Toxicity verses usefulness of Lead (Pb): Lead is indeed known for its toxicity, however, it has historically been used for various practical purposes due to its unique properties. For example, Pb has a relatively low melting point compared to other metals, which makes it suitable for certain manufacturing processes. Due to its high density it is used for shielding during radiation. Owing to the bright colors of Pb compounds and moisture resistant property it is widely used in paints, pigments, cosmetics, spices and cookware. However, in the present world some of the largest sources of Pb pollution are the lead-acid batteries and e-waste recycling.

In 2020, UNICEF and Pure Earth published a joint report titled “Toxic Truth” that found roughly one-third of children worldwide, totaling up to 800 million, possess blood lead levels (BLL) ≥5 micrograms per decilitre (µg/dL): a threshold that warrants global and regional interventions as per World Health Organization (WHO) and the United States Centers for Disease Control and Prevention. Lead is a potent neurotoxin with devastating effects on children's brains. It poses irreparable harm, particularly to infants and those under the age of 5, disrupting brain development and resulting in lifelong neurological, cognitive, and physical impairments. The report links childhood Pb exposure to mental health and behavioral issues, as well as an escalation in crime and violence. Older children face severe consequences, including an elevated risk of kidney damage and cardiovascular diseases later in life. The compiled evidence makes it evident that Pb poisoning poses a significantly greater threat to children's health than previously recognized. While further research is necessary, the emerging data calls for immediate and decisive action, emphasizing the urgency to address this issue promptly.

Why do we care about atmospheric Pb? In the post leaded gasoline world, the centers of emission have shifted from US to the east (South and southeast Asia) and south (sub Saharan Africa and parts of South America) as evidenced by the number of affected children and their average BLL (Figure 1).

Ingestion is the primary pathway of Pb exposure especially amongst children (hand to mouth behavior). Approximately 10-70% of ingested Pb is absorbed by the body (~50% in children and ~10% for adults). Inhalation is the second major pathway however, unlike ingestion, almost all inhaled Pb is absorbed into the body, making this exposure route a bit more serious. Atmosphere is the first recipient of the pollutant. Thus it is imperative to understand the present-day sources of Pb in the atmosphere. Quantifying concentration of Pb in atmospheric aerosols provides valuable information about the extent of pollution, however, it doesn't offer insights into the specific sources of contamination.

The logic and magic of Pb isotope: This is where Pb isotopes become essential as “fingerprinting” tools. Pb has four naturally occurring isotopes out of which ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb are radiogenic daughters of ²³⁸U, ²³⁵U, and ²³²Th, respectively while ²⁰⁴Pb is the least abundant and the sole non-radiogenic isotope. Hence the difference in isotopic signature of Pb bearing minerals arises from the relative proportion of initial U–Th–Pb in the system. Lead is a heavy metal with low relative atomic weight differences between its isotopes resulting in minimal mass-dependent Pb isotope fractionation in  physical, chemical, and biological processes. Lead isotopes do not fractionate during industrial and environmental processes and preserve the source signature even after degradation, processing and transportation.

Atmospheric Pb sources in India and Southeast Asian (SEA) countries: The Pb isotope ratios measured in 341 atmospheric aerosols (PM_2.5, PM₁₀, suspended particulate matter SPM) from India and SEA countries in the last decade are mixture of different sources or end members such as traffic emissions, coal combustion, biomass burning, industrial emissions, natural background etc. This source tracing approach is crucial for environmental monitoring, regulatory efforts, and the development of targeted mitigation strategies to address Pb contamination.

Difficulty arises when two or more end members have overlapping isotope ratios. For example, unleaded fuel and biomass have similar Pb isotope ratios. Gondwana coal and crust have overlapping ratios (Figure 2). Additionally, mass balance based linear mixing model are based on equations (1), (2) and (3) provided in the manuscript. If the number of mixing endmembers are > 3, this becomes an underdetermined system with no unique solution.

Mixing Polygon construction: Lead isotope ratios of most of the plausible end members of SEA and Indian aerosols are well documented in the literature. Previous source apportionment studies in the region used linear mixing models and considered Gondwana coal, Indian and SEA ore, unleaded and leaded fuel, solid waste and biomass burning and upper continental crust as end members of aerosol Pb. With these end members a mixing polygon simulation (Monte Carlo) was created in 3-dimensional space in R. The proportion of iterations in which the mixing envelope includes the isotopic composition of the aerosols within it, represents the probability of the aerosols (n=341) for lying inside the envelope. When analysing in ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb space, the median probability of the aerosols to fall inside the mixing envelope formed by these sources was 32 %, 36 %, 44 % and 40 % for Singapore, Thailand, Vietnam and India respectively. We realized there are missing sources. In the previous literature there were mention of tertiary coal & wood charcoal, ship emission and sea spray as contributors of elemental composition of aerosol. Lead isotope ratios of tertiary coal & wood charcoal were available from India. Sea spray from Malay Peninsula is relevant for the SEA countries. Hence Pb isotope composition and concentration for these sources were adopted from literature. The biggest challenge was obtaining Pb isotope composition of heavy oil used in ship. In one of our previous studies V/Ni ratio of SEA aerosols indicated ship emission as a potential source yet none of the previous studies in the region measured ship soot or heavy oil Pb concentration or isotope ratios. One data point from a study in Vietnam was available in literature and the second data point was from Germany. This remains a gap in our study and we highlighted the need to measure ship soot and/or heavy oil for trace metal composition and isotope ratios. Addition of new sources as end members improved the results. Now the median probability for the aerosols to lie inside the mixing envelope increased to 76 %, 83 %, 73 % and 67 % for Singapore, Thailand, Vietnam and India respectively.

Bayesian mixing model: Once the end members were fixed we used Bayesian mixing model, MixSIAR. MixSIAR has been widely used in the field of ecology to study food webs and infer the diets of consumers based on stable isotope systems such as δ¹³C, δ¹⁵N and δ³⁴S. Recently, MixSIAR has also been used in few source apportionment studies concerning Pb pollution that utilises Pb isotopic compositions of pollution sources.

Bayesian mixing model has several advantages over linear mixing model. For example it incorporates uncertainty in the mixing end members by providing posterior distributions for model parameters, can accommodate non-Gaussian distributions, heavy tails, and other complex distributions within the dataset and deal with small sample sizes. The results were startling. Upper continental crust (UCC) contains approximately 17 ppm Pb. Hence soil formed from weathering of UCC have considerable concentrations of Pb. Natural background (crustal dust and aerosolized soil) was a major contributor of aerosol Pb. This finding is not surprising as crustal dust was always a natural background contributor towards atmospheric Pb even before the Anthropocene Epoch. The second largest contributor in the SEA countries is the leaded gasoline Pb which was surprising. Thailand, Singapore and Vietnam phased out leaded gasoline by 2000. Aerosols were collected between 2012 and 2017. Thus legacy lead is still circulating in the environment. Gasoline Pb was emitted as Pb halides that settled on top soil and road dust. Due to long residence time of soil Pb (100 to 200 years), it is only natural that the legacy Pb is still recirculating in the environment. Besides natural background and legacy Pb, coal combustion from both Gondwana coal and Tertiary coal contributed towards aerosol Pb. Indian ore and coal are important sources for Indian aerosol. Whereas in SEA countries natural background and legacy lead from leaded gasoline are the combined largest source (Figure 5 of manuscript).

Policy implications: Preventing topsoil aerosolization, or the dispersion of soil particles into the air, is important for maintaining soil health and air quality. There are several easy to implement and cost effective measures that have been globally adopted to prevent topsoil aerosolization. For example, i) planting vegetation such as grass, cover crops, helps stabilize the soil, ii) applying a layer of mulch, such as straw, wood chips, or other organic materials, helps protect the soil surface from wind erosion, iii) planting cover crops during periods when the main crop is not growing provides ground cover iv) proper grazing management as overgrazing by livestock can lead to the removal of protective vegetation v) use of soil stabilization techniques, such as applying soil binders or polymers, can help improve the cohesion of soil particles vi) minimizing unnecessary disturbance of the soil, such as through construction activities or off-road vehicle use to mention a few. Additionally the legacy Pb contaminated soil can be covered by fresh uncontaminated soil. The effectiveness of these measures may vary depending on the specific environmental conditions and land use practices. Combining multiple strategies and adopting sustainable land management practices can contribute to effective soil conservation and prevent topsoil aerosolization.

Direction for future study: Two gaps were identified in this modelling study. 1. Thailand ranks 20^th in the world for coal consumption, 23^rd in coal production and 32^nd in coal reserve (https://www.worldometers.info/coal/thailand-coal). Yet there are no Pb data from Thai coal. Two studies on Thai aerosols by Jariya Kayee used regional coal data from Indonesia, India, Vietnam and China as end members. The MixSIAR results revealed tertiary coal emission as a major contributor of Thai aerosol Pb. In absence of local data we postulated that tertiary coal emission are transboundary transport from NE India where there are extensive tertiary coal reserves.

2. Since 1992, the Asian ship traffic has increased by 200%. India and the SEA countries have long coastline. In the MixSIAR analysis, Indian and Thai aerosol demonstrated >10% median contribution from ship emission. Yet ship emission data from the region is lacking, except for one data point from Vietnam ship soot.

Behind the Scene: This manuscript wouldn’t have been possible without three architects behind the scene. Dr. Xianfeng Wang (Associate Professor, Asian School of Environment, Nanyang Technological University, Singapore) supported the authors with intellectual, financial, laboratory and logistics support to work extensively on air pollution in Singapore, Thailand and India since 2016. In my decade long association with Xianfeng I understood the true meaning of the phrase “friend philosopher and guide”. I was familiar with Pb isotope chemistry as a hard rock geochemist to decipher mantle processes. That Pb isotopes can be a powerful tool to decipher environmental processes was introduced to me by my dear friend Dr. Mengli Chen (Research scientist, Tropical Marine Science Institute, National University of Singapore). Mengli has been a constant source of intellectual inspiration. Last but not the least, Dr. Jack Longman’s (Assistant Professor. Department: Geography and Environmental Sciences, Northumbria University) paper “Quantitative assessment of Pb sources in isotopic mixtures using a Bayesian mixing model”  published in Scientific Reports in 2018 was an eye opener when we were struggling with isotope mixing models for Pb. He also served as a reviewer for our manuscript which was a bonus. We hope to meet Dr. Longman in person someday, after all the  world is a small place.