Our scientific questions were framed based on the COVID lockdown when Indian atmosphere transitioned from heavily polluted to relatively cleaner conditions. Though there was significant reduction in emissions from industrial and transportation sectors, power plants continued their operations as essential service providers. Lignite-fired power plant at Neyveli with an installed capacity of 3390 MW, located ~200 km south of Chennai, Tamil Nadu was such an example. As heavy metals such as lead (Pb), cadmium (Cd), etc. concentrate in the PM_2.5 fraction of aerosols, it provides an ideal basis for precise source apportionment of heavy metal emissions from anthropogenic processes. Thus, we collected PM_2.5 from October 2020 to February 2022 along with lignite, fly ash, solid waste & biomass burning ash and topsoil from Neyveli. We hypothesized that during the COVID lockdown period, with minimal to negligible contributions from other major anthropogenic sources, the ambient air quality will provide a more controlled environment. This unique situation will allow for an accurate assessment of the heavy metal content in PM_2.5 directly attributable to coal combustion processes.

The PM_2.5 and the end members were analysed for 13 elements (Mg, Al, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb), many of which are typical markers of coal combustion emission. For example, coal combustion typically has V/Pb ratios of >1, in contrast to vehicle exhaust emissions, where V/Pb < 1, and industrial emissions, where V/Pb ≪ 1. Interestingly, all the PM_2.5 aerosols either had V/Pb <1 or V/Pb <<1 though the fly ash and lignite had V/Pb>1 as expected (Figure 1a). This means that aerosol bound heavy metals collected from the vicinity of a coal fired power plant do not source the metal from coal combustion! We were perplexed. We had to revisit our hypothesis and modify it to understand the sources of the Neyveli PM_2.5.

Isotopes of Pb have long been used as a robust and faithful tool to track anthropogenic pollution sources. Lead has four naturally occurring isotopes: ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb. The ratios of these isotopes vary depending on the source of the Pb, which can be used as a “fingerprint” to trace its origin. Additionally, Pb isotopes do not significantly fractionate during physical, chemical or metallurgic processes such as weathering, atmospheric transport, smelting, combustion etc. This means the isotopic signature remains stable and can reliably trace the source. For example, Pb isotopes from coal combustion emission particles reflect the isotopic composition of the coal. The isotopic signature of leaded gasoline depends on the source of the Pb ore used.

All our aerosols along with the possible source endmembers were plotted in an isotopic space (Figure 1b). Aerosols plotted far away from coal and fly ash. Instead, they were more aligned to the solid waste & biomass burning ash, Indian fuelwood, traffic emission and aerosols from open waste burning site in Delhi. Besides these anthropogenic sources, transboundary aerosol from Thailand, natural sources such as sea spray aerosol and crustal dust also contributed towards aerosol Pb. However, we had to run a couple of statistical analysis to understand significant contributors and their relative contributions. 

The 3D mixing polygon simulation assesses whether all potential aerosol sources are accounted for by evaluating the median probability of aerosols falling within a mixing envelope formed by the sources. The inclusion of various sources such as crust, vehicle exhaust, biomass burning, ore as representative of industrial emission, leaded gasoline as legacy Pb from soil resuspension, seawater and transboundary aerosols increased the median probability of aerosol falling inside the envelope to 0.68. Interestingly, coal and fly ash show minimal influence and their incorporation as sources did not increase the probability further (Figure 2a).

The relative contribution of aerosol sources was quantified using the Bayesian model MixSIAR, revealing seasonal variations (Figure 2b). Transboundary aerosols play a significant role in post-monsoon months, contributing up to 36%. Natural sources (crust + seawater) dominate in summer, contributing up to 49%, compared to 27% in post-monsoon. In summer, air mass back trajectories demonstrate no influence from transboundary aerosols. Instead, the air mass originated predominantly from short ranges where impacts of local burning facilitated dehydrated soil resuspension. Thus, an increase in the relative crustal contribution and, consequently, the contribution of legacy Pb from soil resuspension is observed in the summer months. Noteworthy, solid waste and biomass burning consistently contribute year-round, peaking at 26% post-monsoon and 22% in summer. MixSIAR also confirms negligible influence of coal and fly ash, on aerosol composition near lignite power plants.

Chemical analysis of aerosol samples provides crucial data on the plausible sources of contaminants, but these findings must be contextualized within the physical characteristics of the field site. The study area employs various measures to control emissions, including water sprinklers at lignite storage yards, transferring regions, and roads, as well as dust extraction systems and electrostatic precipitators (ESP) at power plants. The power plant stacks are ≥220 meters high for improved pollutant dispersal. Effective regulatory measures, including bag filters at crusher houses and fly ash storage, have led to minimal coal emissions and reduced heavy metal contamination in the atmosphere, as indicated by the metal composition and Pb isotopes of PM2.5. In contrast, the region experiences pollution from open burning of crop residues, residential fuelwood, and solid waste. The burning of municipal solid waste and biomass is common in rural Tamil Nadu, with a significant portion of households in Cuddalore (a coastal town, approximately 20 to 25 kilometers southeast of Neyveli) relying on biomass for cooking. In 2018, Tamil Nadu generated 31.62 million tons of crop residue, of which 4.11 million tons were burned. Open burning of solid waste and biomass was observed in Neyveli during the sampling period (Figures 3a-d). India ranks among the top countries globally in both waste production and open burning of waste. Open burning of wastes at dumpsites in different cities is a common picture in India (Figures 3e-f). The mass of pollutants emitted from open burning per mass of waste (i.e. the emission factor) is an order of magnitude higher than other controlled processes.

This work highlights both improvements and challenges of air pollution control strategies. While there is a 'cloud' due to the persistent pollution from biomass and waste burning, there is also a 'silver lining' in the form of reductions in coal combustion emissions, indicating progress in certain sectors of air pollution control. In recent years, there has been a noticeable reduction in particulate matter emissions from coal combustion in India, largely due to governmental efforts aimed at curbing pollution. Despite this positive trend, a growing challenge in managing both the disposal of agricultural residues and municipal solid waste looms over India.