Online observation of light non-methane hydrocarbons (C2–C5) over the central Himalayas: Influence of the Indo-Gangetic Plain region

Abstract

Studies pertaining to non-methane hydrocarbons (NMHCs) are very limited in South Asia, particularly at high-mountain sites. Therefore, online in-situ observations of light NMHCs (C2–C5) were conducted in the Central Himalayas (Nainital, 1958 m) from January 2017 to December 2020. Additionally, air samples were collected from an Indo-Gangetic Plain (IGP) site (Haldwani, 554 m), located about 18 km and the nearest source regions for the mountain site. The diurnal variations show daytime higher values at the mountain site, which is typical for a pristine site. Correlation analysis and percentage contributions confirm the major roles of biomass burning at the mountain site and Liquefied Petroleum Gas (LPG) emission, automobile and domestic combustion at the IGP site. Nevertheless, the anthropogenic influences also reaches the Himalayan site, mainly in spring, which is confirmed by the concentrated weighted trajectory (CWT) analysis. Such influences are limited to the IGP site in winter, primarily due to the shallower boundary layer. The estimated photochemical age varies from 9 days in winter to 16 days in summer-monsoon. It is shown that the hydroxyl radical reactivity, ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAFP) are 4–6 times higher at the IGP site compared to the Himalayan site. Propylene, ethylene and n-butane are major contributor in these at both sites. Ethane is found to be most abundant (40%) at the Himalayan site, while propane (27%) at the IGP site. An assessment of percentage contribution of different NMHCs in emission inventories for the Himalayan region is suggested.