Second report of the National Pollutant Release Inventory working group 2001 to 2002: annex C

Annex C: Criteria for Developing the Species Listing

The following describes the weighting criteria as they were applied to develop the specified list for speciation.

Mass

This is the emitted mass (in tones) of the individual VOC species estimated from the total-VOC emissions contained in the RDIS II 1995 database, using VOC speciation profiles from the US EPA SPECIATE 3.0 database.

Reactivity

This is a measure of the ozone-forming potential of the individual species. The quantity considered is the gas-phase OH reactivity (or kOH) expressed in units of molecules cm^-3 s^-1. The product of kOH and a VOC concentration gives an estimate of initial organic-radical (RO2) production. This product is used in this case only as a relative indicator. In actual fact, reactive air-quality models take into account many additional factors such as reaction length, species availability, and the impact of other oxidants, temperature, and sunlight when estimating ozone levels. The reactivity weighting was performed by dividing the emitted VOC species mass by the molecular weight of the species and multiplying it by the kOH reactivity value.

Solubility

Aqueous solubility in water is one of the properties used to examine the PM-forming potential of the individual VOC species. The solubility weighting of the species was estimated by dividing the mass emissions of the species by the molecular weight of the species and multiplying it by the species solubility.

Condensability

The condensability weighting is estimated using the saturation vapour pressure (SVP) of the species. The emitted mass of the species was divided by the molecular weight of the species and then divided by the SVP value to obtain the condensability weighting. As with the other weightings, species-specific SVPs were used when available; otherwise, surrogate values for structurally similar compounds were used.