Health impacts, ozone and secondary organic aerosol formation potential of aromatic hydrocarbons (C₆-C₈) in the Central Himalayas and the Indo-Gangetic Plain

The pristine Himalayas are sensitive to pollutants from different source regions, including its foothills that have adverse effects on air quality and climate. Despite this, there are no observations of aromatic hydrocarbons in the central Himalayas. Thus, online observations of aromatics (C₆-C₈, defined here as BTEX) were conducted for the first time at the mountain site (Nainital, 1958 m) in the central Himalayas during January 2017-December 2022 period. Additionally, observations were made at a foothill site in the Indo-Gangetic Plain (Haldwani, 554 m) to assess the source features. The diurnal variations at the mountain site were characterised by daytime higher BTEX (~ 6 ppbv), in-contrast the IGP foothill site exhibited nighttime elevated levels (~ 19 ppbv). Higher values were exhibited in spring and autumn at the mountain site, while values were higher in winter at the IGP foothill site. These variations were primarily influenced by the transport of diverse air masses to the mountain site, while the role of local emissions was found at the IGP foothill site. The boundary layer processes play a role at both sites, but differently. Xylene was the most abundant (60-65%) at both sites, suggesting the influence of emissions from the IGP region. This contrasts with the composition used in emission inventories for this region. The toluene to benzene ratio at mountain (4.5) and IGP foothill (5.3) sites and ternary plots indicated the dominance of industrial/vehicular sources, with some contributions from biomass burning. The estimated OH reactivity, ozone formation potential and secondary organic aerosol formation potential were found to be 4-6 times greater at the IGP foothill site than those at the mountain site. Xylene played a significant role in these processes at both sites. Furthermore, benzene played a dominant role in the hazard ratio and the lifetime cancer risk (LCR) at both sites. The LCR at both sites crossed the probable risk limit.