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.

Eleven years of hospital admissions data for Auckland, New Zealand for respiratory conditions are analyzed using a Poisson regression modelling approach, incorporating a spline function to represent time, based on a detailed record of haze events and surface air pollution levels over an eleven-year period, taking into account the daily average temperature and humidity, the day of the week, holidays and trends over time. NO 2 was the only pollutant to show a statistically significant increase (p = 0.009) on the day of the haze event for the general population. Ambient concentrations of CO, NO and NO 2 were significantly associated with admissions with an 11-day lag period for the 0-14 year age group and a 5-7 day lag period for the 65+ year age group. A 3-day lag period was found for the 15-64 year age group for CO, NO and PM 10 . Finally, the incidence of brown haze was linked to significant increases in hospital admissions. A lag period of 5 days was recorded between haze and subsequent increases in admissions for the 0-14 year age group and the 65+ group and an 11-day lag for the 15-64 year age group. The results provide the first statistical link between Auckland brown haze events, surface air pollution and respiratory health. Medical institutions and practitioners could benefit from improved capacity to predict Auckland's brown haze events in order prepare for the likely increases in respiratory admissions over the days ahead.