Air quality in Christchurch : an assessment of factors contributing to visibility degradation
Thesis DisciplineEnvironmental Science
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The cause of visibility degradation in Christchurch has been a concern for those responsible for management of the air quality for many years. In the late 1990s, Environment Canterbury, the local air quality regulator, become concerned that the regulatory measures proposed to reduce the 24-hour average mass PM10 concentrations, targeting emissions from domestic fires, would have little impact on reduced daytime visibility. The lack of improvement in visibility may then have advanced the perception that the air plan measures were unsuccessful in reducing PM10 mass. A research programme was designed to determine the causes of reduced visibility in Christchurch, and to examine variations in the composition of summer versus winter haze and implications for the management of reduced visibility in Christchurch. The study also examined the relationship between visibility perception and light extinction. The causes of reduced daytime visibility in Christchurch were assessed based on the results of an air quality monitoring programme. This included the measurement of light scattering by particles (Bsp), light absorption by particles (Bap) and light absorption by gases (Bag), which were summed with the Rayleigh scattering constant to give an estimate of total light extinction. The composition of particulate was measured using a combination of techniques including proton induced x-ray emission, ion chromatography and a series 5400 carbon analyser. Hourly average PM2.5 concentrations were also measured using a TEOM sampler. Results indicated maximum daytime light extinction values of around 1000 Mm⁻¹ during the winter compared to around 400 Mm⁻¹ during the summer months. The main contributor to light extinction was light scattering by particles which accounted for about 64 % on average, compared to 15 % Bap, 17% Bsg and 3 % Bag. The contribution of Bsp and Bap increased during the haze episodes to 71 % and 25 % respectively for the top I 0% of light extinction data. The main sources contributing to poor visibility during both summer and winter were found to be motor vehicle emissions (30% winter, 40% summer) and secondary particles (50% winter, 40% summer). Domestic fires contributed less than 10% on average during the winter. Other minor sources included soil, NO2 (Bag), and light scattering by gases (Bsg). The study indicates that measures proposed by Environment Canterbury to reduce emissions from domestic fires will not result in significant improvements in daytime visibility in Christchurch.