Understanding the origin of carbonaceous aerosols during periods of extensive biomass burning in northern India☆
Introduction
In general, 20%–50% of total aerosol mass is composed of carbonaceous fraction in urban environments, which is considered an important constituent of atmospheric particulate matter (PM) (Contini et al., 2018; Putaud et al., 2010; Villalobos et al., 2015). Carbonaceous fraction includes components such as organic carbon (OC) and light-absorbing refractory elemental carbon (EC), measured using thermal-optical methods, or black carbon (BC), quantified using optical methods (Contini et al., 2018). Total carbon (TC) concentrations in aerosol samples can also include carbonate carbon. Numerous previous studies have already shown that enhanced levels of carbonaceous aerosols in the ambient atmosphere deteriorate air quality, adversely affect human health, and influence radiative forcing that can induce climate change (Chirizzi et al., 2017; Jacobson, 2001; Janssen et al., 2011; Li et al., 2016; Menon et al., 2002; Penner et al., 2003; Velali et al., 2016). Organic carbon, one of the most significant contributors to PM mass concentrations, can be directly emitted from natural (biogenic emissions) as well as anthropogenic (combustion processes) sources, or can be formed through various atmospheric physicochemical transformations involving gaseous precursors (secondary formation) (Pandis et al., 1992; Robinson et al., 2007). On the other hand, EC mainly originates through anthropogenic sources from (i) incomplete combustion of biomass/biofuels by burning agricultural waste or by using them as domestic fuel, and (ii) fossil fuel combustion mainly from vehicular exhaust (Cao et al., 2003; Querol et al., 2013; Sandrini et al., 2014). The OC/EC ratio varies widely in aerosol fractions depending upon the variation in sources and their strength as well as meteorological conditions, which makes the ratio a useful fingerprint to understand sources and atmospheric processes related to the formation of secondary organic compounds (Gentner et al., 2012; Robinson et al., 2007). However, large uncertainties exist with respect to combustion-derived aerosols’ regional and global climate effects due to a lack of spatial and temporal measurement data of combustion sources, their chemical characteristics, and size distribution.
The stable carbon isotope composition [denoted as δ13C, a measure of 13C/12C ratio in the sample relative to the reference Pee Dee Belemnite (PDB) standard, in parts per thousand or ‰] of carbonaceous aerosols has been used to establish source characteristics (Ke et al., 2007). δ13C values of total carbon (TC) have been previously used to identify sources of ambient PM and to understand various atmospheric processes (Garbarienė et al., 2016; López-Veneroni, 2009; Masalaite et al., 2015; Ulevicius et al., 2016; Widory et al., 2004). It is difficult to distinguish and quantify natural and anthropogenic sources’ contribution to atmospheric particles because human-induced emissions result in an enhanced formation of secondary organic aerosols from natural precursors (Bond et al., 2007; Hoyle et al., 2011). However, δ13C values of carbonaceous aerosols, which carry the isotopic signature of their precursors, have been used successfully to identify pollutant sources and, in turn, to assess anthropogenic impact (Cao et al., 2011; Fisseha et al., 2009; Górka et al., 2012; Kawashima and Haneishi, 2012; Kundu and Kawamura, 2014; Widory, 2006). In addition to the contribution from a variety of sources such as fossil fuel (−26.4 to −28.6‰; Widory, 2006) and biomass/biofuel (average δ13CC3 vegetation: −25.5‰; δ13CC4 vegetation: −12‰; Cachier et al., 1989), large variations in δ13C values of ambient aerosols may arise from kinetic fractionation caused by secondary aerosol formation (Irei et al., 2015; Pavuluri and Kawamura, 2016).
The Indo-Gangetic Plain (IGP) in India is one of the hotbeds of air pollution, with a consistent cover of atmospheric haze that advects towards the Bay of Bengal region in wintertime, causing a change in radiation balance over this region (Bikkina et al., 2016; Ram et al., 2012a, Ram et al., 2012b; Ramanathan et al., 2007). It is the most densely populated region of India (∼40% of the total population) and accounts for nearly 45% of the total food production (Badarinath et al., 2006; Gupta et al., 2004). Nearly 20 million hectares of land area are utilized for wheat and rice cultivation, covering Punjab, Haryana, and western regions of Uttar Pradesh (Badarinath et al., 2006; Gupta et al., 2004). Farmers burn large amounts of post-harvest agricultural waste in open fields from October to November (mainly paddy residue) and April to May (wheat crop residue). Such open burning of wheat and paddy residue in IGP results in an emission of ∼300 Gg/y and ∼1400 Gg/y of PM2.5, respectively (Rajput et al., 2014).
Particle concentrations in the ambient air of IGP becomes highly elevated due to these PM emissions in combination with relatively stable meteorological conditions (high relative humidity, low wind speed, and lower boundary layer height). This results in frequent haze formation, visibility reduction, and degradation in air quality in downwind sites such as Delhi, the national capital region (NCR), and the states of Uttar Pradesh, Bihar, and West Bengal (Singh and Dey, 2012; Tiwari et al., 2011). During winter, nearly 40 million people in the NCR are affected by hazardous air pollution by breathing air akin to smoking 30 to 40 cigarettes a day (PM2.5 remains > 250 μg m−3), according to Pant et al. (2015). Since burning agricultural waste causes extreme deterioration of air quality, it is of the utmost importance to study the consequences and processes associated with these emissions. Chemical composition of PM in IGP is dominated by organic aerosols (Rajput et al., 2011; Ram and Sarin, 2011). Several previous studies in the North Indian region have demonstrated the enhanced contribution of secondary organic aerosols and agriculture residue burning to the ambient PM (Rajput et al., 2014a; Rastogi et al., 2015; Sharma et al., 2016). Precursors of secondary aerosols and fine primary aerosols are emitted from fossil fuel combustion and biomass burning (Rastogi et al., 2014; Singh et al., 2014). In this context, formation of secondary aerosols along with significant variability in aerosol emission sources in the IGP presents a challenge with respect to source apportionment; therefore, it is critical to identify and chemically characterize the sources.
In this study, we determined the chemical characteristics (OC, EC, ionic species, and δ13C) of ambient PM2.5 aerosols collected from the town of Beas, located in the state of Punjab, India, as well as aerosols derived from the combustion of paddy, wheat residue, and biofuel. Our major goal is to assess the variabilities of these parameters in the pre-monsoon period (wheat crop residue combustion), post-monsoon period (paddy residue combustion), and monsoon period (relatively cleaner for background values determination) and to determine how much of the observed variability can be quantitatively assigned to their precursor source(s).
Section snippets
Sampling site
Our sampling site (semi-urban) was located in Daulo Nangal village, Beas, Punjab (31.52 °N, 75.29 °E, 238 m above mean sea level [AMSL]), in the northwest region of India in the IGP (Fig. S1). Nearly 1000 mm of rainfall during the southwest monsoon months, from June to September, washes out particles from the ambient atmosphere (Rajput et al., 2014). In this region of IGP, massive post-harvest wheat residue burning occurs from April to May, whereas paddy residue burning occurs from October to
Seasonal variations in the PM2.5 chemical composition influenced by extensive biomass burning periods
Details regarding PM2.5 chemical compositions during post-monsoon (paddy burning period), pre-monsoon (wheat residue burning period), and monsoon periods are mentioned in Table 1. PM2.5 concentrations were comparable during post-monsoon (range: 184–342 μg m−3; median: 290 μg m−3) and pre-monsoon (range: 106–458 μg m−3; median: 229 μg m−3) seasons and significantly higher (p < 0.0001) than concentrations during the monsoon period (range: 23–95 μg m−3; median: 64 μg m−3) (Table 1). The lower PM2.5
Conclusions
This study focuses on the enhanced atmospheric carbonaceous aerosols concentration during periods of post-harvest crop residue burning (pre- and post-monsoon) in Northern India. During pre-monsoon and post-monsoon seasons, overall PM2.5 concentrations were similar, with enhancement occurring at night as compared to daytime. Ambient aerosols during the post-monsoon season (TC/PM) were more carbon-rich as compared to aerosols in other sampling seasons. The OC/EC ratio remained >2 during every
Credit author statement
Gyanesh Kumar Singh: Conceptualization, Investigation, Writing – original draft. Vikram Choudhary: Investigation, Writing – review and editing. Pradhi Rajeev: Investigation, Writing – review and editing. Debajyoti Paul: Supervision in stable isotope analysis and interpretation, Writing – review and editing. Tarun Gupta: Supervision, Writing – review and editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are thankful to Mr. Abhishek Kumar, Department of Earth Sciences for providing assistance in stable isotope analysis. We thank five anonymous reviewers and Prof. Admir Créso Targino (editor) for their constructive comments and insightful suggestions to improve clarity of this manuscript.
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This paper has been recommended for acceptance by Admir C. Targino.