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Spatiotemporal Characteristics of the NMHCs and Their Contribution in Situ Tropospheric Ozone Production: First Results from the Central Himalayan

M. Rajwar1,2, M. Naja1, S. Lal3 and R. K.Tiwari2

1Aryabhatta Research Institute for observational sciencES (ARIES), Department of Science and Technology (DST), Atmospheric Science Division, Govt. of India, Nainital, Uttarakhand, India; 05942-270771, E-mail: mahendar@aries.res.in
2Deen Dayal Upadhyaya Gorakhpur university, Gorakhpur, India
3Physical Research Laboratory, Ahmedabad, India

Our study presents the first time observations of fourteen different NMHCs including BTEX species over a central Himalaya high altitude site (Nainital; 29.4oN, 79.5oE, 1958 m amsl) and a semi-urban site (Kathgodam; 29.3oN,79.5oE, 554 m amsl) in the Indo-Gangetic Plain (IGP). The analysis includes the NMHCs datasets from January 2017 to December 2020. The present analysis finds the diurnal variation with slightly higher values in noon has been observed in n-butane, i-butane, m-xylene, and p-xylene while bi-mode variations have been seen in ethane, propane, benzene and toluene. Noontime higher values indicate the dominance of the photochemistry whereas the importance of the convective boundary layer is reflected in the bi-mode variability. Ethane, toluene and benzene have shown consistent seasonal variation, with wintertime higher values while toluene and o-xylene showed higher values in autumn. In addition to this, with the help of an NCAR-MM photochemical 0-D box model, we test the sensitivity of the ozone formation regime over different levels of NOx and non-methane hydrogcarbons (NMHCs) and also find a greater ozone production rate [P(O3)] for aromatics (>8.5 ppbv/h) followed by alkanes (>6.5ppbv/h) and alkenes (>5ppbv/h) for the IGP site. This is the first time that we are presenting a detailed analysis of long-time systematic and continuous observations of NMHCs from the central Himalayan region.

Figure 1

Figure 1. The investigation of 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 hydroxide (OH) mechanism for NMHCs removal in the atmosphere at both the sites.