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.
Highlights
In the urban centers of the world, high levels of ozone are generated by the emissions of nitrogen oxides and hydrocarbons from automobile and other combustion sources
We delineate the balance of ozone in urban Saudi Arabia based on measurements and model calculations that form a complete set of primary variables affecting its production and loss
During the peak ozone production period, between about 9:00 a.m. and noon, average net production rates range from 60–170 ppb/h. These rates are not sustained and give way to negative net transport, which means that not enough ozone is being produced in the local area to maintain the observed concentrations based on the NOx-volatile organic compounds (VOCs) chemistry
Summary
In the urban centers of the world, high levels of ozone are generated by the emissions of nitrogen oxides and hydrocarbons from automobile and other combustion sources. The air quality standards are established for criteria pollutants, which in the United States and many other countries are: NO2, O3, SO2, fine and coarse particles, SO2 and lead.[1,2,3] In large urban areas continual measurements of these pollutants are necessary to be certain that the standards are being met It is of scientific and practical interest to track the sources and sinks of ozone and the other criteria pollutants.[4] In this paper, we delineate the balance of ozone in urban Saudi Arabia based on measurements and model calculations that form a complete set of primary variables affecting its production and loss. The results provide a deeper look at urban atmospheric chemistry in a desert environment almost devoid of biogenic emissions and with cities isolated from each other making them virtual replicates of the same urban environment
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