During 19–22 January 2018, New South Wales experienced very hot conditions, with maximum temperatures between 34 and 41°C for four consecutive days in western Sydney. The temperature and meteorology on these days were typical of summer ozone episodes observed in western Sydney. However, ozone exceedances were observed only on Friday 19 January and Monday 22 January, and not during the intervening weekend.
This difference in exceedances was attributed to two factors. Firstly, nitrogen oxide (NOx) gases, a major precursor to ozone formation in urban areas, were found to be higher on the exceedance days. The differences in the NOx concentrations were due primarily to differences in emissions between weekdays and weekends, specifically the impact of increased vehicle emissions during weekday morning hours. These higher NOx concentrations were observed across Sydney in the early morning, and in the western regions of Sydney during the afternoon, concurrent with the ozone exceedances. Secondly, a bushfire in the Royal National Park, on the southern outskirts of Sydney, on Monday 22 January, increased ozone precursor emissions, causing a larger and more wide-spread ozone exceedance event than would be expected from regular emissions alone.
