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Biogenic and anthropogenic isoprene emissions in the subtropical urban atmosphere of Delhi
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Original Article
Biogenic and anthropogenic isoprene emissions in the subtropical urban atmosphere of Delhi Prabhat Kashyapa, Amit Kumara, Ram Pravesh Kumarb, Krishan Kumara,* a b
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, 110067, India School of Earth, Environment & Space Studies, Centre University of Haryana, Haryana, 123031, India
A R T I C LE I N FO
A B S T R A C T
Keywords: BVOC Biogenic isoprene Anthropogenic isoprene Gas chromatography Ozone formation potential
This study examines the seasonal and intraday variations in isoprene, benzene and toluene at different sites in Delhi with a view to estimate the anthropogenic and biogenic fractions of isoprene and their ozone formation potential. For this purpose, measurements of ambient isoprene, benzene and toluene were made at selected vegetative and traffic sites of Delhi using Gas Chromatography-Mass Spectrometry. The study reveals that the average isoprene concentrations were higher at vegetative sites (4.40 ± 3.35 μg/m3) as compared to traffic sites (0.56 ± 0.22 μg/m3). The benzene and toluene levels, on the other hand, were found to be higher at the traffic sites (23.22 ± 10.94 and 60.58 ± 20.27 μg/m3) as compared to vegetative sites (8.01 ± 4.56 and 24.75 ± 10.86 μg/m3) respectively. The isoprene concentrations at vegetative sites were found to be the highest in summer (6.51 ± 3.56 μg/m3) followed by post-monsoon (4.80 ± 3.18 μg/m3) and winter (1.82 ± 0.57 μg/ m3). In the context of intraday variation, isoprene concentrations attained maximum values during the afternoon period. However, no significant intraday/seasonal variation was observed in isoprene concentration at traffic sites. Estimation of anthropogenic and biogenic fractions of isoprene revealed that anthropogenic contribution varied between 60 and 70% at traffic sites and less than 10% at the vegetative sites. Upon computation of ozone formation potential of isoprene, benzene and toluene it is seen that isoprene's contribution towards ozone formation is higher at vegetative sites as compared to traffic sites. But, the anthropogenic isoprene's contribution towards ozone formation is found to be minimal in the present study. In view of the existing vegetation cover (~20%) of Delhi, however, the role of isoprene in surface ozone formation cannot be understated.
1. Introduction Volatile organic compounds (VOCs) constitute an important class of organic chemicals, which play a key role in atmospheric chemistry (Kumar et al., 2019; Tong et al., 2013). VOCs are primarily emitted from biological sources i.e. plants, and other living organisms found in terrestrial and aquatic environments (Guenther et al., 1995; Kansal, 2009; Sahu, 2012), collectively known as biogenic VOCs (BVOCs). Anthropogenic sources of VOCs include emissions from industries, motor vehicles, biomass burning and solvent operations (Kansal, 2009; Piccot et al., 1992). However, the contribution of BVOCs to the total global emissions of VOCs, is higher than that from anthropogenic activities (Helmig et al., 2009). Among various BVOCs, Isoprene (2-methyl-1,3-butadiene, C5H8) is a major hydrocarbon with estimated emissions of 535 Tg per year, which constitutes ~50% of the total BVOCs emissions of 1000 Tg per year (Guenther et al., 2012).
Isoprene is a highly reactive compound having atmospheric chemical lifetime of the order of minutes to hours (Achakulwisut et al., 2015; Pike and Young, 2009). Chemical reactions of isoprene with the OH, ozone, and nitrate radical largely influence the oxidative chemistry of the troposphere (Beerling et al., 2007). Isoprene and its oxidation products have significant role in modulating the tropospheric ozone and CH4 concentrations (Pacifico et al., 2009). Isoprene has a crucial role in O3 formation and destruction depending upon the concentrations of nitrogen oxides (NOx = NO+NO2) in the ambient atmosphere. At high NO concentrations, isoprene oxidation leads to formation of NO2, followed by photolysis of NO2 which results in O3 formation. On the other hand, isoprene also directly reacts with O3 at low NO levels, which ultimately reduces the tropospheric O3 (Monson et al., 2007; Sanderson et al., 2003). Besides this, secondary organic aerosols (SOA) are also formed by the oxidation products of the isoprene (Claeys et al., 2004).
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. * Corresponding author. E-mail address: [email protected] (K. Kumar). https://doi.org/10.1016/j.apr.2019.07.004 Received 11 March 2019; Received in revised form 24 June 2019; Accepted 10 July 2019 1309-1042/ © 2019 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V.
Please cite this article as: Prabhat Kashyap, et al., Atmospheric Pollution Research, https://doi.org/10.1016/j.apr.2019.07.004
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monitoring isoprene, benzene and toluene, in the ambient air of Delhi during three different seasons (i.e. post-monsoon, winter and summer) in 2017–18. Sampling of ambient air at all the six locations was conducted during the period from October to November 2017 for the post monsoon, December 2017 to February 2018 for the winter and April to June 2018 for the summer seasons respectively. These seasons were divided on the basis of climatic condition and the analysis of air trajectories reported by many researchers (Kumar and Yadav, 2016; Jain et al., 2005). The sampling locations were divided into two groups based on the abundance of vegetation and traffic density at the site. The first group named as ‘vegetative sites’ consists of three locations namely; Jawaharlal Nehru University (JNU), Yamuna Biodiversity Park (YBP) and Swarn Jayanti Park (SJP). The other group categorized as ‘traffic sites’ also includes three locations viz. Punjabi Bagh (PJB) traffic intersection, Central Road Research Institute (CRI) traffic intersection and Pitampura (PTP) traffic intersection. From each site, nine samples were collected in each season at three different times during the day i.e. morning (8:00 h to 11:00 h), afternoon (12:00 h to 15:00 h) and evening (17:00 h to 20:00 h) on three different days in each season.
The most dominant source of atmospheric isoprene is terrestrial vegetation (greater than 90%) and almost half of it is released from broadleaf trees and a noteworthy portion from various shrubs species (Guenther et al., 2006). The biogenic isoprene emissions are mainly driven by photosynthesis and dependent on a number of environmental factors especially temperature and solar radiation (Reimann et al., 2000). The variability in the isoprene emissions is also dependent on the plant species, geographical location and environmental conditions. The magnitude of isoprene emissions is observed to be higher in the tropical and sub-tropical zones because of high temperature and light flux (Chang et al., 2014). In spite of the predominance of biogenic sources, a significant portion of isoprene is also released from motor vehicles (Borbon et al., 2001; McLaren et al., 1996; Reimann et al., 2000). Since motor vehicle activities are mainly concentrated in urban areas, contribution of anthropogenic sources of isoprene in the urban atmosphere is expected to be significant. For this reason, a number of studies have focused their attention on the estimation of anthropogenic and biogenic fractions of isoprene in the ambient atmosphere in different cities across the world (Borbon et al., 2001; Chang et al., 2014; Reimann et al., 2000; Wagner and Kuttler, 2014; Wang et al., 2013). In the Indian context, some of the early studies (Padhy and Varshney, 2005; Varshney and Singh, 2003) were focused at the estimation of isoprene emissions from biogenic sources through chamber based experiments. Other noteworthy studies on isoprene were conducted for the cities of Mohali (Sinha et al., 2014) and Ahmedabad (Sahu and Saxena, 2015). Despite the few studies mentioned above, examining the anthropogenic contribution to ambient isoprene levels has remained largely ignored in the Indian region. Delhi, being the capital city of India, has witnessed unprecedented growth in the number of vehicles in the last few decades, with its population of registered number vehicles increasing from 1.8 million in 1990-91 to 10.99 million in 2017–18 (Economic Survey of Delhi, 199900, 2018-19). Since substantially high levels of O3 have been reported in the ambient atmosphere of Delhi (Jain et al., 2005; Prakash et al., 2011), it becomes pertinent to evaluate the anthropogenic/biogenic isoprene contributions to ozone formation in Delhi region. However, despite the significant role of isoprene in urban atmospheric chemistry, no such study can be cited in scientific literature in this context. So, the present study examines the seasonal and intraday variations in isoprene, benzene and toluene at different sites in Delhi with a view to estimate the anthropogenic and biogenic fractions of isoprene and their ozone formation potential.
2.2. Sampling method and sample analysis For the analysis of isoprene and other VOCs, air samples were collected with the help of a portable VOC sampler at a flow rate of 100 ml/ min for 3 h, using preconditioned ‘FLM carbopack X’ deactivated stainless steel thermal desorption (TD) tubes (purchased from Supelco Inc., USA). After sampling, the tubes were sealed using the brass endcaps to prevent any contamination. The tubes were then properly labelled and wrapped in the aluminium foil and immediately stored at a temperature < 4 °C. After that, TD tubes were transferred to the laboratory for further analysis within 24 h of sampling. All collected samples were analyzed using Gas Chromatograph-Mass Spectrometry (GC-MS) (Shimadzu QP-2010 Plus). During this process, samples were thermally desorbed from the tubes, for 5 min at a temperature of 250 °C with the help of thermal desorption unit (TD-20, Shimadzu). A constant supply of high purity helium gas (used as a carrier gas) at a flow rate of 60 ml/min was then used for back-flushing the analytes into a cold trap for 5 min at −20 °C. Analytes contained in helium gas were then injected in Rtx 5 MS capillary column (thickness 0.25 μm, internal diameter 0.25 mm and length 30 m) coupled with GC. GC oven was initially programmed at 40 °C and was kept for 5 min at this temperature and then, its temperature was gradually increased at the rate of 15 °C/min to attain a temperature of 200 °C, which was stabilised for 5 min. GC column was directly attached with the MS to perform qualitative analysis of analytes present in the carrier gas using mass/charge ratio of the compounds. The quantification of targeted compounds was done by comparing peak area with standard calibration curve. To prevent any error related to wrong peak detection, every chromatogram was checked individually. The standard calibration curve for isoprene was prepared using the standard of pure isoprene (> 99.5%). For benzene and toluene, a mixture of BTEX (benzene, toluene, ethylbenzene, and xylene) compounds purchased from Supelco Inc., USA (BTEX mix, 1000 μg/ml each in methanol) was used as standard. The calibration curves for each compound were constructed according to the 325B method of US Environmental Protection Agency (USEPA) guidelines. From the original liquid standards, multiple standards of different concentrations (0 ppm, 1 ppm, 100 ppm, 500 ppm, 1000 ppm) were prepared and spiked exactly 1 μl of aliquot from each liquid standard to the separate TD tubes. Further, the mass of each compound was theoretically calculated in μg for each standard tube. The calibration curves were prepared between mass and area under curve and a good linear fit was observed with R2 > 0.99 for each compound. The concentrations (Cm) in μg/m3 of targeted compounds were calculated with the following equation:
2. Material and methods 2.1. Site description The present study was conducted in the capital city of India, Delhi, situated at 28.38°N latitude and 77.17°E longitude on the bank of river Yamuna at an elevation of 216 m above sea level with a total area of ~1482 km2 (Pandey et al., 2012). The city accommodates more than 16.7 million people (Census of India, 2011). The number of registered vehicles in the city is now more than 10 million (Economic Survey of Delhi, 2018-19). The rapid growth of vehicles has developed a rising concern of increase in anthropogenic pollutants in the city. Despite being an urban area, National Capital Territory (NTC) Delhi region is having a total of 192.41 km2 of forest with a tree cover of around 12.97% of the total geographical area of Delhi (India State of Forest Report, 2017). In Delhi, many species of various genera are isopreneemitting species like Ficus religiosa, Ficus virens, Dalbergia sisoo, Eucalyptus globules, Mangifera indica, Butea monosperma etc. (Padhy and Varshney, 2005). Wide diversity of isoprene-emitting species and heavy traffic throughout the year in Delhi makes it a suitable location to assess the biogenic as well as anthropogenic isoprene emission in the city to understand its atmospheric chemistry in a better way. In this study, six different locations (Fig. 1) were selected for 2
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Fig. 1. Map showing the sampling locations in Delhi. Green and red dots represent the vegetative and traffic sites, respectively.
Cm =
Mc X 106 Ts X Fr
(1)
where, Mc is the mass of targeted compound computed from the calibration curve in μg, Ts is the total time of sampling in minutes and Fr is the flow rate of the sampler in ml/min. After analysis, each tube was again conditioned at a temperature of 200 °C to clean them for further reuse. Before sampling, a breakthrough test was also performed in order to know the safe sampling volume for the TD tubes. For this test, two identical TD tubes were placed in series and sampling was conducted. Both these tubes were analyzed in a similar way as other sampled tubes and no breakthrough was observed for the selected compounds. Field blanks were also analyzed where no targeted compounds were observed. Limit of detection (LOD) for each compound was assessed using the replicate analysis of lowest standard run. The LOD for each compound was found to be 0.2 μg/m3. For the reliability of results, analytical precision was also calculated from the relative difference between the absolute values of the two identical samples taken together in the same the environmental conditions with the same flow rate and sampling duration. The precision results gave the variation within 10% and it was also found that the high concentration values have more precision as compared to low concentration values.
Fig. 2. Boxplot representing the isoprene concentration at different sites. The solid dot inside the box denotes the mean concentration while, the box shows the inter-quartile range. The bottom and top of the box indicate the 25th and the 75th percentile. The upper end of whisker represents the maximum value and its lower end corresponds to the minimum value.
to lie in the range 0.14–1.13 μg/m3, 0.30–1.07 μg/m3 and 0.20–0.94 μg/m3 at PJB, CRI, and PTP, respectively. Among vegetative sites, the YBP site was dominated by trees like Ficus religiosa and Holoptelea integrifolia which are reported to be isoprene emitting plant species (Padhy and Varshney, 2005). The JNU site is dominated with trees species like Ficus religiosa, Ficus benghalensis, Ficus virens and Syzygium cumini while the SJP site is abundant with species like Ficus virens and Eucalyptus globulus with a few species of Dalbergia sissoo which are also known to be isoprene emitting (Varshney and Singh, 2003). In case of traffic sites, the most plausible source of isoprene emission was vehicular exhaust (Borbon et al., 2001; Reimann et al., 2000). The mean concentrations of benzene at different sites are represented in Fig. 3. It shows that mean benzene levels and their variability at the traffic sites were significantly higher as compared to those over vegetative sites. The mean concentrations of benzene were found to be the highest at CRI (28.17 μg/m3), followed by PJB (25.09 μg/m3) and PTP (16.80 μg/m3). Among the vegetative sites, the
3. Results and discussions 3.1. Variability of isoprene, benzene and toluene among different sites The measurements of isoprene along with benzene and toluene were made at six locations in Delhi during three seasons. The mean concentration and the overall variability of isoprene at these sites are shown in Fig. 2. It is seen that isoprene levels were significantly higher at the vegetative sites as compared to the traffic sites. The mean concentration of isoprene at vegetative sites were found to be 6.04, 4.53 and 2.65 μg/m3 at YBP, JNU and SJP, respectively while, its values at traffic sites were found to be 0.57, 0.67 and 0.46 μg/m3 for PJB, CRI and PTP, respectively. Further, the figure depicts that the isoprene variation is greater at all vegetative sites as compared to traffic sites. The isoprene concentration at YBP, JNU and SJP sites were found to be in the range of 0.64–12.92 μg/m3, 1.55–13.82 μg/m3 and 0.53–9.57 μg/ m3, respectively. However, its concentration at traffic sites was noticed 3
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mean concentrations of benzene were in the order JNU (9.71 μg/ m3) > SJP (7.65 μg/m3) > YBP (6.68 μg/m3). Similarly, mean concentrations of toluene at traffic sites were also observed to be higher than that over the vegetative sites (Fig. 4). Among the traffic sites, the mean concentrations of toluene were observed as 68.08, 60.64 and 53.87 μg/m3 at CRI, PJB and PTP, respectively. In comparison, the values of mean concentrations of toluene at vegetative sites were found to be 26.33, 26.20 and 21.65 μg/m3 at JNU, SJP and YBP, respectively. Table 1 compares the observed levels of isoprene, benzene and toluene in the present study with the values of the previous studies across the world. The levels of these hydrocarbons measured were found to be comparable to those reported by various studies. In particular, the isoprene concentrations observed in the present study were found to be lower than those in other Indian cities i.e. Mohali (Sinha et al., 2014) and Ahmedabad (Sahu and Saxena, 2015). It is noteworthy that the main source of benzene and toluene in the urban ambient atmosphere is the traffic exhaust (Han and Naeher, 2006). In addition to the traffic source, toluene is also emitted from various solvents and household products which include paints, varnishes, adhesives and other finishing building materials (ATSDR, 2000). Therefore, ratio of toluene to benzene (T/B) has been widely accepted as an indicator of traffic emissions. When the values of T/B ratio lie in the range of 1.5–4.3, it suggests the predominance of traffic sources while values approaching 10 or higher indicate the existence of strong industrial sources nearby (Kumar et al., 2017; Niu et al., 2012). Some studies have also accepted the T/B values lower than 3 as characteristic of traffic emissions (Alghamdi et al., 2014; Brocco et al., 1997; Hoque et al., 2008). In the present study, the mean T/B ratios were found to be 3.6, 3.8, 3.2, 2.8, 2.7 and 3.5 for YBP, SJP, JNU, PJB, CRI and PTP, respectively during the overall study period. These values were comparable to those found in different urban areas across the world viz. Paris, France (2.9–3.4), Provinces of China (3.85), Mortsel, Belgium (3.8–4.4) and Hungary (3.6) (Buczynska et al., 2009; Keymeulen et al., 2001; Tong et al., 2013; Vardoulakis et al., 2002).
Fig. 3. Boxplot representing the benzene concentration at different sites. The solid dot inside the box denotes the mean concentration while, the box shows the inter-quartile range. The bottom and top of the box indicate the 25th and the 75th percentile. The upper end of whisker represents the maximum value and its lower end corresponds to the minimum value.
3.2. Seasonal and intra-day variability of isoprene, benzene, and toluene 3.2.1. Seasonal variations In the present section, seasonal variations of isoprene, benzene and toluene at vegetative and traffic sites have been discussed. Seasonal average concentrations of isoprene, benzene and toluene at vegetative and traffic sites (Fig. 5a and b) were computed by averaging their seasonal concentrations across the three sites belonging to each category. Fig. 5a shows that isoprene concentrations at vegetative sites were
Fig. 4. Boxplot representing the toluene concentration at different sites. The solid dot inside the box denotes the mean concentration while, the box shows the inter-quartile range. The bottom and top of the box indicate the 25th and the 75th percentile. The upper end of whisker represents the maximum value and its lower end corresponds to the minimum value.
Table 1 Comparisons of estimated VOCs concentrations (μg/m3) in present study with the various other studies. City
Isoprene
Toluene
Benzene
References
Delhi (India) Mohali (India) Ahmedabad (India) Copenhagen (Denmark) Athens (Greece) Central/Western site (Hong Kong) Taipei (Taiwan) Beirut (Lebanon) Rio de Janeiro (Brazil) Bogota 3 sites (Colombia) Patras (Greece) Paris city (France)
2.44 5.29 4.46 0.56 0.28 0.49 1.31 0.19 1.39 0.98 2.81 0.25
42.79 10.17 27.51 38.43 3.01 10.42 7.91 11.34 17.74 – 1.06 1.05
15.68 5.53 8.31 10.86 7.03 1.33 1.37 1.86 12.42 1.71 0.38 3.29
Present studya Sinha et al., (2014) b Sahu and Saxena (2015) c Christensen and Palmgren (1999) Panopoulou et al., (2018) c Guo et al., (2007) d Wang et al., (2013) e Salameh et al., (2015) f Martins et al., (2015) d Franco et al., 2015 d Kaltsonoudis et al., 2016 b Baudic et al., (2016) c
a b c d e f
average conc. of summer, winter, post-monsoon. average conc. of summer. average conc. of winter. annual average concentration. average conc. of summer & autumn seasons. average conc. of summer & winter seasons. 4
c
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Fig. 5. Seasonal variation of isoprene, benzene and toluene at (a) vegetative sites (b) traffic sites.
the highest during the summer (6.51 μg/m3), followed by post-monsoon (4.80 μg/m3) and winter (1.82 μg/m3). The seasonal variation observed in isoprene concentrations at vegetative sites is possibly governed by variations in biogenic emissions caused by seasonal variations in solar intensity and temperature (Reimann et al., 2000). However, the mean concentrations of benzene & toluene were observed to be the highest during the winter (9.72 & 25.91 μg/m3) followed by summer (7.81 & 25.31 μg/m3) and post-monsoon (6.47 & 22.38 μg/m3). The higher concentrations of benzene and toluene during winter could be due to stable atmospheric conditions. Relatively lower concentrations of these compounds during summer/post-monsoon might be attributed to unstable conditions. In contrast to the vegetative sites, no significant seasonal variation in isoprene concentrations was observed at traffic sites as the isoprene concentrations remained low at these sites during all the seasons (Fig. 5b). However, benzene concentrations at traffic sites showed seasonal variation similar to that over the vegetative sites, with highest concentrations during the winter (25.76 μg/m3), followed by summer (23.65 μg/m3) and post-monsoon (19.39 μg/m3). In case of toluene, a different trend was observed with its concentrations being highest during the post-monsoon (66.69 μg/m3), followed by winter (60.49 μg/ m3) and summer (55.24 μg/m3). Further, a comparison of Fig. 5a and b reveals that the concentrations of benzene and toluene at traffic sites were significantly higher than those at vegetative sites in all seasons.
In contrast to the vegetative sites, no significant intra-day variation in isoprene concentrations was observed at traffic sites as the isoprene concentrations remained low at these sites during the entire day (Fig. 6b). However, benzene & toluene concentrations at traffic sites showed intra-day variation similar to that over the vegetative sites, with highest concentrations (30.06 & 75.70 μg/m3) during the evening and lowest concentrations (17.28 & 48.47 μg/m3) during the afternoon. This intra-day variation in benzene and toluene is possibly due to the higher vehicular emissions of these compounds during the morning and evening peak traffic hours, as well as due to the diurnal variations in the boundary layer mixing. 3.3. Anthropogenic and biogenic fractions of isoprene In urban settings, isoprene in the ambient atmosphere may be due to contributions from both anthropogenic and biogenic sources (Borbon et al., 2001). In the study on biogenic isoprene emissions in Taipei, Wang et al. (2013) suggest that comparison of the ratios of ambient isoprene/tracers with the ratios of isoprene/tracers that are characteristic of vehicle exhaust could be used for the estimation of anthropogenic contribution to ambient isoprene levels. For this purpose, they suggest that characteristic ratios of isoprene/exhaust tracer could be obtained either (i) by using the ratios from traffic dominated site or (ii) by using the minimum ratio observed among urban sites. In the present study, we have used the minimum isoprene/benzene ratio at traffic sites for the estimation of anthropogenic and biogenic contribution to ambient isoprene. Here, benzene is used as a traffic tracer as burning of fossil fuels in motor vehicles is considered to be its most dominating source and only 5–10% of its emissions in outdoor air, are from nonmobile sources (Bolden et al., 2015). The anthropogenic contribution to ambient isoprene was estimated using the following equation:
3.2.2. Intra-day variations Intra-day (morning, afternoon and evening) average concentrations of isoprene, benzene and toluene at vegetative and traffic sites (Fig. 6a and b) were computed by averaging the respective intra-day values across the three sites belonging to each category. Fig. 6a shows that isoprene concentrations at vegetative sites were significantly higher in the afternoon (6.46 μg/m3) as compared to morning/evening (3.49/ 3.17 μg/m3). This kind of intra-day variation at vegetative sites presumably occurs because biogenic isoprene emissions are primarily governed by solar irradiance and ambient temperature (Guenther et al., 1993; Niinemets et al., 2010; Sharkey and Singsaas, 1995). In contrast, the average concentrations of benzene and toluene were the highest (10.18 & 30.36 μg/m3) during evening and lowest (6.44 & 21.27 μg/m3) during the afternoon.
I Ianthro = ⎛ ⎞ ×B ⎝ B ⎠min _ traffic where, Ianthro is the anthropogenic isoprene,
(2)
()
I B min _ traffic
represents the
minimum isoprene/benzene ratio among the traffic sites and B is the benzene concentration at any given site. Subtracting Ianthro from the total ambient isoprene at a given site gave the biogenic isoprene (Ibio) . Table 2 shows the anthropogenic and biogenic isoprene with their 5
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Fig. 6. Intra-day variation of isoprene, benzene and toluene at (a) vegetative sites (b) traffic sites.
percentage contributions at different sites. It is seen that Ianthro and Ibio averaged over all the seasons, were observed to lie in the range of 0.11–0.15 μg/m3 and 2.53–5.94 μg/m3, respectively at the vegetative sites. In comparison, the Ianthro and Ibio at traffic sites lied in the range 0.26–0.45 μg/m3 and 0.18–0.21 μg/m3, respectively. Overall, the contributions of Ianthro and Ibio to total ambient isoprene were found to be 5.83% and 94.16% at vegetative sites and 66.12% and 33.88% at traffic sites, respectively. 3.4. Ozone formation potential (OFP) As described earlier, VOCs have a key role in the formation of secondary air pollutants namely; ozone, peroxyacyl nitrate, photochemical smog and organic aerosol (Alghamdi et al., 2014; Kumar et al., 2018; Murphy et al., 2010). In the current section, we have examined the ozone formation potential (OFP) of the studied VOCs (isoprene, benzene and toluene) at the vegetative and traffic sites. OFP of the individual VOC (i) using maximum incremental reactivity (MIR) proposed by Carter (1994) is calculated by following equation:
OFP (i ) = Concentration (i) × MIR coefficient (i)
Fig. 7. Ozone formation potential of isoprene (biogenic & anthropogenic), benzene and toluene at different sites.
Fig. 7 illustrates the estimated OFP (μg/m3) of isoprene, benzene and toluene at vegetative and traffic sites. It is clearly noticed that OFP of the isoprene was significantly higher at vegetative sites as compared to traffic sites. On the other hand, OFP of benzene and toluene exhibited higher values at traffic sites in contrast to vegetative sites. OFP of isoprene among the vegetative sites were observed to be the highest for YBP (64.1 μg/m3), followed by JNU (45.5 μg/m3) and SJP (28.1 μg/
(3)
where OFP (i) describes the ozone formation potential of individual VOC (i) and MIR coefficient (i) (dimensionless, gram of O3 per gram of VOC) infers maximum incremental reactivity of compound i. The MIR coefficients used for OFP calculation were obtained from Carter (2009). MIR is also known as good indicator for comparing the OFP of individual VOC.
Table 2 Anthropogenic and biogenic fraction estimation of isoprene (μg/m3) at the sampling sites.
Vegetative
Traffic
a
Sites
Average Estimated Anthropogenic Isoprene
YBP JNU SJP PTP CRI PJB
0.107 0.155 0.122 0.256 0.454 0.394
(3.53%)a (5.52%) (8.45%) (60.97%) (67.49%) (69.90%)
Average Estimated Biogenic Isoprene
Average Total Estimated Anthropogenic Isoprene
Average Total Estimated Biogenic Isoprene
5.937 4.136 2.527 0.185 0.215 0.184
0.128 (5.83%)
4.200 (94.16%)
0.368 (66.12%)
0.195 (33.88%)
(96.46%) (94.48%) (91.55%) (39.03%) (32.51%) (30.10%)
Values in the parenthesis represent the percentage contribution. 6
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m3). However, OFP of isoprene exhibited lower values at traffic sites in the order of CRI (7.1 μg/m3) > PJB (6.1 μg/m3) > PTP (4.7 μg/m3). Further, OFP values of benzene were found to be highest and lowest as 20.3 and 4.8 μg/m3 at CRI and YBP, respectively during the total studied period. Similarly, OFP of toluene exhibited the highest values at CRI (272.3 μg/m3) and lowest at YBP (86.6 μg/m3). Further, it is observed that toluene is the major contributor to OFP at traffic sites. In the context of vegetative sites, however, both toluene and isoprene contribute significantly towards ozone formation. But, it is found that contribution of anthropogenic isoprene towards ozone formation is minimal in Delhi. Economic Survey of Delhi (2018-19) reports that Delhi's vegetation cover increased to 20.6% in 2017. Thus, biogenic contribution to ambient isoprene levels in Delhi is expected to be significant and its role in the formation of surface ozone cannot be understated.
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4. Conclusions The present study reveals that mean isoprene concentrations in Delhi were higher at vegetative sites (2.65–6.04 μg/m3) as compared to traffic sites (0.46–0.67 μg/m3). On the contrary, benzene and toluene were found to have higher concentrations at traffic sites. Seasonal variations in isoprene concentrations at vegetative sites showed highest values during the summer, followed by post-monsoon and winter. The seasonal variation observed in isoprene concentrations at vegetative sites is plausibly governed by variations in solar intensity and temperature. In contrast, no significant seasonal variation in isoprene concentrations was observed at traffic sites as the concentrations remained low and almost similar during all the seasons. In the context of intra-day variation, isoprene concentrations attained maximum values during the afternoon period at vegetative sites. However, no significant intraday variation was observed in isoprene concentration at traffic sites. Estimation of anthropogenic and biogenic fractions of isoprene (Ianthro and Ibio), revealed that contributions of Ianthro and Ibio to total ambient isoprene were found to be 5.83% and 94.16% at vegetative sites and, 66.12% and 33.88% at traffic sites, respectively. Ozone formation potential of isoprene was significantly higher at vegetative sites as compared to traffic sites. But, the anthropogenic isoprene's contribution towards ozone formation is found to be minimal in the present study. Given the substantial vegetation cover of Delhi, however, biogenic contribution to ambient isoprene in Delhi's atmosphere is expected to be significant and its role in the formation of surface ozone cannot be understated.
Acknowledgment The authors would like to thank the support provided by the School of Environmental Sciences, Jawaharlal Nehru University, New Delhi in the form of necessary facilities required for the study. We are also thankful to the Department of Science and Technology, Government of India for providing the Promotion of University Research and Scientific Excellence Grant (DST-PURSE grant) for providing the required consumables for this study. One of the author Prabhat Kashyap is also thankful to theUniversity Grants Commission (UGC), India for providing the financial support in the form of scholarship for the research work. We are also thankful to Advanced Instrumentation Research Facility (AIRF) of Jawaharlal Nehru University for sample analysis and support of Dr. Ajai Kumar is gratefully acknowledged.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apr.2019.07.004. 7
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Kansal, A., 2009. Sources and reactivity of NMHCs and VOCs in the atmosphere: a review. J. Hazard Mater. 166, 17–26. Keymeulen, R., Görgényi, M., Héberger, K., Priksane, A., Van Langenhove, H., 2001. Benzene, toluene, ethyl benzene and xylenes in ambient air and Pinussylvestris L. needles: a comparative study between Belgium, Hungary and Latvia. Atmos. Environ. 35, 6327–6335. Kumar, A., Singh, D., Anandam, K., Kumar, K., Jain, V.K., 2017. Dynamic interaction of trace gases (VOCs, ozone, and NOx) in the rural atmosphere of sub-tropical India. Air Qual. Atmos. Health 10, 885–896. Kumar, A., Singh, D., Kumar, K., Singh, B.B., Jain, V.K., 2018. Distribution of VOCs in urban and rural atmospheres of subtropical India: temporal variation, source attribution, ratios, OFP and risk assessment. Sci. Total Environ. 613, 492–501. Kumar, P., Yadav, S., 2016. Seasonal variations in water soluble inorganic ions, OC and EC in PM10 and PM > 10 aerosols over Delhi: influence of sources and meteorological factors. Aerosol Air Qual. Res 16, 1165–1178. Kumar, R.P., Kashyap, P., Kumar, R., Pandey, A.K., Kumar, A., Kumar, K., 2019. Cancer and non-cancer health risk assessment associated with exposure to non-methane hydrocarbons among roadside vendors in Delhi, India. Hum. Ecol. Risk Assess. 1–15. Martins, E.M., Nunes, A.C., Corrêa, S., 2015. Understanding ozone concentrations during weekdays and weekends in the urban area of the city of Rio de Janeiro. J. Braz. Chem. Soc. 26, 1967–1975. McLaren, R., Singleton, D.L., Lai, J.Y.K., Khouw, B., Singer, E., Wu, Z., Niki, H., 1996. Analysis of motor vehicle sources and their contribution to ambient hydrocarbon distributions at urban sites in Toronto during the Southern Ontario Oxidants Study. Atmos. Environ. 30, 2219–2232. Monson, R.K., Trahan, N., Rosenstiel, T.N., Veres, P., Moore, D., Wilkinson, M., Norby, R.J., Volder, A., Tjoelker, M.G., Briske, D.D., 2007. Isoprene emission from terrestrial ecosystems in response to global change: minding the gap between models and observations. Philos. Trans. A Math Phys. Eng. Sci. 365, 1677–1695. Murphy, J.G., Oram, D.E., Reeves, C.E., 2010. Measurements of volatile organic compounds over West Africa. Atmos. Chem. Phys. 10, 5281–5294. Niinemets, Ü., Arneth, A., Kuhn, U., Monson, R.K., Peñuelas, J., Staudt, M., 2010. The emission factor of volatile isoprenoids: stress, acclimation, and developmental responses. Biogeosciences 7, 2203–2223. Niu, Z., Zhang, H., Xu, Y., Liao, X., Xu, L., Chen, J., 2012. Pollution characteristics of volatile organic compounds in the atmosphere of haicang district in xiamen city, southeast China. J. Environ. Monit. 14, 1144–1151. Pacifico, F., Harrison, S.P., Jones, C.D., Sitch, S., 2009. Isoprene emissions and climate. Atmos. Environ. 43, 6121–6135. Padhy, P.K., Varshney, C.K., 2005. Emission of volatile organic compounds (VOC) from tropical plant species in India. Chemosphere 59, 1643–1653. Pandey, P., Kumar, D., Prakash, A., Masih, J., Singh, M., Kumar, S., Jain, V.K., Kumar, K., 2012. A study of urban heat island and its association with particulate matter during winter months over Delhi. Sci. Total Environ. 414, 494–507. Panopoulou, A., Liakakou, E., Gros, V., Sauvage, S., Locoge, N., Bonsang, B., Psiloglou, B.E., Gerasopoulos, E., Mihalopoulos, N., 2018. Non-methane hydrocarbon
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Atmospheric Pollution Research journal homepage: www.elsevier.com/locate/apr
Original Article
Biogenic and anthropogenic isoprene emissions in the subtropical urban atmosphere of Delhi Prabhat Kashyapa, Amit Kumara, Ram Pravesh Kumarb, Krishan Kumara,* a b
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, 110067, India School of Earth, Environment & Space Studies, Centre University of Haryana, Haryana, 123031, India
A R T I C LE I N FO
A B S T R A C T
Keywords: BVOC Biogenic isoprene Anthropogenic isoprene Gas chromatography Ozone formation potential
This study examines the seasonal and intraday variations in isoprene, benzene and toluene at different sites in Delhi with a view to estimate the anthropogenic and biogenic fractions of isoprene and their ozone formation potential. For this purpose, measurements of ambient isoprene, benzene and toluene were made at selected vegetative and traffic sites of Delhi using Gas Chromatography-Mass Spectrometry. The study reveals that the average isoprene concentrations were higher at vegetative sites (4.40 ± 3.35 μg/m3) as compared to traffic sites (0.56 ± 0.22 μg/m3). The benzene and toluene levels, on the other hand, were found to be higher at the traffic sites (23.22 ± 10.94 and 60.58 ± 20.27 μg/m3) as compared to vegetative sites (8.01 ± 4.56 and 24.75 ± 10.86 μg/m3) respectively. The isoprene concentrations at vegetative sites were found to be the highest in summer (6.51 ± 3.56 μg/m3) followed by post-monsoon (4.80 ± 3.18 μg/m3) and winter (1.82 ± 0.57 μg/ m3). In the context of intraday variation, isoprene concentrations attained maximum values during the afternoon period. However, no significant intraday/seasonal variation was observed in isoprene concentration at traffic sites. Estimation of anthropogenic and biogenic fractions of isoprene revealed that anthropogenic contribution varied between 60 and 70% at traffic sites and less than 10% at the vegetative sites. Upon computation of ozone formation potential of isoprene, benzene and toluene it is seen that isoprene's contribution towards ozone formation is higher at vegetative sites as compared to traffic sites. But, the anthropogenic isoprene's contribution towards ozone formation is found to be minimal in the present study. In view of the existing vegetation cover (~20%) of Delhi, however, the role of isoprene in surface ozone formation cannot be understated.
1. Introduction Volatile organic compounds (VOCs) constitute an important class of organic chemicals, which play a key role in atmospheric chemistry (Kumar et al., 2019; Tong et al., 2013). VOCs are primarily emitted from biological sources i.e. plants, and other living organisms found in terrestrial and aquatic environments (Guenther et al., 1995; Kansal, 2009; Sahu, 2012), collectively known as biogenic VOCs (BVOCs). Anthropogenic sources of VOCs include emissions from industries, motor vehicles, biomass burning and solvent operations (Kansal, 2009; Piccot et al., 1992). However, the contribution of BVOCs to the total global emissions of VOCs, is higher than that from anthropogenic activities (Helmig et al., 2009). Among various BVOCs, Isoprene (2-methyl-1,3-butadiene, C5H8) is a major hydrocarbon with estimated emissions of 535 Tg per year, which constitutes ~50% of the total BVOCs emissions of 1000 Tg per year (Guenther et al., 2012).
Isoprene is a highly reactive compound having atmospheric chemical lifetime of the order of minutes to hours (Achakulwisut et al., 2015; Pike and Young, 2009). Chemical reactions of isoprene with the OH, ozone, and nitrate radical largely influence the oxidative chemistry of the troposphere (Beerling et al., 2007). Isoprene and its oxidation products have significant role in modulating the tropospheric ozone and CH4 concentrations (Pacifico et al., 2009). Isoprene has a crucial role in O3 formation and destruction depending upon the concentrations of nitrogen oxides (NOx = NO+NO2) in the ambient atmosphere. At high NO concentrations, isoprene oxidation leads to formation of NO2, followed by photolysis of NO2 which results in O3 formation. On the other hand, isoprene also directly reacts with O3 at low NO levels, which ultimately reduces the tropospheric O3 (Monson et al., 2007; Sanderson et al., 2003). Besides this, secondary organic aerosols (SOA) are also formed by the oxidation products of the isoprene (Claeys et al., 2004).
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. * Corresponding author. E-mail address: [email protected] (K. Kumar). https://doi.org/10.1016/j.apr.2019.07.004 Received 11 March 2019; Received in revised form 24 June 2019; Accepted 10 July 2019 1309-1042/ © 2019 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V.
Please cite this article as: Prabhat Kashyap, et al., Atmospheric Pollution Research, https://doi.org/10.1016/j.apr.2019.07.004
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monitoring isoprene, benzene and toluene, in the ambient air of Delhi during three different seasons (i.e. post-monsoon, winter and summer) in 2017–18. Sampling of ambient air at all the six locations was conducted during the period from October to November 2017 for the post monsoon, December 2017 to February 2018 for the winter and April to June 2018 for the summer seasons respectively. These seasons were divided on the basis of climatic condition and the analysis of air trajectories reported by many researchers (Kumar and Yadav, 2016; Jain et al., 2005). The sampling locations were divided into two groups based on the abundance of vegetation and traffic density at the site. The first group named as ‘vegetative sites’ consists of three locations namely; Jawaharlal Nehru University (JNU), Yamuna Biodiversity Park (YBP) and Swarn Jayanti Park (SJP). The other group categorized as ‘traffic sites’ also includes three locations viz. Punjabi Bagh (PJB) traffic intersection, Central Road Research Institute (CRI) traffic intersection and Pitampura (PTP) traffic intersection. From each site, nine samples were collected in each season at three different times during the day i.e. morning (8:00 h to 11:00 h), afternoon (12:00 h to 15:00 h) and evening (17:00 h to 20:00 h) on three different days in each season.
The most dominant source of atmospheric isoprene is terrestrial vegetation (greater than 90%) and almost half of it is released from broadleaf trees and a noteworthy portion from various shrubs species (Guenther et al., 2006). The biogenic isoprene emissions are mainly driven by photosynthesis and dependent on a number of environmental factors especially temperature and solar radiation (Reimann et al., 2000). The variability in the isoprene emissions is also dependent on the plant species, geographical location and environmental conditions. The magnitude of isoprene emissions is observed to be higher in the tropical and sub-tropical zones because of high temperature and light flux (Chang et al., 2014). In spite of the predominance of biogenic sources, a significant portion of isoprene is also released from motor vehicles (Borbon et al., 2001; McLaren et al., 1996; Reimann et al., 2000). Since motor vehicle activities are mainly concentrated in urban areas, contribution of anthropogenic sources of isoprene in the urban atmosphere is expected to be significant. For this reason, a number of studies have focused their attention on the estimation of anthropogenic and biogenic fractions of isoprene in the ambient atmosphere in different cities across the world (Borbon et al., 2001; Chang et al., 2014; Reimann et al., 2000; Wagner and Kuttler, 2014; Wang et al., 2013). In the Indian context, some of the early studies (Padhy and Varshney, 2005; Varshney and Singh, 2003) were focused at the estimation of isoprene emissions from biogenic sources through chamber based experiments. Other noteworthy studies on isoprene were conducted for the cities of Mohali (Sinha et al., 2014) and Ahmedabad (Sahu and Saxena, 2015). Despite the few studies mentioned above, examining the anthropogenic contribution to ambient isoprene levels has remained largely ignored in the Indian region. Delhi, being the capital city of India, has witnessed unprecedented growth in the number of vehicles in the last few decades, with its population of registered number vehicles increasing from 1.8 million in 1990-91 to 10.99 million in 2017–18 (Economic Survey of Delhi, 199900, 2018-19). Since substantially high levels of O3 have been reported in the ambient atmosphere of Delhi (Jain et al., 2005; Prakash et al., 2011), it becomes pertinent to evaluate the anthropogenic/biogenic isoprene contributions to ozone formation in Delhi region. However, despite the significant role of isoprene in urban atmospheric chemistry, no such study can be cited in scientific literature in this context. So, the present study examines the seasonal and intraday variations in isoprene, benzene and toluene at different sites in Delhi with a view to estimate the anthropogenic and biogenic fractions of isoprene and their ozone formation potential.
2.2. Sampling method and sample analysis For the analysis of isoprene and other VOCs, air samples were collected with the help of a portable VOC sampler at a flow rate of 100 ml/ min for 3 h, using preconditioned ‘FLM carbopack X’ deactivated stainless steel thermal desorption (TD) tubes (purchased from Supelco Inc., USA). After sampling, the tubes were sealed using the brass endcaps to prevent any contamination. The tubes were then properly labelled and wrapped in the aluminium foil and immediately stored at a temperature < 4 °C. After that, TD tubes were transferred to the laboratory for further analysis within 24 h of sampling. All collected samples were analyzed using Gas Chromatograph-Mass Spectrometry (GC-MS) (Shimadzu QP-2010 Plus). During this process, samples were thermally desorbed from the tubes, for 5 min at a temperature of 250 °C with the help of thermal desorption unit (TD-20, Shimadzu). A constant supply of high purity helium gas (used as a carrier gas) at a flow rate of 60 ml/min was then used for back-flushing the analytes into a cold trap for 5 min at −20 °C. Analytes contained in helium gas were then injected in Rtx 5 MS capillary column (thickness 0.25 μm, internal diameter 0.25 mm and length 30 m) coupled with GC. GC oven was initially programmed at 40 °C and was kept for 5 min at this temperature and then, its temperature was gradually increased at the rate of 15 °C/min to attain a temperature of 200 °C, which was stabilised for 5 min. GC column was directly attached with the MS to perform qualitative analysis of analytes present in the carrier gas using mass/charge ratio of the compounds. The quantification of targeted compounds was done by comparing peak area with standard calibration curve. To prevent any error related to wrong peak detection, every chromatogram was checked individually. The standard calibration curve for isoprene was prepared using the standard of pure isoprene (> 99.5%). For benzene and toluene, a mixture of BTEX (benzene, toluene, ethylbenzene, and xylene) compounds purchased from Supelco Inc., USA (BTEX mix, 1000 μg/ml each in methanol) was used as standard. The calibration curves for each compound were constructed according to the 325B method of US Environmental Protection Agency (USEPA) guidelines. From the original liquid standards, multiple standards of different concentrations (0 ppm, 1 ppm, 100 ppm, 500 ppm, 1000 ppm) were prepared and spiked exactly 1 μl of aliquot from each liquid standard to the separate TD tubes. Further, the mass of each compound was theoretically calculated in μg for each standard tube. The calibration curves were prepared between mass and area under curve and a good linear fit was observed with R2 > 0.99 for each compound. The concentrations (Cm) in μg/m3 of targeted compounds were calculated with the following equation:
2. Material and methods 2.1. Site description The present study was conducted in the capital city of India, Delhi, situated at 28.38°N latitude and 77.17°E longitude on the bank of river Yamuna at an elevation of 216 m above sea level with a total area of ~1482 km2 (Pandey et al., 2012). The city accommodates more than 16.7 million people (Census of India, 2011). The number of registered vehicles in the city is now more than 10 million (Economic Survey of Delhi, 2018-19). The rapid growth of vehicles has developed a rising concern of increase in anthropogenic pollutants in the city. Despite being an urban area, National Capital Territory (NTC) Delhi region is having a total of 192.41 km2 of forest with a tree cover of around 12.97% of the total geographical area of Delhi (India State of Forest Report, 2017). In Delhi, many species of various genera are isopreneemitting species like Ficus religiosa, Ficus virens, Dalbergia sisoo, Eucalyptus globules, Mangifera indica, Butea monosperma etc. (Padhy and Varshney, 2005). Wide diversity of isoprene-emitting species and heavy traffic throughout the year in Delhi makes it a suitable location to assess the biogenic as well as anthropogenic isoprene emission in the city to understand its atmospheric chemistry in a better way. In this study, six different locations (Fig. 1) were selected for 2
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Fig. 1. Map showing the sampling locations in Delhi. Green and red dots represent the vegetative and traffic sites, respectively.
Cm =
Mc X 106 Ts X Fr
(1)
where, Mc is the mass of targeted compound computed from the calibration curve in μg, Ts is the total time of sampling in minutes and Fr is the flow rate of the sampler in ml/min. After analysis, each tube was again conditioned at a temperature of 200 °C to clean them for further reuse. Before sampling, a breakthrough test was also performed in order to know the safe sampling volume for the TD tubes. For this test, two identical TD tubes were placed in series and sampling was conducted. Both these tubes were analyzed in a similar way as other sampled tubes and no breakthrough was observed for the selected compounds. Field blanks were also analyzed where no targeted compounds were observed. Limit of detection (LOD) for each compound was assessed using the replicate analysis of lowest standard run. The LOD for each compound was found to be 0.2 μg/m3. For the reliability of results, analytical precision was also calculated from the relative difference between the absolute values of the two identical samples taken together in the same the environmental conditions with the same flow rate and sampling duration. The precision results gave the variation within 10% and it was also found that the high concentration values have more precision as compared to low concentration values.
Fig. 2. Boxplot representing the isoprene concentration at different sites. The solid dot inside the box denotes the mean concentration while, the box shows the inter-quartile range. The bottom and top of the box indicate the 25th and the 75th percentile. The upper end of whisker represents the maximum value and its lower end corresponds to the minimum value.
to lie in the range 0.14–1.13 μg/m3, 0.30–1.07 μg/m3 and 0.20–0.94 μg/m3 at PJB, CRI, and PTP, respectively. Among vegetative sites, the YBP site was dominated by trees like Ficus religiosa and Holoptelea integrifolia which are reported to be isoprene emitting plant species (Padhy and Varshney, 2005). The JNU site is dominated with trees species like Ficus religiosa, Ficus benghalensis, Ficus virens and Syzygium cumini while the SJP site is abundant with species like Ficus virens and Eucalyptus globulus with a few species of Dalbergia sissoo which are also known to be isoprene emitting (Varshney and Singh, 2003). In case of traffic sites, the most plausible source of isoprene emission was vehicular exhaust (Borbon et al., 2001; Reimann et al., 2000). The mean concentrations of benzene at different sites are represented in Fig. 3. It shows that mean benzene levels and their variability at the traffic sites were significantly higher as compared to those over vegetative sites. The mean concentrations of benzene were found to be the highest at CRI (28.17 μg/m3), followed by PJB (25.09 μg/m3) and PTP (16.80 μg/m3). Among the vegetative sites, the
3. Results and discussions 3.1. Variability of isoprene, benzene and toluene among different sites The measurements of isoprene along with benzene and toluene were made at six locations in Delhi during three seasons. The mean concentration and the overall variability of isoprene at these sites are shown in Fig. 2. It is seen that isoprene levels were significantly higher at the vegetative sites as compared to the traffic sites. The mean concentration of isoprene at vegetative sites were found to be 6.04, 4.53 and 2.65 μg/m3 at YBP, JNU and SJP, respectively while, its values at traffic sites were found to be 0.57, 0.67 and 0.46 μg/m3 for PJB, CRI and PTP, respectively. Further, the figure depicts that the isoprene variation is greater at all vegetative sites as compared to traffic sites. The isoprene concentration at YBP, JNU and SJP sites were found to be in the range of 0.64–12.92 μg/m3, 1.55–13.82 μg/m3 and 0.53–9.57 μg/ m3, respectively. However, its concentration at traffic sites was noticed 3
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mean concentrations of benzene were in the order JNU (9.71 μg/ m3) > SJP (7.65 μg/m3) > YBP (6.68 μg/m3). Similarly, mean concentrations of toluene at traffic sites were also observed to be higher than that over the vegetative sites (Fig. 4). Among the traffic sites, the mean concentrations of toluene were observed as 68.08, 60.64 and 53.87 μg/m3 at CRI, PJB and PTP, respectively. In comparison, the values of mean concentrations of toluene at vegetative sites were found to be 26.33, 26.20 and 21.65 μg/m3 at JNU, SJP and YBP, respectively. Table 1 compares the observed levels of isoprene, benzene and toluene in the present study with the values of the previous studies across the world. The levels of these hydrocarbons measured were found to be comparable to those reported by various studies. In particular, the isoprene concentrations observed in the present study were found to be lower than those in other Indian cities i.e. Mohali (Sinha et al., 2014) and Ahmedabad (Sahu and Saxena, 2015). It is noteworthy that the main source of benzene and toluene in the urban ambient atmosphere is the traffic exhaust (Han and Naeher, 2006). In addition to the traffic source, toluene is also emitted from various solvents and household products which include paints, varnishes, adhesives and other finishing building materials (ATSDR, 2000). Therefore, ratio of toluene to benzene (T/B) has been widely accepted as an indicator of traffic emissions. When the values of T/B ratio lie in the range of 1.5–4.3, it suggests the predominance of traffic sources while values approaching 10 or higher indicate the existence of strong industrial sources nearby (Kumar et al., 2017; Niu et al., 2012). Some studies have also accepted the T/B values lower than 3 as characteristic of traffic emissions (Alghamdi et al., 2014; Brocco et al., 1997; Hoque et al., 2008). In the present study, the mean T/B ratios were found to be 3.6, 3.8, 3.2, 2.8, 2.7 and 3.5 for YBP, SJP, JNU, PJB, CRI and PTP, respectively during the overall study period. These values were comparable to those found in different urban areas across the world viz. Paris, France (2.9–3.4), Provinces of China (3.85), Mortsel, Belgium (3.8–4.4) and Hungary (3.6) (Buczynska et al., 2009; Keymeulen et al., 2001; Tong et al., 2013; Vardoulakis et al., 2002).
Fig. 3. Boxplot representing the benzene concentration at different sites. The solid dot inside the box denotes the mean concentration while, the box shows the inter-quartile range. The bottom and top of the box indicate the 25th and the 75th percentile. The upper end of whisker represents the maximum value and its lower end corresponds to the minimum value.
3.2. Seasonal and intra-day variability of isoprene, benzene, and toluene 3.2.1. Seasonal variations In the present section, seasonal variations of isoprene, benzene and toluene at vegetative and traffic sites have been discussed. Seasonal average concentrations of isoprene, benzene and toluene at vegetative and traffic sites (Fig. 5a and b) were computed by averaging their seasonal concentrations across the three sites belonging to each category. Fig. 5a shows that isoprene concentrations at vegetative sites were
Fig. 4. Boxplot representing the toluene concentration at different sites. The solid dot inside the box denotes the mean concentration while, the box shows the inter-quartile range. The bottom and top of the box indicate the 25th and the 75th percentile. The upper end of whisker represents the maximum value and its lower end corresponds to the minimum value.
Table 1 Comparisons of estimated VOCs concentrations (μg/m3) in present study with the various other studies. City
Isoprene
Toluene
Benzene
References
Delhi (India) Mohali (India) Ahmedabad (India) Copenhagen (Denmark) Athens (Greece) Central/Western site (Hong Kong) Taipei (Taiwan) Beirut (Lebanon) Rio de Janeiro (Brazil) Bogota 3 sites (Colombia) Patras (Greece) Paris city (France)
2.44 5.29 4.46 0.56 0.28 0.49 1.31 0.19 1.39 0.98 2.81 0.25
42.79 10.17 27.51 38.43 3.01 10.42 7.91 11.34 17.74 – 1.06 1.05
15.68 5.53 8.31 10.86 7.03 1.33 1.37 1.86 12.42 1.71 0.38 3.29
Present studya Sinha et al., (2014) b Sahu and Saxena (2015) c Christensen and Palmgren (1999) Panopoulou et al., (2018) c Guo et al., (2007) d Wang et al., (2013) e Salameh et al., (2015) f Martins et al., (2015) d Franco et al., 2015 d Kaltsonoudis et al., 2016 b Baudic et al., (2016) c
a b c d e f
average conc. of summer, winter, post-monsoon. average conc. of summer. average conc. of winter. annual average concentration. average conc. of summer & autumn seasons. average conc. of summer & winter seasons. 4
c
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Fig. 5. Seasonal variation of isoprene, benzene and toluene at (a) vegetative sites (b) traffic sites.
the highest during the summer (6.51 μg/m3), followed by post-monsoon (4.80 μg/m3) and winter (1.82 μg/m3). The seasonal variation observed in isoprene concentrations at vegetative sites is possibly governed by variations in biogenic emissions caused by seasonal variations in solar intensity and temperature (Reimann et al., 2000). However, the mean concentrations of benzene & toluene were observed to be the highest during the winter (9.72 & 25.91 μg/m3) followed by summer (7.81 & 25.31 μg/m3) and post-monsoon (6.47 & 22.38 μg/m3). The higher concentrations of benzene and toluene during winter could be due to stable atmospheric conditions. Relatively lower concentrations of these compounds during summer/post-monsoon might be attributed to unstable conditions. In contrast to the vegetative sites, no significant seasonal variation in isoprene concentrations was observed at traffic sites as the isoprene concentrations remained low at these sites during all the seasons (Fig. 5b). However, benzene concentrations at traffic sites showed seasonal variation similar to that over the vegetative sites, with highest concentrations during the winter (25.76 μg/m3), followed by summer (23.65 μg/m3) and post-monsoon (19.39 μg/m3). In case of toluene, a different trend was observed with its concentrations being highest during the post-monsoon (66.69 μg/m3), followed by winter (60.49 μg/ m3) and summer (55.24 μg/m3). Further, a comparison of Fig. 5a and b reveals that the concentrations of benzene and toluene at traffic sites were significantly higher than those at vegetative sites in all seasons.
In contrast to the vegetative sites, no significant intra-day variation in isoprene concentrations was observed at traffic sites as the isoprene concentrations remained low at these sites during the entire day (Fig. 6b). However, benzene & toluene concentrations at traffic sites showed intra-day variation similar to that over the vegetative sites, with highest concentrations (30.06 & 75.70 μg/m3) during the evening and lowest concentrations (17.28 & 48.47 μg/m3) during the afternoon. This intra-day variation in benzene and toluene is possibly due to the higher vehicular emissions of these compounds during the morning and evening peak traffic hours, as well as due to the diurnal variations in the boundary layer mixing. 3.3. Anthropogenic and biogenic fractions of isoprene In urban settings, isoprene in the ambient atmosphere may be due to contributions from both anthropogenic and biogenic sources (Borbon et al., 2001). In the study on biogenic isoprene emissions in Taipei, Wang et al. (2013) suggest that comparison of the ratios of ambient isoprene/tracers with the ratios of isoprene/tracers that are characteristic of vehicle exhaust could be used for the estimation of anthropogenic contribution to ambient isoprene levels. For this purpose, they suggest that characteristic ratios of isoprene/exhaust tracer could be obtained either (i) by using the ratios from traffic dominated site or (ii) by using the minimum ratio observed among urban sites. In the present study, we have used the minimum isoprene/benzene ratio at traffic sites for the estimation of anthropogenic and biogenic contribution to ambient isoprene. Here, benzene is used as a traffic tracer as burning of fossil fuels in motor vehicles is considered to be its most dominating source and only 5–10% of its emissions in outdoor air, are from nonmobile sources (Bolden et al., 2015). The anthropogenic contribution to ambient isoprene was estimated using the following equation:
3.2.2. Intra-day variations Intra-day (morning, afternoon and evening) average concentrations of isoprene, benzene and toluene at vegetative and traffic sites (Fig. 6a and b) were computed by averaging the respective intra-day values across the three sites belonging to each category. Fig. 6a shows that isoprene concentrations at vegetative sites were significantly higher in the afternoon (6.46 μg/m3) as compared to morning/evening (3.49/ 3.17 μg/m3). This kind of intra-day variation at vegetative sites presumably occurs because biogenic isoprene emissions are primarily governed by solar irradiance and ambient temperature (Guenther et al., 1993; Niinemets et al., 2010; Sharkey and Singsaas, 1995). In contrast, the average concentrations of benzene and toluene were the highest (10.18 & 30.36 μg/m3) during evening and lowest (6.44 & 21.27 μg/m3) during the afternoon.
I Ianthro = ⎛ ⎞ ×B ⎝ B ⎠min _ traffic where, Ianthro is the anthropogenic isoprene,
(2)
()
I B min _ traffic
represents the
minimum isoprene/benzene ratio among the traffic sites and B is the benzene concentration at any given site. Subtracting Ianthro from the total ambient isoprene at a given site gave the biogenic isoprene (Ibio) . Table 2 shows the anthropogenic and biogenic isoprene with their 5
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Fig. 6. Intra-day variation of isoprene, benzene and toluene at (a) vegetative sites (b) traffic sites.
percentage contributions at different sites. It is seen that Ianthro and Ibio averaged over all the seasons, were observed to lie in the range of 0.11–0.15 μg/m3 and 2.53–5.94 μg/m3, respectively at the vegetative sites. In comparison, the Ianthro and Ibio at traffic sites lied in the range 0.26–0.45 μg/m3 and 0.18–0.21 μg/m3, respectively. Overall, the contributions of Ianthro and Ibio to total ambient isoprene were found to be 5.83% and 94.16% at vegetative sites and 66.12% and 33.88% at traffic sites, respectively. 3.4. Ozone formation potential (OFP) As described earlier, VOCs have a key role in the formation of secondary air pollutants namely; ozone, peroxyacyl nitrate, photochemical smog and organic aerosol (Alghamdi et al., 2014; Kumar et al., 2018; Murphy et al., 2010). In the current section, we have examined the ozone formation potential (OFP) of the studied VOCs (isoprene, benzene and toluene) at the vegetative and traffic sites. OFP of the individual VOC (i) using maximum incremental reactivity (MIR) proposed by Carter (1994) is calculated by following equation:
OFP (i ) = Concentration (i) × MIR coefficient (i)
Fig. 7. Ozone formation potential of isoprene (biogenic & anthropogenic), benzene and toluene at different sites.
Fig. 7 illustrates the estimated OFP (μg/m3) of isoprene, benzene and toluene at vegetative and traffic sites. It is clearly noticed that OFP of the isoprene was significantly higher at vegetative sites as compared to traffic sites. On the other hand, OFP of benzene and toluene exhibited higher values at traffic sites in contrast to vegetative sites. OFP of isoprene among the vegetative sites were observed to be the highest for YBP (64.1 μg/m3), followed by JNU (45.5 μg/m3) and SJP (28.1 μg/
(3)
where OFP (i) describes the ozone formation potential of individual VOC (i) and MIR coefficient (i) (dimensionless, gram of O3 per gram of VOC) infers maximum incremental reactivity of compound i. The MIR coefficients used for OFP calculation were obtained from Carter (2009). MIR is also known as good indicator for comparing the OFP of individual VOC.
Table 2 Anthropogenic and biogenic fraction estimation of isoprene (μg/m3) at the sampling sites.
Vegetative
Traffic
a
Sites
Average Estimated Anthropogenic Isoprene
YBP JNU SJP PTP CRI PJB
0.107 0.155 0.122 0.256 0.454 0.394
(3.53%)a (5.52%) (8.45%) (60.97%) (67.49%) (69.90%)
Average Estimated Biogenic Isoprene
Average Total Estimated Anthropogenic Isoprene
Average Total Estimated Biogenic Isoprene
5.937 4.136 2.527 0.185 0.215 0.184
0.128 (5.83%)
4.200 (94.16%)
0.368 (66.12%)
0.195 (33.88%)
(96.46%) (94.48%) (91.55%) (39.03%) (32.51%) (30.10%)
Values in the parenthesis represent the percentage contribution. 6
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m3). However, OFP of isoprene exhibited lower values at traffic sites in the order of CRI (7.1 μg/m3) > PJB (6.1 μg/m3) > PTP (4.7 μg/m3). Further, OFP values of benzene were found to be highest and lowest as 20.3 and 4.8 μg/m3 at CRI and YBP, respectively during the total studied period. Similarly, OFP of toluene exhibited the highest values at CRI (272.3 μg/m3) and lowest at YBP (86.6 μg/m3). Further, it is observed that toluene is the major contributor to OFP at traffic sites. In the context of vegetative sites, however, both toluene and isoprene contribute significantly towards ozone formation. But, it is found that contribution of anthropogenic isoprene towards ozone formation is minimal in Delhi. Economic Survey of Delhi (2018-19) reports that Delhi's vegetation cover increased to 20.6% in 2017. Thus, biogenic contribution to ambient isoprene levels in Delhi is expected to be significant and its role in the formation of surface ozone cannot be understated.
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4. Conclusions The present study reveals that mean isoprene concentrations in Delhi were higher at vegetative sites (2.65–6.04 μg/m3) as compared to traffic sites (0.46–0.67 μg/m3). On the contrary, benzene and toluene were found to have higher concentrations at traffic sites. Seasonal variations in isoprene concentrations at vegetative sites showed highest values during the summer, followed by post-monsoon and winter. The seasonal variation observed in isoprene concentrations at vegetative sites is plausibly governed by variations in solar intensity and temperature. In contrast, no significant seasonal variation in isoprene concentrations was observed at traffic sites as the concentrations remained low and almost similar during all the seasons. In the context of intra-day variation, isoprene concentrations attained maximum values during the afternoon period at vegetative sites. However, no significant intraday variation was observed in isoprene concentration at traffic sites. Estimation of anthropogenic and biogenic fractions of isoprene (Ianthro and Ibio), revealed that contributions of Ianthro and Ibio to total ambient isoprene were found to be 5.83% and 94.16% at vegetative sites and, 66.12% and 33.88% at traffic sites, respectively. Ozone formation potential of isoprene was significantly higher at vegetative sites as compared to traffic sites. But, the anthropogenic isoprene's contribution towards ozone formation is found to be minimal in the present study. Given the substantial vegetation cover of Delhi, however, biogenic contribution to ambient isoprene in Delhi's atmosphere is expected to be significant and its role in the formation of surface ozone cannot be understated.
Acknowledgment The authors would like to thank the support provided by the School of Environmental Sciences, Jawaharlal Nehru University, New Delhi in the form of necessary facilities required for the study. We are also thankful to the Department of Science and Technology, Government of India for providing the Promotion of University Research and Scientific Excellence Grant (DST-PURSE grant) for providing the required consumables for this study. One of the author Prabhat Kashyap is also thankful to theUniversity Grants Commission (UGC), India for providing the financial support in the form of scholarship for the research work. We are also thankful to Advanced Instrumentation Research Facility (AIRF) of Jawaharlal Nehru University for sample analysis and support of Dr. Ajai Kumar is gratefully acknowledged.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apr.2019.07.004. 7
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