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Concentrations of ambient ozone (O-3) and nitrogen oxides (NOx) were measured continuously for a period of 12 months in the city of Jeddah from December 2011 to December 2012. Meteorological parameters, wind speed, temperature, and relative humidity were also monitored. Concentrations of ground O-3 were found to be highly dependent on the NOx diurnal cycle and wind speed. Nitrogen oxides were found to exceed air quality standards, especially in industrial sites, while O-3 concentrations were found to exceed 40 ppb, averaged over 1 h, on more than 24% of the measured days in the rural sites. Furthermore, they exceeded 30% in all other areas (i.e., the urban ones).

O-3 and NOx were inversely related. The highest average NOx concentration (96 ppb) occurred in a rural area downwind of a desalination plant, while the average O-3 concentration peaked in a rural area upwind of a desalination plant, reaching 63.5 ppb, although it also reached 72.6 in another rural area, and we consider this latter result as the background figure in the present study. The seasonal variations of O-3 were more distinct than those of NOx. To the best of our knowledge, this is the first report providing comprehensive background information on air quality in an arid area of the developing world.