Processes governing the surface ozone over a tropical hill station in the Western Ghats

Tropospheric ozone (O3) plays predominant role in atmospheric chemistry, exacerbates the air quality, and contributes to the climate change. Near the Earth's surface, O3 levels exhibit large heterogeneity due to diverse emissions of precursors and effects of meteorological conditions. Nevertheless, the understanding of key dynamical and photochemical processes governing O3 remains limited in the Western Ghats, a tropical region of immense significance in context of the geosphere-biosphere processes. In this regard, we combined surface O3 measurements at Ponmudi (8.758 degrees N, 77.114 degrees E,-1 km above mean sea level-amsl), a high-altitude site in the Western Ghats, with ground-and balloon-borne O3 measurements at western coast of India (Thumba: 8.542 degrees N, 76.858 degrees E,-3 m amsl) and photochemical box model simulations. O3 diurnal variation exhibits a small amplitude (3-9 ppbv) with lower values during the daytime, in contrast to substantial urban O3 build-up (25-37 ppbv) at Thumba. The influence of regional pollution from the urban boundary layer, downdraft of O3-rich air during night, and dry deposition processes are suggested to govern the O3 diurnal pattern over Ponmudi. Lower O3/CO and O3/NO2 ratios indicating lower O3 production efficiency, and dominant role of dynamics are consistently seen over the study region. O3 variability between Ponmudi and Thumba are found to be well correlated (r = 0.7-0.8) during pre-monsoon and post-monsoon seasons as the sea breeze circulation transport the airmass from coast to the Western Ghats during the daytime. Mean O3 levels at Ponmudi are typically lower than those above Thumba showing an effective O3 loss towards Ponmudi. Chemistry involving strong natural emissions (e.g., isoprene) as simulated by photochemical box model, and greater dry deposition tend to supress O3 build up over this densely vegetated tropical region. Unlike other high-altitude sites, O3 seasonal cycle at Ponmudi is associated closely with synoptic wind changes, and peaks during the winter owing to the transport from Indo Gangetic Plain/northern India. Our study highlights the roles of regional pollution as well as natural processes including biogenic emissions in governing the surface O3 variability over the Western Ghats.