Wednesday, September 23, 2009

Frequently Asked Questions: (4) What is Particulate Source Apportionment?

By definition, this is the process of estimating the contribution of various sources to the pollution. It is important to keep in mind two concepts.

One method is via an emissions inventory. Once an emissions inventory is established, through a series of surveys, data collection, and multiplication of emission factors, for each of the sectors involved and for the indirect sources such as fugitive emissions, one could arrive at the proportions of contribution of each of the sectors to each of the pollutants.

Second method is via the ambient concentrations. Let’s assume the PM measured at various hot spots in the city, the samples are then analyzed for the chemical composition, which are then regressed through the profiles for various sources, to arrive at a series of numbers estimating the possible contribution of the known sources.

It is very IMPORTANT to understand that the two methods are different and the numbers they indicate are different, although they are talking about the same PM pollution.
  1. The emissions are not same as ambient concentrations.
  2. The contributions estimated from emissions need not be the same as the contributions estimated via the ambient measurements.
  3. The emissions inventory is usually for the whole of the city or the area of interest, while the later method represents more of the measurement area features.
The emissions undergo advection and chemical transformation, before they appear as a sample measured for the ambient concentrations.

The long range transport plays a key role in the advection scheme, as the tall sources (industries) tend to disperse farther than the roadside emissions and hence the difference in their signatures in the contribution calculations.

When discussing the emissions inventory, each of the pollutants is discussed separately (such as PM, SO2, NOx, CO, etc.) and it is important that the pollutants are NOT added to average the contribution – each of them have a role to play. Whereas, the contribution estimates from PM ambient sampling, is a combination of all the pollutants and sources – primary and secondary.

None the less, both the top-down (ambient) and bottom-up (emissions) methods are very important (and essential) to understand the strength of the sources and their potential to control.

If health impacts are the deciding factors, then the ambient contributions (from the second method discussed above or following the dispersion modeling of emissions) are the most important.

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