Characteristics and Causes of Peak Ozone Concentrations : A Case Study of the Large Maritime Urban Location of Auckland , New Zealand .

Tropospheric ozone (O₃) is a key component of photochemical smog and brown haze that has become a regular feature of the atmospheric environment of the greater Auckland urban area.  Despite generally low surface concentrations, there is growing concern about the region’s susceptibility to high ozone levels in the future. The origin of peak O₃ concentration is investigated using three years of data from a location on the east coast of Auckland urban area. The full O₃ dataset is compared with its two subsets, namely, low O₃ (1 to 10^th percentile) and peak O₃ (>90^th percentile) data. Variation in nitrogen oxides concentration and meteorological variables are examined in the three datasets. The highest frequency of peak O₃ concentrations occur in spring and winter during the afternoon hours under moderate temperature and moderate to high winds speeds. A large number of these peak O₃ events belong to offshore maritime winds coming from Hauraki Gulf. Three processes appear to be responsible for peak O₃ concentration, namely, photochemical reaction, advection via maritime winds, and down welling of O₃ from upper air. Peak O₃ concentration in any given season is the sum of O₃ introduced by one or more of these processes. The results suggest that regional background O₃ is mainly responsible for peak O₃ levels in winter and spring; the known ‘same-day’ and ‘next-day’ mechanism may also have some contribution to peak O₃ events especially in winter. Peak O₃ concentration events in autumn and summer are mainly attributed to local photochemical O₃ via same-day reaction while transport of regional background O₃ through horizontal and down welling from upper troposphere also contributed towards peak O₃ events. The very low incidence of peak O₃ in autumn and summer months is likely due to dry deposition of O₃ over aerosols particles and reduction of background O₃ levels by NO titration of O₃.  The study demonstrates the complex nature of peak ozone events for an urban area in a maritime location. Despite generally low surface concentrations in the Auckland region currently, the results reveal the potential for and susceptibility to high ozone levels in the future.