Characteristic aerosol properties from ecologically sensitive high-altitude region of Western Ghats in India

The study aimed to evaluate the burden of anthropogenically emitted pollutants over an ecologically sensitive region of Munnar in Western Ghats, and quantify a baseline for aerosol measurements for comparisons with highly polluted urban clusters in India. Pre-monsoon sampling of PM₁ aerosols was carried out at the NABHA (Natural Aerosol and Bioaerosol at High Altitude) laboratory (10.09°N, 77.07°E), located ~1600 m above sea level, approximately 90 km east of the Arabian Sea. The collected PM₁ particles were analysed for metals, carbonaceous fractions, inorganic species, and polycyclic aromatic hydrocarbons (PAHs) to identify potential pollution sources and evaluate the toxicity of the sampled airmasses.

The PM₁ concentrations at this ecologically sensitive site were notably elevated, with an average of 21.7 ± 5.5 µg/m³ for the sampling duration. The physio-chemical analysis revealed the composition consisting of approximately 8% crustal dust, 7% sea salt, 20% sulphate, 7% ammonium, 42% organic mass (OM), and 4% elemental carbon (EC). Surprisingly, PM-bound nitrates were below quantifiable levels, likely due to the region's high relative humidity (>80%) and frequent precipitation, which may have scavenged nitrate salts, given their high water-solubility. However, nitrogen in the form of ammonia (1.5 ± 0.9 µg/m³) and water-soluble nitrogen (2.4 ± 0.8 µg/m³) was abundant in PM₁. The concentration of PM₁-bound USEPA priority PAHs was 98.1 ± 11.2 ng/m³. This included 2–3 ring PAHs at 41.5 ± 7.5 ng/m³ and 4–6 ring PAHs at 56.7 ± 7.2 ng/m³.

Both the PM₁ mass concentration and chemical characterization showed significant contribution of anthropogenically derived aerosol mass burden. The elevated OC/EC ratio (~6.6), coupled with a high water-soluble organic carbon (WSOC) fraction (~0.8), suggests a significant contribution from biomass burning emissions and active combustion sources in the region during sampling duration. Emissions from tea processing factories, coupled with the unchecked growth of tourist vehicles, are significant contributors to the regional high levels of sulphate, EC, Zn, Mn, Cu, and heavy metals in the ambient air. The influx of marine aerosols from the Arabian Sea further amplifies the regional pollutant load, particularly sulphate concentrations, formed through the atmospheric oxidation of dimethyl sulphide released by oceanic phytoplankton. These marine air masses additionally carry ship emissions, as evidenced by the significant presence of Ni and V in the aerosol composition.

The insights gained from this data at a strategically important geographic location are pioneering and have the potential to make a significant contribution to the development of pollution control policies in ecologically sensitive and high-altitude areas. Additionally, these findings could serve as a valuable resource for advancing climate change research.