Ozone balances in urban Saudi Arabia

Abstract Net ozone production rates averaging 130 ppb/h lasting 3–5 h are calculated for five cities of urban Saudi Arabia where hourly ozone and NO2 measurements were taken for a whole year. Concentrations of ozone in the cities are not unusually high at ~40 ppb during the peak 4 h, but are 50–60 ppb at altitude throughout the day. A mass balance of ozone reveals that the locations represented by the sites export ozone during the peak production periods before mid-day at average rates comparable to the net production, but later a substantial import of ozone is required to balance the budget. Concentrations of NO are high throughout the day (~14 ppb) and night (~11 ppb) and yet significant ozone concentrations remain during the night (~18 ppb). An explanation is that large amounts of ozone are generated in the surrounding areas and aloft by ozone precursors from the cities. It is brought back during the night by transport processes and reacts with fresh emissions of NO driven by nightlife resulting in an outflow of NO2 to create the observed balances. Over the whole day, the regions represented by the sites are net sinks of ozone. We describe and support this picture of ozone balances in urban Saudi Arabia.