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Acute Health Risk from Irregular Intermittent Air Pollution Sources: Challenges of Definition Boris Balter*,† and Marina Faminskaya†,‡ †
Space Research Institute, Russian Academy of Sciences, Moscow 117997, Russia Russian State Social University, Moscow 129226, Russia that way, so both are, in principle, acceptable, but they can give dissimilar results. This problem has surfaced only recently, because it does not exist for constant sources as well as in the traditional approach to intermittent sources. Traditionally, for acute risk from irregular sources, the worst case scenario is assumed:1 risk for any receptor point is calculated assuming that the moment of emission would coincide at least once with the worst meteorological conditions for this pair source−receptor. However, for infrequent emissions, the chances of having the worst case scenario are slim, so that modeled worst case concentrations/risks are exaggerated. In recent years, this problem has come into focus2 and, to tackle it, the special software EMVAP (Emissions Variability Processor) is being developed as a supplement to the well-known pollutant dispersion model AERMOD3 under U.S. EPA guidance. The approach is based on stochastic Monte Carlo modeling of emissions 4 so that, instead of the absolute maximum concentration/risk for a receptor, we get the mathematical expectation of the maximal value of hourly concentrations/risks, e.g., in a year. It depends on the frequency of emissions. If emission happens just once a year, the expected concentration/ ormally, the main end point of risk assessment for a plant risk is closer to that obtained with the average meteorology is a risk map. It serves to establish the exclusion zone for than with the worst case meteorology, which means huge reductions of acute risk: up to 2 orders of magnitude. territories with an unacceptable hazard. The map is pointwise, Consider a simplified situation. Fix a receptor point and but the decision on the acceptability of the hazard is not: it assume a single pollution source, active only once a year, at a involves the entire situation “plant + surrounding territory”. random moment. It is quite improbable that at this moment the The transition from pointwise values to a decision aggregated wind would come from the source to the receptor point, so the over territory poses no problem for constant pollution sources, worst case situation contributes little to the Monte Carlo but, in our opinion, it does for acute risk from intermittent mathematical expectation value. All other possible situations sources. There is an additional degree of freedom related to contribute zero: the wind blows elsewhere. (We skip the role of randomness of intermittent emissions, so that the usual wind speed and atmospheric stability). Thus, the expected definition of acute individual risk based on the yearly maxima acute risk for this receptor point is small. The same is true for of hourly pollutant concentrations seems incomplete. Decisions all other receptor points. Now let us reference risk to the on acceptability of risk depend on how we combine the source, rather than to any specific receptor point. Whenever the aggregation in time of random emissions and the aggregation in emission occurs, the plume would hit some receptor point, space of exposed territories. Acceptability is usually defined guaranteeing a significant contribution to the Monte Carlo from the worst point on a risk map: we first seek to identify the expectation of the spatial maximum of momentary risk. We call maximum risk in time for each point, and then maximize it over it source-related risk, and it is larger than receptor-related risk. territory. In this approach, rare emissions may present no In our opinion, this means that acute risk definition needs hazard because each point, considered individually, would have extension to make it dependent on the reference f rame. a low chance of being hit by an emission. However, if, We get both reference frames by swapping the order of risk aggregation over time and space. Monte Carlo gives the alternatively, we first look at the entire spatial risk pattern expected maximum over time. It can be done before or after around the plant at the moments of emission, the chance that it maximizing over space (taking the worst receptor), to generate, contains some inadmissible points would be much higher. This expresses a mathematical fact: maxima in time and space, being nonlinear functions, do not commute, and we have to decide Received: September 25, 2014 what to do first. No first principle compels us to proceed this or Published: November 20, 2014 ‡
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© 2014 American Chemical Society
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dx.doi.org/10.1021/es504712x | Environ. Sci. Technol. 2014, 48, 14070−14071
Environmental Science & Technology
Viewpoint
Figure 1. Dependence of the expected yearly maximum of acute risk from an intermittent source on the number of “firings”, hours/year. Two alternative definitions: receptor-wise (maximum over receptors of maximum over time) and source-wise (maximum over time of maximum over receptors) are compared with the worst-case.
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respectively, a receptor-related or a source-related estimate. For an intermittent source, these two approaches can give vastly different results, as illustrated by the following practical example: an emergency waste treatment pond of a chemical plant. There are two end points of interest: the worst risk value on populated areas and on the exclusion zone border. Both are shown as functions of assumed source intermittency (hours per year): from 1 to 8760 (constant source). The source-related case is much worse than the receptor-related case but still better than the worst case scenario: it takes into account the low probability of the worst wind speed and atmospheric stability, although not the low probability of the worst wind direction. For a constant source, both reference frames merge into the worst case scenario. However, for intermittencies of 200−1000 h/year, the situation on the exclusion zone border is admissible in a receptor-related frame, but inadmissible in a source-related frame. How should a decision maker understand this situation? The problem is of practical importance: major contributors to acute risk often are powerful intermittent sources−usually irregular. In addition, similar issues exist in water pollution, intermittent exposure of migrating humans, etc.
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REFERENCES
(1) Risk Management Program Guidance for Offsite Consequence Analysis, EPA 550-B-99-009; U.S. Environmental Protection Agency, 1999. (2) Murray, D. R.; Newman, M. B. Probability analyses of combining background concentrations with model-predicted concentrations. J. Air Waste Manag. Assoc. 2014, 64 (3), 248−254. (3) Paine, R. J.; Heinold, D. W. Single-source dispersion modeling: The state of current and future tools. EM Mag. 2012, Aug, 22−26. (4) Hanna, S.; Paine, R.; Heinold, D.; Kintigh, E.; Baker, D. Uncertainties in air toxics calculated by the dispersion models AERMOD and ISCST3 in the Houston ship channel area. J. Appl. Meteorol. Climatol. 2006, 46, 1372−1382.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The work was supported by the Russian Ministry of Education and Science (task 2014/601). 14071
dx.doi.org/10.1021/es504712x | Environ. Sci. Technol. 2014, 48, 14070−14071
