Correspondence Comment on “Tracking Petroleum Refinery Emission Events Using Lanthanum and Lanthanides as Elemental Markers for PM2.5” SIR: In their 2007 paper, Kulkarni et al. (1) attributed 15.9 µg/m3 PM2.5 at an ambient monitor (averaged over two days) to an emission event at a Houston area oil refinery but they could not explain why the average PM2.5 level at that site was within only 0.7 µg/m3 of the average for three upwind monitors. Absent any larger upwind-downwind PM2.5 gradients, the magnitude of the reported impact can only be explained by inaccurate assumptions or possibly other errors in the execution of their approach. From midday August 1 to midday August 3, 2006 (the authors’ assumed this was the period of impact), the average PM2.5 level was 30.7 µg/m3 at Channelview, the closest downwind monitor and where the highest impact was reported. Over the same period, the average PM2.5 levels at three upwind and two other downwind monitors ranged from 28.9 to 31.6 µg/m3 and 29.6 to 31.5 µg/m3, respectively, indicating a uniform regional haze that raised PM2.5 levels to more than twice the annual averages upwind and downwind of the release. The regional haze is attributed elsewhere to Saharan dust (2). Time series patterns (Figure 2 of Kulkarni, et al.) show that PM2.5 levels remained spatially uniform throughout the episode as the levels at all Houston-area monitors increased and decreased almost simultaneously first with the approach and then passage of the regional haze, respectively. The absence of any distinguishing spike in the Channelview time series attributable to the emission event can only be reconciled with the reported impact by assuming that a hole in the regional haze hovered over that monitor throughout the episode, where PM2.5 concentrations attributable to regional haze were less than half the regional average and, coincidentally, the deficit relative to the regional average was equal to the PM2.5 concentration attributed to the event. Alternatively, the discrepancy between the observational evidence and the authors’ calculations may reflect the uncertainty of their approach. The emission event was caused by a crack in the secondary cyclone of a fluid catalytic cracking unit (FCCU) allowing spent catalyst to be released, almost all to the ground according to a publicly available report (3). The reported source attribution was based on measurements of rare earth elements (REEs) in downwind air samples, previously published catalyst speciation profiles, and three key assumptions: (1) the older speciation profiles fit the catalyst released during the event, (2) all the REE mass detected on sample filters (above background) accumulated over two days, and (3) the emission event was the only source of REE enrichment. While noting these assumptions, the authors did not address the size of potential errors, which as shown below could be an order of magnitude or more. The reported FCCU impacts were estimated by expressing the mass balance of REEs during the emission event as REEambientPM2.5 ) SFCCUcontribution × FFCCUcatalyst + constant (1) where F is the mass concentration of REEs in the catalyst and S is the contribution of FCCU catalyst emissions to 10.1021/es8019726 CCC: $40.75
Published on Web 03/17/2009
2009 American Chemical Society
ambient PM2.5. Given REE ambient concentrations, the value of S is inversely related to the value of F. That is, an underestimate of the REE concentration in the catalyst would produce an overestimate of the catalyst contribution to ambient PM2.5. That is exactly what happened here as the previously analyzed catalysts had an average lanthanum concentration of about 0.8% wt. while the lanthanum content of catalyst released during the event was about 2.5 times greater according to data obtained from refinery personnel. Since lanthanum accounted for most of the REE mass in the spilled catalyst, the underestimated lanthanum concentration produced a roughly 250% overestimate in the FCCU impact. No explanation was given for assuming that the entire REE mass measured in the laboratory (above background) accumulated on the sample filters over just two days other than the selected two days coincided with the regional haze episode and that afterward PM2.5 levels receded to levels “similar to those measured before the haze episode and the simultaneous report of release of material from the FCC Unit.” Actually, regionwide PM2.5 levels remained 1.5-3 times greater than before the start of the episode for several more days while the filter at Channelview that was analyzed for REEs continued to accumulate PM2.5 mass until it was replaced eight days later. Throughout this extended period catalyst that initially fell to the ground may have been resuspended, blown downwind, and captured by the filter. Kulkarni et al. would have attributed the entire collected REE mass to just the initial two days instead of integrating over the correct eight days. The potential error produced by this assumption is another 400%. Alternatively, the authors might have integrated the entire excess REE over the 27 h period of the leak as actually reported in the publicly available account, but the shorter averaging period would have produced an even more absurd impact estimate, roughly equal to the entire measured PM2.5 for the period. By assuming that the catalyst release was the only source of REE enrichment above background, the authors’ neglected to consider the source of the regional haze. REE enrichment has previously been found in Saharan dust samples and REE patterns in soils are commonly used by geochemists as evidence of African dust inputs to soils around the world (4).
Literature Cited (1) Kulkarni, P.; Chellam, S.; Fraser, M. P. Tracking petroleum refinery emission events using lanthanum and lanthanides as elemental markers for PM2.5. Environ. Sci. Technol. 2007, 41, 6748–6754. (2) East Texas Sahara Dust August 1-3, 2006. Texas Commission on Environmental Quality: Austin, Texas.http://www.tceq.state. tx.us/assets/public/compliance/monops/air/sigevents/06/ event2006-08-01txe.html (accessed 2/11/08). (3) Air Emission Event Report for Tracking Number 79352. Texas Commission on Environmental Quality: Austin, Texas.http:// www.tceq.state.tx.us/compliance/field_ops/eer (accessed 2/11/08). (4) Muhs, D. R.; Budahn, J. R.; Prospero, J. M.; Carey, S. N. Geochemical evidence for African dust inputs to soils of western Atlantic islands: Barbados, The Bahamas, And Florida. J. Geophys. Res. 2007, 112, F02009, DOI: 10.1029/2005JF00045.
Albert Hendler URS Corporation Austin, Texas 78729 ES8019726 VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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