Elemental characteristics of atmospheric aerosols using Raman Spectroscopy: a case study from Doon valley during Covid-19 pandemic

This study presents a Raman spectroscopic investigation of total suspended particles (TSP) collected from Wadia Institute of Himalayan Geology, Dehradun Campus, during the COVID-19 lockdown period (April to September 2020). The lockdown created a rare low-emission atmospheric condition, offering an ideal opportunity to examine the chemical nature of ambient aerosols in the absence of usual anthropogenic interference. Raman micro-spectroscopy was used to analyze the Polytetrafluoroethylene (PTFE) filter strip of aethalometer AE51, providing molecular-level insights into their carbonaceous and organic composition. Spectra collected during the pre-monsoon months (April-May) were dominated by well-resolved D (~1350 cm-1) and G (~1580 cm-1) bands, indicating the presence of disordered and graphitic black carbon (BC) from combustion sources. In contrast, spectra from the monsoon months (June-August) exhibited broadened bands with enhanced contributions from oxygenated and aliphatic organic compounds, as evidenced by peaks in the 1700-3100 cm-1 range. These features reflect secondary organic aerosol (SOA) formation through aqueous-phase and photochemical aging processes under high humidity. By September, the onset of post-monsoon conditions led to a resurgence of combustion-derived carbon signatures, mixed with aged oxidized organics. The Raman spectral evolution across seasons reveals the influence of meteorology and emission dynamics on aerosol composition. Pre-monsoon aerosols were dominated by primary BC, monsoon aerosols by aged organics, and post-monsoon aerosols by a combination of both. This work highlights the utility of Raman spectroscopy for molecular fingerprinting of aerosols and provides a seasonal reference for understanding aerosol aging and source transformation processes in the Himalayan foothills.