High ozone at rural sites in India

Abstract. Past observations of O₃ at urban, rural and lower free tropospheric sites in India have shown generally low values rarely exceeding 60 ppbv. We show that this can not be generated to all over India. Surface ozone (O₃) concentrations are obtained from measurements in rural, urban and free tropospheric environments in January 2001 and 2002 as a part of Mobile Lab Experiments (MOLEX) conducted in western India. Elevated O₃ from 70 to 110 ppbv (nmole/mole) are recorded during afternoon hours at rural sites in downwind of major industrial region of Gujarat adjoining the Arabian Sea. Repeated observations during both the years indicate that this is a regular process in this region. The average background ozone is found to be 42±6 ppbv. The elevated ozone in the downwind site is about 60% higher than that in the major urban center and its surroundings and by a factor of 2 higher than the background levels of O₃ in this region. In comparison to the downwind observations; the ozone observed at the continental stations in rural (Gadanki), urban (Ahmedabad) and free tropospheric (Mt. Abu) sites in India are low and rarely exceeded 60 ppbv during the month of January. Forward trajectory analysis shows that the polluted plumes from this urban area can get transported more than 3000 km to the marine boundary layer over the Arabian Sea and the Indian Ocean within a week. Similar transport of pollutants from major urban sites like Delhi and other cities can enhance O₃ in their downwind rural sites and can affect the human health as well as vegetation significantly.