Comparative Assessment of Crystallinity in Coarse Particulates (PM₁₀) in Urban Environments of IGP and Central IHR

The crystallinity of a particle dictates its chemical stability, atmospheric lifetime, and biological toxicity. It increases the surface reactivity and adsorption, thereby increasing the potential toxicity. The present study focuses on seasonal variations in the mineralogy and crystalline size of ambient particulate matter (PM₁₀) collected from three sites in northern India representing different environments and topographies, Delhi (urban megacity, an ecological transition zone between the Thar Desert to the west and the humid plains to the east), Varanasi (in the heart of the Middle-Ganga Valley, a humid subtropical region), and Nainital (subtropical highland, in the Kumaon foothills of the outer Himalayas), from January to December 2022. The analysis was carried out using an X-ray diffractometer (XRD), which helped deduce the crystal sizes and the percentage of crystallinity for each sample (four samples per site). The results indicate significant regional and seasonal differences in mineral richness and crystal structure across the study sites. Delhi samples had a relatively low crystallinity, ranging from 13.92% in the post-monsoon to 18.28% in winter, with mineral peak areas ranging from 15.72 to 26.97 nm. Varanasi showed considerable seasonal variation, with crystallinity ranging from 16.29% to 28.19% and peaking in summer, while the mineral peak areas remained high (90.87-95.98 nm). Nainital exhibited distinct seasonal variation with peak crystallinity observed in summer (25.07%) and winter (19.41%), while the monsoon (9.90%) and post-monsoon (8.48%) seasons demonstrated substantial decreases. Mineral peak areas for Nainital ranged from 40.43 to 92.10 nm. These patterns show that regional environmental settings and seasonal climatic circumstances have a major influence on the crystalline properties of particulate matter. Consistent with previous studies, annual mineralogy and crystal size of Delhi and Nainital are reported, whereas seasonal investigations remain limited. The present observations enhance the understanding of seasonal aerosol crystallinity in the Indo-Gangetic Plain (IGP) and Indo-Himalayan Region (IHR) contexts, with implications for interpreting aerosol sources, atmospheric processing, and potential environmental consequences.