A climatological analysis of the incidence of brown haze in Auckland, New Zealand

Abstract

Brown haze has been observed over a number of urban centres in different geographical locations around the world. It is a clear indication of degradation in air quality above a city; however, little is known about its underlying causes. This study examines the incidence of brown haze over the subtropical city of Auckland, New Zealand between 2001 and 2011. Using photographs to document the incidence and severity of the haze and a combination of near-surface meteorological observations, reanalysis data and three-dimensional modelling approaches, a detailed climatology of the incidence of haze is developed. The results show that haze is observed during weekday mornings in the cool months, most commonly associated with cold, calm conditions. These conditions are most commonly prevalent with the slow eastward movement of anticyclones (classified as Kidson weather types H and HSE). However, the conditional probability of haze events occurring within these classes is less than 25%, and there is little consistency in the thresholds of surface meteorological conditions between classes. Re-analysis of the mean sea-level pressure at regional to synoptic scales reveals that the strength and stability of the anticyclone is important in determining the occurrence and severity of brown haze events. Brown haze development is associated with a strong NE-SW temperature gradient immediately prior to the event, and anticyclonic development is enhanced by a well-developed wave train across the Southern Ocean. Back trajectory modelling shows these conditions are associated with slow moving air masses and suggest that the source of pollution is most likely to be local in origin. The results demonstrate the importance of a detailed understanding of synoptic and regional scale meteorology in predicting urban scale pollution events, even when the source of the pollution is local.