Comparison of Primary and Secondary Emissions from an Internal Combustion Engine - Novel Combustion Concepts for Sustainable Energy Development - page 410

nuclei (<3 nm in size). Nuclei formation is followed by surface growth under which
H molecules is striped off and results in higher C/H ratio. The size of the spherule is
*

20 nm. These spherules are the building blocks for agglomeration and form
agglomerated particles. Spherules form chain and eventually agglomerated particles.
The fifth step is surface oxidation. In the past, several researchers (Li and Wallace

1995 ) have come up with semi-empirical models for studying the soot formation
mechanism.

3 Secondary Organic Aerosol (SOA)

Organic compounds are a dominant portion of the ambient aerosols (Seinfeld and
Pandis 2012 ). Current analytical methods have enabled us to quantitatively evaluate
these organic fractions. The oxidation of volatile organic carbon (VOC) results in
the formation of SOA. The composition and formation mechanism for SOA needs
to be carefully studied to effectively control it. Risk assessment of SOA desires
availability of such information.

3.1 SOA Formation

Oxidation of the reactive organic gases (ROG) results in the SOA formation. Both
natural as well as anthropogenic sources like solvents, vegetation and ocean
emissions, combustion of wood and biomass, wood and fossil fuels etc. are
responsible for ROG formation (Seinfeld and Pandis 2012 ; Jacobson et al. 2000 ).
The most common ROG found in the atmosphere are alkanes, alkenes, aromatics
and phenols.
Oxidation of precursor ROGs results in formation of semi-volatile organic
carbon (SVOC). The species having their saturation vapor pressure below a certain
pressure are known as semi-volatile (Turpin et al. 2000 ). It is required that for SOA
formation, the rate of oxidation of precursor ROGs should be high. Next comes the
partitioning of the SVOCs into the aerosol phase. There should be suf cient amount

of SVOC having low saturation vapor pressure. Thus, SOA formation inherently
depends on reactivity or volatility (Grosjean and Seinfeld 1967 ). Figure 3 shows a
schematic view of SOA formation in the atmosphere.

3.2 Oxidation of Reactive Organic Gases (ROGs)

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(a) Gas Phase Oxidation O 3 , OH radical and NO 3 radical are the 3 major

ambient oxidizing. Photolysis of ozone in presence of moisture produces
hydroxyl radical (OH). There are compounds (oxygenated aliphatics, saturated