Ozone is a secondary pollutant formed through the interaction of VOCs (see Volatile organic compounds) and nitrogen oxides (NOx). The AQI for ozone in all capital cities was deemed ‘good’ from 1999 to 2008, and remained steady during this time. Only Canberra had ozone levels that were decreasing, but this result was based on limited evidence. Maximum levels of ozone in the capital cities have remained stable since SoE 2011. Sydney continues to have the greatest number of exceedances of the 4-hour ozone standard (Figure ATM36) and did not meet the 4-hour standard in 4 of the 5 years from 2010 to 2014 (note that the standard allows 1 exceedance). The past 5 years of the Melbourne timeseries show the trend increasing. The highest concentration, of 0.114 ppm, was recorded in 2014, but this was a single exceedance, and the standard has been met in Melbourne since 2010. Measurements of ozone in Canberra are included for the first time in this plot and were below the standard at all times.