Studies of Non-Methane Hydrocarbons (NMHCs) in Ambient Air over the Central Himalayan and Associated Regions

Abstract

Air pollution is a major environmental challenge, with a substantial release of pollutants into Earth's atmosphere due to rapid urban and industrial growth over recent decades. Non-Methane Hydrocarbons (NMHCs) stand out among these pollutants, playing a crucial role in the formation of surface ozone (O3) and secondary organic aerosols (SOAs). In addition, a diverse range of NMHCs serves as effective atmospheric tracers with lifetimes varying from a few hours to several tens of days. NMHCs exhibit high reactivity, easily oxidized by hydroxyl (OH) radicals and ozone. Their chemical reactivity increases from light to higher hydrocarbons and from saturated alkanes to unsaturated alkenes. A large range of atmospheric lifetimes in NMHCs helps in assessing the atmospheric transport and photochemical aging. Despite observations of NMHCs over the Indian region being scarce and almost non-existent in the central Himalayas. Under these conditions, the present thesis work has attempted to characterize the NMHCs species over the central Himalayan and associated regions and to develop a better understanding of their role in O3 and SOAs formation process. Therefore, a study was conducted based on the first online observations of light NMHCs (C2-C5) at a mountain site (Nainital, 1958 m AMSL) in the central Himalayas and offline observations at an IGP foothill site (Haldwani 554 m AMSL). The diurnal variations showed higher daytime values at the mountain site and higher nighttime values at the IGP foothill site. Average levels of NMHCs were significantly higher at the IGP foothill site, with alkanes, alkenes and alkynes showing notable differences. Biomass burning and LPG evaporation played major roles, with ethane (40%) predominant in the central Himalayas and propane (27%) dominant at the IGP foothill site. Seasonal variations, correlation studies and comparisons with emission inventories highlighted the impact of combustion processes. The study also assessed the dominant role of propylene, ethylene and n-butane in OH reactivity, ozone formation potential (OFP) and aerosol formation potential (SOAFP) at both sites, emphasizing the need for their emission control strategies. Further, in addition to observations of light NMHCs, the health risks, OFP and SOAFP of aromatic hydrocarbons (C6-C8) were also assessed. First-time observations of BTEX (benzene, toluene, ethylbenzene and (m, p & o) xylene) in the central Himalayas (Nainital) revealed that the mountain site exhibited daytime higher values (up to ~6 ppbv), while the IGP foothill site showed elevated nighttime values (up to ~19 ppbv). Most of BTEX showed higher values in spring/autumn and winter in the mountain site and the IGP foothill site, respectively. BTEX levels were significantly higher at the IGP foothill site. Xylene was the most abundant (60-65%) aromatic at both sites, suggesting the influence of emissions from the IGP foothill on the mountain site. This contrasts with the composition used in emission inventories for this region. Analysis of interspecies ratios of toluene to benzene and ternary plots indicated the dominance of industrial sources, with some contributions from vehicular exhaust and biomass burning. The estimated OH reactivity, OFP and SOAFP potential were 4-6 times higher at the IGP foothill site than at the Himalayan site. Xylene played a significant role in these processes at both sites. Furthermore, benzene played a dominant role in the hazard ratio (HR) and the lifetime cancer risk (LCR) at both sites. The LCR at both sites crossed the probable risk limit. Apart from NMHCs, surface ozone behavior in Himalayan foothills was also studied. Doon Valley (Dehradun: 700 m) in the Himalayan foothills acts as one of the links between the Himalayas and the IGP. Surface ozone in the valley exhibited urban behavior with daytime peaks. Ozone showed higher values in spring (49.2±24.8 ppbv in May), driven by biomass burning. About 9-50% enhancement in ozone was found in the high-fire activity period (April-May). Ozone exceeds the 8-hours national air quality standard (50 ppbv) throughout the year, except in July-September. Moreover, a photochemical box model estimated ~41 ppbv and ~8 ppbv of ozone production and loss, respectively. The role of the HO2+NO reaction (85.6%) in ozone production and the O3+HO2reaction (56.2%) in ozone loss were seen. Exposure metrics analysis (M7 and AOT40) estimated an annual loss of 27-37 kilotons of wheat and 14 32 kilotons of rice production due to elevated ozone levels. Furthermore, the HR for BTEX and LCR values for benzene and ethylbenzene exceeded the standard limits (USEPA and WHO), indicating significant health risks to the population. In addition, the CAMS model and satellite based studies demonstrated the NOx-sensitive behavior of ozone production in this Himalayan region, where aromatics exhibited the maximum OFP among different NMHCs. This thesis work emphasizes the role of diverse emission sources and NMHCs distributions at the mountain and the IGP foothill sites, suggesting further comprehensive studies and long-term observations in different Indian regions. The observations presented in the work are deemed crucial for evaluating the impact of emissions in the Asian summer monsoon region, as also seen in the recent campaign (ACCLIP) conducted by NCAR and NASA in the Asian summer monsoon region.