Light NMHCs over the central Himalayas region: Variabilities and Ozone Production Potential

Abstract:

Non-methane hydrocarbons (NMHCs) play a critical and important role through photochemistry in determining ozone concentrations and its variability. They are also important precursors for secondary organic aerosol in the atmosphere. Despite their importance, there are limited measurements of NMHCs over the Indian subcontinent, particularly over the Himalayas. Therefore, first ever online observations of 14 different NMHCs have been initiated over a high altitude site Nainital (29.5^oN,79.5^o E,1958 m amsl) in the central Himalayan region since 2016 with a frequency of 2 to 3 days continue in a week. In addition, air samples are collected from two other sites (Kathgodam 554m amsl, Pantnagar 231m amsl;) in the IGP region about once in a week, representing regions of emission sources for the central Himalayas. The IGP region is known to be one of the major source regions having diverse emissions. Here, we present diurnal and seasonal variations in NMHCs and identification of their emission sources. Two kinds of diurnal variations are seen, one with noontime higher values in n-butane, i-butane and m,p-xylene and other with bi-model variations in ethane, propane, toluene. Noontime higher values indicate dominance of the photochemistry whereas importance of the convective boundary layer is reflected in the bi-model variability of diurnal patterns. Some of the NMHCs (ethane, toluene, benzene etc.) showed consistent seasonal variation, with winter time high except toluene and o-xylene, those showed autumn time high. Inter correlation coefficient between ln((n-butane)/(ethane)) and ln((i-butane)/(ethane)) at Nainital is greater than 0.8 with slope of about 0.9 indicating that these NMHCs are removed by OH oxidation over this region apart from seasonal change in transport pattern. Further, inter correlation between acetylene and ethane suggests for the biomass burning as a major source of these species during the spring season. As expected, higher levels of NMHCs are observed at the IGP sites. Percentage contribution of alkanes is greater at IGP site when compared with the Himalayan site, while aromatic compounds show greater contribution at the Himalayan site. A preliminary estimate using a box model shows greater ozone production rate for aromatics (>8.5 ppbv/h) followed by alkanes (>6.5ppbv/h) and alkenes (>5ppbv/h) for Kathgodam, a semi-urban site. Further details will be presented.