Urban Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 92

Table 4.3. Estimated e -folding lifetimes of reactive organic gases representing alkanes, alkenes, alkynes,
aldehydes, ketones, alcohols, aromatics, and hemiterpenes, respectively, against photolysis and oxidation by
gases at specified concentrations in urban air

Lifetime in polluted urban air at sea level

OH(g) HO 2 (g) O(g) NO 3 (g) O 3 (g)

10 6 10 9 10 4 10 10 10 12

5 × 2 × 8 × 1 × 5 ×

ROG species Photolysis molec cm −3 molec cm −3 molec cm −3 molec cm −3 molec cm −3

n -Butane - 22 h 1,000 y 18 y 29 d 650 y

trans-2 -Butene - 52 m 4 y 6.3 d 4 m 17 m

Acetylene - 3.0 d - 2.5 y - 200 d

Formaldehyde 7 h 6.0 h 1.8 h 2.5 y 2.0 d 3,200 y

Acetone 23 d 9.6 d - - - -

Ethanol - 19 h - - - -

Toluene - 9.0 h - 6 y 33 d 200 d

Isoprene - 34 m - 4 d 5 m 4.6 h

Lifetimes were obtained from rate and photolysis coefficient data. Gas concentrations are typical (but not necessarily average) values for
each region. Units: m, minutes; h, hours; d, days; y, years; -, insignificant loss. ROG, reactive organic gas.

the reactivity and emission rate of an ROG is impor-
tant for prioritizing which organics to control first in an
urban area.
Table 4.4 shows the most important ROGs in Los
Angeles during the summer of 1987, in terms of a
combination of abundance and reactive ability to form
ozone. The table shows that m - and p -xylene, both aro-
matic hydrocarbons, were the most important gases in
terms of generating ozone. Although alkanes are emit-
ted in greater abundance than are other organics, they
are less reactive in producing ozone than are aromatics,
alkenes, or aldehydes.
In the following subsections, photochemical smog
processes involving the chemical breakdown of organic
gases to produce ozone are discussed.

4.3.3. Ozone Production from Alkanes
Table 4.4 shows that i -pentane and butane are important
alkanes in terms of concentration and reactivity in pro-
ducing ozone in Los Angeles air. As in the background
troposphere, the main pathway of alkane decomposition
in urban air is OH(g) attack. Alkane concentrations are
not affected much by photolysis or reaction with O 3 (g),
HO 2 (g), or NO 3 (g) (Table 4.3). Of all alkanes, methane
is the least reactive and the least important with respect
to urban air pollution. Methane is more important with
respect to background tropospheric and stratospheric
chemistry. The oxidation pathways of methane were
given in Reactions 4.16 to 4.20, and those of ethane
were shown in Reactions 4.26 to 4.30.

4.3.4. Ozone Production from Alkenes
Table 4.4 shows that alkenes, such as ethene and
propene, are important ozone precursors in photochem-
ical smog. Mixing ratios of ethene and propene in
polluted air reach 1 to 30 ppbv. Table 4.3 indicates
that alkenes react most rapidly with OH(g), O 3 (g), and
NO 3 (g). In the following subsections, the first two of
these reaction pathways are discussed.

Table 4.4. Ranking of most important species in
terms of chemical reactivity and abundance during
Southern California air quality study, summer 1987

1. m -and p -Xylene 6. i -Pentane

2. Ethene 7. Propene

3. Acetaldehyde 8. o -Xylene

4. Toluene 9. Butane

5. Formaldehyde 10. Methylcyclopentane

4.3.4.1. Alkene Reaction with
the Hydroxyl Radical
When ethene reacts with the hydroxyl radical, the radi-
cal substitutes into ethene's double bond to produce an
ethanyl radical in an OH(g) addition process. The

Ranking was determined by multiplying the weight fraction of
each organic present in the atmosphere by a species-specific
reactivity scaling factor developed by Carter (1991).
Source :Lurmann et al. (1992).