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.

A Statistical Analysis of the Relationship between Brown Haze and Surface Air Pollution Levels on Respiratory Hospital Admissions in Auckland, New Zealand

“…Brown haze is associated with increased concentrations of surface pollutants in Auckland [30,31,33]. The brown haze events are an indicator of elevated levels of all pollutants across Auckland, in particular in central Auckland.…”

“…From 2001 to 2011, a photographic record of brown haze events was created by the former Auckland Regional Council. For this period, every day during the cool season (May through to August) was classified as either a brown haze day or a non-brown haze day based on the intensity and duration of discoloration in the sky [30,31]. Figure 1 provides examples of clear and brown haze days over Auckland.…”

“…Figure 1 provides examples of clear and brown haze days over Auckland. The classification system has been used in previous research [30,31,33]. comprehensive brown haze photographic record, hospital admissions records, surface air pollution for four commonly measured air pollutants (rather than just one), as well as meteorological data.…”

“…Figure 1 provides examples of clear and brown haze days over Auckland. The classification system has been used in previous research [30,31,33]. …”

“…Another investigation, incorporating back-trajectory meteorological modelling, further identified the importance of long-term stable air masses and local pollution sources in its formation [31]. Studies have shown that much of the variance in Auckland's NO 2 levels during the morning rush hour commute can be explained by meteorological conditions, including high atmospheric pressure, cool temperatures and calm winds, conditions also conducive to brown haze formation and observed predominantly during the winter months [32].…”