A multi-scale assessment of the air pollution climatology of Alexandra, New Zealand using a combination of field observations and statistical and physical based modelling

Abstract

This thesis examines the relationship between atmospheric phenomena and particulate matter in Alexandra, a township located within an inland basin in Central Otago. The primary aim of this research is to define the air pollution climatology of Alexandra by characterising those atmospheric processes which control particulate matter concentrations at a variety of spatio-temporal scales. During the winter months, Alexandra suffers from some of the poorest air quality in New Zealand and currently exceeds the Air Quality National Environmental Standards (AQNES) for daily PM10 nearly fifty times a year. This provides a major challenge for the Otago Regional Council (ORC) as they look to meet the AQNES, which is three exceedances a year, by 2016. However, the relationship between emissions and PM10 is complex due to the climate of Alexandra being strongly influenced by its location within an inland basin. This makes the task of meeting the AQNES inherently difficult. Therefore, an understanding of the atmospheric processes that control PM10 is of importance to the ORC as it attempts to improve Alexandra’s air quality. This research utilised both long term meteorological data as well as observations from a field campaign, to represent atmospheric processes occurring at the synoptic, local and micro-scales. Long-term data from the ORC’s monitoring station (2006 – 2013) and a NIWA automatic weather station (2008 – 2013) were analysed to describe the relationship between PM10 and the synoptic and local-scale conditions over the winter months (May-August). A temporary automatic weather station was installed between 14 June 2013 and 20 August 2013 which included eddy covariance measurements in order to make inferences on the micro-scale processes present within the atmospheric boundary layer. Finally, a case study was also undertaken on 23 July 2013 which utilised vertical wind profiles observed by Sodar. Synoptic scale analysis indicated that poor air quality is not only linked to anticyclonic conditions, but was a feature of nearly all synoptic conditions, which highlights the effects of topographic sheltering on Alexandra’s climate. Even during times of strong westerly flow over the South Island, high PM10 concentrations were observed, indicating that the air mass within the Alexandra Basin is frequently decoupled from the gradient flow. The continuation of synoptic conditions was found to be an important control on air quality, with those circulations linked to high PM10 persisting longer and being preceded by synoptic conditions that are also linked to poor air quality. To define the local meteorological conditions that were most important to air quality in Alexandra, a Classification and Regression Tree (CART) analysis was used. A range of meteorological predictor variables recorded over seven winters were recursively partitioned in order to predict whether or not Alexandra’s daily average PM10 concentration exceeded the AQNES standard. The conditions defined by the CART analysis indicated that air temperature and wind speed predictor variables could explain much of the variation in PM10 concentrations, with exceedances most likely to occur during low overnight temperatures and/or calm conditions. Overall, the conditions defined by the CART analysis were shown to correctly predict whether or not an exceedance day occurred 76% of the time, indicating that the CART analysis is a robust tool for air quality assessment. An examination of the local wind field was undertaken in order to understand how local flows might influence dispersion in the Alexandra Basin. A down-valley flow was frequently observed entering the basin from the NE of the township, where the Manuhikirea River enters the basin. This wind was shown to skirt the township before being redirected to the E/SE, as a result of the interaction with both the local topography and the convergence of perpendicular drainage flows. The presence of a flow from the NW was also noted and this was deemed to converge with the down-valley flow over the township. This convergence, coupled with the effects of cold air pooling due to topographical blocking indicates that rather than increasing dispersion, the local wind field encourages the presence of stagnant air, further enhancing reduced dispersion conditions. Scaling parameterisations applied to eddy covariance measurements were used to characterise the micro-scale controls on the atmospheric boundary layer (ABL). It was found that PM10 concentrations in Alexandra were strongly influenced by both the stability and the height of the ABL, particularly during periods when emissions were being released (e.g. 1800h – 0000h and 0600h – 0900h). The characteristics of the ABL were shown to vary considerably throughout the night, moving between strongly stable stratification with a height < 30m to weakly stable stratification with heights exceeding 50m, indicating turbulent motion within the nocturnal boundary layer is present, which influenced PM10 concentrations. Finally, a case study assessing the atmospheric processes present during the occurrence of one of Alexandra’s bimodal evening peak events was undertaken using a multi-scaled approach. Using vertical wind profiles captured by Sodar, the presence of a Low Level Jet (LLJ) was observed, occurring above the local drainage flows. As the onset of the LLJ coincided with a return from strongly stable to weakly stable stratification, the LLJ is thought to produce wind shear above the Stable Boundary Layer (SBL), resulting in the introduction of downward turbulence from aloft. This phenomenon coincided with a decrease in PM10 after the first evening peak, and it is proposed that the shear introduced from the LLJ results in a re-coupling between the Residual Layer (RL) and the SBL, introducing clean air to the surface from above. The LLJ was observed to dissipate after reaching its evening maxima and it is thought that the re-coupling of the RL and the SBL reduces the valley pressure gradient by forming one, colder air mass at the end of the basin that leads to a decrease in the circulation. Once the LLJ subsided, a second evening peak was observed, which is credited to the re-accumulation of emissions that continued throughout the evening.