Spatio-temporal variation of atmospheric pollutants during the winter of 2011 in Nelson South

Abstract

High concentrations of atmospheric pollutants in small urban areas are recognised as a significant problem around the world. Periods of poor air quality in small urban areas generally occur in winter where emissions from domestic space-heating dominate and the atmosphere is stable. However, the main drivers for this specific, small urban area air quality problem are poorly understood. A field campaign (Nelson ’11) was launched in Nelson South in the South Island of New Zealand during the winter of 2011 (June, July, and August) to determine why some small urban areas su er from degraded air quality. Nelson was chosen as it is small city set within complex coastal terrain which experiences episodes of poor air quality as a consequence from domestic biomass combustion. To investigate this air quality issue, three ground-level PM10 monitoring sites were deployed in Nelson South with a fourth monitoring site operational 25 meters aloft when wind conditions allowed. Supplementary black carbon monitoring also occurred with portable and mobile monitoring equipment. Additionally, periods of intensive vertical monitoring were conducted with rope and pulley and tethered balloon systems within Nelson South’s nocturnal boundary layer. PM10 concentrations showed a distinct diurnal cycle with elevated concentrations in the evenings and nights. Significant local scale variability was observed among the monitoring locations in Nelson South. The complex, hilly terrain bounding Nelson’s urban area sheltered and contained pollutants within the Nelson South area by reducing horizontal pollutant advection. The topography of the area was therefore an important attribute for Nelson South’s air quality. During periods of poor air quality, a very stable nocturnal boundary layer was observed. The extreme stability extended to a height of 300 meters above the surface and reduced vertical pollutant mixing. Nocturnal low level jets were observed and their presence coincided with the subsequent removal of the pollutants at the surface. The suppressed and ine ective dispersion of biomass burning pollutants was concluded to be the key parameter to understand why poor air quality in Nelson South is often present during the winter. The complex relationships between topography, meteorology, and emissions on urban air quality were well demonstrated.