Water-soluble inorganic ions in atmospheric particles in New Zealand urban areas

Abstract

An extensive research program investigating atmospheric particles has been conducted in three New Zealand cities (namely Auckland, Christchurch and Hamilton), employing internationally recognised high volume and low volume samplers. The collected airborne particles have been studied in terms of seasonal changes, site correlations and differences, compositional differences between coarse and fine particles, differences between workdays and weekends, and day-night variations, based on analyses of the particulate concentrations of eight major water-soluble ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3- and SO42-) using ion chromatography. Site and seasonal differences were observed, with Hamilton having the lowest particulate ion concentrations, Auckland the strongest contribution of sea-salts, and Christchurch and Hamilton in winter a stronger contribution from home-heating to PM10. Four major types of sources to PM10 were identified in these cities by principal component analysis (PCA), namely sea-salts, wood burning, secondary aerosols, and resuspended soil. It has been found that meteorological conditions had large effects on the contribution of these sources. An extra chloride source from coal burning was also recognised in Christchurch and Hamilton winters through an enrichment factor study. PCA and multiple linear regression have been performed for source apportionment of PM10 in Christchurch winter and wood burning and secondary aerosols were found to have the largest contribution. Hence it is suggested that intensive wood and coal burning for home-heating, the sheltered geographic location, and the relatively stable atmospheric conditions are the major causes responsible for the serious air pollution problem in Christchurch winter. It is proposed that secondary aerosols have only major local sources in these three cities, and that fine secondary aerosols may be formed mainly via gas-to-particle conversion while in the Auckland and Christchurch winters coarse NO3- and nss-SO42- mainly on the surface of coarse sea salt particles. It is also proposed that fine particles in Christchurch winter might be more aged than those in other seasons and might be also more aged than those in the other cities. It is suggested by an ion balance study that in certain samples coarse particles may be neutral or alkaline but fine particles neutral or acidic. Significant chloride loss was detected with fine particles having more chloride depletion.