Vertical distribution of tropospheric ozone over Cairo, Egypt.
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Cairo is a key location that is situated on the southern and eastern boundaries of the Mediterranean Basin, where summer tropospheric ozone levels are known to be elevated as a result of the persistence of a semi-stationary anticyclone that favours northerly flow from Europe, where anthropogenic emissions of ozone precursor gases are high. Strong levels of insolation, the absence of wet removal mechanisms, and low deposition velocities over the Mediterranean Sea further promote the summer enhancement of ozone. Ozone profiles at Cairo, recorded by MOZAIC (Measurement of OZone and wAter Vapor aboard In-service AirCraft) aircraft, were examined with a view to assessing the relative influence of a range of factors on the vertical distribution of tropospheric ozone. These included long-range transport of ozone and precursor gases from Europe, North America and Asia, assessed through back trajectory analysis with the aid of the HYSPLIT (Hybrid Single Particle Integrated Trajectory) modelling programme. The influence of local pollution sources was determined using local pollution monitoring data, satellite measurements of nitrogen dioxides (N02) and MOZAIC carbon monoxide (CO) data. Results show that lower and mid-tropospheric ozone values at Cairo are enhanced in summer relative to other seasons, with high upper tropospheric values occurring in February and April. The upper tropospheric variability is attributed to stratospheric intrusions during the movement of the tropopause which is consistent with the known springtime enhancement due to stratospheric-tropospheric exchange (STE). The lower tropospheric summer enhancement is linked to the effects of local pollution and polluted air masses originating from Europe. This summer ozone enhancement extends to a height of 8 km, which is fairly unusual for the region. The mid-tropospheric ozone enhancement appears to be a unique feature observed over Cairo, as other Mediterranean cities such as Athens, Greece usually display peaks in the upper and lower troposphere only. Therefore this enhancement is of considerable interest as it is unique to the region. iii In the mid-troposphere mean ozone values in summer (JJA) range between 70-80 ppbv, with values approaching 100 ppbv on individual days. Investigations into the probable causes of this enhancement suggest that the enhanced ozone is not created in the midtroposphere due to low levels of ozone precursor gases occurring in the mid-troposphere. Further, convective uplift of near-surface ozone is unlikely to occur as local pollution is confined to below 1000 hPa. It is therefore suggested that the enhanced ozone in the midtroposphere is being brought into the region by the long-range of polluted air masses from distant sources. Hierarchical classification of ozone profiles using the Statistical Package for the Social Sciences (SPSS version 11, 2001) programme allowed for the determination of least and most polluted profiles to emerge, which when related to air mass origins, highlights the significant role of long-range transport to mid-tropospheric ozone summer enhancement.