Assessment of Metals in Fugitive Dust for Permitting in Ontario
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As part of the Ministry of Environment, Conservation and Parks’ (MECP) Procedures for permit application reports (Guideline A10 – Procedure for Preparing an Emission Summary and Dispersion Modelling (ESDM) Report) the A-10 Guideline, for air quality assessment consultants to follow, provides a chapter (Chapter 7) on how to assess the significance of contaminants and sources. The main aim of this chapter is to provide guidance on screening out insignificant sources and contaminants from air quality impact assessments and allows the proponent to focus on only those sources and contaminants released into the air that will be significant.
One particular section, Section 7.4, focuses on fugitive dusts; that is, dusts emitted from operations in open situations (not emitted through an identifiable vent or stack). This would include road dust, outdoor storage piles and outdoor material handling (e.g., at a gravel pit operation).
In particular, section 7.4 identifies the need for air quality impact assessment consultants to identify metals in fugitive dusts in certain facilities, as shown below:
In order to consider metals in fugitive dusts, if the facility already exists, then on-site sampling can be conducted to provide the proportion of total dust emitted which consists of metals. If sampling from road dust (paved or unpaved roads) for an air quality impact assessment the US EPA provides sampling protocols in their AP-42 set of documents to appropriately take samples of the “silt” (or suspensible) portion of road dust. Likewise, the silt portion of other fugitive dust sources can be sampled using sieving techniques as described in AP-42 Appendix C.1 and C.2.
However, once the silt portion of the fugitive dust source is separated there is still a need to chemically analyze the silt portion and identify and quantify the metals in the dust silt sample.
The usual method followed by air quality assessment consultants for bulk samples is to use methods developed by the US EPA in their “Compendium of Methods for the Determination of Inorganic Compounds in Ambient Air”. Especially the methods in Methods IO-3 “Overview of Chemical Species Analysis of Filter-collected Suspended Particulate Matter (SPM)” are often used for bulk samples in the absence of methods more specific to bulk samples.
Most of the methods used require significant digestion of the sample in preparation for analysis. For example, Method IO -3.4 “Determination of Metals in Ambient Particulate Matter Using Inductively Coupled Plasma (ICP) Spectroscopy” requires aggressive acid digestion of the dust sample before Inductively-coupled plasma (ICP) analysis. This aggressive digestion dissociates molecules allowing only the ionic concentration of metals to be determined. However, in that disassociation, information is lost on which compounds the metal atoms were originally part of. Therefore the results obtained will only provide the ionic concentration of metals and not tell us what compounds they were originally in. This may lead to incorrect further analysis, as explained below, in air quality impact assessment.
Environmentally Exposed Metals
Even if, in the facilities mentioned above, metallic-form metal particles are released it is unlikely they will remain in pure metallic form for very long, but likely converted to environmentally stable forms, mostly oxides. Therefore when this dust drifts off-site in the blowing winds any human inhalation will be exposed to (most likely) the oxides of the metals rather than the pure metallic forms.
One example would be release of aluminum-containing dust from facilities that manufacture aluminum parts. If the facility includes processes that cause the release of fine aluminum dust particles (say from aluminum part grinding or drilling operations) these may be released onto the grounds around the facility and then secondarily released from ground level by vehicle traffic over paved or unpaved roads. Sampling of road dusts would be conducted, by air quality assessment consultants, to analyze for Al content and, using the ICP technique, levels for the ionic concentration of Al would be provided. This value would then be plugged into emissions and dispersion calculations to determine off-site impacts of Al, as part of the air quality impact assessment.
However, it would likely be incorrect to treat off-site impacts as due to metallic Al as Al oxidizes rapidly to form a layer of oxide on the surface (termed “passivation” and contributes to Al’s characteristic of corrosion resistance), or Al2O3. So, in terms of human inhalation exposure, our lungs would really be exposed to Al2O3 rather than metallic Al. This has significant implications for demonstrating compliance as the air quality benchmark for metallic Al is 12 µg/m3, much lower than the benchmark for the oxide (120 µg/m3)(one must also account for mass conversion via stoichiometric relationships). It may be significantly more difficult, and in fact incorrect, to try to demonstrate air impacts compliance against the metallic form rather than the oxide form. Therefore, accounting for the conversion of the metallic to the environmentally more stable form is an important consideration in air quality impact assessments for facilities that are supposed to account for metals in those fugitive dusts.
Another, and perhaps more common example, is when metallic iron is exposed to the environment. Especially in fine dust form it converts rapidly to the oxide form (ferric oxide), aka “rust”. Again, the oxide form has a higher (easier) compliance point (25 µg/m3) versus the more difficult compliance benchmark for metallic iron of 4 µg/m3. These considerations, for air quality assessment consultants, also apply to welding fumes that exit a facility as a process fugitive (i.e., exit through open doors or windows in an unorganized way and not through dedicated vents). This is often the exit mode of welding fumes during summer operations at welding shops, where doors and windows are kept open to dilute welding fumes and to reduce worker exposure.
The overall conclusion for air quality impact assessment is that it is important to carefully consider the environmental chemistry occurring and not to simply treat lab results for metals analysis as the final answer and assume that the ionic concentrations provided are what is emitted in the fugitive dust cloud. One must consider the environmental chemistry and oxidizing atmosphere in converting, usually in an oxide and use the environmental compound instead where appropriate.