ATMOSPHERIC CHEMISTRY 125
sources for OH and HO 2 radicals depends on the concentra-
tions of the different species present, the location (urban or
rural), and the time of day.Organic ReactivityAtmospheric organic compounds have a wide range of reac-
tivities. Table 4 lists calculated tropospheric lifetimes for
selected volatile organic compounds (VOCs) due to photolysis
and reaction with OH and NO 3 radicals and ozone (Seinfeld
and Pandis, 1998). All of the processes identified in the table
lead to the formation of organic peroxy radicals that oxidize
NO to NO 2 , and hence lead to ozone production. But we can
see that in general the reaction of the organic molecule with
the hydroxyl radical is the most important loss process.
The most important chain-terminating process in the
urban atmosphere is the reaction of OH with NO 2. Hence,
comparing the relative rates of the OH reaction with VOCs
to that of OH with NO 2 is important for assessing the pro-
duction of ozone. Seinfeld (1995) found that the rate of the
OH reaction with NO 2 is about 5.5 times that for the OH
reactions with a typical urban mix of VOCs, where NO 2 con-
centrations are in units of ppm and VOC concentrations are
in units of ppm C (ppm of carbon in the VOC).^ If the VOC-
to-NO 2 ratio is less than 5.5:1, the reaction of OH with NO 2
would be expected to predominate over the reaction of OH
with VOCs. This reduces the OH involved in the oxidation
of VOCs, hence inhibiting the production of O 3. On the otherTABLE 4
Estimated tropospheric lifetimes for selected VOCs due to photolysis
and reaction with OH and NO 3 radicals and ozone
Lifetime Due to Reaction with
OHa O 3 b NO 3 c hn-Butane 5.7 days — 2.8 yr
Propene 6.6 h 1.6 days 4.9 days
Benzene 12 days — —
Toluene 2.4 days — 1.9 yr
m-Xylene 7.4 h — 200 days
Formaldehyde 1.5 days — 80 days 4 h
Acetaldehyde 11 h — 17 days 5 days
Acetone 66 days — — 38 days
Isoprene 1.7 h 1.3 days 0.8 h
-Pinene 3.4 h 4.6 h 2.0 h
-Pinene 2.3 h 1.1 days 4.9 h
Camphene 3.5 h 18 days 1.5 days
2-Carene 2.3 h 1.7 h 36 min
3-Carene 2.1 h 10 h 1.1 h
d-Limonene 1.1 h 1.9 h 53 min
Terpinolene 49 min 17 min 7 min
Source: From Seinfeld and Pandis (1998). With permission.
a 12-hour daytime OH concentration of 1.5 × 10^6 molecules cm^3 (0.06 ppt).
b 24-hour average O 3 concentration of 7 × 10^11 molecules cm^3 (30 ppb).
c 12-hour average NO 3 concentration of 2.4 × 10^7 molecules cm^3 (1 ppt).^TABLE 5
Maximum incremental reactivities (MIR) for some VOCsVOCMIRa
(grams of O 3 formed per
gram of VOC added)Carbon monoxide 0.054
Methane 0.015
Ethane 0.25
Propane 0.48
n-Butane 1.02
Ethene 7.4
Propene 9.4
1-Butene 8.9
2-Methylpropene (isobutene) 5.3
1,3-Butadiene 10.9
2-Methyl-1,3-butadiene (isoprene) 9.1
-Pinene 3.3
-Pinene 4.4
Ethyne (acetylene) 0.50
Benzene 0.42
Toluene 2.7
m-Xylene 8.2
1,3,5-Trimethylbenzene 10.1
Methanol 0.56
Ethanol 1.34
Formaldehyde 7.2
Acetaldehyde 5.5
Benzaldehyde 0.57
Methyl tert-butyl ether 0.62
Ethyl tert-butyl ether 2.0
Acetone 0.56
C 4 ketones 1.18
Methyl nitrite 9.5
Source: From Finlayson-Pitts and Pitts (2000). With permission.
a From Carter (1994).hand, when the ratio exceeds 5.5:1, OH preferentially reacts
with VOCs, accelerating the production of radicals and hence
O 3. Different urban areas are expected to have a different mix
of hydrocarbons, and hence different reactivities, so this ratio
is expected to change for different urban areas.
Carter and Atkinson (1987) have estimated the effect of
changes in the VOC composition on ozone production by use
of an “incremental reactivity.” This provides a measure of the
change in ozone production when a small amount of VOC is
added to or subtracted from the base VOC mixture at the fixed
initial NO x concentration. The incremental reactivity depends
not only on the reactivity of the added VOC with OH and
other oxidants, but also on the photooxidation mechanism,
the base VOC mixture, and the NO x level. Table 5 presents a
table of maximum incremental reactivities (MIR) for several
VOCs. The concept of MIR is useful in evaluating the effect
of changing VOC components in a mixture of pollutants.C001_008_r03.indd 125C001_008_r03.indd 125 11/18/2005 10:15:21 AM11/18/2005 10:15:21 AM