South Asia, particularly the Indo-Gangetic Plains and foothills of the Himalayas, has been found to be a major source of pollutant gases and particles affecting the regional as well as the global climate. Inventories of greenhouse gases for the South Asian region, particularly the sub-Himalayan region, have been inadequate. Hence, measurements of the gases are important from effective characterization of the gases and their climate effects. The diurnal, seasonal, and annual variation of surface level O3 measured for the first time in northeast India at Dibrugarh (27.4° N, 94.9° E, 111 m amsl), a sub-Himalayan location in the Brahmaputra basin, from November 2009 to May 2013 is presented. The effect of the precursor gases NO x and CO measured simultaneously during January 2012-May 2013 and the prevailing meteorology on the growth and decay of O3 has been studied. The O3 concentration starts to increase gradually after sunrise attaining a peak level around 1500 hours LT and then decreases from evening till sunrise next day. The highest and lowest monthly maximum concentration of O3 is observed in March (42.9 ± 10.3 ppb) and July (17.3 ± 7.0 ppb), respectively. The peak in O3 concentration is preceded by the peaks in NO x and CO concentrations which maximize during the period November to March with peak values of 25.2 ± 21.0 ppb and 1.0 ± 0.4 ppm, respectively, in January. Significant nonlinear correlation is observed between O3 and NO, NO2, and CO. National Atmospheric and Oceanic Administration Hybrid Single-Particle Lagrangian Integrated Trajectory back-trajectory and concentration weighted trajectory analysis carried out to delineate the possible airmass trajectory and to identify the potential source region of NO x and O3 concentrations show that in post-monsoon and winter, majority of the trajectories are confined locally while in pre-monsoon and monsoon, these are originated at the Indo-Gangetic plains, Bangladesh, and Bay of Bengal.