Emission Source Tracer Elements in Ambient PM: Demonstrating Chemical Complexity and the Implications for Health Effects
TERESA MORENO (1), Xavier Querol (1), Andrés Alastuey (1), Wes Gibbons (2)
(1) Institute of Environmental Assessment and Water Research, IDAEA, CSIC, Barcelona, Spain (2) AP 23075, Barcelona 08080, Spain
Abstract Number: 68
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Last modified: October 13, 2009
Working Group: sq3
It is increasingly clear from analytical datasets on particulate matter obtained from standardised air monitoring stations that there is great variability in the chemical composition of inhalable inorganic aerosols both in space and time. This chemical complexity is best demonstrated using selected elements which, combined with source apportionment techniques such as positive matrix factorisation, allow differentiation between geological and anthropogenic sources. The commonest geological elements in particulate matter are associated with felsic silicates and so include Si, Al, K, Na, Ca, Rb and Li. However, some of the more useful tracer elements for crustally-derived aerosols are those present in small, durable accessory minerals such as zircon (Zr, Hf), rutile (Ti, Nb, Ta), and monazite (La, Ce). With regard to technogenic tracer elements, these are usually best selected from a range of metals and metalloids which are themselves commonly toxic and therefore implicated in health effects. Thus, among the most useful industrially-derived tracers are Zn, Pb, As, Cu, Sb, Mn and Cd, all of which can consistently be demonstrated to concentrate in the finer, more deeply inhalable PM fraction of ambient air, and are therefore more likely to become involved in inflammation and oxidative stress after inhalation.
Tracer elements are sometimes highly specific in their origin and as such can be used to identify particular pollutant sources, such as Mn from ferroalloy plants or Sr from firework combustion. In such cases monitoring campaigns must be targeted at individual plumes, although given the fact that urban PM may derive from large numbers of potential sources a strong case can usually only be made using a large chemical database supported by detailed meteorological data: in the México City Metropolitan Área for example there are an estimated 30,000 industries, many of which use metallurgical processes. It can also be useful to examine element ratios, such as Cu/Sb which differs between traffic brake emissions and waste incineration fly ash. Similarly, some technogenic processes produce distinctive “unnatural” geochemical fractionation patterns, as is the case in emissions from oil refineries using La-rich zeolitic fluid catalytic converters (FCC) and from power stations burning the same refined oils. Such FCC-related emissions will show La/Ce values much higher than the 0.5 typical of uncontaminated geological materials, in direct contrast to traffic tailpipe particles derived from the abrasion of Ce-rich vehicle catalytic converters which show the opposite trend.
Given the chemical complexity of ambient atmospheric particulate matter, legislation concerned only with measuring physical PM mass concentrations fails to address health effects linked to the variability in potential toxicity of inhalable urban aerosols.