Delhi, and the improvements in fuel and vehicle quality to lower emissions are some of the recent commendable steps towards curtailment of air pollution. Moreover, the increasing number of studies related to this field shows the importance of research on this subject. Several studies have assessed the trends of air pollutant emissions from different sources across several cities in India. However, there is an urgent need for a comprehensive review of the existing issues in the Indian scenario. More focus is needed on studying the impacts of these pollutant emissions on various forms, such as the ecosystem, biodiversity, buildings and materials, and primarily the health risks that people are vulnerable to due to breathing foul air.
A comprehensive review is done to understand the current scenario in the Indian context. The following section comprises a detailed review focusing on air pollution studies in India, the various sources, and the effects of the pollutants on the ecosystem, biodiversity, materials and buildings, and on human health, which are discussed in the later sections of this article. The various air quality standards followed by countries worldwide are included as well. The Discussion section of the article consists of the mitigation strategies adopted for emission control in India, the challenges posed by various sectors in the Indian scenario, and the research gaps that have been identified from the available literature. The key conclusions and a few recommendations form part of the last section.
Reviewing Literature
The present review is divided into three sub-sections: The first sub-section discusses the literature that focuses on air pollution in India on a national scale; the next segment highlights the various sources of air pollution and the effects of the pollutants. The major sources are categorised into seven sectors. Thereafter, the various effects of pollutant emissions are pointed out. The air quality standards adopted by various countries for controlling air pollution have been discussed in the later sections of this article.
Studies on air pollution in India
Though various studies have addressed the issue of air pollution and its impacts on urban Indian cities, most of these studies are limited to specific cities and do not necessarily give a complete picture of the situation. Some of the highlights of these studies are discussed in the following paragraphs.
Pandey and Venkataraman (2014) evaluated the effects of emissions from various modes of transport in India. Their study inferred that on-road transportation contributed over 97% of the estimated emissions in India, when compared to other modes of transport, such as railways, waterways, and airways. Gurjar, Ravindra, and Nagpure (2016) did a comprehensive study on various anthropogenic emission sources in Indian megacities, such as Delhi, Mumbai, and Kolkata. The global impact of urban pollution is also discussed in their study. Upadhyay, Dey, Chowdhury, et al. (2018) evaluated the major anthropogenic sources of PM2.5 and the potential benefits to human health, if sufficient control measures are applied to curb emissions. A recent study by Jat, Gurjar, and Lowe (2021) examined the extent of pollution during the winter months in India. The study used a WRF-Chem model, that is, Weather Research and Forecasting (WRF) coupled with chemistry, to evaluate the concentrations of pollutants, such as PM2.5, oxides of sulphur (SOX), oxides of nitrogen (NOX), black and organic carbons, and non-methane volatile organic carbons (NMVOCs) that were identified for the winter months. The various sources of air pollution can be classified into seven major sources and the consequent effects are discussed in this article.
Sources of air pollution
The various sources of air pollution are classified into seven major sectors, which include transportation, industries, agriculture, power, waste treatment, biomass burning, residential, construction, and demolition waste.
Vehicular/Transport Emissions
The transportation sector is the main contributor of air pollutants in almost every city, but this phenomenon is worse in urban cities (Gurjar, Aardenne, Lelieveld, et al. 2004). This could be due to the increased number of vehicles when compared to the existing infrastructural facilities, e.g., roads, fuel stations, and the number of passenger terminals provided for public transport. In India, the amount of motorised transport increased from 0.3 million in 1951 to 159.5 million in 2012 (Gurjar, Ravindra, and Nagpure 2016). A significant share of vehicular emissions comes from urban cities, such as Delhi, Mumbai, Bengaluru, and Kolkata. Carbon monoxide (CO), NOX, and NMVOCs are the major pollutants (>80%) from vehicular emissions (Gurjar, Aardenne, Lelieveld, et al. 2004). Other trace emissions include methane (CH4), carbon dioxide (CO2), oxides of sulphur (SOx), and total suspended particles (TSPs).
In an urban environment, road traffic emissions are one of the prime contributors of air pollution. Road dust is a major contributor to PM emissions in Delhi (37%), Mumbai (30%), and Kolkata (61%). Road transport is the largest source of PM2.5 in Bengaluru (41%), Chennai (34%), Surat (42%), and Indore (47%) (Nagpure, Gurjar, Kumar, et al. 2016). In the Indian context, some of the essential factors of high traffic emissions include extreme lack of exhaust measures, the highly heterogeneous nature of vehicles, and poor quality of fuel.
Industrial Processes
Over the last few decades, India has witnessed large-scale industrialisation. This has degraded the air quality in most urban cities. The Central Pollution Control Board (CPCB) has categorised the polluting industries into 17 types, which fall under the small and medium scale (Gurjar, Ravindra, and Nagpure 2016). Out of these categories, seven have been marked as 'critical' industries that include iron and steel, sugar, paper, cement, fertiliser, copper, and aluminium. The major pollutants comprise SPM, SOX, NOX, and CO2 emissions.
The small-scale industries, which are not regulated like the major industries, use several energy sources apart from the primary source of state-provided electricity. Some of these fuels include the use of biomass, plastic, and crude oil. These energy sources are neglected in the current emission inventory studies. In Delhi, after the intervention of the judiciary in 2000, many industries were relocated from urban areas to adjacent rural areas (Nagpure, Gurjar, Kumar, et al. 2016). In Delhi, a major fraction of the pollution load comes from the brick manufacturing industries, which are situated at the outskirts of the city. Rajkot (42%) and Pune (30%) are the two cities where industries play a prominent role in contributing to the highest amount of PM2.5 (Nagpure, Gurjar, Kumar, et al. 2016).
Agriculture
Agricultural activities produce emissions, which have the potential to pollute the environment. Ammonia (NH3) and nitrous oxide (N2O) are the key pollutants released from agricultural activities. The other agricultural emissions include methane emissions from enteric fermentation processes, nitrogen excretions from animal manure, such as CH4, N2O, and NH3, methane emissions from wetlands, and nitrogen emissions from agricultural soils (N2O, NOX, and NH3) due to the addition of fertilisers and other residues to the soil (Gurjar, Aardenne, Lelieveld, et al. 2004). Agricultural processes, such as 'slash and burn' are prime reasons for photochemical smog resulting from the smoke generated during the process. Crop residue burning is another process that results in toxic pollutant emissions. This is how neighbouring cities of Delhi contribute to the agricultural pollution load. This is an example of how external sources contribute to the menace of air pollution in the city (Nagpure, Gurjar, Kumar, et al. 2016).
Power Plants
The contribution of power plants to air emissions in India is both immense and worrisome. The thermal power plants manufacture around 74% of the total power generated in India (Gurjar, Ravindra, and Nagpure 2016). According to The Energy and Resources Institute (TERI), the emissions of SO2, NOX, and PM increased over 50 times from 1947 to 1997. Thermal power plants are the main sources of SO2 and TSP emissions (Gurjar, Aardenne, Lelieveld, et al. 2004), thereby contributing significantly to the emission inventories. In Delhi, power plants contributed 68% of SO2 emissions and 80% of PM10 concentrations over a period from 1990 to 2000 (Gurjar, Aardenne, Lelieveld, et al. 2004). Thus, there is an urgent need to adopt alternative power sources including green and renewable resources for meeting the energy requirements.
Waste Treatment and Biomass Burning
In India, about 80% of municipal solid waste (MSW) is still discarded into open dumping yards and landfills, which leads to various GHG emissions apart from the issues of foul odour and poor water quality at nearby localities. The lack of proper treatment of MSW and biomass burning has been responsible in causing air pollution in urban cities. In Delhi alone, around 5300 tonne of PM10 and 7550 tonne of PM2.5 are generated every year from the burning of garbage and other MSW (Nagpure, Gurjar, Kumar, et al. 2016).
Methane (CH4) is the major pollutant released from landfills and wastewater treatment plants. Ammonia (NH3) is another by-product, which is released from the composting process. The open burning of wastes, including plastic, produces toxic and carcinogenic emissions, which are a grave concern (Gurjar, Aardenne, Lelieveld, et al. 2004).
Domestic Sector
Households are identified as a major contributor of air pollution in India. The emissions from fossil fuel burning, stoves or generators come under this sector, thereby affecting the overall air quality. Domestic energy is powered by fuels, such as cooking gas, kerosene, wood, crop wastes or cow dung cakes (Gurjar, Aardenne, Lelieveld, et al. 2004).
Though liquefied petroleum gas (LPG) is used as an alternative source of cooking fuel in most urban cities, the major share of India's rural population continues to rely on cow dung cakes, biomass, charcoal or wood as the primary fuel for cooking and other energy purposes and demands. These lead to severe implications on air quality, especially the indoor air quality, which could directly affect human health. According to HEI (2019), about 60% of India's population was exposed to household pollution in 2017.
Construction and Demolition Waste
Another major source of air pollution in India is waste, which is an outcome of construction and demolition activities. Guttikunda and Goel (2013) inferred from their study that around 10,750 tonne of construction waste is generated in Delhi every year. Even after the construction phase, these buildings have the potential to be the major contributors of GHG emissions. Nowadays, the increasing interest in green building technologies and the application of green infrastructure and materials during construction could tackle this issue to a large extent, thereby preserving our biodiversity and maintaining cleaner air quality.
Impacts
On the Ecosystem
The terrestrial ecosystem is widely affected by ground air pollution. The ill-effects include respiratory and pulmonary disorders in animals and humans (Stevens, Bell, Brimblecombe, et al. 2020). The effects on the marine ecosystem include acidification of lakes, eutrophication, and mercury accumulation in aquatic food (Lovett, Tear, Evers, et al. 2009). These processes may indirectly affect the health of the living beings. Similarly, soil acidification is another phenomenon that is common in forest ecosystems as a result of long-term pollutant accumulation. The deposition of sulphate, nitrate, and ammonium is the main reason for soil acidification. Bignal, Ashmore, Headley, et al. (2007) inferred that traces of heavy metals were found in soil samples in areas adjacent to roadways due to cumulative deposition of pollutants. Soil pollution indirectly affects the ecosystems of plants and animals that are reliant on soil for nutritional intake. Nitrogen deposition in wet and dry forms on vegetation and soil surfaces can occur from vehicular and agricultural activities (Driscoll, Whital, Aber, et al. 2003). The results of these activities on the ecosystem have long-term environmental implications, such as global warming and climate change (Lovett, Tear, Evers, et al. 2009). A recent study by Stevens, Bell, Brimblecombe, et al. (2020) discussed four threats to the global ecosystem from pollution, namely, the effects of primary pollutants, such as SO2 and NO2 in a gaseous state, the consequences of wet and dry depositions from SOX and NOX emissions, effects of eutrophication by nitrogen deposition, and the impact of ground-level ozone concentrations.
On Biodiversity
The ill-effects of air pollutant emissions could impact the biological diversity. Though it is evident that air pollution contributes to ground-level emissions, limited studies have been conducted to address the effects on our biodiversity. Acid rain, which is a result of air pollution, is caused by the oxidation and wet deposition of SO2 and NOX emissions in the atmosphere (Rao, Rajasekhar, and Rao 2016). Therefore, acid rain can have harmful effects on our biodiversity.
Nitrogen deposition on plants is a serious outcome of air pollution (Lovett, Tear, Evers, et al. 2009). Bignal, Ashmore, Headley, et al. (2007) investigated three sites adjacent to roadways in the UK to study the impact of pollution on the health of oak and beech trees. Several damages, such as increased defoliation, discolouration, poorer crown condition, and increased pest attacks were observed during the study. It was inferred that significant effects on plants could be found within 100 m from the roadways due to NO2 emissions.
Ozone is another pollutant which is toxic to both plants and animals. Ozone results in reduced photosynthesis and slower growth in plants. In animals and humans, ozone can affect the lung tissues causing respiratory conditions, such as asthma (Stevens, Bell, Brimblecombe, et al. 2020). The effect of ground-level ozone on the crop yield was studied by Sharma, Ojha, Pozzer, et al. (2019), where the researchers evaluated the pan India losses in crop yield and financial problems incurred during 2014–15 due to the ozone. Poor air quality and exposure to anthropogenic pollution had a negative effect on the health of animals as well (Isaksson 2010).
Moreover, the reproductive performance of animals also gets affected due to increased oxidative stress (Isaksson 2010), thereby impacting the population of any species. This may not prove healthy especially for the endangered species. Considering the rapid urbanisation, more focus should be given to this study area in the future.
On Materials and Buildings
SOX and NOX emissions can harm the flora, fauna, material surfaces, and even damage buildings and structures. The negative effects may be in the form of discolouration, loss of material, structural failing, and soiling. This can reduce the service life of buildings and can severely damage historical monuments and structures. One such example is India's white-marble Taj Mahal, which is turning yellow as a result of being exposed to SOX emissions from industries and acid rain. Another historical monument in India is Hyderabad's Charminar, which is turning black due to it being situated in a highly polluted area (Rao, Rajasekhar, and Rao 2016). The erosion of such heritage zones poses a grave concern.
On Human Health
People residing in areas exposed to poor air quality and high pollution levels are prone to hazardous health risks. Such deleterious implications can lead to both minor respiratory disorders and fatal diseases (Gurjar, Jain, Sharma, et al. 2010). Molina, Molina, Slott, et al. (2004) inferred that the studies conducted worldwide had similar conclusions regarding the impact of pollutants on humans. Emissions such as PM, O3, SOX, and NOX have the potential to damage the cardiovascular and respiratory systems of humans. In recent years, the study of human health risks as an outcome of poor air quality has been of prime focus. Gurjar, Jain, Sharma, et al. (2010) evaluated the health risks people in urban areas were prone to due to air pollution in terms of mortality and morbidity. However, there are several limitations associated with the application of this health risk assessment methodology, which must be addressed in the future studies. The HEI (2019) assessed the impact of PM2.5 concentrations in India and concluded that around 1.1 million deaths in 2015 were a result of being exposed to air pollution. Upadhyay, Dey, Chowdhury, et al. (2018) inferred that a total of 92,380 lives would have been saved if control measures were applied in the transport, residential, industries, and energy sectors, which are some of the prominent contributors of air pollution.
Gurjar, Ravindra, and Nagpure (2016) concluded in their study that around 30% of Delhi's population complained of respiratory issues due to air pollution in the selected year. Another study by Nagpure, Gurjar, and Martel (2014) evaluated that the mortality rate due to air pollution had doubled between 1990 and 2010 in the capital city. According to Gurjar, Mohan, and Sidhu (1996), the number of premature deaths in Mumbai due to air pollution was recorded at 2800 in 1995, which later increased exponentially to 10,800 in 2010 (Gurjar, Ravindra, and Nagpure 2016). In Kolkata, the premature deaths were estimated to be around 13,500 in 2010. Similarly, Delhi reported about 18,600 premature deaths per year (Lelieveld, Evans, Fnais, et al. 2015).
Air quality standards
The acceptable threshold level of air pollution in terms of its potential impacts on health and environment is defined as the ambient air quality standards. These standards are adopted and enforced by a regulatory body or authority. Every standard should have a standalone definition and its threshold values should be justified appropriately (Molina, Molina, Slott, et al. 2004). The air quality standards may vary for different countries due to various factors, such as economic conditions, technological know-how, and indigenous air pollution-related epidemiological studies. These are known as the National Ambient Air Quality Standards (NAAQS) in countries, such as India, China, and the US. However, in Canada and the European countries, the limit values are predefined (WHO 2005). Table 1 gives a representation of the different standards adopted by different countries (WHO 2005).
For India, the NAAQS developed by the Central Pollution Control Board (CPCB 2009) are given in Table 2.
Discussion
Mitigation strategies for emission control in India
In India, the central and state governments have taken several steps to control air pollution and improve the ambient air quality. Various initiatives, such as the use of compressed natural gas (CNG) as an alternative fuel, the odd-even measures implemented in Delhi, the introduction of Bharat Stage VI vehicle and fuel standards, the Pradhan Mantri Ujjwala Yojana (PMUY), and the National Clean Air Programme (NCAP) are some examples in this endeavour. The CPCB ensures the monitoring and regulation of the NAAQS in the cities, towns, and industrial areas with the cooperation of the respective state pollution control boards (SPCBs). Under these plans, various sector-wise measures have been implemented in the urban cities of India. For the transport sector, for instance, some of these measures include the use of electric vehicles (EVs) as a mode of public transportation, development of cycling infrastructure, use of bioethanol as fuel, and the construction of multi-level car parking facilities and peripherals to tackle congestion. Within the industrial sector, some of the measures undertaken comprise the implementation of zig-zag technology for the stack emissions from brick kilns, online monitoring of discharges through the Online Continuous Emission Monitoring Systems (OCEMS), and the installation of web cameras in highly polluting industries. To tackle the problem of open burning of garbage and household wastes, door-to-door collection of segregated wastes has been introduced and several compost pits have been established in urban cities. In the residential sector, the government has set a target of achieving 100% usage of LPG for cooking purposes. Further, to control the concentrations of particulate matter (PM) and dust particles, various steps, such as the green buffer around cities, maintenance of 33% green cover around urban areas, installation of water fountains across the cities have been taken over the years (Ganguly, Kurinji, and Guttikunda 2020; Sharma, Mallik, Wilson, et al. 2018; Sharma, Rehman, Ramanathan, et al. 2016).
Other potential mitigation strategies
Air quality management in megacities is a four-stage process that involves problem identification, formulation of policies, their implementation, and control strategies (Molina, Molina, Slott, et al. 2004). The various management tools to ensure emission control and attainment of air quality standards include, air quality modelling, emission inventories, monitoring the concentration of pollutants, and source apportionment studies. These methodologies involve a complex analysis of extensive data sets for the effective management of air quality standards. Due to the lack of transparency and unavailability of data, uncertainties are introduced in the estimation of atmospheric concentrations. Minimising these uncertainties with our scientific understanding is one of the major challenges towards addressing the issues related to air quality (Gurjar and Ojha 2016).
The increase in private vehicles is the prime contributor of air pollution in Indian cities (Molina, Molina, Slott, et al. 2004). Therefore, there should be some policy norms that would set a certain limit to private vehicle ownership. Second, the age of vehicles degrades the air quality and such ageing vehicles should be phased out over a period of 10 years or so. Threshold limits should be imposed on emissions from all sources, primarily vehicles and industries, and the violators should be penalised.
Infrastructural modifications to limit traffic in polluted areas, development of efficient public transport facilities, such as the Bus Rapid Transit (BRT) system or other public transit systems, improved facilities for walking, biking, and public transport, and relocation of point sources out of urban centres could help curb emissions significantly.
Technology modifications, such as the introduction of hybrid vehicles or fuel cell vehicles or fuel modifications, such as ultra-low sulphur fuels, or alternative fuels like CNG, methanol in Brazil or hydrogen fuel in Japan (Molina, Molina, Slott, et al. 2004) could reduce air pollution levels. In recent years, owing to the reduced sulphur fractions in the fuels, decreasing trends in SOX have been observed (Gurjar, Ravindra, and Nagpure 2016) and such a development could further control air pollutant emissions.
Control points should be identified and prioritised in urban areas that would help reduce pollutant emissions significantly. The development of sustainability matrices could help monitor and regulate the emissions. Emission trading, also known as cap and trade, is another control strategy that could be applied in urban cities, a practice already prevalent in the US, where economic incentives are offered to reduce the pollutant concentrations (Molina, Molina, Slott, et al. 2004). Congestion pricing, as followed in London, where a driver is charged each time they enter the peak zones of a city could be another avenue to explore within the Indian context as well. However, such a strategy would require strong public awareness and support to become successful.
A combination of effective policies, technologies, and land-use planning could help develop a control strategy for emission control. Stricter emission standards, cleaner fuels, advancements in engines, manufacture of cleaner and green vehicles, and post-emission treatment technologies could curtail pollution levels in urban areas to a great extent. Concrete policy measures could be imposed that would further limit the exposure of people to pollutant emissions. Relocation of industries to the outskirts of the city is a fine example (Molina, Molina, Slott, et al. 2004) to consider in this regard.
Limiting the emissions from combustion sources could curb pollution. One such example was the use of CNG-fuelled vehicles in Delhi from 2001 to 2006, which had reduced the emissions of PM, CO, NOX, and SO2 levels considerably.
Challenges in the Indian scenario
Air pollution poses serious risks to human health, economic assets, and the overall environment (Gurjar, Butler, Lawrence, et al. 2008). In the current Indian scenario, urban cities are mostly polluted by vehicular emissions, industries, and thermal power plants (Gurjar, Ravindra, and Nagpure 2016). Nagpure, Gurjar, Kumar, et al. (2016) studied and inferred that vehicular emission is the major contributor of increasing pollution in Delhi. Gurjar, Aardenne, Lelieveld, et al. (2004) had earlier indicated that there is a lack of India-specific emission factors for several air pollutants, which could be a major concern towards developing realistic emission inventories for Indian cities. Further, Nagpure, Sharma, and Gurjar (2013) observed that neither ratio nor realistic numbers are available for two-stroke and four-stroke two-wheelers or for light and heavy commercial vehicles. Similarly, for evaluating the utilisation factors for vehicles, which indicate how frequently a vehicle is being used in a given period of time, the escalating travel demand in the country is not considered. This results in uncertainties in the estimated emissions of air pollutants.
Over the last decades, industrialisation has boomed and India ranks among the top 10 industrialised countries, globally (Gurjar, Ravindra, and Nagpure 2016). Guttikunda and Calori (2013) studied and listed the improvements that could be made in the emission estimates from Indian cities by monitoring capacity, regular documentation of pollutant sources, fuel usage patterns, and receptor modelling studies.
In India, the methodologies associated with emission estimation from biomass burning have certain limitations. For instance, Gurjar, Aardenne, Lelieveld, et al. (2004) exempted several sources from estimation due to non-reliable data sets pertaining to biogenic emissions. Guttikunda and Calori (2013) estimated that the burning of roadside garbage and the landfill fires have an uncertainty of ±50%. The study also estimated that the data on fuel used for cooking and heating in the domestic sector have an uncertainty of ±25%. This uncertainty of fuel usage data for the in-situ generators used in large institutions, hospitals, and hotels was ±30% for the year 2010.
As discussed in the previous section, the implementation of strict policy measures and the use of advanced technologies and infrastructure could tackle the problem of air pollution to a great extent. Though stringent measures and policies are being adopted to curb vehicular and stack emissions, most Indian cities lack the technological and infrastructural wherewithal. In a developing country like India, financial constraints faced during the timely planning and implementation of advanced urban infrastructural changes could pose a serious hurdle to air pollution mitigation strategies (Gurjar and Nagpure 2016).
Irresponsible human behaviour is another major issue that makes the existing challenges difficult to overcome. The lack of public interest in the emission control measures and inefficient traffic management system are major hurdles to realising the goal of clean air. The lack of public interest in certain measures taken by the government could result in significant losses of investments in infrastructural facilities (Gurjar and Nagpure 2016).
Research Gaps
Several studies focus on air pollutant emissions in Indian urban cities and industrial clusters. However, India-specific emission factors are either unavailable or difficult to interpret for various sources in most cases (Gurjar, Aardenne, Lelieveld, et al. 2004). Also, there is a lack of adequate research on the extent of pollution concentrations in medium-scale cities, which are likely to expand in the near future. For a country like India, nearly 68% of population (Chandramouli 2014) resides in rural areas and is dependent on domestic cooking fuels, such as wood and cow dung cakes. Moreover, practices such as biomass and crop burning create additional point sources of air pollution. This further gives an opportunity to evaluate the strategies to reduce emissions from such sources.
A recent national-level emission inventory for India at fine resolution is not available in the public domain and research on policy measures using regional air quality modelling mostly depends on global emissions inventories, which are at coarser resolutions. For Indian cities with limited or no air quality monitoring infrastructure, researchers and authorities are dependent on the data available through secondary sources. However, these data sets are non-reliable and the accuracy of such data is also uncertain (Gurjar, Jain, Sharma, et al. 2010)Risk of Mortality/Morbidity due to Air Pollution (Ri-MAP).
With the increasing rate of industrialisation, Gurjar, Aardenne, Lelieveld, et al. (2004) discussed the lack of factual data on industrial production and fuel statistics for Indian cities.
The urban population in India is anticipated to increase exponentially and the number of cities will grow as well. This suggests that the MSW generation will also increase, which must be managed efficiently. However, in India, proper MSW management and treatment techniques need to be implemented other than the current practices of landfilling and composting. Moreover, data sets on detailed MSW statistics regarding the amount of wastes collected, segregated, stored, and treated were absent (Gurjar, Aardenne, Lelieveld, et al. 2004).
Over the years, indoor air quality (IAP) has become an area of scientific interest and researchers worldwide are studying the threats IAP poses to human life. However, in the Indian context, there are limited studies which have stressed on the impact of indoor air pollution concentrations.
Conclusion and Recommendations
An effective and successful emission control strategy should be holistic (Molina, Molina, Slott, et al. 2004). It must be a combination of successfully applied scientific ideas and technological advancements; should support the economy and be supported by the public. Various steps taken by the Government of India to control air pollution in Indian cities have been highlighted in the previous sections. These measures have the potential to tackle pollution only if implemented successfully in the coming years.
India is facing serious issues of poor air quality in many urban areas. Apart from the much discussed megacities, like Delhi, various reports suggest that several medium-scale cities are equally at the brunt of filthy air. The ill-effects could impact human health in a negative way, also affecting the biodiversity, other life forms, heritage, cultural buildings and even climate in the longer term. It is about time that the government comes forward to support cities for the development of infrastructure and treatment facilities.
The control strategies adopted to tackle air pollution must be sustainable in nature. For example, the urban air pollution control strategy should depend mainly on sustainable means of public transportation modes, such as BRTs, metros, trams, cycle lanes and well-connected pedestrian facilities, which can further ensure minimum use of private vehicles, thereby reducing air pollution levels. People must be motivated to opt for an efficient public transport system instead of relying on private vehicles. Similarly, some strict laws must be enforced, such as emission trading and congestion pricing, which have the potential to reduce emissions drastically. Apart from these, the use of alternate fuels and e-cars, e-bikes and hybrid vehicle types must be promoted by the government. All these measures could reduce city emissions significantly.
The residents of rural areas are seldom aware of the harmful effects of air-borne pollutants and their consequence to human health. Public awareness programmes should be initiated by the government in every city, both rural and urban, highlighting the importance of managing air pollution at source and the various control measures that could be adopted to reduce pollutant emissions. Such initiatives could significantly reduce the activities, such as open burning of wastes, crop burning, use of biomass as a fuel for cooking and burning of plastic and rubber materials during winters. A holistic approach incorporating all of the mentioned measures could be beneficial to attain cleaner air quality in Indian cities and guarantee a healthier place to inhabit.
In this context, the NCAP launched by the Government of India appears to be a timely intervention. It is based on a long-term, time-bound, national-level strategy to tackle air pollution in a comprehensive manner with targets to achieve 20–30% reduction in particulate matter (PM) concentrations by 2024, keeping 2017 as the base year for the comparison of concentration levels. A total of 122 non-attainment cities have been identified across the country based on the 'Air Quality' data obtained for the period 2014–2018 under NCAP. The city-specific action plans are being prepared which, inter-alia, include measures for strengthening the monitoring network, developing emission inventories, carrying out source apportionment studies, reducing vehicular/industrial emissions, and generating public awareness, among others. It is expected that such initiatives by the central and state governments along with the participation of local bodies and other stakeholders comprising academia, research institutions, and public interest groups would result in ensuring better air quality in India.
Dr Bhola Ram Gurjar is Professor of Civil (Environmental) Engg., and Dean of Resources & Alumni Affairs (DORA), Indian Institute of Technology, Roorkee. He can be reached at bhola.gurjar@ce.iitr.ac.in. This article was originally published in the January to March 2021 issue of Energy Futures magazine.
Acknowledgements
I thank my students, who have helped me in conducting the literature survey and compiling the necessary information from various bibliographical resources.
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