Effect of atmospheric stability on the impact of domestic wood combustion to air quality of a small urban township in winter

In the winter of 2011, a field campaign was undertaken in the small township of Nelson, New Zealand to measure the vertical and horizontal distribution of concentrations of airborne particulate matter. The aim of this campaign was to improve our understanding of the causal factors which result in periods of very high concentrations of particulate pollution in small townships during winter where emissions are dominated by the combustion of wood for domestic heating. The results showed that mean hourly surface concentrations of particulates throughout the airshed were characterized by a distinctive diurnal cycle, with two peaks in concentration (one in the late evening and then, unusually, a second mid-morning). Although the timing and magnitude of hourly peak concentrations was variable throughout the valley, there was no evidence to suggest that regional or topographic flows played a significant role in the build-up of pollutants at any given location.Analysis of vertical profiles of black carbon showed that high concentrations of particulates were confined to the lowest 50 m of the boundary layer. Concentrations decreased with increasing height within this polluted surface layer. The atmosphere was very stable during the evening period. After midnight, a period of increased mixing was consistently identified throughout the lowest 100 m of the boundary layer and associated with the sudden cleansing of the surface and lower layers of the boundary layer. Throughout the observational period there was no evidence for the storage of pollutants aloft. Thus the vertical mixing of pollutants to the surface could not account for increased pollutant concentrations during the morning period. However, at this time the boundary layer remained stable and concentrations of black carbon were mixed through a very shallow layer. This suggests that despite lower domestic heating emissions in the morning, the reduced mixing volume is a likely cause of the observed marked peak in morning surface concentrations.