Correspondence Comment on “Identifying Fly Ash at a Distance from Fossil Fuel Power Stations” SIR: Flanders (1) makes a very useful and important contribution to the application of magnetic techniques to the study of air pollution particulates, using the coercive force field Hc to identify power station fly ash in dust samples. In general terms, we agree with his approach and also his findings that the Hc of dust samples may indicate the proportion of fly ash relative to the total amount of airborne magnetic material (1), but we would add some cautionary notes. First, there is a wealth of environmental magnetic results that show the difficulties of quantifying the magnetic component where there is evidence of variable grain sizes and mineral types. For example, the rapid oxidation of magnetite into haematite in fly ash (2) may be expected to affect the Hc values described. Other magnetic parameters, such as the ‘hard’ remanence expressed as the difference between the saturation (10 kOe) remanence and a moderate (1 kOe) or high (3 kOe) back field remanence can be used to semiquantify the haematite component (3, 4). Second, samples of dust in and around urban areas, especially close to ground level, contain magnetic particles from various sources (e.g., fly ash, vehicle exhaust, degradation products, emissions from metallurgical industries, surface soil) (5, 6). Our knowledge of each is presently inadequate, but it cannot normally be assumed that fly ash or surface soil magnetic particles dominate the magnetic properties. Third, the claim that distance-dependent deposition of magnetic spheres represents only a minor control on particle-size distribution and Hc values is counter to other evidence. For example, the mean particle size of fly ash in the first 30 km around a large coal-fired power station in Spain (2) was >10 µm, reducing to
