Classification of chemicals into emission-based impact categories: a first approach for equiprobable and site-specific conceptual frames

The classification step is systematically neglected in the calculation of life cycle impact assessment indicators: for several direct contribution of one pollutant to several impact categories, the distribution between these impact categories is not accounted. The proposed approaches of non-redundant classification are based on the probability that an emitted substance and its chemically degraded forms are involved into several environmental impacts.

Two types of repartitions are examined in this paper: (1) one called equiprobable classification, based on identical probability of contributions to several impact categories, and (2) and one called zone classification based. Both methods are based on a classification coefficient alpha and require categorization of chemical pathways (reactive, suspensive, direct, indirect). The first method, the equiprobable classification, is a quick method that allows avoiding multiple counting of pollutants. The second method, the zone classification method, is based on two steps: (1) the first step requires defining an impacted zone around the source, inside which the emitted chemicals are expected to majorly diffuse or spread, and (2) in the second step, the score of the chemical is set according to the occurrence of the chemical target inside the impacted zone.

Both methods are applied on a case study of NOx emissions in Paris. Results highlight big differences (33% to 47%) when using or not the classification coefficient. Coefficients calculated using equiprobable or zone classifications are of the same order of magnitude except for indirect contributions.

The zone classification method is compared to other site-specific life cycle assessment methods. The choices of impacted zone and of chemical targets are discussed with the aim to meeting paradoxical constraints: local parameters must be sufficiently detailed in the majority of cases, but generic enough to avoid time consuming researches. The availability of data is also discussed as well as the possibility to include indirect impacts into the environmental system.

The conceptual frame suggested for classification of pollutants into emission-based impact categories is a first approach for a scientific question that was raised in the 90s. Equiprobable classification is easy to apply but not entirely satisfying from a physical and chemical background. Zone classification relies upon the specificity of emission compartments and occurrence of chemical targets in the impacted zone. It still needs improvements. It has important consequences in terms of data collection, system boundaries, and only practice will show its actual feasibility.