The relationship between Brown haze, atmospheric boundary layer structure, and air pollution in an urban area of complex coastal terrain

Brown haze, observed over some cities during the winter months, has been found to be associated with poor surface air quality and negative health outcomes. While the link between the atmospheric boundary layer (ABL) structure and surface air quality is well-established, the degree to which boundary layer structure influences local brown haze formation is unknown. Using continuous ceilometer data from seven winters, we investigate the influence of the ABL structure in relation to surface air pollution and brown haze formation over an urban area of complex coastal terrain in the Southern Hemisphere city of Auckland, New Zealand. When days with severe brown haze are compared with those when brown haze is expected but not observed (based on favorable meteorology and high surface air pollution levels), the presence of severe brown haze is found to coincide with a significantly shallower boundary layer during both the daytime convective phase (similar to 48% lower) and the earlymorning nocturnal phase (similar to 28% lower). Severe brown haze is also found to be associated with significantly reduced morning transition growth rates (70 m h(-1)) compared to days on which brown haze is expected but not observed (170 m h(-1)). Our results also suggest the entrainment of pollutants stored in the residual layer may contribute to the severity of the haze. This study illustrates the complex interactions between the ABL structure, air pollution, and the presence of brown haze, and demonstrates the utility of a ceilometer instrument in understanding and predicting the occurrence of brown haze events.