Particle Number Emissions and Source Signatures of an Industrial Facility L. MORAWSKA,* G. R. JOHNSON, C. HE, G. A. AYOKO, M. C. H. LIM, C. SWANSON, Z. D. RISTOVSKI, AND M. MOORE † International Laboratory for Air Quality and Health, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia The work presented was conducted within the scope of a larger study investigating impacts of the Stuart Oil Shale project, a facility operating to the north of the industrial city of Gladstone, Australia. The aims of the investigations were threefold: (a) the identification of the plant signatures in terms of particle size distributions in the submicrometer range (13-830 nm) through stack measurements, (b) exploring the applicability of these signatures in tracing the source contributions at locations of interest, at a distance from the plant, and (c) assessing the contribution of the plant to the total particle number concentration at locations of interest. The stack measure- ments conducted for three different conditions of plant operation showed that the particle size distributions were bimodal with average modal count median diameters (CMDs) of 24 (SD 4) and 52 (SD 9) nm. The average of all the particle size distributions recorded within the plant sector at a site located 4.5 km from the plant, over the sampling period when the plant was operating, also showed a bimodal distribution. The modal CMDs in this case were 27 and 50 nm, similar to those at the stack. This bimodal size distribution is distinct from the size distribution of the most common ambient anthropogenic emission source, which is vehicle emissions, and can be considered as a signature of this source. The average contribution of the plant (for plant sector winds) was estimated to be (10.0 ( 3.8) × 10 2 particles cm -3 and constituted approximately a 50% increase over the local particle ambient concentration for plant sector winds. This increase in particle number concentration compared to the local background concentra- tion, while high compared to the clean environment concentration, is not significant when compared to concentrations generally encountered in the urban environment of Brisbane. Introduction Emissions from stationary sources such as industrial plants, power plants, and refineries are a major contributor to global atmospheric pollution. Inventories of emissions in relation to the pollutants regulated by air quality legislation are conducted routinely in most developed countries, and therefore, there is a relatively large body of information available concerning the relative contributions of individual facilities to the total emissions of these pollutants. However, there is much less information available on the contribution of individual industrial facilities to the actual airborne concentrations of the pollutants on different spatial scales, including in immediate proximity to and at various distances from a plant. There is even less information available on the contributions of industrial facilities to ambient concentra- tions of unregulated pollutants which are nonetheless potentially associated with health and/or environmental impacts, as is the case for submicrometer particles. The reason for the former is that source contribution to the pollutant concentrations is not spatially uniform owing to the effects of local meteorological conditions and topography. Such information could be obtained through extensive modeling; however, such modeling is not always conducted in association with, or as a part of, emission inventories. Validation of pollutant dispersion and concentration models would also require an adequate body of monitoring data. Yet, such information is of importance in relation to human exposure and risk assessment and, in turn, risk control. The need for information on airborne submicrometer particles has been highlighted in recent years by the emergence of scientific evidence that these very small particles may be more significant from the point of view of health than the larger particles or gaseous pollutants (1). A large variety of industrial facilities involve combustion of fossil fuels or biomass, and combustion processes are the most significant source of these particles in the air. Submi- crometer particles are normally measured in terms of their number concentration: the numbers of particles emitted and present in the air are large, yet their mass is very small. Measurements of particle number concentration require state of the art instrumentation, and there are no standard procedures for conducting such investigations. Despite the complexities involved in measuring airborne submicrometer particles, the value of such measurements lies in the fact that they are conducted in real time and therefore provide data suitable for the investigation of spatial and temporal variations in particle concentrations. In ad- dition, the size distribution of the particles can be used as a source signature, thus providing insight into contributions from individual sources. Source apportionment is compli- cated by the fact that ambient air contains a dynamic mixture of pollutants emitted from various sources. Therefore, it is a mixture which undergoes continuous change in time as the interactions between the pollutants take place and as the components of the mixture are removed from the air due to the presence of various sinks. This difficulty is further complicated by the fact that specific emission characteristics are rarely unique to a particular source. The particle physical characteristic which is most ap- plicable as a source signature is the number size distribution. When a single pollution source is investigated and when it operates under steady-state conditions (for example, steady parameters of the combustion process), the size distribution obtained is likely to have one or two distinctive peaks. Those peaks are called modes of the distribution. In the case of a mixture of particles from various sources and of different size distributions, the measured size distribution may or may not display individual peaks from the contributing sources and thus may or may not be used for source identification. In addition to the mixture problem, processes including coagulation and atmospheric reactions * Corresponding author phone: +61 7 3864 2616; fax: +61 7 3864 9079, e-mail: l.morawska@qut.edu.au. † National Research Centre for Environmental Toxicology, 39 Kessels Rd., Coopers Plains, Brisbane, QLD 4108, Australia.