Characterizing changes in surface ozone levels at an urban site in Xi´an, China

The surface ozone levels over Xi´an, China were monitored every 5-min using the Model ML/EC9810 ozone analyzer from March 23, 2008 to January 12, 2009. The daily ozone concentrations ranged from 0.43 ppbv to 64.23 ppbv, averaging a yearly mean value of 16.04 ppbv. Seasonal variation of ozone levels showed a clear summer maximum and winter minimum pattern. The diurnal variation of ground ozone showed a typical unimodal distribution, which peaked between 1500 and 1600 of local standard time (LST), and had the minimum value between 0700 and 0800 LST. Significantly positive correlation was found between ozone concentration and ambient temperature, which indicated that intensity of solar radiation was one of the major factors influencing on the surface ozone production. In contrast, high relative humidity was unfavorable to the photochemical reaction on ozone produce. No weekend effect for the ozone concentration was found in Xi´an, which was mainly because the difference of vehicle number between weekday and weekend was not evident.