The geographical distribution of meteorological parameters associated with high and low summer ozone levels in the lower troposphere and the boundary layer over the eastern Mediterranean (Cairo case)

Abstract

In continuation of previous research for evaluation of the high ozone levels observed during summer time over the eastern Mediterranean, MOZAIC profiles collected at the airport of Cairo from 1994 to 2008 are analysed. Average profiles corresponding, respectively, to the highest and the lowest ozone mixing ratios for the 0-1.5km layer over Cairo in summer (JJA) (94 profiles) are examined along with their corresponding composite maps of geopotential height (and anomalies), vertical velocity (and anomalies), specific humidity anomalies, precipitable water anomalies, air temperature anomalies and wind speed at 850 hPa. In addition, backward trajectories arriving in the boundary layer over Cairo during the days with highest or lowest ozone mixing ratios are examined. During the 7% highest ozone days at the 0-1500m layer over Cairo, very high ozone concentrations of about 80 ppb on average are observed from the surface up to 4-5 km altitude. The difference in ozone concentrations between the 7% highest and the 7% lowest ozone days reaches maximum values around 60 ppb close to the ground. During the highest ozone days for both 1.5-5 and 0-1.5km layer, there are extended regions of strong subsidence in the eastern Mediterranean but also in eastern and northern Europe and over these regions the atmosphere is dryer than average. In addition, characteristic profiles with the highest ozone concentrations in the 0-1500m layer are examined in order to assess the influence of atmospheric transport and photochemistry on the ozone concentrations over the area.