Pollutant-specific scale of multimedia models and its implications for the potential dose

The spatial range is a generic indicator for how far pollutants are likely to travel. It also indicates the appropriate, pollutant-specific area of a multimedia model, which is the square of the spatial range. Formulations of the spatial range can be based on advective or dispersive transport. They differ in whether they take the extent and shape of the earth's surface into account. We suggest the common element of the different approaches is that all account for the persistence and mobility of pollutants. The mobility is the expected travel speed and depends on the partitioning. This paper extends the concept of a pollutant-specific model scale through the introduction of a characteristic atmospheric scale height. It is the height of the atmosphere that would be needed to contain all the pollutant if the entire atmosphere had ground-level concentration, taking into account deposition and degradation. We define the spatial range as the expected advection-driven travel distance of a pollutant molecule released to a specific compartment. This novel analytical formulation is more comprehensive but encompasses all previous advection-based proposals of a spatial range. We evaluate the spatial range and scale height of 288 chemicals for releases to air, surface water, and surface soil. We find a strong correlation between the spatial range for air releases and the scale height because both depend on persistence. We investigate the effect of the spatial scale on calculations of the human toxicity potential, a screening-level risk indicator based on toxicity and potential dose. The product of model area and potential dose is found to be the same for calculations using a fixed model area and those using the pollutant-specific spatial scale. The introduction of the scale height, however, can change the potential dose by more than 1 order of magnitude.