Precursor relationships, ozone formation and midday meteorology in Sydney

Precursor concentration distributions and the derived relationships which characterize the spatial distribution of emission sources in the Sydney airshed in 1975/6/7 are compared with precursor data obtained during the 1981/2 summer. The 1981/2 data shows that: 1. (i) NMHC point sources no longer dominate the character of Sydney's moring time urban air parcels and 2. (ii) morning time meteorology and/or overall 6:00–10:00 a.m. mass emission rates have changed, resulting in an increase in the frequency of occurrrence of moderate to high NHMC and NO x concentration events. The absence of dominant morning time NMHC point sources balances the increased precursor concentrations and gives rise to a predicted ozone concentration distribution for 1981/2 which is similar to that reported elsewhere for 1975/6/7. The marked reduction in the monitored frequency of occurrence of high ozone events since 1975/6/7 cannot be explained by changes in the characteristics of morningtime air parcels and it is suggested that the observed reduction may be due to a change in the midday (10:00 a.m.–4:00 p.m.) meteorology since 1975/6/7.