The emissions were found to be more than doubled than 2011 levels. Buses and trucks were found to be the biggest contributors. Simulation runs with HYSPLIT showed that black carbon emissions from the traffic in Guwahati to be transported and deposited in the Himalayas. 2. Keywords:Black carbon, Guwahati, Source apportionment, HYSPLIT, deposition, emission inventory, Himalayas3. Introduction:Black carbon is a component of soot and often defined as the combination of elemental carbon and condensed organics (Shrestha et al. , 2010). BC particulates are formed from the incomplete combustion processes (Cooke and Wilson, 1996). In outdoor environments they are formed mainly due to the burning of fossil fuels such as coal, gasoline, diesel etc. , while they are contributed mainly by the burning of biomass such as wood and crop residue in the indoors (Andreae and Crutzen, 1997). These BC particles present in the soot are responsible for its absorbing nature of solar radiation (Bond and Bergstrom, 2006) and thus plays an important role in the global climate system (Wang et al. , 2016). BC is considered to be the second largest contributor to global warming after CO2 (Surendran et al. , 2013). BC particulates due to its long atmospheric residence time (Babu and Moorthy, 2001) are amenable to long range transport (Tiwari et al, 2013). The proximity of India and China and they being the largest emitters of BC(Qin and Xie, 2012), the Himalayas are considered as one of the most vulnerable regions to the effects of climate change. When BC is deposited over the snow and ice including those present over the glaciers, it darkens the snow and reduces the surface albedo, thereby increasing the absorption of solar radiation enhancing its melting rate (Warren and Wiscombe, 1985). The deposited BC particles are reported to be causing significant melting in the Himalayas (Menon et al. , 2010).In the meantime, it can affect the biosphere through several pathways including long-term change in weather pattern such as the Indian monsoon (Lau et al. , 2008; Lau and Kim, 2006). The emissions of black carbon have been increasing worldwide. Though BC emissions in the United States is projected to have a declining trend (Kirchstetter et al. , 2017), the global emissions deviate from this trend. The bulk of the global BC generated are from the regions showing significant economic growth with China and India being the greatest sources contributing to 25-35% of the global BC emissions (Center for Climate and Energy Solutions, 2010). Guwahati is the largest city of the North-East region of India and considered to be oneof the fastest growing cities in India. Being the gateway to almost all the states in the North-East, the city has grown tremendously in the past few decades.
There are two estimations, ‘High’ and ‘Low’ as followed in Baidya and Borken-Kleefeld (2009)4. 3. 2. Activity of VehiclesActivity of vehicles is the distance travelled by a vehicle per unit time. For generating the activity data, vehicles have been assumed to run within Kamrup (Metropolitan) district. The reason behind this assumption is that when a vehicle is registered in a certain district there is a high probability that the vehicle will be driven primarily in that area. As per 2011 population census, Guwahati under Kamrup(Metropolitan) district contains approximately 77% of the total population of Kamrup(Metropolitan) district. As it is a common fact that vehicular activity is more dominant in areas with higher population, hence the city of Guwahati was taken as the domain for activity calculation for vehicles registered in the mentioned district. An online approach was considered for activity data estimation as no readily available data was found. There are several websites which provides the platform for selling used vehicles. The data provided on the website includes the total distance travelled by a vehicle as well as the age of the vehicle.
Meteorological data was obtained from the website of Air Resources Laboratory website of NOAA for 2017-2018. The simulations were run month-wise from January to December for deposition. Simulations runs give the total depositions in µg/m2 in each month. The simulations were run for both low and high estimation of emissions assuming that the rate of emission will remain constant for a particular year. 5. Results and Discussion5. 1. Analysis of Collected DataThe mean concentrations are shown in figure 2. Considerable concentration differences was found between the two locations in different seasons especially in pre-monsoon, monsoon and post-monsoon where the concentrations are differ by a factor of approximately.
The concentration at UB are found to be less than at LN at all times. Most likely this is due to the difference in the vehicle and other anthropogenic activity at the two sites as found in other studies also.
The correlation between the BC concentrations and the meteorological parameters are given in Table 1. With the exception of monsoon season at Uzanbazar, temperature, wind speed and solar radiation are found to have significant negative correlations with BC concentrations while relative humidity having positive values. Solar radiation is responsible for temperature and temperature is responsible for temperature. Higher temperature leads to higher buoyancy of air parcels and expands the atmospheric boundary layer, contributing to dispersion of the pollutants. So, it’s no wonder why all these three meteorological parameters have negative correlations with BC concentrations. Relative humidity generally have an agglomerating effect on the pollutants especially particulates which is not seen here, which is evident from the positive correlation values.
For rainfall, significant correlation of moderate strength is only found during the pre-monsoon season at UB. In other cases, it’s seen that rainfall is found to be non-significant parameter for affecting BC concentration. Seasons with unavailability of rainfall are shown by dashes. In this case, it may either be due to hydrophobic nature of BC particulates or the rainfall intensity might not be sufficiently strong. Thus, from this temperature, wind speed and solar radiation are found to be the prime parameters affecting BC concentration in the atmosphere. Seasonal variations in concentrations were also found to follow a pattern at both the locations with the maximum being in winter, which is followed by pre-monsoon, post-monsoon and monsoon. This pattern however is not an isolated one and is seen in other studies also. At LN, average temperatures were found to be lowest in winter and successively increasing in pre-monsoon, post-monsoon and monsoon as seen in Table 2. The exact reverse trend was seen in concentrations at that location. At UB however, similar average temperatures during pre-monsoon, monsoon and post-monsoon with significant difference with winter. Hence, similar concentrations were observed during pre-monsoon, monsoon and post-monsoon. The diurnal variation of average concentration and the concentration range are shown in figure 3. In most of the cases, concentration peaks are observed in the morning and the evening. This is most likely due to the low boundary layer height and increasing emission activity during this periods of time, as seen in other studies also. Also it is seen thatin most of the seasons at both the locations, the concentrations vary significantly during the morning and the night time. The reason for these was found to be the fluctuations in availability of wind during these period in different monitoring days.
It is seen from figure 4(a) and figure 4(b) that fossil fuel burning is the dominant contributor to the total BC having a share greater than 80% in all the seasons in both locations. However, such values aren’t unexpected since the monitoring has been performed in an urban area where traffic is the dominant source. The proportion of BCbb could have been expected to be larger had the site been located in a rural area where mostly wood is used for cooking and agriculture residues are burnt to clear the fields for subsequent cultivation.
Development of Emission Inventory
Figure 5(a) shows the yearly total number of vehicle registrations. An increasing trend in total registrations was observed from 2001 and peaking in 2015. Thereafter it is on a decreasing trend. The decrease in 2016 and 2017 may be the effects of government policies such as implementation of Goods and Service Tax and demonetization by the Govt. of India. Two-wheeler and cars are dominating the registration numbers as seen in figure 5(b).
The emission inventory was first developed for the year 2011, in which the emissions ranged from 0. 19-0. 27 Gg/year. Paliwal et al. (2016) had estimated the emissions to be from 0. 15-0. 30 Gg/year from the same region. The same assumptions were then used to develop the latest emission inventory (2018) and it ranged from 0. 45-0. 55 Gg/year.
The estimated on-road vehicles at the end of 2017 and the total emissions per day are shown in figure 6. Trucks and buses are found to be the biggest contributors to BC emission.
Deposition of BC
It is observed that throughout the year BC particles are depositing in the surrounding region (figure 7) throughout the year. Also it is seen that Guwahati is seen to be contributing to the black carbon deposits in the Himalayas which is the main objective of the study. The units though in µg/m2, the effect of gradual accumulation along with the increasing emissions and time, the total depositions might be significant. Hence, immediate mitigations are the need of Conclusion
Based on the analysis and simulation results, the following conclusions can be drawn:
The concentration of BC at UB is found to be much lower than at LN which elucidates the contribution of traffic to BC.
BC is seen to have significant negative correlation with temperature, wind speed and solar radiation. Relative Humidity has positive correlation, while rainfall is found to be a non-significant meteorological parameter.
BC concentrations are found to be maximum in winter, followed by pre-monsoon, post-monsoon and the least in monsoon. In monsoon however, rainfall is not found to be the factor for reduced concentrations.
BC concentration peaks are observed in the morning and evening/night time. The concentrations vary highly during the morning/night/evening time mostly and little variation is seen during the day time.
Fossil fuel BC is found to be the major source of BC with contributions greater than 85%at both locations in all the seasons.
The BC emission inventory generated for 2018 ranges from 0. 45-0. 55 Gg/year which is more than twice the emissions in 2011. Trucks and buses are the biggest contributors to the emissions.
HYSPLIT model simulations show that BC emissions in Guwahati are found to be carried and deposited in the nearby region including the Himalayas.