Temporal evolution of winter smog within a nocturnal boundary layer at Christchurch, New Zealand

In July 2000 (South Hemisphere winter), the (C) under bar hristchurch (A) under bar ir ($) under bar ollution (S) under bar tudy (CAPS2000) was performed in order to establish a comprehensive data set for documentation and analysis of nocturnal winter smog conditions in the Christchurch area. Field activities included meteorological surface measurements, tethersonde ascents, radiosoundings and sodar measurements. Air pollutant monitoring included CO, NO, NOx, O-3, PM10, PM2.5, and black carbon measurements near the surface, and for the first time vertical CO-profile measurements in the nocturnal boundary layer up to 100 m height. A prerequisite for nocturnal winter smog conditions is the evolution of stable stratification before the evening traffic and domestic heating reach a maximum. When stable stratification persists during domestic heating and road traffic in the morning, a second pronounced maximum of air pollutants evolves at around 0800 NZST. The meteorological measurements also revealed a complex nocturnal surface wind field, dominated by drainage winds from the Port Hills to the south and from the Canterbury Plains to the west of the Christchurch city area. A resulting convergence zone forms over the central parts of the city and is accompanied by low wind speeds. The position of the convergence zone varies during the night. These low winds over the city centre, in conjunction with stable stratification, favour the accumulation of air pollutants in the lowest tens of metres. The nocturnal winter smog situation ends with the erosion of the surface inversion at about 1100 NZST. It is shown from analysis of the vertical CO profiles that the level of air pollution in the Christchurch area depends on the height of the stable nocturnal boundary layer, which itself is governed by variations in the local wind systems.