Environ Sci Pollut Res (2015) 22:2846–2855 DOI 10.1007/s11356-014-3531-2

RESEARCH ARTICLE

Determination of wood burning and fossil fuel contribution of black carbon at Delhi, India using aerosol light absorption technique S. Tiwari & A. S. Pipal & A. K. Srivastava & D.S. Bisht & G. Pandithurai

Received: 5 April 2014 / Accepted: 28 August 2014 / Published online: 14 September 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract A comprehensive measurement program of effective black carbon (eBC), fine particle (PM2.5), and carbon monoxide (CO) was undertaken during 1 December 2011 to 31 March 2012 (winter period) in Delhi, India. The mean mass concentrations of eBC, PM2.5, and CO were recorded as 12.1±8.7 μg/m3, 182.75±114.5 μg/m3, and 3.41±1.6 ppm, respectively, during the study period. Also, the absorption Angstrom exponent (AAE) was estimated from eBC and varied from 0.38 to 1.29 with a mean value of 1.09±0.11. The frequency of occurrence of AAE was ~17 % less than unity whereas ~83 % greater than unity was observed during the winter period in Delhi. The mass concentrations of eBC were found to be higher by ~34 % of the average value of eBC (12.1 μg/m3) during the study period. Sources of eBC were estimated, and they were ~94 % from fossil fuel (eBCff) combustion whereas only 6 % was from wood burning (eBCwb). The ratio between eBCff and eBCwb was 15, which indicates a higher impact from fossil fuels compared to biomass burning. When comparing eBCff during day and night, a factor of three higher concentrations was observed in nighttime than daytime, and it is due to combustion of fossil fuel (diesel vehicle emission) and shallow boundary layer conditions. The contribution of eBCwb in eBC was higher between 1800 and 2100 hours due to burning of wood/biomass. A significant correlation between eBC and PM2.5 (r=0.78) and eBC and CO (r=0.46) indicates the similarity in location sources. The mass concentration of eBC was highest (23.4 μg/m3) during the month of December when the mean Responsible editor: Constantini Samara S. Tiwari (*) : A. K. Srivastava : D. Bisht : G. Pandithurai Indian Institute of Tropical Meteorology, Prof. Ram Nath Vij Marg, R-Block, New Rajinder Nagar, New Delhi 110060, India e-mail: [email protected] A. S. Pipal Department of Chemistry, University of Pune, Pune 411007, India

visibility (VIS) was lowest (1.31 km). Regression analysis among wind speed (WS), VIS, soot particles, and CO was studied, and significant negative relationships were seen between VIS and eBC (−0.65), eBCff (−0.66), eBCwb (−0.34), and CO (−0.65); however, between WS and eBC (−0.68), eBCff (−0.67), eBCwb (−0.28), and CO (−0.53). The regression analysis indicated that emission of soot particles may be localized to fossil fuel combustion, whereas wood/biomass burning emission of black carbon is due to transportation from farther distances. Regression analysis between eBCff and CO (r=0.44) indicated a similar source as vehicular emissions. The very high loading of PM2.5 along with eBC over Delhi suggests that urgent action is needed to mitigate the emissions of carbonaceous aerosol in the northern part of India. Keywords Black carbon . Fine particle . Carbon monoxide . Fossil fuel . Biomass burning

Introduction Aerosol particles which are present in the atmosphere perturb the Earth’s radiation balance and exert significant impacts on the climate system directly and indirectly through scattering and absorbing the solar radiation. They can also affect cloud formation, optical properties, and lifetimes of clouds by acting as cloud condensation nuclei (Xu et al. 2012). These particles also play a crucial role in visibility degradation (White and Roberts 1977) and adverse health effects. Pope and Dockery (2006) and Jerret et al. (2005) have reported that the long-term exposure to combustion-related fine particulates is an important environmental risk factor for cardiopulmonary and lung cancer mortality. The direct and indirect effects of aerosols on the radiative heat balance of the earth remain one of the greatest sources of uncertainty in the assessment of global climate change (IPCC 2007). In order to better understand

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and evaluate the many impacts that aerosol particles have on the environment, it is necessary to measure aerosol properties with reasonable spatial and temporal resolution (Van et al. 2010). Due to their light-absorbing properties, black carbon aerosol (soot particles) can create a strong perturbation in the atmosphere by absorbing solar radiation. Black carbon particles are the second largest heating agents after carbon dioxide (Forster et al. 2007). Black carbon (BC) is usually released from the burning of biomass and fossil fuels, e.g., automobile and aircraft emissions and forest fires, etc. Biomass burning is also one of the major sources of anthropogenic aerosols (fine particles: PM2.5) and gases like carbon monoxide (CO) in the tropics (Andreae et al. 2008; Beegum et al. 2008; Venkataraman et al. 2005). Generally, BC is a component of fine particle (10 μg/m3) over urban locations in Indian cities such as Delhi, Kanpur, Agra, and Kolkata (Soni et al. 2010; Tiwari et al., 2009, 2013a; Safai et al. 2008; Tripathi et al. 2005; Pani and Verma 2010). However, at Allahabad, Hyderabad, Bangalore, Pune, Udaipur, Trivandrum, and Kullu, the eBC mass concentration (~5 μg/ m3) was approximately half of the present study (Dumka et al. 2010a; Safai et al. 2007, 2013; Awasthy et al. 2010; Babu and Moorthy 2002; Kuniyal 2010; Vyas 2010). Several studies have also been reported over hilly regions in India (Nainital, Mukteshwar, Sinhagad, and Himalayan foothills), indicating approximately six times lower mass concentrations (~2 μg/ m3) as compared to that of Delhi (Pant et al. 2006; Dumka et al. 2010a, 2010b; Hyvarinen et al. 2009; Raju et al. 2011). Nair (2007) and Beegum et al. (2009) reported that the total aerosol and eBC mass concentrations were 5–10 times higher over the IGP region compared to non-IGP regions. Carbon monoxide showed a considerable daily variability from 0.79 to 11.32 ppm with a mean value of 3.41 ppm. This is substantially higher (~double) than the value 1.72 ppm stipulated by the NAAQS. Incomplete combustion of fossil fuels and biomass burning is one of the major sources of carbonaceous aerosols and CO in the tropics (Andreae 1991). Several studies have shown significant correlations

Environ Sci Pollut Res (2015) 22:2846–2855 2.25

(a)

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Fig. 1 Day-to-day variability of absorption Angstrom exponent (a) and mass concentrations of eBC (b) over Delhi during the winter of 2011–2012

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between the mass concentrations of eBC and CO (gaseous) associated with biomass burning and fossil fuel (Chen et al. 2001; Baumgardner et al. 2002; reference therein). In the present study, the correlation between eBC and CO is 0.44, indicating that the sources that emit these pollutants are closely located. Chen et al. (2001) reported a significant correlation (0.7–0.9) between CO and EC and suggested common and proximate sources, likely traffic emissions. Beig and Sahu (2010) reported the relative contributions of CO emission from power, industrial, transport, and residential sectors by 0.04 % (0.29 Gg/year), 1.6 % (10.92 Gg/year), 60.8 % (427.55 Gg/year), and 37.6 % (264.41 Gg/year), respectively, over the National Capital Region of Delhi. However, in the case of eBC, it was similar to CO as power (0 %), industrial (40 %), transport (46 %), and residential (14 %) sectors. AAE was derived from the wavelength dependence of aerosol optical absorption coefficient of light for the study period based on Kirchstetter et al. (2004). Aerosol types were distinguished based on AAE values, higher AAE (~2) for biomass smoke aerosols due to their strong absorption efficiency in the lower wavelengths and lower AAE (~1) for aerosols from vehicle emissions (Russell et al. 2010). The occurrence of AAE less than unity was ~17 % whereas greater than unity was about 83 %, which indicates the dominance of particles from fossil fuel burning over Delhi during winter period. The large variation of AAE during daytime indicates changes in the composition of eBC and its source contributions depending upon the monitoring site. Earlier studies reported that a value of AAE (α) of approximately 1 denotes soot particles from fossil fuel burning; in the cases of biomass burning, it will be between 1.5 and 3; and for dust, it will be between 2 and 3 (Kirchstetter et al. 2004; Sandradewi et al.

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2008a, 2008b). In southern India, Ganguly et al. (2005) reported a value of α of 1.52, indicating combined emissions from biofuel and fossil fuel burning. Bond (2001) suggested that a value of α between 1 and 2.9 indicates that residential biofuel is the source of black carbon. Aruna et al. (2013) reported the value of alpha around 1.1 (varying from 0.9 to 1.1 for a metropolitan station Chennai), which is almost similar to our findings. Pandithurai et al. (2008) reported a very low spectral dependence of aerosol optical depth α (0.33) during post-monsoon season due to higher loadings of coarse mode particle over Delhi. The variability of AAE at the present study was from 0.63 to 1.97, which clearly indicates that the observed BC is influenced from different sources such as fossil fuel and wood burning emissions. In monthly analysis of AAE, it was observed higher in the month of February (1.14); however, it was lower in January (0.80). The lower AAE in January may be due to the occurrence of fog which brings a lot of fine aerosols (Safai et al. 2008). During foggy condition, BC mixes with the sulfate aerosol (Soni et al. 2010; Rastogi and Sarin 2005). Apart from this, the burning of biofuels also release huge amount of carbonaceous aerosols especially organic carbon (Tiwari et al. 2013a; Pipal et al. 2014). Goyal and Sidhartha (2003) reported that the major source of fine particles during winter is the burning of fossil fuels over Delhi. Singh et al. (2005) also suggested that besides these sources, biomass and biofuel burning and transported dust from the Great Indian Thar Desert has contributed to the aerosol characteristics in Delhi. A significant negative correlation (−0.34) between eBC and AAE was observed in the overall study. High-resolution data (5 min) of eBC along with AAE for a period of 5 days from 28 January 2012 to 02 February 2012 were plotted to view the relationship between them as depicted in Fig. 2. From the

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Fig. 2 Day-to-day variability of black carbon and absorption Angstrom exponent over Delhi during 28 January 2012 to 2 February 2012

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above observation, the source apportionment of eBC was estimated on the wavelength dependence of AAE absorption, as characterized by the source-specific AAE. For diesel exhaust, which is black, the value of the AAE lies very close to 1. Wood or biomass smoke contains aromatic compounds which absorb heavily in the blue and UV part of the light spectrum. This higher absorption at low wavelengths leads to a higher AAE; for wood smoke, the value lies around 2. These two different and specific values of absorption coefficients at 470 and 950 nm allow for a construction of apportionment of eBC to fossil fuel (traffic: eBCff) and biomass (wood combustion: BCwb) with time resolution of the Aethalometer measurements, respectively. The contribution of eBCff and eBCwb in the present study was 94 and ~6 %, respectively (Fig. 3), which indicates that the major source of eBC is from fossil fuel combustion over Delhi during winter period (Sandradewi et al. 2008a). Diurnal variability in eBC, eBCff, and eBCwb The diurnal variations of near-surface eBC, eBCff, and eBCwb are plotted (Figs. 4 and 5), which indicate a significant diurnal variation of two (morning and evening) distinct and prominent peaks in eBCwb; however, in the case of eBC and eBCff, it was higher during nighttime. In the morning (0700–0900 hours), the eBC mass concentration was 9.97±0.14 μg/m3, thereafter gradually decreases with a minimum eBC (2.36±0.2.7 μg/m3) during the afternoon hours (1300–1600 hours). These sharp enhancements during morning seem to be driven by vehicular emissions, as they are coincident with the morning traffic rush hours and the decrease in traffic circulation during afternoon. Before the evening rush hour, and the typical daytime, development of a convective boundary layer condition which leads to a faster dispersion results in a dilution near the surface and leads to a decrease in the eBC mass concentrations. However, the evening

(1900–2100 hours) peak was not as dominant as the morning one, having slightly lower concentrations (6.66±0.86 μg/m3) but still it is very high. In Fig. 4, it is also seen that eBCff tracked as eBC, but in the case of eBCwb, it was found to vary significantly. In nighttime (midnight to 0600 hours) and during daytime (1100 to 1800 hours), the concentrations of eBCwb were much lower (~350 ng/m3) as compared to that of between 0700 and 1100 hours (684 ng/m3) and 1900 to 2300 hours (1,078 ng/ m3), indicating the impact of wood combustion during the evening and morning over Delhi. Ganguly et al. (2006) reported that the increase in the production of eBC aerosol around 1700 hours is due to increased household activities and gradual formation of a surface-based inversion opposing vertical mixing in the atmosphere. Another reason for an increase of eBC in the evening is open burning of solid wastes such as dry leaves and other garbage materials, particularly during the post-monsoon and winter seasons (Pathak et al. 2010). In addition to waste burning, wood and shrubs are also burnt at night during cold winter months (Ali et al. 2004). In the late night, between 2200 and 0200 hours, very high mass concentrations of eBc were observed (10.71±0.08 μg/m3); it is due to shallower nocturnal boundary layer and lower wind speeds during night, leading to a rapid reduction in the ventilation effects and consequently confining the aerosols causing the high concentrations of eBC (Kunhikrishnan et al. 1993). As night advances, there is a progressive and strong reduction in the traffic density and eBC generation, while the existing particles closer to the surface are partly lost by sedimentation. Thus, the concentration gradually decreases early in the morning between 0300 and 0600 hours. Similar diurnal variations of BC concentrations were reported by Beegum et al. (2009) over an urban coastal site; Latha and Badarinath (2005) for an urban, industrial station; and by Pathak et al. (2010) and Safai et al. (2007) for other arid continental locations in India. Stull (1998) and Kunhikrishnan et al. (1993) reported that atmospheric aerosol concentration is

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Fig. 3 Source contribution of black carbon from fossil fuel (eBCff) combustion and wood burning (eBCwb)

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affected by the stability of the atmospheric boundary layer, which is active during the daytime due to surface temperature increase and stable at night. It has been shown that the nocturnal boundary layer is shallower than its daytime counterpart by a factor of about 3. In general, the winds are higher during daytime and lower during nighttime. Babu and Moorthy (2002) and Tiwari et al. (2013a) have reported that ventilation coefficient rapidly reduces during the night, resulting in the confinement of aerosols. Diurnal contribution of eBCwb and eBCff in total BC (eBC) was estimated and plotted (Fig. 5), which indicates exactly the opposite variation with time due to different emission sources. In the overall study, the contribution of eBCff was higher by a factor of 15 than the eBCwb. The higher contribution of eBCwb in eBC

was between 1800 and 2100 hours; however, in the case of eBCff, it was opposite and indicates higher during nighttime from 2200 to 0600 hours due to combustion of diesel during night. In Delhi, heavy diesel vehicles are prohibited to enter Delhi between 0600 and 2200 hours. After 10 pm (2200 hours) only they are allowed to enter through the urban area of Delhi. Meanwhile, the aerosol concentration is also dependent on the stability of the boundary layer, which is usually unstable during daytime and stable at nighttime. The boundary layer heights over Delhi showed a diurnal pattern with low mixing heights at nighttime and higher during the daytime (Tiwari et al. 2013b). Due to these reasons, the concentrations of eBCff were observed higher in nighttime over Delhi.

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Fig. 4 Diurnal variability in total black carbon (eBC), eBCff, and eBCwb

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Fig. 5 Diurnal variation of eBCwd/eBC and eBCff/eBC

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Impact of meteorological parameters on mass eBC During the study period, the mean WS was 2.89±0.68 m/s and varied from 1.64 to 4.91 m/s. The average VIS was 2.30± 1.06 km and varied from 0.37 to 4.83 km. The diurnal variability of the mass concentration of eBC, CO, and VIS are plotted in Fig. 6. It was observed that the visibility was highest (VIS≥4 km) around 1700 hours local time (LT) and the corresponding value of eBC was 2.1 μg/m3; however, the lowest (VIS≤1 km) was around ~0800 hours LT when eBC was highest (10.4 μg/m3). A large variability was seen from afternoon to evening, while low visibilities occurred in the morning till 0900 hours during winter. CO was observed at lower

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Fig. 6 Diurnal variation of atmospheric visibility and eBC and CO concentrations during the winter season of 2011–2012 over Delhi

concentration as compared to eBC during nighttime; however, during daytime, it shows a similarity with higher concentrations. During the study period, the monthly mean VIS was 1.31, 1.69, 2.99, and 3.26 km during December, January, February, and March while the corresponding eBC was 23.4, 8.7, 6.1, and 9.9 μg/m3, respectively. Very good agreement between eBC and VIS was seen from December to January except March. The higher eBC in the month of March may be due to long-range transport of aerosols from the northwestern part of India during crop burning period (Awasthy et al. 2010; Badarinath et al. 2009). This analysis supports the previous discussions in “Dayto-day variability of mass eBC, PM2.5, and gaseous CO” that the higher AE in the month of March is due to long-range transport

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of pollutants. Regression analysis between VIS vs BC and CO was studied, and significant negative correlations were seen between VIS vs eBC(−0.65), eBCff (−0.66), eBCwb (−0.34), and CO (−0.65); however the correlation observed between WS (wind speed) and eBC (−0.68), eBCff (−0.67), eBCwb (−0.28), and CO (−0.53), respectively.The regression analysis indicated that fossil fuel emission of soot particles may be localized, whereas biomass eBCwb is due to transportation from distant sources (Tiwari et al. 2014b). Regression between eBCff and CO (r=0.44) indicated similar sources such as vehicular emission. The lower visibility clearly indicated the impact of aerosol (eBC and CO) through absorption of light (Deng et al. 2011; Zhao et al. 2011). The influence of air pollutants (both gaseous and aerosols) on visibility in Delhi have previously been assessed rather qualitatively and empirically (e.g., Goyal and Sidhartha 2003; Tiwari et al. 2012; Tiwari et al. 2014b). The atmospheric visibility is deteriorated by light-absorbing components such as soot particles which are present in the atmosphere in higher concentrations over the northern part of India (Tiwari et al. 2013b; Srivastava et al. 2012). Sloane and White (1986) suggested that the loss of VIS is an easily measured manifestation of air pollution, arising from a loss of contrast between the object and the background and attenuation of the light signal from the object due to scattering and absorption of light by fine particulates and gaseous pollutants. Regression analysis between visibility and eBC showed a correlation coefficient of −0.65, which indicates that the rapid growth of anthropogenic emissions especially fossil fuel consumption (Tiwari et al. 2009; Tiwari et al. 2013a) and biomass burning over the study region is responsible for this very strong negative relationship. Generally, during winter in the northern part of India, for most of the period, calm conditions with lower temperature, thick foggy weather, and shallow boundary layer heights were seen (Tiwari et al. 2014b; Pipal et al. 2014). In such conditions, pollutants could not be dispersed and mix with free troposphere. The impact of such conditions is discernible as poor visibility and high levels of pollutants in this region (Mohan and Bhati 2009). In a recent study, Xu et al. (2012) have also reported a negative impact of BC on visibility, having a significant correlation of −0.79 between atmospheric visibility and optical properties at an urban site in Shanghai, China.

Conclusions Due to the poor air quality during winter periods, real-time measurements of eBC (effective black carbon) and PM2.5 (fine particles) along with CO (carbon monoxide) were carried out from 1 December 2011 to 31 March 2012 over Delhi, a mega city located in the northern part of India. The measured mean concentrations of eBC, PM2.5, and CO were observed as 12.1 ±8.7 μg/m3, 182.75±114.5 μg/m3, and 3.41±1.6 ppm. The AAE was also estimated from the measured absorption

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coefficients and varied from 0.38 to 1.29 with a mean value of 1.09±0.12. The occurrence of AAE less than unity was ~17 %, whereas the occurrence greater than unity was about 83 % during winter period. The mass concentrations of eBC were found to be higher by ~34 % of the average value of eBC (12.1 μg/m3) during the study period. The source contribution of eBC has been evaluated, and the contribution of fossil fuel (eBCff) and wood burning (eBCwb) were ~94 and ~6 %, respectively. The ratio between eBCff and eBCwb of 15 indicates a higher impact from fossil fuels compared to biomass burning. At nighttime, the eBCff was higher with a factor of 3 in comparison to daytime values due to combustion of fossil fuel, especially diesel oil and shallow boundary layer conditions. The contribution of eBCwb to eBC was higher between 1800 and 2100 hours due to burning of wood or biomass. The mass concentration of eBC was highest (23.4 μg/m3) during the month of December when the mean visibility was 1.31 km. Regression analysis among WS, VIS, soot particle, and CO was studied, and significant negative relationships were seen in VIS and eBC (−0.65), eBCff (−0.66), eBCwb (−0.34), and CO (−0.65); however, between WS and eBC (−0.68), eBCff (−0.67), eBCwb (−0.28), and CO (−0.53). The regression analysis indicated that fossil fuel emission of soot particles may be localized whereas biomass BCwb is due to transportation from distant sources. Regression between BCff and CO (r=0.44) indicated similar sources such as vehicular emission. The high loading of eBC aerosol over Delhi suggests that action will be required to mitigate the emission of carbonaceous aerosol in the northern part of India and IGP region.

Acknowledgments The authors gratefully thank Prof. B. N. Goswami, Director, IITM Pune for his encouragement and support.

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