New Study Uses Micro-PIXE Spectroscopy to Trace Pollution Sources in Indian Air Samples

Air pollution is one of the current, ongoing environmental issues that many nations around the globe. A recent study published in the journal Nano Micro Small explores this topic. Led by Bibhudutta Rout of the University of North Texas, the research team tested the effectiveness of micro-particle-induced X-ray emission (micro-PIXE) spectroscopy in pinpointing pollution sources in urban and indoor environments (1).

One of the most common pollution sources in urban and indoor environments is particulate matter, and this mixture has been found in the air of many cities dealing with air quality issues. Particulate matter is a mixture comprised of tiny solid particles and liquid droplets suspended in the air (1). Because it contributes to air pollution and airborne diseases, there is a big incentive to reduce the amount of particulate matter in the air. Understanding the elemental composition of PM is critical in identifying its sources and formulating strategies for pollution control (1). Rout and his team have showcased how micro-PIXE when combined with advanced data analysis tools like GeoPIXE, offers a non-destructive and highly sensitive approach to elemental mapping (1).

India is one country that is dealing with air quality issues. A recent article documented the challenges of combatting urban air pollution in Delhi, India (2). NBC News has also reported that the Indian government has instituted several reforms to combat this issue, including mandating government workers to work remotely 50% of the time to reduce traffic congestion and factory emissions (3).

People walking in the streets of Delhi amidst smog | Image Credit: © saurav005 - stock.adobe.com

In this study, Rout and the team investigated the elemental composition of particulate matter (PM) collected from two vastly different environments in India: the bustling outdoor market in Old Delhi and indoor settings within Panjab University hostels. Using a 2-million electronvolt energetic scanning proton micro-beam with an approximate diameter of just 1 square micrometer, the researchers were able to produce high-resolution elemental maps of selected regions of interest (ROI) within each air sample (1). This enabled them to not only identify the presence of various elements but also understand the concentration ratios and co-localization patterns among them (1).

Comparing the indoor samples with the outdoor samples showed that the elements detected differed. For example, in the indoor samples from the Panjab University hostel, the elements silicon (Si), calcium (Ca), and potassium (K) were found in significant concentrations (1). According to the study, these elements are typically associated with natural sources such as soil erosion and biomass burning—specifically the seasonal crop burning prevalent in northern India (1). The outdoor samples collected from the Old Delhi market exhibited high levels of sulfur (S), pointing to emissions from coal-fired power plants, a major contributor to urban air pollution in the region (1).

The outdoor samples also revealed the presence of scandium (Sc), titanium (Ti), chromium (Cr), manganese (Mn), and zinc (Zn). The presence of these elements indicates that the particulate matter is a result of vehicle exhaust, industrial emissions, and other anthropogenic sources (1). Meanwhile, the indoor air also showed trace quantities of vanadium (V), cobalt (Co), and copper (Cu), suggesting occasional infiltration of outdoor air pollutants or contributions from indoor activities and building materials (1).

The GeoPIXE software played a pivotal role in the data interpretation process. GeoPIXE allowed researchers to highlight pixels that corresponded to specific elemental concentration ratios, offering insight into how different elements are distributed spatially within the samples (1). This level of analysis is crucial in environmental forensics because it links particular patterns of elemental presence to distinct pollution sources.

One of the main features of micro-PIXE is its ability to provide quantitative results without the need for external standards. Moreover, the technique requires no sample destruction, preserving the integrity of the material for further study or archival purposes (1).

Rout's team asserts that this study paves the way for more widespread adoption of micro-PIXE in environmental monitoring, especially in regions with complex pollution dynamics like India (1).

References

1. Saini, D. K.; Byers, T. A.; Verma, S.; et al. Unraveling Various Sources of Particulates Matter in Air-Dust Samples Collected from North India Through Elemental Mapping and Concentration Correlation Using Micro-Particle-Induced X-Ray Emission Spectroscopy. Nano. Micro. Small 2025, 21 (8), 2409264. DOI: 10.1002/smll.202409264
2. Wetzel, W. The Critical Role of Chromatography in Addressing Urban Air Pollution. LCGC International. Available at: https://www.chromatographyonline.com/view/the-critical-role-of-chromatography-in-addressing-urban-air-pollution (accessed 2025-04-07).
3. Qadir, G. New Delhi’s Air Pollution Leaves Residents Gasping For Breath. NBC News. Available at: https://www.nbcnews.com/news/world/india-new-delhi-air-pollution-rcna56535 (accessed 2024-10-03).