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

Tropospheric ozone (O<inf>3</inf>) plays predominant role in atmospheric chemistry, exacerbates the air quality, and contributes to the climate change. Near the Earth's surface, O<inf>3</inf> 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 O<inf>3</inf> 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 O<inf>3</inf> measurements at Ponmudi (8.758° N, 77.114° E, ∼1 km above mean sea level–amsl), a high-altitude site in the Western Ghats, with ground- and balloon-borne O<inf>3</inf> measurements at western coast of India (Thumba: 8.542° N, 76.858° E, ∼3 m amsl) and photochemical box model simulations. O<inf>3</inf> diurnal variation exhibits a small amplitude (3–9 ppbv) with lower values during the daytime, in contrast to substantial urban O<inf>3</inf> build-up (25–37 ppbv) at Thumba. The influence of regional pollution from the urban boundary layer, downdraft of O<inf>3</inf>-rich air during night, and dry deposition processes are suggested to govern the O<inf>3</inf> diurnal pattern over Ponmudi. Lower O<inf>3</inf>/CO and O<inf>3</inf>/NO<inf>2</inf> ratios indicating lower O<inf>3</inf> production efficiency, and dominant role of dynamics are consistently seen over the study region. O<inf>3</inf> 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 O<inf>3</inf> levels at Ponmudi are typically lower than those above Thumba showing an effective O<inf>3</inf> loss towards Ponmudi. Chemistry involving strong natural emissions (e.g., isoprene) as simulated by photochemical box model, and greater dry deposition tend to supress O<inf>3</inf> build up over this densely vegetated tropical region. Unlike other high-altitude sites, O<inf>3</inf> 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 O<inf>3</inf> variability over the Western Ghats.