Characterising and Assessing Fine Particle Concentrations in the Hunter Valley

Particle, or dust, emissions from open cut coal mining have been the subject of significant investigation over a number of years. Previous ACARP projects investigated the issues surrounding particle emissions from coal mining in the Hunter Valley (Holmes Air Sciences 1999; Richardson 1999; Bridgman et al. 2002; Richardson 2002). The most recent of these, ACARP project C13036, investigated the use of optical samplers and used meteorological data to provide information on the likely origin of particles observed in the PM10 and PM2.5 size components. However, with the increasing focus on the human health impacts of atmospheric fine particles, and the suggestion that the finest particles may be more significant for human health impacts, the current project has focused on providing data on PM1 composition for the Hunter Valley.

The objectives of the current were:

• extend the measurement program from C13036 for a further 12 months to provide a more robust set of PM10 and PM2.5 data for source determination;
• obtain detailed PM1 chemical composition and SEM/TEM determination of size and composition of particles within the PM1 fraction in order to provide guidance as to their likely sources;

The major findings from the project include:

OSIRIS and TEOM measurements:

• Further data were obtained with which to correlate the optically based instrument the OSIRIS with a TEOM for PM2.5. The results showed a variation in the correlation for the Singleton site compared with the data from Nelson et al.,(2007) , but showed reasonable agreement with the results for Glenville. As indicated in Project C13036 further work is required to determine corrections factors to adjust TEOM, GRIMM and OSIRIS data with regard to the Reference Method, for the Hunter Valley.
• The data showed that the annual NEPM PM2.5 advisory target of 8 µg/m3 is likely to have been exceeded at all sites during the three year time frame of this study. These results indicate the difficulties which may be encountered in  the Hunter Valley in achieving the annual PM2.5 target were it to be adopted in the future as a compliance guideline. However, these results need to be treated with some caution due to the lack of direct harmonisation between the OSIRIS results and the reference method.

PM1composition

Crustal Material

The PM1 samples collected and analysed for the major crustal elements Al and Si show two important features. These are;

• The Al and Si concentrations are reduced in the PM1 component relative to the 1<PM<2.5 μm component showing that the contribution of crustal material decreases in the finer particle component

• The Singleton data show lower concentrations of these two elements when compared with data from Glenville. The Glenville site is closer to mining operations.

The SEM data show that larger crustally derived particles have generally disappeared from the samples at the 0.56 μm cut-off.

Seasalt

The changes which occur when seasalt aerosols react were discussed in some detail in the previous ACARP project report ((Nelson et al. 2007).  The information in the earlier report was primarily based on the bulk chemical analyses of PM filters by multi-element ion beam analysis. The SEM studies in the current project have confirmed the size related Cl depletion of seasalt (NaCl) particles. As examples, in the 3-4 µm cubic seasalt crystals captured on the MOUDI stage 2, almost all have a stoichiometric Na:Cl atomic ratio of ~1:1. Conversely, those salt derived particulates captured on the finer impaction stages all show the effects of significant chlorine depletion and replacement by the (NO3)-ion.

Carbon

Two types of carbon commonly contribute to particle concentrations. These are elemental carbon and organic carbon. The sources of both types of carbon are generally combustion processes. In the Hunter Valley another potential source of carbon is from coal dust, however, as this is produced from mechanical attrition processes this source is likely to be insignificant in the PM1 component of fine particles. This is largely supported by the SEM observations which show little evidence of coal like particles in the smallest size fractions.

The PM1 samples subjected to EC and OC analysis generally shows a higher ratio of EC/OC at Singleton compared with Glenville and also show an increase in the ratio for the PM1 component compared with the 1<PM<2.5 µm component at Singleton. However, the limited data obtained from the current study make it difficult to draw broader conclusions

Secondary aerosol

Secondary aerosol in the atmosphere forms from chemical and physical processes involving species initially formed as gases reacting to form new species which can condense onto existing particles or form new ones. This is a very complex area of science with significant research over the past decade attempting to quantify the formation and significance of secondary aerosol.

Potential sources of precursors for secondary particle formation include both anthropogenic and biogenic sources. Evidence for secondary aerosol formation has come from the SEM data in the current study and suggests that the secondary aerosol (identified as soft aerosol in the current study) dominates the 0.56 µm cut-off stage and lower.

Bioaerosols

Bioaerosols are ubiquitous in the atmosphere and include pollens and spores, fungi, algae and fragments of other living matter (Matthias-Maser et al. 2000). A wide range of this biological detritus was observed on all MOUDI stages.  These ranged in size from material collected on the 18 µm MOUDI stage down to identifiable material at ~ 0.3 µm.  

Overall findings for Hunter Valley PM1

In broad terms the following comments can be made concerning the PM1 component of the airborne particle distribution in the Hunter Valley:

• Particles contributing to the greater than PM1 component of airborne particles are more likely to arise from
• mechanical attrition processes, whether natural (windblown) or anthropogenic (including mining, agriculture and road dust)
• power generation
• seasalt and its degraded products and  
• bioaerosols
• Particles contributing to the PM1 component also contain material from the above sources but increasingly become dominated by the products of combustion processes or direct gas to particle conversion processes that occur naturally in the atmosphere.

In the Hunter Valley direct sources of fine particles from combustion processes include motor vehicle exhaust (diesels in particular, which would include road transport and possibly a significant contribution from mine vehicles and machinery), domestic wood burning and bush fire smoke. While secondary particle formation will also be occurring, the importance of this source is still the subject of intense scientific investigation worldwide and beyond the scope of the current project. Nevertheless the current project has shown that, soft aerosols, believed to be the products of atmospheric chemical processes form the bulk of the ultra fine material.

Requests for this project report  will also receive a copy of C13036.