Diurnal and seasonal variabilities in surface ozone at a high altitude site Mt Abu (24.6°N, 72.7°E, 1680 m asl) in India

Measurements of surface ozone, CO, CH4 and oxides of nitrogen have been made at a high altitude site Mt Abu (24.6°N, 72.7°E, 1680 m asl), India for the period 1993–2000. Diurnal patterns in ozone do not show daytime in situ photochemical buildup at this site throughout the year. On the contrary, lower ozone mixing ratios are observed during the day. Interestingly, diurnal pattern changes significantly after the northeast monsoon period (winter) and shows a unique double hump in spring at this site. Seasonal variation in ozone shows a pronounced maximum (monthly average about 46 ppbv) in the late autumn and winter, unlike many other global sites. Local pollutants are major contributors to the ozone levels during this period, while regional transport plays a role in spring. Lowest ozone mixing ratio (monthly average about 25 ppbv) is observed during southwest monsoon period (summer), when clean marine air from the Arabian Sea and Indian Ocean reaches to this site. The unique meteorology over this region seems to play an important role in seasonal as well as in diurnal variations in ozone. Background and continental ozone levels are estimated to be 33.4±13.3 and 48.1±9 ppbv, respectively, over this region of India. A correlation study between ozone and CO indicates possibility of incomplete photochemical processes over Asia. Annual average mixing ratios of CO, CH4 and oxides of nitrogen are observed to be about 131.4±35.8 ppbv, 1.63±0.04 ppmv and 1.5±1.4 ppbv, respectively.