Application of WRF Model for Simulation of High Ozone Episodes in Islamabad, Pakistan

Elevated ozone levels in ambient air of Islamabad have been observed frequently over a period of five years. High ozone episodes were identified by analysis of hourly ozone concentrations and exceedances than the standard limit of 66 ppbv. These episodes have been observed to be lasting for many days which is quite alarming. Two high ozone episodes were selected for thorough analysis; the first episode occurred during June 09-15, 2009 and the second high ozone event was observed during June 19-23, 2010. Observations of ozone precursors, meteorological parameters, back trajectories computed with HYSPLIT model and WRF model simulations have been incorporated in order to understand the origin and contributing sources of these events. The synoptic analysis has been conducted in order to appropriately assess the role of meteorology to high level of ozone during selected durations for Islamabad city. The regional background O₃ concentration for Islamabad has been estimated to be 31ppbv which shows the contribution of air advection into Islamabad that is not influenced by direct emissions in the city.

Contribution of emissions from the road transport to ozone formation seems significant as the atmospheric concentration of nitric oxide ( 41 to 120 μgm^-3) have been observed to exceed National Environmental Quality Standards. Along with the availability of precursor gases, high ozone levels also depend on the photochemical processes depending on meteorology. Transport of pollutants from the adjacent city of Rawalpindi adds to the degree of ozone formation in Islamabad. It is revealed that the stagnant meteorological conditions led to accumulation of ozone in the area causing high pollution episodes.