Diurnal and seasonal variability of surface ozone and NOx at a tropical coastal site: Association with mesoscale and synoptic meteorological conditions

Simultaneous measurements of near-surface ozone, NOx (NO + NO2), and meteorological parameters were carried out at the tropical coastal location of Trivandrum (8.55 degrees N, 77 degrees E) in India from November 2007 to May 2009. The data have been used to investigate the diurnal and seasonal patterns of ozone and its precursor, NOx, and also the interdependence of these two chemical species. The diurnal pattern is found to be closely associated with the mesoscale circulation (sea breeze and land breeze) and the availability of NOx. The daytime peak in ozone extends until the onset of land breeze, which brings in NOx for titration of ozone. Near-surface ozone concentration reaches peak values during the postmonsoon or winter months and shows minima during the summer or monsoon season. The high ozone concentration during winter is due to the presence of northeasterly winds that transport precursor gases to the site. The daytime concentration of ozone is found to be directly linked to the nighttime level of NOx. The present analysis reveals that one molecule of NOx or NO2 is responsible for the formation of about seven to nine molecules of ozone. A study of satellite-derived tropospheric ozone and total ozone has shown that tropospheric ozone contributes 8%-15% of total ozone over this site and near-surface ozone contributes 34%-83% of tropospheric ozone. The seasonal pattern of tropospheric column ozone is similar to that of tropospheric NO2.