A review on nanoparticle dispersion from vehicular exhaust: Assessment of Indian urban environment

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A review on nanoparticle dispersion from vehicular exhaust: Assessment of Indian urban environment Tandra Banerjee∗, R.A. Christian Environmental Laboratory, Civil Engineering Department, S V National Institute of Technology Surat, 395007, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Air pollution Vehicular emission Nanoparticles Dispersion Urban environment Regulatory norms

A comprehensive review is reported on the extent of release of ultrafine and nanoparticles from vehicular exhaust on Indian roads, the mechanism of evolution of these particles and the influence of key fuel and meteorological parameters on their evolution and dispersion. Consolidated understanding developed based on the available literature on nanoparticle formation and transformation processes is presented pictorially in the form of a schematic diagram. Influence of various parameters on the evolution of nanoparticles is elaborated using the present scientific understanding of dispersion mechanisms in the near and far field of vehicular exhausts. Inferences are drawn for the influence of Indian road conditions and atmospheric conditions on the dispersion of these evolved nanoparticles. Broad based suggestions are outlined for the Indian regulatory authorities so as to minimize the hazardous influence of such particulate emissions to urban population.

1. Introduction Indian cities have been graded in terms of their air pollution status as per the National Air Quality Index (NAQI) released by (CPCB, 2016a). Table 1 summarizes the grading criteria and air quality index of some of the cities of India. It is observed that the air quality in most of the Indian cities is graded between very poor to poor. Exhaust from vehicles is a major source of air pollution in urban India (Mahalakshmi et al., 2014). This is because of exponential growth of vehicles on urban Indian roads. The number of cars in India is projected to be in the range of 45–60 million by 2025 (TERI, 2014). The expected number of cars would be about 10 million by 2025 in capital city of Delhi alone (Ramachandra and Shwetmala, 2009; TERI, 2014). The increased number of vehicles in Indian cities is resulting in alarming rate of rise of air pollution levels in these cities. Major constituent of vehicular pollutants include hydrocarbon compounds, carbon monoxide, nitrous oxides, carbon dioxide and airborne particulate matters majority of whose sizes are significantly less than 1 μm. The particulate matter emission from vehicular exhaust represents a mixture of fine, ultrafine, and nanoparticles. Toxicologists define ultrafine particle as those with sizes below 100 nm, fine particles as those below 1000 nm and coarse particles as those above 1000 nm (Oberdörster et al., 2005). Particles below 300 nm are usually referred as nanoparticles (Kumar et al., 2009). Regulatory agencies however use terms such as PM10, PM2.5 and PM1 for mass of particulate matters

below 1000 nm, 250 nm and 100 nm respectively. Mass concentration of 10 μg/m3 for PM2.5 contains as many as 2.4 million 20-nm particles/ cm3 (HEI, 2013). Smaller diameters of these particles enhance their probability to penetrate into the human respiratory and cardiovascular systems thereby causing lung diseases and increase in blood coagulability (Donaldson et al., 2005; Jonathan et al., 2012; Pope et al., 1995; Pope and Dockery, 2006; Zhang et al., 2008). Since nanoparticles have higher surface area per unit mass, these particles interact easily with other biological systems and help toxic chemicals to penetrate cell membrane thereby affecting non-respiratory organs in human body like kidney, brain, liver, spleen and even skin (Forbe et al., 2011; Kreyling et al., 2006; Maier et al., 2006; Mohan et al., 2013; Oberdörster et al., 2005). Study carried out in Delhi suggested that exposure to nanoparticles (Kumar et al., 2011a) and other pollutants (Gurjar et al., 2010) emitted from vehicles causes∼11250 excess deaths in Delhi every year. Fig. 1(a and b) shows the status of acute respiratory illness and death due to respiratory diseases and lung cancer in India and especially in Delhi due to increase in concentration of PM10 during 2009–2012 as reported in TERI (The Energy and Resources Institute) policy report 2014 (TERI, 2014). In addition, particulate matters are one of the most hazardous pollutants in context to their strong influence on global climate (Strawa et al., 2010). Particulate matter of size 100–1000 nm are comparable to the wavelength of visible light and are responsible for reduction in urban visibility (Hujia et al., 2013; Jung et al., 2009; Kim et al., 2006;

Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. E-mail address: tandra_banerjee@rediffmail.com (T. Banerjee). http://dx.doi.org/10.1016/j.apr.2017.10.009 Received 27 June 2017; Received in revised form 24 October 2017; Accepted 25 October 2017 1309-1042/ © 2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved

Please cite this article as: Banerjee, T., Atmospheric Pollution Research (2017), http://dx.doi.org/10.1016/j.apr.2017.10.009

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Some important questions that arise in context to Indian urban planning are: how the toxicity of these particulate matters are to be measured (or monitored) and how are the regulatory laws to be framed to reduce their release in urban Indian cities. Present practice is to control the particulate matter in terms of their mass concentration with limiting values for PM10 or PM2.5 (CPCB, 2016b). Even for a small mass concentration, the particulate number concentration for smallest size particles can be significantly large. Smallest diameter particles with more surface area per unit mass can absorb more carcinogenic organic compounds and have largest capability to penetrate the cell membrane. Larger number of such particles is thus hazardous to human (Forbe et al., 2011; Mohan et al., 2013). These smallest size particles (≤100 nm) make up about 99% concentration of the total number of nanoparticles in ambient air with negligible mass (Kittleson, 1998) and 85% of this number concentration comes from vehicles in the urban environment (Kumar et al., 2011b). Thus particle number concentration becomes an important parameter to be measured and monitored. Modification in emission standards to Bharat stage IV in many metro cities and associated improvement in fuel technology, advancements in engine design and after treatment emission systems have brought down the concentration of mass of emitted nanoparticles. However the number concentration is yet not tackled. No emission inventories for nanoparticles are presently available for developing countries like India (Krishna, 2012). Thus to combat the effect of nanoparticles released by the vehicles in countries like India and to develop an exhaustive emission inventory for the same, it is essential to understand the process of evolution of these nanoparticles from the exhaust of vehicle and the influence of various factors which affects the evolution and dispersion of these nanoparticles in Indian urban environment. The present article is an attempt in this direction. The major components of the present paper are:

Table 1 Average Air quality Index (AQI) of Indian cities and their respective grading (CPCB, 2016a). Sr. No

Cities

Months

Average AQI

Grading

1

Delhi

2

Patna

3

Lucknow

4

Kanpur

5

Agra

Jan-16 Feb-16 Mar-16 Jan-16 Feb-16 Mar-16 Jan-16 Feb-16 Mar-16 Jan-16 Feb-16 Mar-16 Jan-16 Feb-16 Mar-16

370 293 238 390 290 198 363 317 179 364 261 194 377 233 143

Very Poor Poor Poor Very Poor Poor Moderately Very Poor Very Poor Moderately Very Poor Poor Moderately Very Poor Poor Moderately

polluted

polluted

polluted

polluted

Good

Satisfactory

Moderate

Poor

Very Poor

Severe

0–50

51–100

101–200

201–300

301–400

> 401

1. A pictorial representation of the present scientific understanding of the complex transformation processes associated with the evolution of nanoparticles from vehicular exhaust and their subsequent dispersion in the ambient air. 2. Analysis of the influence of fuel, fuel conversion efficiency, vehicular motion and traffic congestions in Indian cities on the evolution and dispersion of nanoparticles and to comment on the influence of implementation of vehicular norms on nanoparticle mass and number concentration. 3. Analysis of the influence of meteorological parameters on the evolution process of nanoparticles from vehicle exhaust, their size and modal distribution and subsequent dispersion and correlate them to meteorological factors in Indian urban cities. 4. Assessment of seasonal characteristics of nanoparticles released from vehicle and associated health hazards in terms of exposure to Indian urban environment. 5. Assessment of the existing mitigation measures and control standards in urban India and suggest new measures and control standards to regulatory authorities of India. In what is discussed in this article, section-2 deals with data associated with release of nanoparticles from vehicular exhaust in urban India, section-3 deals with transformation processes associated with nanoparticle evolution. Section-4 deals with the influence of key fuel parameters on the formation of nanoparticles. The consequences of Indian meteorological conditions on distribution of size and chemical composition of nanoparticles is discussed in section-5. Effect of vehicular dynamics, road conditions and seasonal variations on nanoparticle dispersion is discussed in Section-6. Seasonal characteristics of nanoparticles released from vehicle are discussed in section 7. Section 8 is an assessment of Indian regulatory control measures. Section-9 deals with policy changes associated with number concentration. Section-10 summarizes the future research needs in India to combat the vehicular nanoparticles.

Fig. 1. (a) Status of acute respiratory illness and (b) deaths due to respiratory diseases and lung cancer in Delhi during 2009–2012 due to increase in PM10 (TERI, 2014).

Seinfeld and Pandis, 2006). Also the particulate matter emission increases with local atmospheric temperature and may result in catastrophic phenomena like acid rains. Particulate matters may be carried by wind over a long distance and get deposited on ground or water. These harmful implications are required to be controlled. 2

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Table 2 Indian cities which ranked among top 20 cities in the world in terms of annual mean PM2.5 (μg/m3) concentration (WHO, 2014).

Table 3 24 h average and maximum value of PM2.5 for a typical monitoring station (Dwarka) in Delhi (CPCB, 2016a).

Rank

City

Annual mean PM2.5, μg/m3

Limiting Value for annual mean in ug/m3

Sr. No

Months

Average Value (μg/m3)

Maximum Value (μg/m3)

1 2 3 4 9 10 11 13 14 15 18 19 20

Delhi, India Patna, India Gwalior, India Raipur, India Ahmedabad, India Lucknow, India Firozabad, India Kanpur, India Amritsar, India Ludhiana, India Allahabad, India Agra, India Khanna, India

153 149 144 134 100 96 96 93 92 91 88 88 88

10 10 10 10 10 10 10 10 10 10 10 10 10

1 2 3

Aril 2015–June 2015 July 2015–September 2015 October 2015–December 2015 January 2016–March 2016

66 54 139.79

106 123 348.0

152.38

379.37

4

level of PM2.5 has reached close to 380 μg per cubic meter in January 2016. Thus, Indian cities cannot continue to add high volume of PM especially from vehicles whose numbers are exponentially growing on urban Indian roads. Uncontrolled vehicular traffic seems to be the primary reason for India being in the group of countries that has the highest particulate matter (PM) levels (community.data.gov.in, 2016). In India very little research data is available on the concentration, dispersion and impacts of nanoparticles in air. In the present paper thus data available on PM10 and PM2.5 particles are used as means to interpret possible behavior of the nanoparticles. It is reported that road vehicles are the highest contributor of all sources of incidental nanoparticles, contributing up to 90% of total particles by number in Indian urban environments. (Kumar et al., 2010, 2012). In order to make an assessment of vehicular nanoparticle dispersion in Indian urban environment, first the scientific community's present understanding on nanoparticle evolution from vehicular exhaust are distilled in the form of a schematic layout.

2. Fine particle release in Indian urban environment World Health Organization (WHO, 2014) has compiled the concentration of fine particles suspended in the atmosphere of more than 1600 cities for the years 2008–2013. This report advises that particles of less than 2.5 μm in diameter (PM2.5) should not exceed 10 μg per cubic meter. Indian cities feature among some of the world's most polluted cities in terms of particulate matter. Also, self-cleaning capabilities of Indian urban areas are restricted by the densely packed highrise buildings. In addition, the ever growing number of on-road vehicles, re-suspension of the dust, and anthropogenic activities aggravate the pollution levels. It is reported by WHO that 13 of the top 20 most polluted cities in the world are from India (WHO, 2014). Thus air quality must be improved in Indian cities so that the residents can have a cleaner environment. Table 2 shows the top 13 most polluted cities in India (along with their rank) in terms of PM2.5 concentration as compiled by WHO (WHO, 2014). Fig. 2 shows the annual mean PM2.5 released in these cities. The continuous ambient air quality data available from the Central Pollution Control Board (CPCB, 2016b) shows critical levels of PM2.5 levels recorded in more than half of cities in India. Table 3 shows the maximum and 24 h average values of PM2.5 released in different months at a particular monitoring station (Dwarka) in Delhi during April 2015 to March 2016. It can be observed from Table 3 that the

3. Nanoparticle evolution from vehicular exhaust Nanoparticle evolution process is very complex and dynamic and takes place within a very short time and spatial scale. The formation of nanoparticles depends on various factors like sulphur content in fuel and lubricating oil, operating conditions of the engine, ambient conditions and exhaust after-treatment and dilution. Though the mechanisms for evolution and transformation processes are reported by various authors across the world, a complete understanding is still lacking in absence of precise measurements. Essentially no or very little research data exists to date regarding concentration of vehicular nanoparticles and their dispersion in Indian urban environment (Krishna, 2012). Measurements reported by CPCB (CPCB, 2016b) are only limited to Fig. 2. Annual mean PM2.5 released in Indian cities (WHO, 2014).

3

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(HEI, 2013). Thus these particles do not pose health threat to people living at a distance from the roadside but are hazardous to on road commuters because of their higher mobility and number concentration. Large Kelvin effect of organics does not allow them to condense on the fraction of very small size non-volatile cores. But H2SO4eH2O under certain conditions if not able to self-nucleate, condenses on these solid cores and helps them to grow by a process called heterogeneous nucleation. As these particles grow in size and are able to overcome Kelvin effect, they allow the organics (semi and low volatile) to condense on to them and make them grow to ∼10 nm size. Thus nucleation is main transformation process in stage-I and it happens at a very fast rate and is negligible after 1 s. Maximum mass of nucleation mode particles are contributed by semi-volatile and low-volatile organics while H2SO4eH2O contributes only 3% of the total mass. Similarly for particles in the nucleation mode which are formed due to heterogeneous nucleation of non-volatile solid cores, semi-volatile and low volatile organics dominates the mass composition with nonvolatile solid core material contributing ∼3% of the total mass. In the formation of soot, molecule of polycyclic aromatic hydrocarbon (PAH) plays a significant role. Formation of soot is not very prevalent from gasoline exhaust as in case of diesel engines, but is formed in large quantities under relatively rich air-fuel ratios. Stage-II begins when the exhaust disperses further from roadside to ambient say at a distance of about 150 m from the exhaust point. Evaporation of semi-volatile organics starts immediately due to dilution while low volatile organics may continue to condense or evaporate depending on the condition of partial pressure of the surrounding. Thus some particle will shrink in size while some will continue growing to the size ranging from 30 to 300 nm that is the size which is observed in the accumulation mode. In addition the newly formed particles are continuously scavenged by soot particles reducing their size. Soot particles themselves undergo oxidation leading to formation of fragmented soot particles. In the accumulation mode, the mass of the particle is dominated by soot particles. Condensation and evaporation/ dilution are the important transformation which takes place in this stage. Nanoparticles emitted from vehicles are classified based on the mode of formation as: nucleation mode particle (in the size range of 1–30 nm), Aitken mode particle (in the size range of 20–100 nm) and accumulation mode particle (in the size range of 30–300 nm) (Kumar et al., 2008a,b). All these modes are subjected to changes (both temporal and spatial changes) due to the influence of various processes and parameters. Influence of important fuel parameters, meteorological parameters and vehicular dynamics on nanoparticle evolution and size distribution are discussed in the subsequent sections.

PM2.5. Thus, based on the inventory for particles (PM10 and PM2.5) by CPCB and data reported by different researchers across the globe, inferences are drawn in this paper to demonstrate the influence of various parameters on the evolution and dispersion of vehicular nanoparticles on Indian roads. Particles formation takes place during the process of combustion, dilution and cooling. During early stage of combustion carbonaceous particles are formed most of which gets oxidized (Khalek et al., 1998). Carbonaceous particles may also form as a result of lubricating oil getting entrained into fuel. Very rich fuel zone in the combustion chamber results in the formation of nonvolatile carbon species and polycyclic aromatic hydrocarbons (PAHs) (Flynn et al., 1999). In compression ignition (CI) engines and direct injection spark ignition (DISI) engines, the fuel injection is in the form of a spray. Very complex interaction between the fuel spray and the gases inside the cylindrical chamber influences the formation of nanoparticles. Evidences show that gas phase compounds which are formed from the metallic additives present in the lubricating oil during combustion, undergo gas-to-particle conversion (nucleation) due to expansion and cooling of products of combustion at the tailpipe exhaust. Most of these engine-formed particles end up as particles in the accumulation mode. Nucleation of ash may however occur when sufficiently high ratio of ash to carbonaceous accumulation mode particles happens (Jung et al., 2005; Khalek et al., 1998; Lee et al., 2006). On exiting the exhaust tailpipe, the dilution and cooling of exhaust results into nucleation (both homogeneous and heterogeneous) and adsorption and condensation tales place on existing particles (Sakurai et al., 2003a,b; Tobias et al., 2001; Ziemann et al., 2002). According to literature (Jacobson and Seinfeld, 2004; Diu and Yu, 2008) the transformation processes happens on dilution and cooling of exhaust can be divided into two stages. Stage-I start from exit of tailpipe and extend till roadside while the Stage-II extends from roadside to ambient air. A comprehensive schematic layout is prepared and presented in Fig. 3 utilizing a widespread review of literature (Guan et al., 2015; Diu and Yu, 2008; Jacobson and Seinfeld, 2004; Pirjola et al., 2015; Robinson et al., 2007; Sakurai et al., 2003a,b) available on various mechanisms of transformation of nanoparticles from vehicular exhaust. Different color and shape represent different constituents released from the exhaust. Interaction among these constituents and resulting transformations are shown by variation in size, shape, color and arrows. Stage-I begins immediately after the exhaust is released from tailpipe of a vehicle. Composition of typical vehicular exhaust (Guan et al., 2015; Robinson et al., 2007; Sakurai et al., 2003a) which leads to particulate matter formulations is a mixture of SO3 and water vapor, low volatile organics from lubricants and semi-volatile organics. The hot exhaust gas experiences rapid dilution with simultaneous oxidation of SO3 and H2O resulting into formation of H2SO4eH2O vapor. As the exhaust cools down the H2SO4eH2O vapor becomes supersaturated initializing the process of Binary Homogeneous Nucleation (BHN) (Pirjola et al., 2015). The nucleated nanoparticles are of very small diameter (less than 3 nm) initially and are difficult to detect or measure. These particles being so small are unable to overcome Kelvin effect and allow other materials to condense on them. A fraction of these nanoparticles continue to grow due to self-coagulation and condensation to size of around 3–4 nm, overcome Kelvin effect and allow condensation of low volatile and semi volatile organics on to them. As a result particles grow to a size ranging between 10 and 20 nm generally known as nucleation mode particles. Other fraction of nanoparticles having size less than 3 nm are unable to overcome the Kelvin effect and due to shortage of H2SO4 do not grow in size. These particles are subjected to coagulation scavenge by larger soot particles or ambient particles resulting in a very short life span of these particles. These particles remain suspended for periods of time ranging from minutes to days. Eventually, these particles may settle to the ground washed out during rain, impact and adhere to objects, or are inhaled by people

4. Influence of key fuel parameters on the formation of nanoparticles Evolution of nanoparticles from vehicular exhaust is influenced by several key fuel parameters. These parameters include sulphur content in the fuel, sulphur to sulphuric acid conversion efficiency, organic species and non-volatile cores in the fuel. The parameterization of binary homogeneous nucleation (BHN) of H2SO4–H2O (Vehkamaki et al., 2003) developed specifically for engine exhaust and measurements on field and laboratory scale reported in (Arnold et al., 1999; Biswas et al., 2007; Kittelson et al., 2004, 2006a,b; Kuhn et al., 2005a,b; Ronkko et al., 2007, Sakurai et al., 2003a; Shi et al., 1999; Tobias et al., 2001; Transport policy.net, 2016; Zhu et al., 2002) provide appreciable information on various parameters affecting nanoparticle number concentration, size, and composition. The influence of these parameters on nanoparticle dispersion is discussed in this section. 4.1. Fuel sulfur content Sulfur content of fuel is one of the most important parameters 4

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Fig. 3. Schematic representation of evolution and growth of nanoparticles from vehicular exhaust.

cities while others are still with Bharat Stage-III (BS-III) norms (Transport policy.net, 2016). This result in difference in fuel sulfur content between those cities with BS-IV norms compared to the rest of the country. The diesel-operated heavy-duty vehicles meeting Bharat-III standards often refuel with high-sulfur diesel. Since their after-treatment systems are not estimated to function properly under these conditions, their PM emissions cannot meet Bharat IV standards. Several countries in the past had introduced fuel with low Sulphur content in the major cities first and then made it mandatory for the rest of the country. The revised standard for sulphur in fuels in Mexico was introduced on the US-Mexico border, which was followed by metropolitan areas (in 2009) and implemented in the rest of the country by 2010 (PCFV, 2017). Similar practices were also implemented in Brazil prior to 2009 where the nationwide specification was one-fourth the metropolitan specification for fuel sulphur content. However, introducing fuel with low sulphur content in only parts of the country was found to result mis-fueling and contamination due to different after treatment technologies used; leading to more air pollution including

influencing particulate matter emissions from vehicles. From very high sulphur content of 10,000 ppm in most of the country in 1999, India has reduced it to a maximum of 350 ppm in cities which follows BS-III norms and 50 ppm in cities with BS-IV norms. A total of 63 cities in India receive supply of 50-ppm-sulfur fuel (Transport policy.net, 2016). Fig. 4 shows the variation of nanoparticle number concentration with fuel sulphur content at plume age (time < 1 s) based on Binaryhomogenous model proposed by Vehkamaki et al. (2003). The variations reported in Fig. 4 are for different ambient conditions and fixed sulphur to sulphuric acid conversion of 1.0% and soot concentration of 107 cm−3. Fig. 4 shows highly sensitive relation between H2SO4eH2O binary-homogenous nucleation rates to fuel sulphur concentration. At ambient temperature of 298 K and relative humidity of 80%, rate of nucleation decreases by about four orders of magnitude, when the fuel sulphur concentration (FSC) is decreased from a value of 400 to 200 ppm. For 100 ppm or less of fuel Sulphur content, H2SO4eH2O binary homogenous nucleation rate is too small. In India Bharat Stage-IV (BS-IV) has been implemented in some 5

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Fig. 4. Variation of nanoparticle number concentration with fuel sulphur content at plume age of time < 1s (Arnold et al., 1999; Vehkamaki et al., 2003; Biswas et al., 2007).

particles are not emitted and sulfuric acid nucleates. If the traps were absent, the same sulfuric acid will be formed but it condenses on soot particles, increasing the mass concentration due to larger size of soot mode particles and negligible number of smallest diameter nucleation mode particles. Thus with application of inappropriate CRDPFs, 90% of the particle mass in the nucleation mode are observed to be sulphate (Kittelson et al., 2006a,b). Another factor responsible for nanoparticle formation is the sulphate deposit on CRDPFs (Arnold et al., 2006; Giechaskiel et al., 2012). As the storage sites gets saturated, sulfates which initially gets deposited on the surface of oxidation catalyst/filters, are evaporated back into the exhaust. This process is augmented by the acceleration of vehicle which increases the in-exhaust H2SO4 concentration (Ronkko et al., 2007; Vaaraslahti et al., 2005). On Indian roads, where the traffic congestion leads to continuous acceleration and deceleration of vehicles (Agarwal et al., 2015; Sharma and Swami, 2012), the sulphate deposit is predicted to increase the nanoparticle number concentration. The remedy thus lies not only with reduction in fuel sulphur content but also with correct selection of after treatment devices. Mayer et al. (2000) had reported about excellent efficiency of Diesel Particulate Filter (DPF) greater than 99% in the mitigation of ultrafine particle emissions (UFP) by urban buses. Several studies also reported higher efficiency of Diesel Particulate Filters (more than 90%) in mitigation of ultrafine particles (Biswas et al., 2009; Liu et al., 2012; Poppel and

particulate matter release. The air quality in Indian urban cities would therefore greatly improve if diesel with 50-ppm-sulfur is sold in the entire country (CSE, 2017). Also, with up-gradation of emission standards from BS-III to BS-IV and subsequently to BS-VI by 2020 (NAFM, 2015), Indian vehicles are being equipped with particulate traps in vehicular exhaust to inhibit the release of particulate matter. Efficient operation of these particulate traps is very sensitive to the fuel sulphur content. Research reported by (Maricq et al., 2002; Reichl et al., 2012; Vaaraslahti et al., 2005) suggested that if the fuel sulphur content is of the order of 3 ppm, these particulate traps can achieve 95 per cent efficiency. But the efficiency may tend to zero for a larger value of 150 ppm sulphur content in fuel, and particulate emissions may double for 350 ppm fuel sulphur content. Accordingly, with present BS-IV norm of 50 ppm sulphur content in fuel and 10 ppm in subsequent BS-VI (by 2020), the nanoparticle emission are anticipated to be high from Indian road vehicles if appropriate after treatment devices are not used. In addition, although the particulate traps are successful in controlling particle mass emissions, several literature based on experiments carried out in laboratory and real field measurements by (Giechaskiel et al., 2012; Kittelson et al., 2006b; Ronkko et al., 2007) demonstrate high nanoparticle number concentrations from vehicles using even ultra-low sulfur fuel with particulate trap. According to these researchers, the reason number concentration goes up with traps is: with traps being present, the soot 6

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Lenaers, 2005; Tartakovsky et al., 2015). Many researchers have reported about the effects of various after treatment technologies on particle mass (PM) and particle number (PN) emissions (Biancotto et al., 2004;, Biswas et al., 2009; Lim et al., 2012; Mayer et al., 2000; Mayer et al., 2004; Richards et al., 2004; Stepien et al., 2011; Thompson et al., 2004). Engine oil consumed during combustion process results in the formation of incombustible ash. The DPF traps the ash in the same way it traps particulate matter. The accumulation of ash in DPFs is an important factor limiting the service life of these filters (Konstandopoulos et al., 2000). Ash accumulates in the DPF over extended use and may also reduce the regeneration efficiency in catalyzed systems (Cummins, 2017). Low ash oil help extend the ash cleaning interval to its maximum level. Some studies have also highlighted the importance of vehicle maintenance for effective operation of after treatment devices (Edesess, 2011; Lau et al., 2015; Ning et al., 2012). Thus, use of low-ash lubricant in addition to low-sulfur (< 50 ppm) fuel, correct selection of after treatment devices (meeting a vehicle type and operating conditions) along with appropriate vehicle maintenance will be the key issues for Indian auto industry in future in order to mitigate the hazardous influence of nanoparticles. 4.2. Sulphur to sulphuric acid conversion efficiency Fig. 5 shows variation of nanoparticle number concentration with efficiency of sulphur conversion at time < 1s of plume age (Vehkamaki et al., 2003). From Fig. 5 it can be seen that nanoparticle number concentration is very sensitive to efficiency of sulphur conversion, especially for smaller efficiency. Number concentration increases by six orders of magnitude when the efficiency of conversion increases from 1.0% to 4.0% for fuel sulphur content of 100 ppm and same order of magnitude in case of 30 ppm when efficiency of conversion increases from 5% to 10%. Efficiency of sulphur conversion is thus a vital parameter which controls the evolution of nanoparticle from engine exhaust. Data shown in Fig. 5 corresponds to soot concentration of 107 cm−3, ambient temperature and relative humidity of 283 K and 60% respectively. Kinetic characteristics in the engine decide the sulphur to sulphuric acid conversion. It is established that higher engine loading associated with higher temperature of exhaust leads to higher SO3 formation from vehicles which ultimately results in higher H2SO4 concentration (Arnold et al., 2006). At higher engine loads, UFP number concentrations is directly proportional to engine load as reported by (Mayer et al., 2004). Cheung et al. (2008) and Kittelson and Kraft also observed that particle mass depended on engine load and increased with increase in engine load which could be attributed to reduction of the air-excess factor. According to this, in Indian cities where most of the heavy diesel vehicles are highly loaded, the nucleation of sulphate particulate is likely to be higher resulting in more number of nucleation mode nanoparticles (Srivastava et al., 2011). More recently however, Tartakovsky et al., 2015 has shown that at constant engine speed, UFP number concentrations decreased with increase in power. A possible explanation for the increase particle number concentration at low loads is attributed to the low nozzle flow rates leading to deterioration of penetration and fuel spray atomization and lowering of engine torque. This fact leads to an increase in a number of particle nucleation sites. This assumption was also indirectly supported by their experimental results of particle number concentration dependence on the engine speed at low constant power. In addition to gases like CO, CO2, NOx, SOx, NH3 and water vapors, exhaust of vehicles in India releases volatile organic compounds (VOC), hydrocarbons (HC) and polycyclic aromatic hydrocarbon (PAH), halogenated organics and organic/inorganic acids and dioxins (CPCB, 2010). To reduce the emission of all these compounds after treatment technologies like Diesel oxidation catalysts (DOC) are presently being used in Indian BS-IV vehicles (ICRA, 2016). DOCs work by oxidizing Carbon monoxide, Hydrocarbons, soluble fraction to carbon dioxide

Fig. 5. Variation of nanoparticle number concentration with efficiency of sulphur conversion at time < 1s of plume age (Arnold et al., 1999; Vehkamaki et al., 2003; Kittelson et al., 2004; 2006a,b; Biswas et al., 2007).

and water. But at the same time it also oxidises SO2 to SO3 which subsequently gets oxidised to H2SO4 initializing binary homogenous nucleation. This leads to formation of nucleation mode particles in large numbers. Thus incorporation of DOCs enhances the sulphur to 7

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(< 20 nm) nanoparticles. A closed filter must be regenerated frequently. Different regenerating technologies are required depending on varying driving pattern of vehicle. Also at times (based on driving pattern) the filter is required to be cleaned to prevent the filter being blocked by soot, which otherwise results in undue back pressure to the exhaust line. Cleaning should be done only by trained personnel using proper equipment since soot is very hazardous (Dinex, 2017). Selection of proper regeneration technology and cleaning of filters will be other issues associated with these DPFs being installed in forthcoming BS-VI vehicles in Indian context.

sulphuric acid conversion efficiency of diesel vehicles leading to the evolution of huge number particles in the nucleation mode from the vehicle exhaust. Literature (Giechaskiel et al., 2012; Kittleson et al., 2006a,b; Maricq et al., 2002; Reichl et al., 2012; Vaaraslahti et al., 2005) also suggests enhanced sulfur conversion efficiency due to application of DOCs. It is therefore not uncommon that the number count of diesel particulate matter increases after the passage through the DOCs (Strom, 2011). Continuously regenerating diesel particulate filter (CRDPF) consists of a diesel oxidation catalyst (DOC) followed by a ceramic particulate filter and are very effective technology for the reduction of PM emissions (Liu et al., 2012). CRDPF utilizes NO2 to combust the soot accumulated in the particulate filter (Kittelson et al., 2006a,b). The CRDPF requires regular regeneration and periodic ash removal to evade obstruction of exhaust line (Lehtoranta et al., 2007, 2009). This is especially important for vehicles in India, because of relatively high emission level and also because Indian vehicles are mostly operated at low velocity and load for long durations. So for vehicles in India and other developing countries there might be need of additional means of active regeneration. In order to evade these blocking risks and to avoid complex regeneration, particle oxidation catalyst (POC) was developed. POC uses honeycomb structure with no risk of clogging and ensures trouble-free operation (Rens and Wilde, 2005). A detailed experimental study by Liu et al. (2012) assessed the impact of two CRDPFs and a particle oxidation catalyst (POC) on particle emissions. Their results showed removal of more than 90% of total particulate matter by both the CRDPFs. However, the POC could remove only partially carbonaceous particles of size less than 30 nm. This is because honeycomb structure provided inadequate residence time for oxidation of solid particles. The emission results from CRDPFs however indicated that more NO2 generating in DOC resulted in higher removal efficiency of particles. However this augments the risk of NO2 exposure. Selective catalytic reduction (SCR) is primarily designed to reduce NOX emissions. Thus integrated systems including DOC, Catalyzed DPF and SCR will be required for both particulate matter and NOX emissions in the next generation of low sulphur fueled vehicles in India as per BS-IV and BS-VI regulations. Survival of the after treatment system will also depend on DPF systems operational issues of periodic cleaning of ash accumulation and reliable backpressure monitoring and logging capabilities.

4.4. Influence of organic species on particle growth in vehicular exhaust Once the exhaust is released from tailpipe, the nucleation of sulfuric acid vapor to nanoparticles happens within a very short time duration (of the order of 0.2–0.5 s) and nucleated particles continue to grow to around 3–4 nm diameter due to condensation and self-coagulation of sulfuric acid. But further growth of these particles happen only when condensation of low and semi volatile organics takes place onto these particles after they have acquired big enough a size to overcome the Kelvin effect. These particles then grow to geometrical mean sizes ranging from 6 nm to as big as 20 nm (Ronkko et al., 2007). Thus the nucleation mode particles can grow only to around 3–4 nm size due to sulphuric acid vapour but their further growth to a bigger size range of 10–20 nm will depend on the organic compounds released from the exhaust due to unburned fuels and lubricating oils. Organics contribute maximum to the mass of these grown particles with sulphuric acid contributing to only 3% of the total mass. Unburned fuel and lubrication oil in the Indian vehicles are significantly higher due to inadequate maintenance of the vehicles (Gawande and Kaware, 2013). This is estimated to augment the growth of nucleated particles to size ∼10 nm and larger increase in their number concentration. Proper vehicle maintenance will be of significant importance in Indian context to reduce the growth of these nucleated particles. Also, evaporation of semi-volatile organics which are condensed onto the nanoparticles begins as the exhaust reaches the roadside (Sakurai et al., 2003a,b, Jacobson et al., 2005; Robinson et al., 2007). Further shrinking of nucleation mode particles happen away from roadway as the evaporation of low and semi-volatile organics continue. Thus, in addition to nanoparticles in the size range of ∼10 nm, a high number of nucleation mode particles in the size range of 2–3 nm as a result of shrinking are also reported in the literature (Jacobson et al., 2005; Robinson et al., 2007). These small particles are thermodynamically stable and are composed of H2SO4 and H2O. They are bigger than the critical size but are still smaller to overcome the Kelvin effect and allow organics to condense on them in absence of sufficient H2SO4 vapors. Being very small in size, they cannot be measured by currently available instruments and their residence time in ambience is significantly large (around∼ 300 s). They do not pose a health hazard to people living far away from the road. But due to their high number and mobility, they can be hazardous to on-road commuters. Considering large number of on-road commuters in Indian cities (Mohan, 2002), these smaller particles are anticipated to have considerable health threat to the commuters. It is to be noted that the accuracy of particle number measurement worsens when particles smaller than 20 nm are measured (Giechaskiel et al., 2017). Estimation of effectiveness of regenerating closed type DPFs to reduce the number concentration for smallest size nanoparticles (∼10 nm) remains a technological challenge to be addressed prior to implementation of forthcoming BS-IV norms in India.

4.3. Effect of soot particle concentration Fig. 6 shows variation of nanoparticle concentration with soot concentration evaluated based on Binary-Homogenous model of Vehkamaki et al. (2003). The data shown in Fig. 6 corresponds to ambient relative humidity of 60% and efficiency of conversion of 1%. For soot concentration greater than 107 cm−3, the effect of soot scavenging is negligible. It is however significant if the soot concentration is less than 107 cm−3. Soot particle reduction from 108 to 107 cm−3 increases the nanoparticle formation by an order of magnitude of five (NYSERDA, 2012). During dilution and cooling of the vehicular exhaust, there exists a competition between nucleation and adsorption. Rapid dilution and low soot particle concentration favors nucleation over adsorption onto the existing particles. The exhaust temperature is low enough under low load and idling conditions, to assist nucleation of volatiles and thereby result in more soot mode particle release due to incomplete combustion (Kittelson et al., 2006a). Also installation of soot filters reduces the accumulation mode soot release but may lead to increase nucleation of volatile nanoparticles. This results in more particle number concentration of volatile nanoparticles (of size < 20 nm) which are likely to be highly carcinogenic due to absorption of inorganic/organic acids. The closed type DPFs made from highly porous ceramics or metal are reported to remove at least 90% of particulate mass and particulate number. They allow very high efficiency of particle number reduction even for the smaller sized

4.5. Influence of non-volatile cores on nanoparticle formation Non-volatile cores play important role in nanoparticle formation when binary homogenous nucleation of H2SO4 is insufficient. This was established first by Sakurai et al., 2003a who has shown that 8

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Fig. 6. Variation of nanoparticle number concentration with soot concentration (Vehkamaki et al., 2003).

correct DPF technology is expected to provide efficient reduction of ultrafine particle number concentrations.

nanoparticles in the size range of 12–30 nm when heated up to 200 °C did not completely evaporate and retained residual non-volatile cores of very small size varying usually between 2 and 3 nm. They reported that these residual particles consisted of refractories like metal oxide and non-volatile carbon. Kuhn et al., 2005a,b, Kittelson et al., 2006a reported presence of non-volatile mode at idle condition of a diesel vehicle. Biswas et al., 2007 reported that refractory particles contribute to 2% of mass concentration of nucleation mode particles and are comparable to that of H2SO4 in terms of volume fraction. To reduce soot emission from diesel engines, metal additives are used as catalysts and these metal additives are also used for diesel particulate trap regeneration (Lamotte et al., 2017). These are projected to increase the number concentration of nanoparticles in the size range of ∼ 2–3 nm which are more lethal for human cardiovascular system and are mostly not detectable. Also, use of low sulphur content diesel with advancement of regulatory norms (BS III to BS IV and subsequently to BS VI) in Indian cities, the natural lubricity of the fuel will be reduced. Lubricants and additives used to augment the lubricity may releases nonvolatile cores which are responsible for nanoparticle formation due to heterogeneous nucleation. Thus selection of correct lubricity improvers along with low sulfur fuel, low-ash lubricating oil and

5. Influence of meteorological factors on nanoparticle formation The meteorological parameters include ambient temperature and relative humidity, wind speed and direction and precipitation. The influence of meteorological parameters on nanoparticle dispersion is discussed in this section. 5.1. Ambient temperature and relative humidity Variation of number concentration of nanoparticles (time < 1 s) behind the exhaust evaluated based on Binary Homogenous Model (Vehkamaki et al., 2003) is shown in Fig. 7 as a function of ambient temperature at four different relative humidity conditions of ambient air (RH). Figure shows that lower ambient temperature and higher relative humidity enhances the nanoparticle formation. The effect of relative humidity (RH) on nanoparticles of detectable size (diameter > 3 nm) is more prominent under high ambient temperature condition. The values shown in Fig. 7 correspond to fuel sulphur content (FSC) of 9

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Fig. 7. Variation of nanoparticle number concentration with ambient temperature and relative humidity at plume age (time < 1 s) (Vehkamaki et al., 2003).

the course of a day. Further growth of nucleated particles takes place by diffusion of vapor molecules due to humid environment (Kulmala, 2003; Chate and Murugavel, 2010). Relatively higher ambient temperature and humidity of air in Indian urban environment enforces the use of air conditioning in vehicles. The air conditioning unit contributes to increase in load of the engine. Using air conditioning can increase fuel consumption by up to 20% because of the extra load on the engine (Khan et al., 2006). An increase in load is expected to increase the particulate matter emission from the vehicle (Farrington and Rugh, 2000). Vehicle Data Collection (CAFEE) with WVU Transportable Laboratory (Khan et al., 2006) has demonstrated an increase of about 10% of particulate mass for PM50. However, the air conditioning unit in a vehicle influences the temperature and humidity

330 ppm, sulphur to sulphuric acid conversion efficiency of 1% and soot concentration of 107 cm−3. The relative humidity demonstrates a diurnal anti-correlation with ambient temperature. This means that an increase in temperature during the day is associated with a decrease in relative humidity. In their study Olivares et al., 2007 has shown that the particle number concentration almost doubled when there was sharp decrease in temperature. They have also demonstrated the effect of relative humidity on nucleation particles with high concentrations observed during very humid periods. Higher atmospheric water vapor content favors the occurrence of binary homogeneous nucleation of H2SO4 and H2O. Also, Kulmala et al., 2004, has reported that during winter freshly nucleated particles typically grow to 10–100 nm at rates of 1–20 nm h−1 during

Table 4 Variation in measured value of PM2.5 with relative humidity and ambient temperature at Delhi (Dwarka) during summer and winter (CPCB, 2016b). Jan-2016 (winter)

RH (%)

Temp (°C)

PM 2.5 (μg/m3)

Apr-2015 (summer)

RH (%)

Temp (°C)

PM 2.5 (μg/m3)

01/01/2016 08/01/2016 15/01/2016 22/01/2016 31/01/2016

71.41 86.18 82.45 80.6 78.83

13.94 15.23 12.38 10.59 16.78

346.92 172.51 246.8 255.58 234

01/04/2015 08/04/2015 15/04/2015 22/04/2015 30/04/2015

63.33 56.57 61.03 34.43 41.3

20.94 22.77 22.44 29.2 17.34

98.39 79.63 38.04 64.16 53.68

10

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influence of precipitation on nanoparticle formation and dispersion for sizes below 150 nm. This is especially so for Indian cities where the variation in rainfall and the raindrop sizes are highly sporadic.

of air inside the vehicle and reduces the nanoparticle number concentration inside the cabin (Jain, 2017). Table 4 shows variation in mass concentration of PM2.5 monitored at Dwarka in Delhi by CPCB for one month each during winter and summer season. As can be observed from the Table, there is a marked variation in relative humidity and ambient temperature between these two months. During the winter month relative humidity is high, ambient temperature is low and mass concentration of PM2.5 is high and it is vice versa for the summer month. Although the data is not for nanosized particle but for a particle size very small, it reflects the trends similar to those described by Olivares et al. (2007). It is to be mentioned here that the monitoring in India is still limited to a particle size of 2.5 μm. Also monitoring is done only for mass concentration and not number concentration. The higher value of particle mass concentration coupled with high relative humidity leads to visibility impairment and suffocation or breathing trouble in people forcing the authorities to opt for measures like odd even formula in Delhi.

6. Effect of vehicular dynamics and road conditions After release, the vehicular exhaust undergoes a series of physicochemical transformation processes influenced by the vehicle dynamics and road condition. The nanoparticle number concentrations in Indian urban roads vary over a wide range of 104 and 106 per cm3. This variation is associated with several vehicle flow characteristics. The influence of these flow characteristics on nanoparticle evolution and dispersion is discussed in this section.

6.1. Influence of vehicle speed on road and away from road

5.2. Influence of wind speed

Kittelson et al., 2004 has demonstrated that higher speed of vehicle is characterized by greater particle number concentration of smaller sized particle. The particle volume or mass concentration is however observed to be lower as compared to the number concentration and the distribution of particle size. At roadside, the nanoparticle number concentrations majorly consist of nucleation mode particles. The nucleation rate increases with increase in traffic rate (Virtanen et al., 2006). Traffic rate in Indian cities are significantly higher owing to inadequate mass transport and urban planning. Thus Indian roads are anticipated to have more nucleation mode particles which pose a considerable health hazards for road commuters. Noteworthy number of particles on the roadside is of size in the range of 3–7 nm. This is due to homogeneous nucleation (Shi et al., 1999). Studies by Hitchins et al. (2000); Morawska et al., 1999; Shi et al., 1999; Zhu et al., 2002, report a decrease in particle number with distance away from the road. These studies showed exponential (or power law) decrease in particle number concentration with the distance from the road. Number concentration of smaller size (between 9.6 nm and 352 nm) particles at busy road was found to be 3.6 times higher than a site at 30 m away from road and beyond 300 m away from roadside the particle number concentration and distribution of size tend to approach closer to the background concentration. The exponential decay of particle number concentration with distance is reported by Kittelson et al. (2004) and is true for near highways in India (Ahmed et al., 2004). However in Indian urban environment, the rapidly rising buildings and street canyons poise significant threat to this decay with distance.

Wind speed affects dispersion, atmospheric mixing and re-suspension of nano particles. Nano particle concentration decreases with dilution. Nano particle number concentrations decreases exponentially with wind speed having a minimum value at wind speeds > 5 ms−1. This is due to the fact that higher wind speeds leads to higher coagulation rate and better mixing. Phenomena of deposition and scavenging are observed at higher wind speed leading to more particle losses (Kumar et al., 2008b; Arnold et al., 1999). Particles of size greater than 100 nm depict a “U-shaped” variation with speed of wind in the range of 5–10 m s−1. Number concentration of particles in the size range of 30–100 nm decreases by 10000 normalized counts per cubic centimeter with increase in wind speed (Arnold et al., 1999). Due to high temperatures in summer and high boundary layer providing more volume for mixing of particles, the number concentration decreases linearly with wind speed (Kumar et al., 2008c). In Indian urban cities where high rise buildings have led to restriction in wind motion and significant rise in summer temperature, the evolution of nanoparticles from vehicular exhaust is likely to be high and its dispersion negligible. This leads to accumulation of nanoparticles in Indian urban street canyons. Indian urban planners are therefore required to develop means to evacuate/disperse the nanoparticles from the streets of urban India and protect the inhabitants from the harmful influence of accumulated nanoparticles. 5.3. Precipitation Particles are predicted to be removed from atmosphere due to precipitation. This is also established by Deshmukh et al. (2013), for Indian city of Raipur where they observe increased rainfall precipitation contributed to decreasing particulate matter levels in the monsoon. However, exactly opposite effect was observed by Easter and Peters (1994), for particle sizes below 150 nm. It was reported that number concentration of nanoparticles increased when it rained with diameter of rain drops larger than 0.4 mm compared to the case when the rain drops were of less than 0.2 mm. The highest value of nanoparticle number concentration was measured within one hour after the rain. This might be explained by the reduced levels of temperature during precipitation. Reduced temperature results in higher value of saturation ratio for semi volatile species. Higher saturation ratio along with low surface area of particles favors formation of new particles. This leads to a significant increase of particle number concentration. Real-time wet scavenging of major chemical constituents of aerosols and role of rain intensity in Indian region has been reported by Kulshrestha et al. (2009). They reported that after rain events, the levels of SO4 aerosols were noticed to be substantially higher (more than double) within 24 h. Considerable research input is however required to exactly establish the

6.2. Influence of traffic density Studies demonstrate that particle number concentration varies with traffic density with highest number concentration measured on weekdays particularly during peak traffic hours (Charron and Harrison, 2003; McMurry and Woo, 2002; Paatero et al., 2005; Ruuskanen et al., 2001). Two peaks can be observed for nanoparticles on weekdays showing traffic peak hours in terms of office going and office returning hours (Hussein et al., 2004; Morawska et al., 2002). A study carried out for variation of mass concentration of PM10, PM2.5 and PM1.0 for Indian city of Chennai shows daily two peaks corresponding to morning and evening peak hour traffic (Srimuruganandam and Shiva, 2010). During off or lean traffic hours a reasonably wide peak is observed. This trend was prominent for nucleation and Aitken mode particles with lesser particles in the accumulation mode (Stanier et al., 2004). Number concentration showed diurnal variations while volume concentration shows strong seasonal variations (Pirjola et al., 2006). Also traffic emissions, secondary particle formation affects the diurnal trends of nanoparticle concentration. 11

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7. Seasonal variation of nanoparticle evaluation from vehicle exhaust

gasoline vehicles as well. Bharat stage IV implemented in India involves gasoline direct injection technology (GDI) in petrol vehicles. Vehicle tests in reports more particle number concentration emission from gasoline direct injection (GDI) engines without gasoline particulate filters (GPFs) than diesel engines (Gang et al., 2014; TE, 2013). Particulate emission due to direct fuel injection from the gasoline direct injection technology increases because of several factors such as incomplete volatilization of fuel, presence of rich zones of fuels, and impingement of fuel to the surface of piston and cylinder (Bonandrini et al., 2012; Maricq et al., 1999a,b; Sementa et al., 2012; Mathis et al., 2005). Several researchers reported release of partly charged spherical amorphous carbon particles of mean size ranging between 10 and 20 nm which are highly toxic in nature (Maricq et al., 1999a,b; Harris and Maricq, 2001; Khalek et al., 2010; Barone et al., 2012; Sementa et al., 2012; Sgro et al., 2012). As per National Auto Fuel Policy and Auto Fuel Vision and Policy 2025 by Ministry of Petroleum and Natural Gas (NAFM, 2015; GOI, 2014), the government of India is planning to leap frog directly to BS-VI norms skipping BS-V altogether to control rapidly deteriorating air quality standards. In the forthcoming BS-VI norms, in addition to imposing threshold limit for mass concentration for nanoparticles for diesel as well as gasoline driven vehicles, Indian policy makers should provide special emphasis to nanoparticle number concentration particularly for gasoline driven vehicle. The legislation limit for particle number will in turn enforce modifications in technologies associated with GDI engines (Panu Karjalainen et al., 2014). Nanoparticle number concentrations in Indian urban environment vary over a wide range from 102 to 107 per cm3 depending on the ambient environment and source of release. The greatest concentration is likely in that part of the urban area where street canyons are formed by surrounding buildings on both sides of the road. The surrounding buildings restrict the evacuation of vehicular exhaust emissions from the street canyon (Dingenen et al., 2004). Street canyons thus trap the pollutants released from the vehicles. Kumar et al., 2008b reported measurement of nanoparticle concentrations for a diesel lorry parked in a street canyon with its engine idling over a time period of few seconds. When they averaged this data over a period of 20 s, they found that this averaged value of number concentration was thousand times as compared to hourly averaged value recorded under normal circumstances. Events of idling vehicles standing at traffic junctions are common in urban Indian road condition. India urban areas of cities like New Delhi are equipped with instruments for measurement of particulate matter at several traffic locations. However influence of such parked idling vehicles does not get recorded due to limitations in the frequencies of sampling. In India air quality data is recorded either every half an hour or every hour basis. Serious regulatory attention is required for such peak concentration exposure since such exposure may intensify respiratory and cardiovascular conditions of the commuters (Brugge et al., 2007). The regulatory authorities must be able to represent the toxic effects of nanoparticles for effective control of nanoparticle dispersion and their associated health hazards. For consideration by the regulatory authorities, literature identifies several characteristics of nanoparticles which include size, geometry, chemical composition, mass concentration and surface area. No consensus is yet to be reached on the most important characteristics to be adopted by the regulatory authorities. Toxicological studies and epidemiological studies suggest that exposure to such particles in the nano range have most critical health effects (Mohan et al., 2013; Donaldson et al., 2005, Murr and Garza, 2009; Ibald-Mulli et al., 2002; AQEG, 2005). The exact biological mechanism involved with the penetration of these particles to human body is a field of serious research for Indian regulatory authorities to identify the most critical characteristics for which threshold limit is to be set (ICRP, 1994; USEPA, 2002). Though there is a lack of standard application guidelines and methods for nanoparticle measurements, several advanced instruments for measurement of particle number concentration and size variation

In winter the mixing layer height is lower and the atmosphere is more stable. This leads to increase in nanoparticle number concentration. In addition, low temperature enhances nucleation particle formation in the vehicle exhaust. Increase in particle concentration occur mostly in morning traffic rush in winter when the mixing height is the lowest which is associated with lowest wind speed in addition to low temperature (Pirjola et al., 2006; Virtanen et al., 2006). Studies showed that the average concentrations in winter were 2–3 times higher than summer (Pirjola et al., 2006; Virtanen et al., 2006; Wehner and Wiedensohler, 2003). Similar results were reported by Balakrishnaiah et al. (2011), for aerosol mass concentration at a tropical semiarid station in India. Thus during summer lowest total number concentration and also nucleation particle concentration are observed while they are the highest during spring and autumn (Hussein et al., 2004; Balakrishnaiah et al., 2011; Laakso et al., 2003; Kanawade et al., 2014). This is also observable from Table 4 in terms of mass concentration for PM2.5 measured at a typical location of Delhi (Dwarka). Nucleation is minimum in summer due to its dependence on saturation vapor pressure which in turn is temperature dependent while in spring season low temperature favors higher nucleation of exhaust gases and minimum mixing due to low boundary layer. 8. Mitigation measures and directives for Indian regulatory authorities Effective reduction of nanoparticle emission from vehicles is only possible with advanced after treatment systems. Diesel exhaust after treatment systems include DOC, DPF, and SCR. Usually a combination of DOC, DPF and SCR are required for the simultaneous removal of particulate matter from engine exhaust and meeting the emission standards of ensuing BS-VI. DPFs suffers from ash accumulation as they trap ash which does not easily gets oxidized. Also, with use of low sulphur content fuel, the natural lubricity of the fuel is reduced. Thus choice of appropriate lubricity improvers with ultra-low sulfur fuel, lubricating oil with low ash content and appropriate after treatment device/s (based on vehicle type and vehicles operating conditions) along with regular vehicle maintenance will be the main issues for Indian auto industry in future in order to alleviate the nanoparticles emission. For the complete elimination of particulate matter emissions, further studies and researches on the after treatment emission control systems should be intensified. The influence of biofuels on the performance of after-treatment technologies like DPFs needs to be evaluated. The information available in literature shows improvement in performance of current after-treatment systems with the use of biofuels (Muncrief et al., 2008). Better soot oxidative reactivity with alternate fuels results in reduction of fuel penalty levied by the soot accumulation on the filter surface (Muncrief et al., 2008). The mitigation measures in future Indian vehicles should therefore involve blending of bio-fuels with conventional fuel to improve DPF performance. Luo et al., 2015 has demonstrated that ethanol-gasoline blended fuels reduce particle number concentration at higher load operating conditions. It has been established that particulate emissions are significantly higher from diesel vehicles compared to gasoline driven vehicles. However recent studies advocate limitation to be implicated for the number concentration of gasoline vehicles as well (Banerjee and Christian, 2017). In the United States, since 2004 regardless of the fuel, same standards have been applied to all vehicles (DieselNet, 2017). As a result, threshold limits for mass concentration of particles have been applicable for gasoline vehicles also. Limits for mass concentration of particles released from direct injection gasoline engines were made in Euro 5 in the year 2009. In Euro 6 norms, controls were applied to restrict the particle number concentration (Kumar et al., 2011b). Thus, globally there is a development for regulating the emission limits for 12

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are now available in the market. Calibration and standardization of these instruments is required to be carried out by the regulatory authorities before the accuracy of their measured results can be relied upon and used for implementing control measures. Most of the available instruments cannot detect particle sizes below 3 nm size but these particles play a very important role for secondary particle formation. Thus more technologically advanced instruments are required to take care of this issue.

3.

4.

9. Summary Laboratory scale experimentation, on-field measurements and modeling have immensely contributed to better understanding of nanoparticle formation from vehicular exhaust. In this paper, we have carried out a detailed reviewed of literature to develop a schematic representation of transformation mechanisms involved in evolution and dispersion of nanoparticles from both gasoline and diesel powered engines. Nanoparticles formation happens during combustion, and also in the exhaust duct and post-tailpipe. The influence of key fuel parameters, meteorological parameters and vehicle dynamics on evolution of nanoparticles from Indian vehicles is discussed in detail including seasonal variations. Major parameters influencing the formation of these nanoparticles are presence of sulphur and metals in the fuel and lubricating oil. Thus along with low sulfur content in fuel, Indian vehicles must use appropriate lubricity improvers and low-ash content lubricating oil to meet the ensuing needs of BS-VI standards. It is established from the present review that except smallest sized sulfate particles, most of the nanoparticles are efficiently mitigated by modern after treatment devices. However the driving conditions, traffic conditions, meteorological conditions and seasonal variations in Indian urban area augments the nucleation of sulphate particulate resulting in more number of smallest sized nucleation mode nanoparticles. Mitigating these smallest sized nucleation mode particles will be huge challenge for Indian regulating authorities. Correct combination of DOC, DPF and SCR are required for the meeting the emission standards of BS-VI. Because of relatively high emission level and also because Indian vehicles are mostly operated at low velocity and load for long durations, Indian vehicles will need additional means of active regeneration of diesel particulate filters. It is also established that without the use of gasoline particulate filters (GPF), more particles are emitted from GDI engines than diesel engines. Thus without proper particle filter technology, the current GDI engines will not be able to match the projected BS-VI particle number standard. Although considerable development has been made, more on-field and experimental research is necessary to completely understand the complex transformation mechanisms associated with nanoparticle evolution and dispersion. This is required to improve the current filter technology to alleviate the smallest sized sulphate particles which are the greatest threat in Indian urban environment.

5.

6.

modifications of fuel and lubricating oil so that effective reduction of nanoparticle emissions is obtained. For a developing country like India, effective and low cost technologies for reduction of nanoparticle number concentration will be emphasis of research in future. This will involve development of low cost technologies for active regeneration of particulate filters and better and frequent cleaning mechanisms. The accuracy of particle number measurement worsens when particles smaller than 20 nm are measured. No instrument available till date can capture nanoparticle size less than 3 nm. It is possible that number concentration of such particles is huge and can cause catastrophic health effect. Thus significant research is required for development of low cost and precise instruments for measurement of number concentration of smallest size nanoparticles. Noteworthy toxicological research is required to understand the chemical, carcinogenic and other health associated issues pertaining to the deposition of metal and inorganic compounds on the nucleated sulphate particles released from the vehicle exhaust in Indian urban roads. Indian urban planners must emphasize on the means to evacuate/ disperse the nanoparticles from the streets of urban India and safeguard the residents from the hazardous influence of these particulate matter concentration. Urban planners must work in consultation with the environmental regulatory/protection agencies towards development of environmental hazard free design of new smart cities being proposed by the government of India.

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