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 (O 3) and nitrogen oxides ( NO x ) 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 O 3 and NO x ( NO + NO 2 ) 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 O 3 and NO x concentrations. The intermediate class was the most persistent but had less significant effect on O 3 and NO x concentrations. In cyclonic conditions, NO x was low due to increased ventilation. NO x concentration under anticyclonic conditions with cold and relatively dry air from Auckland urban areas was large, while that of O 3 was generally small, as O 3 is efficiently removed as a result of chemical titration with NO in the stable boundary layer. Under these conditions O 3 can also be expected to be removed by dry deposition and NO x removed by the heterogeneous hydrolysis of dinitrogen pentoxide (N 2O 5). In cyclonic conditions and during times when winds from over the ocean prevailed, the concentration of O 3 was relatively high, apparently due to a very weak O 3 titration reaction and intrusion of regional-scale background O 3. Temperature and humidity had no significant effect on nocturnal O 3, while both affected NO 2 concentrations.