Light-NMHCs in the Central Himalayas and associated IGP Region: Role in Ozone and Secondary Organic Aerosols-Formation

Non-methane hydrocarbons (NMHCs) are important precursors of tropospheric ozone and secondary organic aerosols (SOAs). The air quality in South Asia is rapidly deteriorating due to increasing pollution levels, and transporting these pollutants to pristine regions in the Himalayas also exacerbates the problem. Despite this, air quality studies are very limited in South Asia, particularly in remote Himalayan regions. This study presents first time, a comprehensive analysis of light NMHCs (C₂-C₅) at the central Himalayas mountain site (Nainital; 29.37°N, 79.45°E, 1958 m a.m.s.l.) and an Indo Gangetic Plain (IGP) site (Haldwani; 29.22°N 79.51°E, 554 m a.m.s.l.). Observations were made from January 2017 to December 2020 using a Thermal Desorption Gas Chromatograph equipped with Flame Ionization Detectors (TD-GC-FID). The continuous online observations showed diurnal variation in light-NMHCs with higher values in the daytime throughout the year except for the summer/monsoon months. The mixing levels of alkanes, alkenes and alkynes vary from the lowest level of 1.96±0.77 ppbv, 0.29±0.06 ppbv, and 0.22±0.20ppbv respectively, to the highest levels of 4.43±0.84 ppbv, 1.03±0.39 ppbv, and 0.75±0.40ppbv in November, respectively. However, an IGP site showed much higher levels at nighttime than in the daytime, where alkanes, alkenes and alkyne showed 19.24±0.24 ppbv, 2.88±1.76 ppbv, 1.41±1.21 ppbv levels during winter and 13.41±9.33 ppbv,1.88±1.65 ppbv, 0.67±0.59 ppbv. Among eight light-NMHCs, the observed levels of ethane, ethylene, propane, n-butane and acetylene were highest during winter and spring and minimum in summer/monsoon at both sites. Ethane is most dominant at the Himalayan site, while propane is at the IGP site. The investigation of the natural logarithmic ratio between two different pairs (ln([n-butane]/[ethane]) to ln([i-butane]/[ethane]) and ln([Propane]/[ethane]) to ln([n-butane]/[ethane]) suggested the role of oxidation of OH mechanism for light-NMHCs removal in the atmosphere at both the sites and globally compared results showed a heterogeneous nature of these light NMHCs in the atmosphere. There is a strong inter-correlation among ethane, i-butane, propane, and n-butane acetylene, which supports the influence of natural gas, LPG leakage and biomass burning. Additionally, a good correlation of combustion tracer carbon monoxide (CO) with ethane, propane, and acetylene reconfirmed that biomass burning is the source of these light-NMHCs at the central Himalayas site, especially during spring. The OH reactivity, ozone formation potential (OFP) and secondary organic aerosol potential (SOAP) is also studied. The OH-reactivity is minimal at a mountain site compared to an IGP site. Propylene (25%-30) and ethylene (10-25%) strongly contributed to OH-reactivity. Both sites have maximum OFP during the winter and a minimum. The OFP during the summer/monsoon for all the seasons. Propylene (23%-35%) and ethylene (18- 22%) are species dominated by OFP throughout the year at both sites. SOAP showed wintertime maxima and springtime minima with the dominance of propylene (35%-45%) and i-butane (15%-38%) at the Himalayan site, whereas propylene (%) and n-butane (%) dominance at the IGP site. Further, these datasets can be used to develop emission inventories and validate various chemical transport models. The results from these studies are also useful for policymakers in formulating and implementing effective emission reduction strategies.