Encyclopedia of Environmental Science and Engineering, Volume I and II

710 MOBILE SOURCE POLLUTION

Much of the particulate is in the form of polynuclear aro-

matic (PNA) hydrocarbons and tars. As for gaseous products,

standard federal cycles have been designed to test for particu-

late emissions.

Lead particulate matter is emitted at a rate of about

0.16g/mile. The average particle size emitted is slightly less

than a micron, but sizes are substantially distributed over

the 0.10–10 micron range. In Table 5, the effect of mileage

on particle distribution may be observed. As the number of

miles driven increases, the segment of particles greater than

9 microns also increases, probably because of deposition

buildup on exhaust pipes.

Using 500 microns as a divider, about 30% of the lead

burned over the lifetime of a car is in the form of fi nes 40%

in the form of coarse particulate.

Research studies^27 showed that a relationship exists

between traffi c volume, proximity to the highway engine

acceleration vs. constant speed, wind direction and the

amount of lead in the air. Lead values can be expressed as

a quadratic function of time and a linear function of traf-

fi c volume. Ter Haar^28 and coworkers found that for Federal

7-mode cycle conditions (Z) cars vary widely in the amounts

and composition of their particulate emissions; (2) cold cycle

operation gives 2–8 times more particulate than hot engine

operation; (3) lead compounds represent less than one-third

of total particulates, the remainder being carbon compounds

along with ammonium and nitrate ions and unknown mate-

rials; (4) carbon emission for stabilized cars using leaded

gasoline varies widely but averages about 35% of the total;

(5) suspended particulate emissions are nearly equal with

new cars whether or not lead is present; (6) exhausted lead

varies with the condition of the exhaust system and ranges

between 7 and 30% of the lead consumed by the engine;

(7) fuel additives affect the amount of emitted particulates;

(8) probe sampling techniques underestimate by a large

factor of amount of particulates emitted by vehicles; and

(9) trapping systems offer potential for greatly reducing the

emission of suspended total particulates. Nonleaded-fuel

cars were found to exhaust 0.165g of particulate per mile

while leaded-fuel cars exhausted 0.152g/mile. The reason

for this is that nonleaded fuel is more aromatic in nature

and that the percentage emission for aromatic hydrocarbons

is considerably greater than that for paraffi nic material. An

increase in the volume of aromatics in the fuel from 10 to

70% doubles the amount of carbon particulates emitted. It

has been claimed^29 that a reduction of 80–90% of the lead

emitted is possible by trapping systems.

Two simple traps have been compared for particulate

removal and the results presented in Table 6.

More complex traps are likely to be very successful in

reducing both lead and total particulate matter.

A more detailed chemical analysis of particulates has

been described under Federal emission test conditions.^30 This

investigation found that a decrease in the total mass of partic-

ulates especially particles of very small size, occurred when

using nonleaded fuel. The discrepancy with the aforemen-

tioned work^28 may be a result of differences in exhaust depo-

sition for the two systems. Particles emitted in the vehicle

exhaust varied in size from 0.01 to 5000 microns, the latter

for such products as rust scale. Typical chemical composi-

tions for the particulate exhausts are presented in Figure 5.

The ratio of Pb to Br is relatively constant 2.1:1. The

ratio on impactor plates was somewhat lower, indicating

that the chemical composition of lead salts may be related

to particle size. Considering some of the differences in the

fi ndings for different particulate matter studies it is critical

that such investigations be perfected and intensifi ed. An

interesting additional discussion of the characterization of

particulate lead in vehicle exhaust may be found in the work

of Habibi^31 and Mueller.^32

TABLE 4

Partial list of odorifous compounds

General classification Compounds

Indans and tetralins Methyl indan

Tetralin

Dimethyl indan

Methyl tetralin

Dimethyl tetralin

Trimethyl tetralin

Alkyl tetralin

Alkyl-substituted naphthalenes Methylnaphthalenes

Dimethylnaphthalenes

Indenes, acenapthenes, and Alkyl-substituted indenes

benzothiophenes Dimethylbenzothiophenes

Acenaphthene

General class Carbon range

Alkenone C5 to C11

Furan C6 to C10

Diene/ C9 to C12

Furfural C6 to C7

Methoxy benzene C8 to C9

Phenol C7 to C12

Benzaldehyde C7 to C10

Benzofuran C8 to C9

Indanone C6 to C10

Indenone C9 to C12

Naphthol C10 to C14

Naphthaldehyde C11

TABLE 5

Lead particulate emissions as a function of size and mileage^25

Average

mileage

Average lead salt emissions, g/mile

>9 microns <1.0 microns <0.3 microns

5,000 0.04 0.07 0.05

16,000 0.06 0.05 0.04

21,000 0.06 0.05 0.03

28,000 0.10 0.03 0.02

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