Synoptic weather types and morning rush hour nitrogen oxides concentrations during Auckland winters

Abstract

A synoptic climatological approach is applied to the study of weather air quality relationships in Auckland. Ten synoptic weather types are identified using a new, two-stage classification scheme that consists of obliquely rotated T-mode principal component analysis (PCA) followed by K-means clustering via Varimax rotated S-mode PCA. The weather types are analysed in relation to local meteorology, and to nitrogen oxides concentrations at two monitoring sites. Two anticyclonic weather types, characterised by calm, cold and relatively humid morning rush hour local conditions, are related to both high regional pollution and severe pollution episodes. Weather types associated with strong and persistent local westsouthwesterly winds help maintain good regional air quality due to enhanced ventilation. Under unstable cyclonic synoptic types, characterized by moderate to strong northerly winds, warm, humid, cloudy and rainy local conditions, emissions from the Auckland Central Business District can have significant air quality impacts on downwind residential areas. It is also suggested that the synoptic determinants of solar radiation intensities as well as temperatures appear important in controlling morning rush hour nitrogen oxides conversion chemistry. The weather type-air quality relationships vary to some extent from site to site due to the coupling of local meteorology and spatial variations in major emission sources. The findings demonstrate that the weather typing approach is not only capable of reflecting the effects of conditions associated with both emissions and local meteorology, but also provides an integrated evaluation of the impacts of synoptic circulation on both regional meteorology and air quality.