Gaseous and Particulate Matter Emissions from Road Transport: The Case of Kolkata, India


 Indian cities are highly dependent on road transport for freight and passenger traffic movements. The estimated road transport led yearly emission inventory of pollutants for 25 million-plus population cities of India indicates vehicle stock as the critical contributor to air pollution in cities. During 2025 the city of Kolkata will be responsible for the emission of 21 668.24 Gg of CO2 followed by 272.81 Gg of CO, 98.21 Gg of NOX, 16.9 Gg of CH4, 93.39 Gg of SO2, 8.6 Gg of PM, and 38.55 Gg of HC due to its 2.79 million vehicles. The total vehicle stock of 25 leading Indian cities increased by 19 % during 2015–2017, and during the same period, Rajkot and Vadodara had the highest rise of 97 % and 94 % respectively. Out of 25 cities total CO2 (220 560 Gg) and CO (3185 Gg) vehicular emissions during 2017, Delhi was the highest contributor with 22 % and 20 % respectively followed by Bengaluru (12 %, 12 %), and Chennai (9 %, 8 %). The GHG emission per unit area of Kolkata during 2017, due to on-road vehicular emission, was the highest amongst the 25 cities of India. For Kolkata, cars were responsible for 35 % for CO2, 55 % of CO, 75 % of CH4, 27 % of PM, omnibuses for 41 % for NOX emission, taxis for 83 % of SO2, and two-wheelers for 36 % of HC emissions.


INTRODUCTION
Kolkata city, the erstwhile capital of the British Indian Empire, is a fast-growing metropolis now where along with the growth of population and economic activities, the level of air pollution is also increasing significantly [1]. Recent research, covering 109 Indian cities, has put Kolkata under the severely vulnerable tag due to the higher status of ambient particulate matter of diameter less than or equal to 2.5 micrometres (PM2.5) and thereby indicating the urgency on prioritizing a clean-air action plan for the city at the earliest [2]. The World Health Organisation (WHO) report covering 4000 cities of 100 countries for the year 2016, released during June 2018, revealed that Kolkata was only second after Delhi in terms of highly polluted cities of India. The air quality of Kolkata worsened more rapidly than that of Delhi [3]. The Central Pollution Control Board (CPCB) of India also expressed concern about the faster deterioration of air quality of Kolkata which is the most important city of eastern India in terms of economic reasons [4].
Indian cities, in general, are characterized by the high motor vehicle population as they are excessively dependent on their road network for freight and passenger traffic movements. The World Bank [5] estimated that about 60 % of freight and 85 % of passenger traffic movements usually depend on on-road transportation only in India. Accordingly, cities are

METHODOLOGY
In general pollutants and gases like particulate matter (PM), carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), nitrous oxide (N2O), sulphur dioxide (SO2), methane (CH4), non-methane volatile organic compounds (NMVOC), and hydrocarbon (HC) are ____________________________________________________________________________ 2021 / 25 720 associated with the vehicular exhaust. Depending upon the type of engine, fuel used, vehicle age, and condition of using the emissions from vehicles vary. Road transport is a critical contributor to air pollution in the Indian city context because of excess use of diesel as fuel and non-availability of environment-friendly fuel like CNG gas in many Indian cities including Kolkata.

Assessment of Motor Vehicle Population and Projection
The category-wise motor vehicle population data of Kolkata, for twelve years (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) were collected from the Indiastat [35] and Open Government Data (OGD) platform of the Government of India [36]. The category-wise motor vehicle population data of 24 other million-plus cities, for three years (2015-2017), also collected from the same sources. The online sources reported data in two broad categories, i.e., transport and non-transport. The transport category includes multi-axle/articulated, vehicles like trucks & lorries (TL), light motor vehicles (goods) or LMVG, buses, taxis, light motor vehicles, (passenger) or LMVP. The non-transport category includes two-wheelers (TW), cars, jeeps, omnibuses, tractors, trailers, and other miscellaneous vehicles. The two-wheelers were then subdivided into TW motorcycles (4-Stroke TW or 4S-2W) and TW scooters (2-Stroke TW or 2S-2W). Indiastat and OGD data of TW do not provide the population of 4S-2W and 2S-2W separately. The proportion of 4S-2W and 2S-2W out of the total of TW was taken as 72:28 based on MoRTH report [37]. We used future growth forecasting techniques like the compound annual growth rate (CAGR) of respective category vehicles for the period 2006-2016 of Kolkata in projecting the total motor vehicle population of the city up to the year 2025.

Calculation of Emission Load
In order to calculate the emission load due to road transport for the period 2015-2025, this study has used the following IPCC equation, Eq. 1 [38]. , , where Ei Total emissions load (i); Vehj Number of different types of city vehicles (j); Dj Kilometer traveled/vehicles/year (j) km; EFi,j,km Emission of pollutants (i) from vehicle type (j) per travelled kilometre.
We used the motor vehicle utilization rate (average kilometre travelled per year) in India from the CPCB study [39]. The CPCB study found out the vehicle utilization rate based on a primary survey of six Indian cities. This study used the average utilization rates of different types of automobiles to reflect pan India status.

Quantification of Emission Factors
The Automotive Research Association of India, ARAI [41] undertook a pioneering attempt during 2007 to develop the emission factors for automobiles plying on the Indian road. This project aimed at the development of typical values of pollutants emitted by different types of automobiles to the atmosphere. The automobiles of five different generations (viz, 1991-96, ____________________________________________________________________________ 2021 / 25 721 1996-2000, post-2000, and post 2005) were considered for understanding the relationship between technology and emission factors and after that use of the relationships to indicate the emission factors for automobiles plying on Indian roads. Different researchers in the field, as well as agencies from time to time, contributed to further update the emission factors as applicable in India. Emission factors, usually expressed in gram per kilometre (gm per km), depends on various factors like actual driving condition, road unevenness, the laden weight of the automobile, acceleration or de-acceleration, vehicle age, fuel condition, ambient temperature, and vehicle maintenance. It is not possible to have a pan world standardized emission inventory as the ground conditions are different. Different pieces of the literature on the even subject also used these emission factors for different types of automobiles plying in the country. It is challenging to have the exact break up of different types of two-wheelers plying on the road, so in the case of two-wheelers, the average of emission factors (4S-2W and 2S-2W) have been considered for generalization. The emission factors of different types of vehicles in India considered for this study can be seen in Table 1.

Calculation of Road Transport Emission of Cities
Measurement and quantifications of city-level road transport emission, both city-wise total emission and per unit area wise emissions, is essential in order to understand the pollution contribution and effect changes in technologies in emission parameters [14]. Emissions of primary pollutants (PM, CO, CO2, NOx, SO2, CH4, and HC) by road transport during 2017-2025 for Kolkata and during 2015-2017 for other 24 Indian cities were calculated out using the IPCC equation for different category of vehicles following their respective India specific mass emission factors [38]. The GHG gas emissions per unit of the geographical area-wise, expressed in gigagrams (Gg) or megagrams (Mg), for all the 25 cities for the period 2015-2017 was then calculated to assess emission intensity per unit city area by using Eq.  where Areai = geographical area of municipal administration of the i th city (km 2 ) and j th type of vehicle. The geographical area of the cities was collected from respective city municipality websites. Vehicle type-wise emission analyses were also carried out for five major Indian metropolitan cities using the IPCC equation for the year 2017, including Kolkata, to understand the contribution of dominant sources of emission in the total emissions of the cities in comparative terms.

Vehicular Emission in Kolkata
The emissions of PM, CO, CO2, NOx, SO2, CH4, and HC during 2015-2025, for different vehicle categories in respect of Kolkata city, have been calculated out using IPCC, 2006 equation. The rising trend of annual emission of CO2 in Kolkata from 2015-2025 can be seen in Fig. 3A. The emission of CO2 will be 21 668.24 Gg during 2025 in India, which is about 104 % more than the 2015 value of 10 603.14 Gg. The increase in CO2 can be attributed mainly to a 107 % increase in the total number of cars during the period 2015-2025. Cars alone will be responsible for about 75 of the total CO2 emission during 2025, followed by other types of automobiles like omnibuses (8 %), taxi (7.6 %), and LMVG (3 %). The CO emission inventory of 126.15 Gg during 2015 will be increased to 272.81 Gg during 2025 by registering a rise of about 116 %, as can be seen in Fig. 3B. The cars will be the main contributor with 53 % of the total emission of CO emission during 2025 followed by LMVG (25 %) and 4S-2W (8 %). The projected increase of two-wheelers by 163 % during 2015-2025 will surely be a contributor to CO presence in the atmosphere as evident from the fact that they will be contributing to about 9 % of the total CO emission of 272.81 Gg during 2025.
The 52.66 Gg of NOx emission during 2015 will be increased to 98.21Gg by 2025 due to vehicular road traffic as can be seen in Fig. 3C. The highest contribution of 42 % of total NOx emission during 2025 will come from omnibuses, followed by 17 % from LMVG, 15 % from cars, and about 11 % from the buses.
During 2015 the CH4 emission from road vehicles was 7.85 Gg which will shoot up 16.9 Gg during 2025 in Kolkata, the increase being 115 %. Four types of automobiles like 4S-2W (5 %), 2S-2W (2 %), Cars (73 %), and LMVG (14 %) will be responsible for about 94 % of 725 CH4 emission during 2025 as can be seen in Fig. 3D. As it is evident from Fig. 3E, there will be 93.39 Gg SO2 emissions in Kolkata due to the vehicle population by 2025, which is about 102 % more than that of 2015 emissions. The major contributors to the pool of SO2 will be three types of vehicles like taxis, cars, and omnibuses, and out of these three, taxies alone responsible for 86 % of the total SO2 emission inventory of the city by 2015.
During 2025 there will be 8.60 Gg of PM pollution in the city due to vehicle population, as shown in Fig. 3F, which will be about 110 % higher than that of 2015 emissions. The main contributor to PM pollution in Kolkata during 2025 will be four types of automobiles viz. LMVG (31 %), Car (25 %), Omnibuses (23 %), and Taxies (6.4 %) constituting about 84.5 % of the total during 2025. HC emission also includes other volatile organic compounds (VOCs) which may not be directly harmful to health at the concentration found in the ambient air. However, they support the formation of hazardous NO2 and O3 in the atmosphere. During 2015 the road vehicles contributed to about 19.6 Gg of HC formation which is expected to increase to 38.55 Gg during 2025 indicating a considerable sharp rise of about 97 % as can be seen in Fig 3(G). The major contributor of HC will include emissions from 4S-2W (20 %), 2S-2W (13 %), cars (47 %) during 2025. Interestingly, two-wheelers (both 4S-2W and 2S-2W) together will be responsible for 33 % of the total HC pollution coming out from the road vehicular transport sector in the city.

GHG Emissions of Kolkata and 24 Other Indian Cities
The IPCC, 2006 equation and category wise break up of total vehicles in the cities were used to calculate GHG emission for 25 leading million-plus Indian cities, including Kolkata. GHG emissions for 25 leading million-plus Indian cities for the year 2017 and category-wise vehicles of the cities responsible for the emissions can be seen in Table 2. Table 2 Table 2.
As the economy progressed, the population of the cities also increased which led pro-rata increase in total passenger travel demand with time. Accordingly, the transport emission load of Indian cities also increased with time. During 2017, all the 25 leading Indian cities emitted 220 560 Gg of CO2 (Table 2). Also, during 2017, across all the 25 cities, the leading CO2 emitting city was Delhi with 22 % of total emission (48317 Gg,), followed by Bengaluru  Table 2 that Kolkata was in the 5th rank, shared jointly with Mumbai, for PM (5 Gg, 6 %) emission while for HC emission, Kolkata ranked 8th with 24 Gg (4 %) emission amongst the 25 cities.

GHG Emissions Per Unit Area of 25 Cities
The city-wise emission per unit area for 25 cities for the year 2017 in respect of PM, CO, CO2, NOx, SO2, CH4, and HC due to on-road vehicle stock is summarized in Table 3. It can be seen from Table 3  All the 25 major million-plus cities are very important from the viewpoint of the Indian economy as they are the centre of many commercial and industrial establishment and providing livelihood and to a large chunk of the Indian population since the independence of the country. A recent report, Global Metro Monitor [8] indicates that higher CO2 emitting cities are also in the list of top ten wealthiest cities of India in terms of their Gross Domestic Product (GDP) contribution towards the national economy. The economic progress has also contributed towards total passenger travel demand and hence surge in the vehicle population. Fig. 4 Table 3

Emission from Different Vehicle Types of Metropolitan Cities
Emission contributions from different types of vehicles during 2017 for the top five metropolitan cities, i.e., Bangalore, Mumbai, Delhi, Chennai, and Kolkata were calculated using the IPCC equation and summarized in Table 4. It can be observed that in Bangalore, out of 26 143 Gg of CO2 emissions during 2017, the highest emission came from cars (71.18 %, 18 609 Gg), followed by taxis (7.06 %, 1845 Gg), truck and lorries (5.06 %, 1323 Gg) and buses plus omnibuses combination (7.61 %, 1991 Gg). Similarly, the highest contributor of CO in Bangalore was cars (47.4 %, 164. Comparison across five cities reveals that cars were responsible for 49 % of CO2 emission for Chennai followed by 40

CONCLUSION
City-wise emission inventory, exclusively for the road transport sector, is of utmost importance for India as the country depends heavily on the road network for freight and passenger traffic movements. Benefits from city-wise emission inventory would be in terms of timely policy prescription mitigation of vehicular emission and reduction of GHG emissions. The road transport led emission inventory for a representative Indian city, i.e., In terms of GHG emissions per square kilometre of the city area, Kolkata topped the list amongst 25 million-plus cities. The CO2 emission contribution per unit area was highest in Kolkata with 62 Gg km −2 during 2017, which was significantly higher than the 25-city average of 24 Gg km −2 . The city also outpaced the other 24 cities by a significant margin with 254 Mg km −2 SO2 emission, 23 Mg km −2 PM emission, and 290 Mg km −2 for NOx during 2017. Kolkata occupied the second position in terms of CO emission per unit area (737 Mg km −2 ), the seventh position for CH4 emission (51 Mg km −2 ), and the eighth position for HC emission per unit area (106 Mg km −2 ) amongst the 25 cities considered in this study.
Emission analysis based on pollution contribution of different types of road transport vehicles for five metropolitan cities, i.e., Bangalore, Mumbai, Delhi, Chennai, and Kolkata reveal that cars were primarily responsible for CO2, CO, NOx, CH4, and PM emission in these five cities during 2017. At the same time, taxies were responsible for SO2 emission and twowheelers for HC emission. For Kolkata, cars were responsible for 35 % for CO2, 55 % of CO, 75 % of CH4, 27 % of PM, omnibuses for 41 % for NOx emission, taxis for 83 % of SO2, and two-wheelers 36 % for HC emissions during 2017. The identification of vehicle categories responsible for major emissions in cities will be handy for the city policymakers to come up with appropriate policy options to curb emissions from the road transport sector.
Alarmed by the huge vehicular pollution in cities, the central government has taken several initiatives like increasing the CNG network to cover more cities, expansion of metro rail networks, and introducing electric vehicles in the cities. However, most remarkable was the recent decision to leapfrog from BS-IV fuel to BS-VI fuel without shifting to BS-V fuel (Indian equivalent to Euro-IV to Euro-VI) and hence directed only BS-VI compliant vehicles should be sold in the country from April 1st, 2020. No doubt, this drastic step was the need of the hour for Indian cities, and it is likely to improve the city air quality. However, the existing BS-IV compliant vehicles have been allowed to remain operational for their entire duration of registration. Both BS-VI fuel and BS-VI compliant vehicles will surely bring down vehicular emission resulting in better air quality in cities. This study, therefore, is very timely and vital from the point of view that it would provide a benchmark to the policymakers and researchers for comparing the emission improvement in the 25 representative cities of India resulting from new initiatives to clean city air in the years to come. Our study has a few limitations as well. The non-availability of updated (type-wise) vehicle utilization rates and emission factors for diesel and gasoline vehicles separately under different engine standards like BS-I to BS-V impaired the total emission estimation. Further, the non-availability of data regarding the numbers of the new vehicles added up in the city stock with technologies per year and year-wise phasing out of vehicles with old technologies made it difficult to bring out the benefits of technological improvements in emission reduction at different point of time.