Abstract:
Four years (April 2003 - July 2007) of ozonesonde observations over Ahmedabad have been studied for the first time using ten days backward trajectories in the boundary layer (lower 2 km) and lower troposphere (2.5-4 km). Ozone data are classified according to the residence times of air-masses over the North-Western India (NWI), marine and Northern Africa/Southern Europe (NASE) regions. Ozone increases linearly with increasing residence days over the NWI region for about six days with maximum increase rate (boundary layer ~4.5±1.1 ppbv/day, lower troposphere ~3.4±0.8 ppbv/day) during spring and minimum during winter (boundary layer ~0.7±0.8 ppbv/day, lower troposphere ~0.8±0.7 ppbv/day). The analysis of surface ozone over Ahmedabad confirms that ozone increase with residence days is largely due to photochemical build up. The estimated background ozone corresponding to zeroth residence day is found to be significantly lower during summer-monsoon (~26.3±3.3 ppbv)than winter (~47.7±3.2 ppbv) within the boundary layer. The air masses mainly influenced by NWI region, marine and NASE regions are termed as regionally polluted, marine and long range transport (LRT) respectively. The regionally polluted ozone is found to be higher than the average ozone during spring and summer-monsoon by 22-41% within the boundary layer and by 9-12% in the lower troposphere.
The marine air shows lower ozone by 38% and 10% during spring and summer seasons respectively
in the boundary layer. LRT plays a significant role in the lower troposphere during spring and
summer seasons with an ozone enhancement of 9% and 27% respectively. The present work suggests that regional pollution and long range transport have significant influence on the seasonal distribution of ozone in the lower troposphere whereas the background ozone levels in summer-monsoon are mainly influenced by marine air mass over this region.
