Non-methane hydrocarbon (NMHC) variability in rural Himalayan atmospheres: impacts on ozone, secondary organic aerosols, and human health.

This study presents the first year-long observations (January 2021-June 2022) of 17 non-methane hydrocarbons (NMHCs) at a high-altitude rural site in the Himalayas (Munsyari: 2200 m a.m.s.l.), a tourist destination influenced by nearby emission sources. Total NMHCs exhibited seasonal variability, with the lowest levels in winter and summer- monsoon (159.5-174.5 ppbC) and the highest levels in spring and autumn (174.5-197.8 ppbC). Aromatic hydrocarbons dominated the NMHC composition throughout the year, contributing about 85-89% of the total NMHCs. This seasonal variability suggests the combined influence of local emissions, boundary layer dynamics, temperature-driven emissions, upslope winds, and biomass burning processes. Further, hydrocarbon ratios highlighted the role of hydroxyl (OH)-driven oxidation processes in the removal of NMHCs. Correlation analysis and ternary plot also indicate contributions from local emission sources such as liquefied petroleum gas (LPG), diesel fuel emissions, and possible solvent use. In addition, the relatively higher levels (4-6 times) compared with other high-altitude sites (e.g., Mt. Abu and Nainital), together with variability-lifetime analysis, suggest the dominance of local sources at this site. However, levels were 3-14 times lower than those at Kathmandu and Indo-Gangetic Plain (IGP) sites. Reactivity-based analyses showed that p-xylene, m-xylene, isoprene, and toluene were major contributors to propylene-equivalent concentration (PEIC), ozone formation potential (OFP), and secondary organic aerosol formation potential (SOAFP). Health risk assessment indicated that non-cancer hazard ratios remained below the threshold, whereas lifetime cancer risk exceeded the USEPA limit, with benzene being the dominant contributor. Overall, these findings provide new insights into the distribution, sources, and atmospheric implications of NMHCs at a high-altitude rural site in the Himalayan region.