Application of synoptic weather typing to an investigation of nocturnal ozone concentration at a maritime location, New Zealand

Relationships between weather types and the air pollutants ozone (O3) and nitrogen oxides (NOx) at night are explored through the analysis of two years of data from the east coast of the Auckland urban area. Principal component analysis is used to identify synoptic classes and corresponding weather types, while principal component regression is used to assess relationships between these and O3 and concentrations. Three synoptic classes (labelled cyclonic, anticyclonic and intermediate) consisting of seven discrete weather types were identified. Anticyclonic and cyclonic synoptic conditions were strong determinants of O3 and NOx concentrations. The intermediate class was the most persistent but had less significant effect on O3 and NOx concentrations. In cyclonic conditions, NOx was low due to increased ventilation. NOx concentration under anticyclonic conditions with cold and relatively dry air from Auckland urban areas was large, while that of O3 was generally small, as O3 is efficiently removed as a result of chemical titration with NO in the stable boundary layer. Under these conditions O3 can also be expected to be removed by dry deposition and NOx removed by the heterogeneous hydrolysis of dinitrogen pentoxide (N2O5). In cyclonic conditions and during times when winds from over the ocean prevailed, the concentration of O3 was relatively high, apparently due to a very weak O3 titration reaction and intrusion of regional-scale background O3. Temperature and humidity had no significant effect on nocturnal O3, while both affected NO2 concentrations.