4.1. Predictied PM2.5 Concentrations
The average PM
2.5 concentrations modelled for the CTM GMR domain (9-km × 9-km) for the entire calendar year of 2008, summer (December, January and February) and winter (June, July and August) for various modelling scenarios (as defined in
Table 1) are illustrated in
Figure 4.
The “base case” scenario depicts the areas of higher PM
2.5 concentrations which generally co-locates with populated areas, i.e., Sydney, Newcastle and Wollongong regions. Annual average PM
2.5 concentrations are below 7 µg/m
3. PM
2.5 over the Sydney region is generally higher in winter. In Newcastle, PM
2.5 is higher in summer, while it does not vary much over Wollongong between seasons. Annual average PM
2.5 concentrations are modelled to exceed the Australian National Environment Protection Measures (NEPMs) ambient air quality standard of the annual average PM
2.5 of 8 µg/m
3 over Upper Hunter region (areas of PM
2.5 greater than 8 µg/m
3 are highlighted with the red contour in
Figure 4a). The area of elevated PM
2.5 is consistent between summer and winter.
The contribution of natural sources including biogenic, sea salt and wind-blown dust emissions to the average PM
2.5 concentrations are shown in
Figure 4b. Natural sources contribute 2–4 µg/m
3 to the annual average PM
2.5 in land, 3–5 µg/m
3 to the summer average PM
2.5 and less than 3 µg/m
3 to the winter average PM
2.5, respectively. Contributions from human-made sources to average PM
2.5 concentrations are shown in
Figure 4c, with localised elevated PM
2.5 concentrations generally coinciding with populated areas consistent with Base case predictions (
Figure 4a). Human-made sources are predicted to contribute significantly to PM
2.5 concentrations in the Upper Hunter region (areas of PM
2.5 greater than 8 µg/m
3 are highlighted with red contour in
Figure 4c). They are also found to contribute significantly to localised elevated PM
2.5 in Sydney, Newcastle and Wollongong, with a greater extent projected in winter than summer.
Figure 4d–h depict the spatial distributions of contributions from several major human-made source groups defined in
Table 1. Industry, which includes all other industrial sources except power generations from coal and gas was modelled to contribute significantly up 7 µg/m
3 (and 8 µg/m
3) to the summer (winter) average PM
2.5 concentrations in the Upper Hunter region (
Figure 4d). PM
2.5 concentrations attributed to power station emissions are more elevated in localised regional areas coinciding with power station locations, and with contributions less than 1 µg/m
3 to the annual average PM
2.5 shown in
Figure 4e in those areas, with a slightly more contributions in summer than in winter. Contributions from power stations to average PM
2.5 are also modelled to be spatially dispersed over the NSW GMR due to emissions occurring from high stacks and the time taken for chemical transformation of precursors and secondary particle formation (
Figure 4e). The major human-made sources contributed to annual average PM
2.5 in the Sydney region can be easily identified as wood heaters (
Figure 4f) and on-road mobile vehicles (
Figure 4g). The wood heaters obviously only have contributions to PM
2.5 concentrations in winter and they contribute up to 3 µg/m
3 and 1 µg/m
3 in the populated area of Sydney and Newcastle, respectively. Contributions to average PM
2.5 concentrations from on-road mobile vehicles appear slightly higher in winter in Sydney (0.5–0.6 µg/m
3) compared to that in summer (0.4–0.5 µg/m
3). It was suggested to attribute to more emissions from cold-start operations in winter and also a better dispersion condition with stronger winds and higher boundary layers predicted in summer. At last, apart from industrial sources, the non-road diesel and marine sources demonstrate relatively significant contributions up to 1 µg/m
3 on average PM
2.5 concentrations in the Upper Hunter region (
Figure 4h), and they also have 0.2 µg/m
3 contributions to average PM
2.5 off the coastline near Sydney and Newcastle.
4.2. Major Chemical Components of PM2.5
To quantify the temporal and spatial variations in major chemical components of PM
2.5, five key chemical species of PM
2.5, such as particulate nitrate, particulate sulphate, particulate ammonium, sodium and elemental carbon, as well as total PM
2.5 mass were extracted from CCAM-CTM model predictions in the CTM GMR domain (9-km × 9-km) for the entire calendar year of 2008 at eighteen NSW OEH air quality monitoring stations (shown as red dots in
Figure 2) located in the Sydney East (Chullora, Earlwood, Lindfield, Randwick and Rozelle), Sydney Northwest (Prospect, Richmond, St Marys and Vineyard), Sydney Southwest (Bargo, Bringelly, Liverpool, Macarthur and Oakdale), Illawarra (Albion Park, Kembla Grange and Wollongong) and Newcastle (Newcastle) regions, respectively.
A regional average of total PM
2.5 concentrations and the major chemical species concentrations at each of the five regions for summer (December, January and February) and winter (June, July and August) in 2008 are shown in
Figure 5. The regional average total PM
2.5 concentrations in summer are between 5.2–6.1 µg/m
3, while the lowest PM
2.5 is found in the Sydney Southwest region and the highest one in the Newcastle region. Particulate sulphate contributes 0.79−0.95 µg/m
3 (15–16%) to the total PM
2.5 mass, followed by sodium and particulate nitrate that account for 0.63–1.06 µg/m
3 (12–18%) and 0.39–0.44 µg/m
3 (7–8%) to the total PM
2.5 mass, respectively. Elemental carbon had less contribution to the total PM
2.5 mass (0.04–0.18 µg/m
3 or 1–3%) compared to other species shown in
Figure 5a, while ammonium also plays a very minor role (0.01–0.04 µg/m
3 or less than 0.8% to the total PM
2.5 mass) in total PM
2.5 composition. The rest of other chemical species including primary and secondary organic aerosols, as well as chloride, calcium, magnesium, and potassium are integrated into the “other” category, as shown in
Figure 5a, which account for around 60% (3.23–3.59 µg/m
3) of the total PM
2.5 mass.
The regional average total PM
2.5 concentrations in winter shown in
Figure 5b are generally lower than those in summer. The most significant drop of total PM
2.5 mass from summer to winter is found in the Illawarra region (2.17 µg/m
3 or 38%). However, one exception with significant increase of total PM
2.5 mass is in the Sydney East region (1.77 µg/m
3 or 30%), which is apparently related to the increase of concentrations of elemental carbon as well as “other” species (including primary and secondary organic aerosols) in the PM
2.5 compositions. The trend of increase in elemental carbon mass and decrease in particulate sulphate mass from summer to winter is clear at each of the five regions. The mass of sodium also decreases across regions from summer to winter although it still accounts for nearly 20% of total PM
2.5 mass in the Newcastle region. The rest of other chemical species (“other” category) has similar proportion around 60% to the total PM
2.5 mass.
Particulate nitrate, sulphate and ammonium are produced by chemical reactions in the atmosphere from gaseous precursors (e.g., NO
2 and SO
2 from human-made sources) and they are dominant species in secondary inorganic aerosols. The regional average of total mass of particulate nitrate, sulphate and ammonium across regions predicted in summer of 2008 were 1.22–1.39 µg/m
3 and accounted for ~23% to the total PM
2.5 concentrations. The regional average secondary inorganic aerosols mass from our modelling results is slightly lower than what was found in Sydney Particle Characterisation Study, where the average mass of ammonium and sulphate was 1.61 µg/m
3 [
15]. The ratio between secondary inorganic aerosols to the total PM
2.5 mass in our study (23%) is close to what was found in chemical transport modelling works done in the Sydney Particle Study, which was around 20% in summer of 2011 [
21], however it is significantly higher than the ratio of 15% estimated from field particle measurements conducted at the same time.
Sodium is a major marker specie for source of sea salt. The sea salt emissions from waves breaking in the open ocean and the coastal surf breaks are begin transported to coastal regions where sodium is found to be accounted for up to 18% of total PM
2.5 mass in the Newcastle and Illawarra region in both summer and winter in our study. The sodium contributions to the total PM
2.5 mass are slightly lower in the three sub-regions across Sydney (9–12%), while the ratio is very close to what was found in Sydney Particle Characterisation Study, where average chemical composition of salt was 12% for all four sites in Sydney [
15]. It should be noted that chloride concentration has not been considered in this analysis and with that the contribution of sea salt to the total PM
2.5 discussed here may be underestimated.
The contributions from elemental carbon to total PM
2.5 mass in our study are generally below 5% in summer of 2008, and they are very close to the ratio of 6% estimated from field particle measurements conducted in summer of 2011 in the Sydney Particle Study [
21]. The elemental carbon contributions to total PM
2.5 mass increased significantly from summer into winter in our study, especially in the Sydney East and Sydney Northwest regions. The elemental carbon contributions in both regions are below 3% in summer but increase up to 13% in winter. The findings are consistent with the increased wood heater usage in winter in populated areas across Sydney.
4.3. Source Contributions of PM2.5
To investigate the major source contributions to the total PM
2.5 mass and the major chemical components discussed in
Section 4.2, the predictions from CCAM-CTM modelling for the entire calendar year of 2008 under various modelling scenarios (as shown in
Figure 4) are extracted at NSW OEH eighteen air quality monitoring stations across the Sydney East, Sydney Northwest, Sydney Southwest, Illawarra and Newcastle regions. The major source groups discussed in this section includes: power stations, wood heaters, on-road motor vehicles, non-road diesel and marine, industry, human-made other, biogenic and natural-other sources as their corresponding modelling scenarios listed in
Table 1.
The regional average major source group contributions (%) to the total PM
2.5 mass at each of the five regions in summer and winter of 2008 is shown in
Figure 6. In
Figure 6a, the natural sources apparently dominate the contributions to the total PM
2.5 mass in summer of 2008. Biogenic and natural-other sources together contribute 71% to the total PM
2.5 mass in the Sydney Northwest region and up to 85% in the Newcastle region. Among human-made sources, power stations and on-road motor vehicles together contribute around 15% to the total PM
2.5 mass in three Sydney sub-regions. While the industry has larger contribution of 12% to the total PM
2.5 mass in the Illawarra region. Non-road diesel and marine generally has contribution less than 3% to the total PM
2.5 across regions. In winter of 2008, as shown in
Figure 6b, natural sources are still the dominating contributors which contribute more than 60% to the total PM
2.5 mass in the Sydney Southwest, Illawarra and Newcastle regions. However, the wood heaters apparently make significant contributions and account for up to 40, 26 and 18% to total PM
2.5 mass in Sydney East, Sydney Northwest and Sydney Southwest regions, respectively in winter. Followed by the emissions from power stations and on-road motor vehicles that together contribute up to 16% of total PM
2.5 mass in winter. There is no significant change in average percentages of contributions from industrial sources (12–14%) and from non-road diesel and marine (3–5%) to total PM
2.5 mass between winter and summer.
The major source groups’ contributions to the secondary inorganic aerosols (including particulate nitrate, particulate sulphate and ammonium) are further illustrated in
Figure 7,
Figure 8 and
Figure 9. In
Figure 7, more than 40% of particulate nitrate mass come from human-made sources, while on-road motor vehicles and non-road diesel and marine certainly are the major sources among human-made sources that have greater contributions to particulate nitrate mass. Power stations have similar regional average contributions (below 5%) in most of the regions across summer and winter. Industry generally has more contributions across regions in winter than in summer.
In
Figure 8, natural-other sources (including sea salt and wind-blown dust) contribute generally around 75% of particulate sulphate mass across regions in summer, while the contributions from natural-other sources slightly decrease in winter. Power stations and the industry are the two most significant contributors from human-made source groups, together they are found to be contributed to 25% of particulate sulphate mass in the Newcastle region in winter and 21–23% of sulphate mass in the Illawarra region. It is shown clearly in
Figure 9 that human-made sources are the major sources of ammonium mass. More than 60% of ammonium mass are contributed from power stations and industry in summer across regions, while the relatively higher contributions from on-road motor vehicle are found in winter.
As shown in
Figure 10a, it is obvious that on-road motor vehicles and non-road diesel and marine are the two major contributors to elemental carbon mass across regions in summer. Emissions from on-road motor vehicles play more important role in contribution of elemental carbon in three Sydney sub-regions and Illawarra, while non-road diesel and marine is the leading contributor of elemental carbon in the Newcastle. Emissions from wood heaters become a more significant source of elemental carbon in winter (
Figure 10b), it contributes to 48–57% of elemental carbon mass across three Sydney sub-regions, while the regional average contribution percentage from on-road motor vehicles and non-road diesel and marine seem decrease compared to that in summer.
It is a challenge when comparing results from source-based methods in this study and from receptor-based methods (e.g., [
9,
14,
15]) because of the mismatching in source categories. As summarised in
Table 3, the top three major source groups contributed to total PM
2.5 mass, particulate nitrate, particulate sulphate, ammonium and elemental carbon at each of the five regions in the NSW GMR are highlighted in order with red, blue and green, it become obvious that apart from natural sources (“biogenic” plus “natural-other” source groups), “power station” is the first or second major source contributing to sulphate and ammonium mass in both summer and winter across regions, and it is also generally the third major contributors to the total PM
2.5 mass in summer. “Wood heaters” is the first or second major source contributing to total PM
2.5 and EC mass across three Sydney sub-regions in winter. “On-road mobile vehicles” is the top contributor to EC mass across regions, and it also has significant contributions to total PM
2.5 mass, particulate nitrate and sulphate mass in the Sydney East region. “Non-road diesel and marine” plays a relatively important role in EC mass across regions except Illawarra. Similar results are also found with “industry”, which is the first or second major contributors to sulphate and ammonium mass across regions, it is also the second or third major contributor to total PM
2.5 mass across regions.
4.4. Population-Weighted Annual PM2.5 Concentrations
The CCAM-CTM modelling results from various emission scenarios discussed in
Section 4.1 were applied to the method described in
Section 2.3 to estimate the major source contributions to population-weighted annual average PM
2.5. As summarised in
Figure 11a and
Table 4, the natural and human-made sources contribute 60% (3.55 µg/m
3) and 40% (2.41 µg/m
3) to the population-weighted annual average PM
2.5 (5.96 µg/m
3). The contributions from total human-made sources were further broken down into major source groups, i.e., wood heaters, industry, on-road motor vehicles, power stations and non-road diesel and marine as shown in
Figure 11b, while each of these major source groups contributed 0.75, 0.61, 0.46, 0.42 and 0.15 µg/m
3 to population-weighted annual average PM
2.5 and accounted for 31%, 26%, 19%, 17% and 6% in the total human-made sources contribution (40% in
Figure 11a).
The population of our study region (shown as NSW GMA in
Figure 2) was about 5.25 million in 2016 and densely populated in Sydney, Newcastle and Wollongong. Exposure to ambient air pollution is a major risk factor for global disease. From the study of Brauer et al. [
41], 87% of the world’s population lived in areas exceeding the World Health Organization Air Quality Guideline of 10 μg/m
3 PM
2.5 (annual average) in 2013, however, the decreases in population-weighted mean concentrations of PM
2.5 were evident in most of high income countries between 1990 and 2013. Australia was among one of those country who had the lowest country-level population-weighted estimates of ≤6 μg/m
3. The population-weighted annual average PM
2.5 modelled for 2008 in the NSW GMA was 5.96 µg/m
3 from our study, slightly lower than the most recent value for PM
2.5 mean annual exposure in Australia was 6.126 µg/m
3 as of 2016 [
42] based on the methods discussed in [
41]. The exposure modelling work presented in this section provides quantitatively estimation of the amount of PM
2.5 that people were exposed to. Results from exposure modelling are also a critical component and primary inputs to for health impact assessment (HIA).