The summer observations identified the major components of fine particles (PM2.5) as:
- Sea salt emissions from waves breaking in the open ocean and coastal surf breaks (34% of measured average PM2.5)
- Organic matter, including primary particles released from sources such as car exhaust, and secondary organic particles formed in the atmosphere (34% of measured average PM2.5). Up to 70% of organic matter assessed through the study could be secondary organic particles formed from gases released by biogenic sources.
- Secondary inorganic particles (15%), soil (11%) and elemental carbon (6%) were also present in significant amounts.
Secondary inorganic particles are fine particles of sulfate, nitrate and ammonium produced by chemical reactions in the atmosphere from sulfur dioxide, oxides of nitrogen and ammonia emissions. Elemental carbon is emitted directly to the air from sources such as vehicles, wood heaters and bushfires.
During the autumn observation program:
- Organic matter was identified as the major component of fine particles (PM2.5), including primary particles released from sources such as car exhaust, and secondary organic particles formed in the atmosphere (57% of measured average PM2.5).
- Elemental carbon (16%), secondary inorganic particles (15%), soil (7%) and sea salt (5%) were also observed to be present in autumn PM2.5 particles.
Compared with summer readings, levels of sea salt in fine particles (PM2.5) were lower during the autumn study. Organic matter comprised a larger portion of the PM2.5 particles measured. The contribution from elemental carbon was also larger compared with summer, while the secondary inorganic contribution was about the same.
Chemical transport modelling
Chemical transport modelling indicated that the release of volatile organic compounds (VOCs) from vegetation was a major source of secondary organic particles during summer, whereas wood heaters were a dominant source of such particles in autumn.
In summary, this combined monitoring-modelling study showed seasonally varying composition of fine particles, with natural emissions such as sea salt and biogenic emissions (VOCs from vegetation and bushfires) contributing significantly to background PM2.5 concentrations. Secondary particles formed in the air were modelled to account for over 40% of the PM2.5 concentrations measured during the autumn and summer observational periods.