Approach to Vehicle Emission Modelling

Rajesh B. Biniwale Scientist

Air Pollution Control Division, NEERI, Nagpur-440 020

Introduction

Vehicle emission estimation is essential for calculating vehicle pollution

load and its impact on ambient air quality. The emission inventory technique is

used for estimation of pollutant loads. The dispersion modeling gives the

resulting ambient air quality. Source apportionment studies are carried out using

receptor modeling. This paper discusses all three approaches in brief. Common

models used for this purpose are also mentioned

Emission Inventory Models

The objective of emission inventory preparation should be very clearly

spelt out to facilitate selection of the appropriate model. The major objective of

emission inventory preparation, in general is estimation of emission loads under

various scenarios with technological or policy interventions planned.

Several models are available for preparation of emission inventories.

Some of the models are however, CORINAIR, IPIECA tool kit, and few more are

specific for the prediction of pollution load.

CORINAIR takes into account emissions from point, line and area sources

and gives percentage-wise or mass emission load per year from 11 different

categories of sources.

The IPIECA toolkit is a PC-based emission inventory model. The Toolkit

allows users to develop comprehensive emission inventories and assess the

effect of a range of future energy, transport, commercial and industrial scenarios

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on emission. The model is highly flexible and can be adapted to characterize

emissions within a city, region or country and at various temporal scales up to

one hundred years. The Toolkit, using built-in options and equations, can also be

used where only incomplete sets of local data are available. The model provides

a sound balance between complexity, accuracy and user friendliness.

Two approaches can be employed to evaluate source contribution from

source emissions data and ambient monitoring data : source-oriented models

and receptor-oriented models. Source-oriented models use emissions data and

fluid mechanically explicit transport calculations to predict pollutant

concentrations at specific receptor air monitoring locations. This type of model is

validated by comparison of the predicted spatial and temporal distribution of

pollutant concentrations against measured concentrations. Receptor oriented

models infer source contributions by determining the best fit linear combination of

emission source chemical composition profiles needed to reconstruct the

measured chemical composition of ambient air.

Dispersion Modelling - Source Oriented Models

Dispersion models may be used to predict the concentration of pollutants

at various locations due to dispersion under prevailing meteorological conditions.

The dispersion models could be source-type specific e.g. for line source, models

which may be used are Caline, Mobile 5 etc. For point sources ISCST3 may be

used. The dispersion models suggested do not account for the regional scenario.

Mesoscale meteorological models such as MM5 in combination with models such

as Calpuff facilitate detailed regional air quality modeling.

Using emission inventory and dispersion modeling estimates of emission

loads and concentration of various pollutants in ambient air may be predicted.

However, the above approach needs to be integrated using receptor modelling.

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Chemical Mass Balance (CMB) - Receptor Model

The CMB model uses the chemical composition of ambient pollution samples

to estimate the concentration of different source types to the measured pollutant

concentrations. The model quantifies contributions from chemically distinct

source types rather than contributions from individual emitters. Sources, which

have similar chemical composition, can not be separated by the model. The

particle characteristics must be such that;

i. they are present in different proportion in different source emissions,

ii. these proportions remain relatively constant for each source type,

iii. change in these proportions between sources and receptor are negligible,

iv. chemical species do no react with each other, i.e., they add linearly, and

v. the number of sources is less than or equal to the number of chemical

species

The contribution of automotive exhaust to ambient air quality depends on

engine type, traffic movement and fuel quality. A detailed study is, therefore,

needed to develop vehicle emission norms keeping in view all these aspects.

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