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OCR for page 99
Atmospheric
Transformations of
Automotive Emissions
ROGER ATKINSON
University of California, Riverside
Components of Atmospheric Pollution / 100
Physical and Chemical Transformations Under Atmospheric
Conditions / 101
Physical Removal Processes / 101 Chemical Removal
Processes / 102
Atmospheric Lifetimes, Fates, and Products of the Atmospheric
Transformations of Automotive Emissions / 105
Atmospheric Lifetimes / 105 Atmospheric Transformations / 109
Analytical Techniques / 125
Summary / 126
Summary of Research Recommendations / 126
Air Pollution, the Automobile' and Public Health. (it) 1988 by the Health Effects
Institute. National Academy Press, Washington, D.C.
99
OCR for page 100
100
Atmospheric Transformations of Automotive Emissions
Components of Atmospheric
Pollution
A wide spectrum of inorganic and organic
chemical compounds are emitted from au-
tomotive use. These emissions arise from
combustion as well as evaporative ~oro-
cesses. They include the obvious water
vapor and carbon dioxide (CO2), as well as
carbon monoxide (CO), oxides of nitrogen
(NOx), oxides and oxyacids of sulfur, re-
duced sulfur compounds, a wide variety of
volatile organic compounds comprising
fuel components and partially oxidized
products of combustion, and particulate
matter. The identities of these emissions
and a quantitative understanding of their
emission rates are the focus of "Auto-
motive Emissions" Johnson, this volume).
In highly urbanized regions, automotive
emissions contribute a significant, and of-
ten major, fraction of the overall emission
burden of NOx, volatile organic com-
pounds, and elemental carbon and/or par-
ticulate organic matter. For example, the
1979 mobile and stationary source contri-
butions of NOx, volatile reactive organic
gases (ROG), oxides of sulfur (SOx), total
suspended particulate matter (TSP), CO,
and lead (Pb) to the overall emissions in the
Los Angeles South Coast Air Basin of
California are given in table 1. In this
particular urban air basin, mobile source
emissions are predominantly automotive
(since aircraft and ship emissions are rela-
tively minor) and are major contributors to
the overall emission inventory of NOx,
ROG, CO, and Pb.
Some of these emissions have a direct
impact on the ecosystem, including human
health. In addition, most of them can un-
dergo chemical transformations in the at-
mosphere (see, for example, Atkinson and
Lloyd 1984; Atkinson 1986), sometimes
leading to the production of more toxic
products. The possible chemical transfor-
mations and physical loss processes that
occur in the atmosphere during transport of
these primary automotive emissions from
source to receptor are the main subjects of
this chapter. The time scales of these atmo-
spheric transformations and physical loss
processes vary widely, with chemical life
times ranging from '1 min for some
highly reactive organic compounds to
months or even years for other much more
inert emissions (Atkinson 1986~. For exam-
ple, the Los Angeles urban plume has been
identified by ambient air monitoring mea-
surements at Niwot Ridge, Colorado, and
the transit time estimated at approximately
four days (Roberts et al. 1984~.
To understand effects on health and to
assess risk, it is necessary to know the
identities, the ambient concentrations, and
the distributions between gaseous and con-
densed phases of the chemical compounds
impacting human receptors. Thus, it is
necessary to determine the chemical and
physical changes that primary automotive
emissions undergo during their transport
through the atmosphere, and the threats, if
any, that the resulting products pose to
human health.
It must be borne in mind that automotive
emissions cannot be considered in isolation.
Synergistic chemical and physical interac-
tions occur between automotive emissions
and emissions arising from, for example,
stationary sources and vegetation, giving
rise to a further multitude of product spe-
cies. Clearly, changes in emission rates or
chemical characteristics of these nonauto-
motive emissions can lead to changes in the
photochemical reactivities of the overall
atmospheric pollutant mixtures. The eluci-
dation of the effect of automotive emissions
on human health necessitates a complete
knowledge of the emission inventories, the
physical and chemical transformations,
transport, and ambient atmospheric mea
surements ot automotive, stationary
source, and vegetative emissions, all com-
bined within the framework of local, ur
Table t. Average Emission Rates of NOX,
ROG, SOx, TSP, CO, and Pb in the South
Coast Air Basin of California During 1979
Emissions (tons/day)
Source NOX ROG SO.r TSP CO Pb
91 7,060 9.1
522588 0.6
6137,650 9.7
Mobile
Stationary
Total
837853
406680
1724317533
73
201
274
SOURCE: Adapted with permission from South
Coast Air Quality Management District 1982.
OCR for page 101
Roger Atkinson
101
ban, or regional photochemical computer
models.
This chapter assesses the atmospheric
lifetimes of the various classes of automo-
tive emissions, which, for compounds of
low volatility, may vary markedly with
their distribution between the gaseous and
particulate phases. The state of knowledge
about products formed by chemical reac-
tions under atmospheric conditions, in-
cluding indoor environments, is reviewed
and discussed. In many cases, the products
formed during the photodegradation of
primary automotive emissions are not pres-
ently known, and studies are needed to
determine the general nature, and toxicity
to humans, of these products. A list of
research recommendations to obtain the
necessary data base about these atmo-
spheric transformations is presented.
Physical and Chemical
Transformations Under
Atmospheric Conditions
Two decades of laboratory, environmental
chamber, and ambient atmospheric mea-
surements have revealed the physical and
chemical processes that transform and/or
remove chemical compounds emitted into
the atmosphere. These atmospheric emis-
sions are partitioned between the gas and
particulate phases, and the atmospheric loss
processes for both phases must be evaluated
separately.
Physical Removal Processes
The physical removal processes can be de-
fined as accretion (or coagulation) of parti-
cles, and dry and wet deposition of gases
as well as particles. Removal of gases and
particles at ground surfaces including
snow-covered ground and other moist sur-
faces is known as dry deposition, whereas
removal of these species by raindrops is
referred to as wet deposition. These pro-
cesses are dynamic, and we do not yet have
a quantitative understanding of them (see,
for example, Eisenreich et al. 1981; Graedel
et al. 1982; Slinn 1982; Colbeck and Harri
son 1985; Dolske and Gatz 1985; Jonas and
Heinemann 1985; Ligocki et al. 1985a,b;
Sehmel et al. 1985; Terry Dana et al. 1985;
van Noort and Wondergem 1985~.
Dry Deposition of Gases and Particles.
Gas-phase species and particles can be re-
moved from the atmosphere by an overall
process that involves downward transport
from the atmospheric boundary layer to the
ground surface. The complex atmospheric
physical mechanisms that deliver gaseous
and particulate species to the surface are
generally combined with the chemical pro-
cesses of mass transfer at the surface by use
of a "deposition velocity" Vat. The dry
deposition rate, F. is
F = Vat [C]
(1)
where [C] is the concentration of the spe-
cies at some reference height (generally
defined as 1 m). The deposition velocity
depends on the specific gaseous chemical
and/or particle species, the surface to which
the species is being deposited, and the
reference height. It also depends on the
atmospheric stability, being highest during
unstable conditions (see, for example,
Cadle et al. 1985; Colbeck and Harrison
1985~.
The deposition velocity is often defined
by three "resistance" terms
all = (A + rb + rS) (2)
where ra is the resistance between the ref-
erence height and the laminar sublayer near
the receiving surface; rb is the laminar sub-
layer resistance; and rS is the surface resis-
tance, specific to each pollutant and surface
type. For certain species (for example, gas-
eous nitric acid), the surface resistance rS is
effectively zero, and transport to the surface
becomes rate determining (Huebert and
Robert 1985~.
The deposition velocities of particles de-
pend on the particle size, exhibiting a min-
imum for particles of mean diameter of
~ O.1 ,um. It should also be noted that, for
particles, a constant adsorption and desorp-
tion of chemicals occurs, characterized by
their Henry's law properties. Thus there is
a dynamic equilibrium between the gaseous
and adsorbed (or particulate) phases which,
OCR for page 102
102
in accordance with the Henry's law con-
stants, depends on temperature, properties
of the individual particles, vapor pressure,
and liquid adsorption properties.
Wet Deposition of Gases and Particles
(Rainout). In addition to dry deposition,
. .
wet c .epos1tlon can remove gaseous com-
pounds and particles from the atmosphere.
This process occurs during precipitation.
Slinn and coworkers (1978) showed that a
falling raindrop attains equilibrium with a
gaseous chemical over a distance of~ 10
m. As described by Eisenreich et al. (1981),
the wet removal of gaseous chemicals arises
from equilibrium partitioning, and a wash-
out ratio, W. defining the scavenging eff~-
ciency of a gas-phase species, is given by
W = Crajs,/Cair = RT/H (3)
where R is the gas constant, T is the
temperature (°K), H is the Henry's law
constant, and Grain and Cair are the concen-
trations in rain and air, respectively. The
deposition rate, F. is then given by
F = wJcair (4)
where ~ is the precipitation rate.
Wet removal of gases is clearly most
important for chemicals highly soluble in
water, such as hydrogen peroxide, nitric
acid, and phenols. Thus, following a pre-
cipitation event, the atmospheric concen-
trations of highly water soluble species may
fall to near zero. For most gas-phase or-
ganic chemicals, however, it is likely that
wet removal is of minor importance. Atlas
and Giam (1981) calculated atmospheric
residence times ranging from ~ 60 days for
phthalates and hexachlorohexanes up to
~ 6 years for the polychlorinated biphenyl
mixture Aroclor 1242.
Clearly, wet deposition is episodic.
Thus, only average wet deposition veloci-
ties can be ascribed, and these are strong
functions of the climatological conditions
at the particular geographic location in
question.
Chemical Removal Processes
Many chemical processes contribute to the
removal of compounds emitted into the
Atmospheric Transformations of Automotive Emissions
troposphere. For gas-phase chemicals,
these removal processes involve
· photolysis during daylight hours;
· reaction with hydroxyl (OH) radicals
during daylight hours;
· reaction with ozone (O3) during daytime
and nighttime;
· reaction with hydroperoxyl (HO2) radi-
cals during, typically, late daytime and
early nighttime hours;
· reaction with the gaseous nitrate (NO3)
radical during nighttime hours;
· reaction with dinitrogen pentoxide
(N205) during nighttime hours;
· reaction with NO2 during daytime and
nighttime hours; and
· reaction with gaseous nitric acid (HNO3)
and other species such as nitrous (HNO2)
and sulfuric (H2SO4) acids.
Additionally, the following processes are
likely to contribute to the degradation of
chemical compounds present in the ad-
sorbed phase:
· photolysis;
· reaction with O3;
· reaction with N2O5 during nighttime
hours;
· reaction with NO2, typically present
throughout a full 24-hr period;
· reaction with H2O2;
· reaction with HNO3, HNO2, and
H2SO4.
Synergism may be important in certain of
these reactions involving adsorbed auto-
motive emissions. For example, the pres-
ence of HNO3 together with NO' may
lead to enhanced nitration of adsorbed
polycyclic aromatic hydrocarbons (PAHs)
(see, for example, Pitts 1983~.
In addition to photolysis and reactions of
automotive emissions in the gaseous and
adsorbed states with a variety of atmo-
spherically important species, the chemical
· . . . . . .
reactions ot automotive emlsslons 1n rain,
cloud, or fog water with other reactive
components of these aqueous systems must
be considered. This subject has recently
received much attention because of an in-
creasing emphasis on acid deposition (see,
for example, Calvert 1984~. Reactions of
chemicals in the aqueous phase with reac
OCR for page 103
Roger Atkinson
tive intermediates such as H2O2 and vari-
ous radical and ionic species have been dealt
with in some detail in connection with
these acidic deposition studies (Graedel and
Weschler 1981; Chameides and Davis 1982;
Graedel and Goldberg 1983; Jacob and
Hoffmann 1983; Chameides 1986; Graedel
et al. 1986~.
Photolysis. Automobile emissions can be
removed from the atmosphere by pho-
tolysis. This process requires that a chem-
ical compound absorb light in the actinic
portion of the spectrum (that is, the wave-
length region from ~ 290 to 1,000 nm)
and, after absorption of a photon, undergo
chemical change (Calvert and Pitts 1966~.
For most compounds, breakage of a~chem-
ical bond requires an energy in excess of
~ 40 kcal/mole (Benson 1976~. Therefore,
photolytic wavelengths of ' 700 nm are
necessary. One fundamental tenet is that
absorption of a single photon (referred to
hereafter as he cannot photodissociate more
than one molecule (Calvert and Pitts 1966~.
Formation of Ozone. Ozone is formed in
the troposphere from the Photolysis of
NO2
NO2 + hv~ NO + 0 (3P) (5)
followed by reaction of the ground-state
oxygen atom, CROP), with O2
M
0(3P) + O2 ' O3 (6)
where M denotes a third body, air in this
case. Tropospheric O3 is also transported
downward from the stratosphere (Logan
1985~. In the clean troposphere, O3 mixing
ratios are typically 30 + 10 parts per billion
(ppb) at3ground level (~ 7 x 10~ mole-
cules/cm ), and increase with altitude
(Logan 1985~. The relative contributions to
tropospheric O3 of photochemical forma-
tion and downward transport from the
stratosphere are discussed by Logan (1985~.
Formation of Hydroxyl Radicals. The
OH radical is the major reactive species in
the troposphere (Logan et al. 1981), and is
formed by Photolysis of 03, Photolysis of
HNO2, and reaction of the HO2 radical
103
with nitric oxide (NO) (DeMore et al.
1985~.
Photolysis of Ozone. Ozone photodisso-
ciates at wavelengths of < 319 nm to yield,
in part, electronically excited oxygen at-
oms, O(~D),
O3+hu ~O(~D)+O2 (7)
which react with water vapor (eq. 8) or N2
and O2 (eq. 9)
O(iD) + H2O ~ 2 OH (8)
O('D) + N2, Of ~ 0(3P) + No, O2 (9)
For a relative humidity of~ 50 percent at
298°K, ~ 0.2 OH radicals are formed for
each O(~D) atom formed.
Photolysis of Nitrous Acid. Nitrous acid,
which is present during nighttime hours in
urban atmospheres (Platt et al. 1980a;
Harris et al. 1982; Pitts et al. 1984a), is
rapidly photolyzed at wavelengths of < 400
nm (eq. 10) during daylight hours to yield
OH radicals (DeMore et al. 1985),
HNO2 + hv~ OH + NO (10)
with a lifetime due to Photolysis of~ 1~15
min under noontime conditions.
Reaction of Hydroperoxyl Radicals with Ni-
tric Oxide. Hydroperoxyl radicals, formed
from the photodissociation of aldehydes
and other photochemical processes (see be-
low, and Atkinson and Lloyd 1984), react
with NO to yield the OH radical
HO2 + NO ~ OH + NO2 (1l)
In the troposphere, under conditions where
NO concentrations are less than 5 x 108
molecules/cm3, HO2 radicals are expected
to react with HO2 and alkyl peroxy (RO2)
radicals in competition with reaction with
NO. Hence a knowledge of tropospheric
NO concentrations is important for assess-
ing the conversion of HO2 to the more
reactive OH radical (Logan et al. 1981;
Crutzen 1982; Logan 1983~.
Formation of Hydroperoxyl Radicals.
Hydroperoxyl radicals are produced under
tropospheric conditions from the pho-
tolysis of aldehydes (Atkinson and Lloyd
1984), for example from formaldehyde
(HCHO),
OCR for page 104
104
Atmospheric Transformations of Automotive Emissions
HCHO + ho {:H + HCO lengths between 400 and 650 nm (Graham
H2 + co (12) and Johnston 1978; Magnotta and Johnston
1980),
followed by the rapid reactions of hydro
gen atoms and HCO radicals with O2
M
H + O2 ~ HO2 (13)
HCO + O2 ~ HOT + CO (14)
The higher aldehydes also photodissociate
to ultimately yield HO2 radicals
RCHO + ho ~ R + HCO
~ O2
HO2 + CO (15)
(Here and below, R. R', R" . . ., represent
unspecified groups and the dot () repre-
sents an incomplete chemical bond or un-
paired electron.)
Additionally, HO2 radicals are formed
from reactions of alkoxy and a-hydroxy
radicals, which are reactive intermediates
produced during the photooxidation pro-
cesses of most organic compounds (At-
kinson and Lloyd 1984). For example,
R:
CHO + Of ~ RCOR' + HOT (16)
R''
RCHOH + Of ~ RCHO + HOT (17)
Daytime tropospheric HO2 radical concen-
trations are calculated to range from ~ 108
to 109 molecules/cm3 (Hov and Isaksen
1979; Stockwell and Calvert 1983a).
Formation of Nitrate Radicals. The gas-
eous NO3 radical has been shown to be an
important constituent of nighttime atmo-
spheres (see, for example, Winer et al.
1984; Atkinson et al. 1986b). Nitrate radi-
cals are formed by the reactions
NO2 + O3 ~ NO3 + O2 (18)
NO2 + NO3 ~ N2Os ( 1 9)
N2Os ~ NO2 + NO3 (20)
NO2 + 0 (3P)
NO3 + by tNo + O2 (21)
with a lifetime of~ 5 sec at noontime
(Magnotta and Johnston 1980), and react
rapidly with NO,
NO3 + NO ~ 2 NOT (22)
NO3 radical concentrations are essentially
negligible during daylight hours. After
sunset, they can rise rapidly to levels of ups
to ~ 400 parts per trillion (ppt) (~ 1 x 10
molecules/cm3) over continental areas (see,
for example, Platt et al. 1984; Atkinson et
al. 1986b). For example, at several semiarid
and desert sites in southern California, Platt
and coworkers (1984) consistently observed
nighttime NO3 radical concentrations of~ 2
x 108 to 2 x 109 molecules/cm3.
Formation of Dinitrogen Pentoxide. As
shown above (eq. 19 and 20), N2O5 is in
equilibrium with NO2 and the NO3 radi
cal. Maximum nighttime N2O5 concentra
tions of~ (2-3) x 10~ molecules/cm3 can
be inferred from the equilibrium constant
for these reactions, which is uncertain by a
factor of + ~ 1.2-1.5 (Graham and John
ston 1978; Tuazon et al. 1984b; Burrows et
al. 1985a; Perner et al. 1985); and concen
trations greater than ~ 2 x 10~° mol
ecules/cm3 were calculated to be exceeded
~ 30 percent of the nights for which data
are available (Atkinson et al. 1986b).
Formation of Gas-Phase Acids. Chemi
cals that are basic can react with gas-phase
acids to form their salts. As presently un
derstood, the major gas-phase acidic spe
cies are HNO3 and HNO2. Nitric acid is
formed in the gas phase from the reaction
of OH radicals with NOB
OH + NO2 ~ HNO3 (23)
and can be formed, probably in the ad
with N2O5 being in relatively rapid (< 1 sorbed phase, from the heterogeneous hy
min at 298°K and 760 torr total pressure) drolysis of N2O5
equilibrium with NO2 and the NO3 radical
(DeMore et al. 1985).
Since NO3 radicals photolyze at wave
heeerogeneous
N2Os + H2O > 2 HNO3 (24)
OCR for page 105
Roger Atkinson
105
though this initially adsorbed HNO3 may
be desorbed back into the gas phase (Tua-
zon et al. 1983; Atkinson et al. 1986a).
Nitrous acid is formed from the reaction
of OH radicals with NO
OH + NO ~ HNO2 (25)
although its rapid photolysis during day-
light hours (eq. 10) leads to a low ambient
daytime concentration. However, HNO2
has been identified and measured in night-
time Los Angeles atmospheres at up to 8
ppb (~ 2 x 10i ~ molecules/cm3) (Harris et
al. 1982~. Indeed, nighttime HNO2 levels
of~ 2 x 10~° to 2 x 10~ molecules/cm3
are probably typical of many, if not most,
urban environments. In environmental
chambers and indoor environments, HNO2
has been shown to be formed from the
heterogeneous hydrolysis of NO2 (Sakamaki
et al. 1983; Pitts et al. 1984c, 1985d).
For automotive emissions associated
with the particulate phase, reactions with
NO2, HNO3, HNO2, N2O5, and O3 must
be considered (see, for example, Pitts
1983~. Many, if not most, of these reactions
probably proceed by reaction of the ad-
sorbed automotive emissions with ad-
sorbed, rather than gas-phase, reactive
atmospheric species. Additionally, photol-
ysis of adsorbed automotive emissions also
occurs and may be highly important
(Behymer and Hites 1985~.
Recommendation 1. Study is required
on the physical removal processes leading
to wet and dry deposition of gases and
particles. Investigations of the processes
occurring on and in particulate and aerosol
matter should focus on gas-to-particle con-
version processes and the chemical pro-
cesses that occur within aerosols (including
fogs and clouds).
Atmospheric Lifetimes, Fates,
and Products of the Atmospheric
Transformations of Automotive
Omissions
This section reviews and summarizes the
present status of knowledge concerning the
atmospheric loss processes and atmospheric
lifetimes of automotive emissions and the
products formed from them under atmo-
spheric conditions. Reference is made
whenever possible to existing reviews and/or
evaluations, from which further details can
be obtained.
Atmospheric Lifetimes
Data obtained during the past two decades
have provided a comprehensive view of the
chemical and physical removal processes
that occur in the troposphere, and of the
reaction rate constants for many of these
processes. Table 2 lists the rate constants at
room temperature (298°K, 77°F) for the
known tropospheric chemical removal re-
actions for selected automotive emissions.
The corresponding calculated lifetimes in
the lower troposphere of these chemicals
due to reaction with each of the atmospher-
ically important reactive species listed in
table 2 are given in table 3.
Although the individual rate constants
are known to a reasonable degree of accu-
racy (sometimes to within + 25 percent,
and in most cases to within a factor of two),
the calculated atmospheric lifetimes are
much more uncertain because of the larger
uncertainties about the ambient atmo-
spheric concentrations of several of these
key tropospheric species. For example,-the
ambient atmospheric OH radical concen-
trations at any given time and/or location
are uncertain to a factor of at least five, and
more likely ten (Hewitt and Harrison
1985~. The tropospheric diurnally and an-
nually averaged OH radical concentrations
are better known, to within possibly a
factor of two (Crutzen 1982), being ~ 5 x
105 and ~ 6 x 105 molecules/cm3 in the
northern and southern hemispheres, re-
spectively. Similar arguments apply for the
ambient nighttime tropospheric concentra-
tions of the NO3 radical and of N2O5
(Atkinson et al. 1986b).
In addition to these chemical loss pro-
cesses of automotive emissions in the tro-
posphere, physical loss processes must also
be taken into account. Tables 4 and 5 give
selected examples from the literature of dry
deposition velocities and of washout ratios
for several inorganic and organic species.
OCR for page 106
106
Atmospheric Transformations of Automotive Emissions
Table 2. Room Temperature Rate Constants at Atmospheric Pressure of Air for the Gas-Phase
Reactions of Selected Automotive Emissions with Atmospherically Important Intermediate Species
Rate Constant [cm3/(molecule see)]
O3 NO3 HO2 N2Os
Emlsslon
OH
1.1 x 10-11
6.6 x 10-12
6.6 x 10-'2
1.3 x 10-13
9 x 10-13
1.6 x 10-13
2.2 x 10-"
3 x 10-14
4.7 x 10-12
3.3 x 10-'l
1.7 x 10-12
1.2 x 10-12
2.5 x 10-'2
8.7 x 10-'2
2.2 x 10-13
2.5 x 10-13
8.5 x 10-12
2.6 x 10-11
3.1 x 10-"
6.4 x 10-"
7.8 x 10-13
6.1 x 10-'2
2 x 10-11
9.0 x 10-12
1.6 x 10
1.3 x 10-11
2.0 x 10-"
3.6 x 10
1.9 x jo-"
2.3 x 10-13
1.0 x 10-'2
3.0 x 10-'2
9 x 10-13
2.9 x 10-12
4.6 x 10-13
~ 1.8 x 10-13
1.3 x 10-'2
6.2 x 10-12
2.5 x 10-1'
4.0 x 10-'
2.8 x 10-'1
2.2 x 10-"
5.2 x 10-"
7.7 x 10-"
3.2 x 10-1'
1.3 x 10-'°
NO2a
NO
HNO2a
HNO3b
SO2
NH3C
CH3NH2C
HCN
H2S
CH3SH
H2O2a
Propane
e-Butane
e-Octane
1,2-Dichloroethane
1,2-Dibromoethane
Ethene
Propene
1-Butene
trans-2-Butene
Acetylene
Propyne
Butadiyne
Formaldehydea
Acetaldehydea
Benzaldehydea
Acrolein
Crotonaldehyde
Methyl vinyl ketone
Acetonea
2-Butanone`'
Dimethyl ether
Methanol
Ethanol
Formic acid
Methyl nitritea
Benzene
Toluene
m-Xylene
1 ,2,4-Trimethyl-
benzene
Phenol
Naphthalene
2-Methylnaphthalene
2,3-Dimethylnaph-
thalene
Phenanthrene
Anthracene
3.2 x 10-17
1.8 x 10-'4
< 5 x 10-19
<2 x 10-22
2.1 x 10-2°
< 2 x 10-2°
< 6 x 10-24
< 1 X 10-23
1.8 x 10-'8
1.1 X 10-17
1.1 X 10-~7
2.0 x 10-16
8 x 10-21
1.4 x 10-2°
6 x 10-2°
2 x 10-24
6 x 10-2~
2.8 x 10-~9
9.0 x 10-~9
4.8 x 10-'8
1.3 x 10-2°
7 x 10-23
1.5 x 10-22
6 x 10-22
1.3 x 10-21
2 x 10-19
4 x 10-~9
4 x 10- i9
1.2 x 10-'2
3.0 x 10-"
< 7 x 10-2'
<3 x 10-'4
1 X 10-12
< 2 x 10- 15
3.6 x 10-17
9.9 x 10-~7
1.1 X 10-'6
7.5 x 10-~5
9.7 x 10-~5
3.8 x 10-'3
s 2.3 x 10-~7
9.4 x 10-~7
5.8 x 10-'6
2.4 x 10-~5
2.0 x 10-~5
< 3 x 10- ~s
< 6 x 10- 16
<9 x 10-'6
<2 x 10-~7
3.6 x 10-~7
1.3 x 10-'6
9.7 x 10-'6
3.8 x 10-'2
1.4 x 1()-~2
8.3 x 1()-~2
< 1 x 10-'8 < 4.2 x 10-23
< 4 x 10-'8
~ 1 x 10-'4
1.4 x 10-~7
4.2 x 10-~7
5.7 x 10-~7
a Photolysis also occurs at a significant rate (see table 3).
b Also reacts with NH3 to form NH4NO3.
c Also reacts with gaseous HNO3 to form nitrate salts.
OCR for page 107
Roger Atkinson
107
Table 3. Calculated Atmospheric Lifetimesa for the Gas-Phase Reactions of the Selected
Automotive Emissions with Atmospherically Important Intermediate Speciesb
Atmospheric Lifetime Due to Reaction with
EmissionOHO3NO3HO2 ho'
NO22 days12 hr1 hr2 hr 2 min
NO4 days1 min3 min20 min
HNO24 days> 33 days ~ 10 min
ANON180 days
SO2e26 days> 200 yr> 4.5 x 104 yr> 600 yr
NH3f140 days
CH3NH2f12 hr2 yr
HCN2 yr
HIS5 days> 2 yr> 4 days
CH3SH8 hr 1 hr
H20214 days > 60 days 36 hr
Propane19 days> 7,000 yr
e-Butane9 days> 4,500 yr9 yr
e-Octane3 days 3 yr
1,2-Dichloroethane100 days
1,2-Dibromoethane90 days
Ethene3 days9 days3 yr
Propene11 hr1.5 days15 days
1-Butene9 hr1.5 days12 days
trans-2-Butene4 hr2 hr4 hr> 150 yr
Acetylene30 days6 yr-14 yr
Propyne4 days3 yr3.4 yr
Butadiyne1 day~ 270 days
Formaldehyde3 days> 2 x 104 yr210 days23 days 4 hr
Acetaldehyde1 day> 7 yr50 days 60 hr
Benzaldehyde2 days 60 days
Acrolein1 day60 days
Crotonaldehyde8 hr18 days
Methyl vinyl ketone1 day3 days
Acetone100 days 15 days
2-Butanone23 days
Dimethyl ether7 days > 40 days
Methanol26 days > 190 days
Ethanol8 days > 130 days
Formic acid50 days
Methyl nitrite~ 120 days3 yr 8 min
Benzene18 days600 yr> 16 yr
Toluene4 days300 yr9 yr
m-Xylene11 hr75 yr2 yr
1,2,~Trimethylbenzene7 hr35 yr120 days
Phenol10 hr 20 min
Naphthalenee1 day> 80 days
2-Methylnaphthalenee5 hr> 40 days
2,3-Dimethylnaphthalenee4 hr> 40 days
Phenanthrene9 hr
Anthracene2 hr
a The time for the compound to decay to 37 percent of its original concentration.
b For concentrations of OH, 12-hr average of 1 x 106 molecules/cm3 (Crutzen 1982); 03, 2thr average of 7 x
10~' molecules/cm3 (Singh et al. 1978); NO3, 12-hr average of 2 x 108 molecules/cm3 (Platt et al. 1984); HO2, 12-
hr average of 108 molecules/cm3 (Hov and Isaksen 1979).
c For solar zenith angle of 0°.
Also reacts with NH3 to form NH4NOs.
e Lifetimes due to gas-phase reaction with a 12-hr average concentration of N~Os of 2 x 10'° molecules/cm3
(Atkinson et al. 1986b) are SO2, > 7.5 x 104 yr; naphthalene, ~80 days; 2-methylnaphthalene, ~ 30 days; 2,3
dimethylnaphthalene, ~ 20 days.
f Also reacts with gaseous HNO3 to form nitrate salts.
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108
Atmospheric Transformations of Automotive Emissions
Table 4. Dry Deposition Velocities for
Several Inorganic and Organic Chemicals
Depositing Species
Mean
Deposition
Velocity
(cm/see)
o3
Particulate sulfur
Particles
0.18-,um median diameter
0.25-m median diameter
Calcium sulfate (CaSO4) particles
1-,um diameter
2-,um diameter
5-,um diameter
10-,um diameter
SO2
HNO3
Tetrachloroethene
Nitrobenzene
Polychlorinated biphenyls
(PCBs)
0.3_0.5a
0.08 - 0.91b
0.17a
0.16a
0.35a
o.olc
0.o3c
0.44c
4.6c
2.la
2.5a
~ 10-4
~ 10
~ 0.1-0.5
a From Dolske and Gatz 1985, with grass as the
surface.
b From Colbeck and Harrison 1985, with grass as the
surface.
c From Jonas and Heinemann 1985, with grass as the
surface.
~ From Sehmel et al. 1985.
e From Eisenreich et al. 1981.
Table 5. Washout Ratios for Selected Organic Chemicals
For the particle-associated chemical species,
the washout ratios W given in table 5 reflect
the loss of the particles. Thus, as discussed
by Eisenreich et al. (1981), the washout
ratios W for aerosols are typically ~ 105 to
106, in comparison to values of~ 10° to 104
for gaseous chemicals.
For particles, the atmospheric lifetimes
due to dry deposition are of the order of
several days for 0.1-1 ,um diameter parti-
cles, and table 6 gives the average lifetimes
of atmospheric particles as a function of
their diameter. The dry deposition lifetimes
of many organic compounds are also weeks
or months (see, for example, Atlas and
Giam 1981~. However, for certain chemi-
cals that have relatively slow gas-phase
chemical loss rates, such as HNO3 and
SO2, dry deposition can be the major loss
process under typical atmospheric condi-
tions. Because of the potential importance
of the dry deposition atmospheric removal
process, measurements of the deposition
velocities of gaseous and particulate species
need to be carried out for a variety of
terrains. This is a difficult and time-con-
suming task, and emphasis must be given
to extending the presently available exper-
imental techniques and to developing and
testing new experimental, and possibly the-
oretical, approaches (see, for example,
Dolske and Gatz 1985~.
. , .
Phase Chemical Washout Ratio (W)
Gas Ethene oxide 4-6a
Phenol (0.7-25) x 1o4b
Nitrobenzene (2-4) x 103a
Naphthalene 100-300C
Phenanthrene 2, 000-4, 000C
Pyrene 3 000-9 000C
Benz ~a j anthracene 7,000-22,000C
Hexachlorobenzene 1,500
Particle PCBs ~ (1-10) x 104
Particles
0.1-1.0-,um diameter ' 10S~
10-,um diameter ~ 106~
Tricosane through hexacosane ~ 2 x 104e
a From Terry Dana et al. 1985.
b From Leuenberger et al. 1985.
c From Ligocki et al. 1985a.
From Eisenreich et al. 1981.
e From Ligocki et al. 1985b.
OCR for page 109
Roger Atkinson
109
- J
For nonpolar organic compounds, wet
deposition appears to be of minor impor-
tance as an atmospheric loss process. How-
ever, for highly water-soluble gases such as
HNO~ and H2O2, wet deposition can be
important (Jacob and HofEmann 1983:
Chang 1984), and in fog and cloud systems
this process leads to removal of these and
other compounds from the gas phase into
the aqueous phase where reactions can oc-
cur that lead ultimately to the formation of
acids and other oxygenated products.
Wet deposition rates are somewhat better
understood, with the experimental results
for gas-phase chemicals agreeing to within
a factor of~ 10 or better with theoretical
expectations (see, for example, Ligocki et
al. 1985a,b; Terry Dana et al. 1985~. Fur-
ther research is needed to provide a wider
data base concerning the washout ratios of
chemical compounds present in the has
phase and of aerosols and particles. These
data will then allow the importance of this
wet deposition process to be better evalu-
ated, both as a loss process for primary
automotive emissions as well as for the
formation and deposition of acid species
resulting from aqueous-phase reactions.
The major atmospheric loss process for
most of the automotive emissions present
in the gas phase is by daytime reaction with
the OH radical. However, for certain
classes of automotive emissions, pho-
tolysis, reaction with NO3 radicals during
nighttime hours, and reaction with O3 can
be important removal routes. Furthermore,
reactions that are relatively minor removal
processes may need to be considered if they
generate products with potential health
risks to humans. For example, the reactions
of gas-phase N2O5 with PAHs appear to be
of minor significance as a PAH loss proc-
ess, but they form toxic nitropolycyclic
aromatic hydrocarbons (nitro-PAHs) (Pitts
et al. 1985b; Sweetman et al. 1986~.
Clearly, a knowledge of the atmospheric
loss processes and lifetimes for automotive
. . . . . , . ~ .
emissions Is important, since t nese lifetimes
determine the geographic extent of the influ-
ence of the parent automotive emission.
Thus, a short lifetime leads to local exposure,
whereas a long lifetime leads to regional or
global exposure at lower concentrations.
Table 6. Average Atmospheric Lifetimes for
Particles Due to Dry Deposition
Diameter (,um)
Lifetime (days)
0.002
0.02
0.2
2
20
200
0.01
10
10
1
0.01
SOURCE: Adapted with permission from Graedel
and Weschler 1981.
The atmospheric lifetimes of automotive
emissions present in the particulate phase
are less well known. Dry and wet deposi-
tion constitute the physical loss processes,
and photolysis and/or reaction with gas-
phase and coadsorbed reactive intermedi-
ates constitute the possible chemical loss
processes. These chemical processes are
substrate dependent, with photolysis, reac-
tion with O3, reaction with NO2 and/or
HNO3, and reaction with N205 occurring
(see, for example, Nielsen et al. 1983; Pitts
1983~. However, due to the dependence of
these loss processes on the nature of the
substrate, it is presently impossible to cite
any meaningful atmospheric lifetimes for
adsorbed automotive emissions, except to
remark that the reaction of PAHs with O3
may lead to lifetimes on the order of hours,
photolysis is probably slow, and reaction
with N2O5 though slow as a loss proc-
ess leads to the formation of direct-acting
mutagenic and possibly carcinogenic nitro-
PAH products.
_ ~1
Atmospheric Transformations
Oxides of Nitrogen. Oxides of nitogen
emitted into the atmosphere as a result
of automotive use comprise NO, NO2,
N2O, HNO2, and possibly HNO3. N2O
has been shown to be chemically inert in
the troposphere, being transported into the
stratosphere where it photodissociates at
wavelengths of ~ 220 nm (Liu et al. 1977),
NATO + he ~ No + OILED) (26)
and reacts with electronically excited oxy-
gen atoms, O(iD), leading to formation of
NO.
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122
Atmospheric Transformations of Automotive Emissions
o
11
OH + CH3COCH3
o
11
CH3COCH2 + H2O
O2
NO- ~ NO2
O O
11 11
CH3COCH2O ~ CH3CO- + HCHO
1 O2 CH3 + CO2
o
11
CH3COCHO + HO2
(105)
Recommendation 4. Investigations,
under atmospheric conditions, of the reac-
tion products for partially oxidized auto-
motive emissions and their health impacts
on humans are needed. This area of re-
search includes the atmospheric transfor-
mations of methanol and ethanol, formal-
dehyde and acetaldehyde coemissions, and
any other emissions associated with their
use as alternative fuels. In addition, the
atmospheric transformation products and
associated health implications of aldehydes,
ketones, cr,,l3 unsaturated carbonyl com-
pounds, carboxylic acids, and other prod-
ucts of incomplete combustion should be
determined.
Monocyclic Aromatic Compounds. The ar-
omatic hydrocarbons such as benzene, tol-
uene, ethylbenzene, the xylenes, and the
trimethylbenzenes are important constitu-
ents of gasoline and diesel fuel, as well as
being major constituents of exhaust emis-
sions. In addition, oxygen- and nitrogen-
containing aromatic compounds, such as
phenol, the cresols, and aromatic amines,
may also be emitted.
The monocyclic aromatic hydrocarbons
are removed from the atmosphere solely by
reaction with the OH radical (table 3).
These OH radical reactions proceed by two
pathways: (1) a minor pathway involving
hydrogen-atom abstraction from C H
bonds of, for benzene, the aromatic ring, or
for alkyl-substituted aromatic hydrocar-
bons, the alkyl-substituted groups; and (2)
c~3
OH +~-
a major reaction pathway involving OH
radical addition to the aromatic ring. For
example, for toluene, these reaction path-
ways are
CH2
- ~ H2O +
CH3 OH
~H
and other isomers
~(106)
The hydrogen-atom abstraction pathway,
top, leads mainly to the formation of
aromatic aldehydes (Atkinson and Lloyd
1984)
CH7 NO CH2O
+~
O2
NO2
CHO
~ + HO2
Benzaldehyde (107)
Subsequent reactions of these aromatic al-
dehydes with OH radicals lead to the for-
mation of peroxybenzoyl nitrates and ni-
trophenols (see above, and Atkinson and
Lloyd 1984). This hydrogen-atom abstrac-
tion pathway is minor, accounting for c 10
percent of the overall OH radical reaction
for benzene and the alkyl-substituted aro-
matic hydrocarbons (Gery et al. 1985; At-
kinson 1986).
The products arising from the OH rad-
ical addition reaction pathways are not
well understood. Under atmospheric con-
ditions, the initially formed OH-aro-
matic adduct is expected to react mainly
with O2, again by two reaction pathways.
For example, for the toluene-OH ad-
duct,
OCR for page 123
Roger Atkinson
123
CH3
+ O2
CH3
~1' + HO2
CH3
OH
7 ~ ~. {-)-
The hydrogen-atom abstraction reaction of
the OH-aromatic adduct to yield phenolic
compounds has been shown to be relatively
minor, accounting for ~ 20 percent of the
overall OH radical reaction mechanism for
toluene (Atkinson and Lloyd 1984; Gery et
al. 1985; Leone et al. 1985~. The major
reaction pathway involves other reactions
of the OH-aromatic-O2 adducts, and these
have been shown to involve ring cleavage.
Thus, the cY-dicarbonyls glyoxal, methyl-
glyoxal, and biacetyl have been identified
and quantified from benzene and the
methyl-substituted benzenes (Bandow et
al. 1985; Bandow and Washida 1985a,b;
Tuazon et al. 1986), and reaction pathways
that lead to these products have been pro-
posed (Atkinson et al. 1980; Killus and
Whitten 1982~. The reaction pathways that
form a-dicarbonyls, phenolic products,
and hydrogen-atom abstraction products,
however, fail to account for ~ 3~50 per-
cent of the overall reaction products. The
recent semiquantitative or qualitative, but
highly important, product studies of Dum-
dei and O'Brien (1984) and Shepson et al.
(1984) have identified a variety of other
bifunctional ring cleavage products from
toluene and o-xylene, which include,
from toluene, CH3COCOCH-CH2,
CHOCOCH CH2, CH3COCH-CH2,
CH3COCH-CHCH CH2, CHOC (OH)
CHCHO, and CH3COCH-CHCH-
CHCHO.
Much less information is available for the
other aromatic compounds either directly
emitted from automobiles, or formed as
products from the primary aromatic emis-
sions during their atmospheric transport.
Indeed, most of the information has been
derived from kinetic rather than direct
product studies. For example, the phenolic
compounds, which are known to be re-
moved from the atmosphere primarily by
chemical reaction with OH and NO3 radi-
cals (tables 2 and 3; Atkinson and Lloyd
1984) and by wet deposition from the gas
phase (Leuenberger et al. 1985), react with
OH radicals mainly by initial OH radical
addition to the ring. However, the NO3
radical reaction appears to proceed by hy
(108) drogen-atom abstraction from the substitu
ent-OH group (Atkinson et al. 1984~.
OH
NO3 + ~
HNO3 + [~ (109)
1 NO2
o- and p-nitrophenol
For other classes of monocyclic aromatic
compounds, product data are not available.
Product yields under atmospheric condi-
tions are reliably known for only a few of
the many aromatic hydrocarbons emitted
from automotive use. Moreover, the health
effects of most of these compounds are not
known, although methylglyoxal has re-
cently been reported to be mutagenic
toward Salmonella typhimurium strain
TA100 (Shepson et al. 1985b). Since the
observed product yields typically account
for ' 50 percent of the overall reaction
products, an understanding of the remain-
ing products and their health effects is
necessary. This will include products
formed in the particulate and gas phases.
Recommendation 5. The products
arising from the OH radical-initiated reac-
tions of the aromatic hydrocarbons, a ma
. . . .
Jor emlsslon category trom automotive
use, need to be identified. These studies are
important because of the relatively high
reactivity of aromatic hydrocarbons, and
will involve the identification and quantifi-
cation of a plethora of bifunctional organic
compounds, many of which will probably
be present in low yield.
Polycyclic Aromatic Hydrocarbons (PAHsJ
and Their Derivatives and Analogues. A
large number of these chemical com
OCR for page 124
124
Atmospheric Transformations of Automotive Emissions
Table 7. Vapor Pressures at 298°K for a
Series of PAHs
PAH
Vapor Pressure at
298°K (torr)
Naphthalene
Phenanthrene
Anthracene
Fluoranthrene
Pyrene
Bench janthracene
Benzota~pyrene
Chrysene
8.0 x 10 a
1.2 x 10 - 4 a
6.0 x 10
9.2 x 106 a
4.5 x 106 a
2.1 x 10a
5.6 x 10
6.4 x 10
a From Sonnefeld et al. 1983.
b From Yamasaki et al. 1984.
pounds, including PAHs (such as naphtha-
lene, phenanthrene, anthracene, fluoran-
thene, pyrene, perylene, and benzo~a]
pyrene), and their alkyl-substituted or ox-
ygen-, sulfur-, and nitrogen-containing de-
rivatives, as well as oxygen-, nitrogen- and
sulfur-containing heterocyclic analogues,
are, or may be, emitted from combustion
sources. Although these compounds are
relatively minor components of automo-
tive emissions, they have assumed a "spot-
light" position in automotive-related health
risk assessments because of their potential
toxicity.
The PAHs and their analogues and de-
rivatives have relatively low vapor pres-
sures (table 7), and are distributed between
the gas and particulate phases, with this
distribution being highly temperature de-
pendent.
As presently understood, the atmo-
spheric transformations of these PAHs and
their derivatives are highly dependent upon
the phase with which they are associated.
The available data show that for the PAHs
present in the gas phase, reaction with the
OH radical predominates, leading to atmo-
spheric lifetimes of a few hours or less
(table 3~. The nighttime reaction with
N205 is of minor significance as a PAH
loss process (table 3) (Pitts et al. 1985a), but
may be important for the formation of
nitro-PAHs (see below, and Arey et al.
1986~.
Recent ambient atmospheric data from
Norway, Denmark, and the United States
show that 2-nitrofluoranthene and 2-ni
tropyren~ nitro-PAHs not observed from
combustion sources are the major nitro-
PAH components of particulate organic
matter (POM) (Nielsen et al. 1984; Pitts et
al. 1985c; Ramdahl et al. 1986~. Since these
two nitro-PAHs are not formed during the
collection of POM, they must be formed in
the atmosphere from the parent PAH dur-
ing transport from source to receptor
(Nielsen et al. 1984; Pitts et al. 1985c; Arey
et al. 1986; Ramdahl et al. 1986; Sweetman
et al. 1986; Zielinska et al. 1986~. Indeed, it
now appears that the majority of the nitro-
PAHs present in ambient POM are formed
via atmospheric transformations during
transport from source to receptor.
xecent env~ronmenta~ chamber studies
have shown that 2-nitrofluoranthene as
well as 2-nitropyrene are formed from the
gas-phase reactions of fluoranthene and py-
rene with OH radicals in the presence of
NOx (Arey et al. 1986~. 2-Nitrofluoran-
thene is also formed from the gas-phase
reaction of N2O5 with fluoranthene
(Sweetman et al. 1986; Zielinska et al.
1986~.
Since many PAHs and their analogues
and derivatives are partitioned primarily
into the adsorbed phase under atmospheric
conditions, a large number of experimental
studies have been performed to delineate
the reaction processes occurring for the
adsorbed-phase compounds. However,
most of these studies have been done using
nonatmospherically realistic adsorbents
such as glass fiber and Teflon-impregnated
glass fiber and silica surfaces. The data
obtained from these and from more realis-
tic surfaces, such as carbon black and fly-
ash, show that the reactions (including
photolysis) are strongly dependent on the
nature of the adsorbent species (see, for
example, Pitts 1983; Behymer and Hites
1985~.
Photolysis, reaction with 03, NO2, and/
or HNO3, and N2O5 have all been shown
to lead to loss of PAHs on several sub-
strates (see, for example, Pitts 1983; Pitts
et al. 1985b, 1986~. Certain of these reac-
tions, in particular, those with NO2 and
HNO3 and with N2O5, appear to be rela-
tively slow under atmospheric conditions
(Pitts et al. 1985b). However, because of
OCR for page 125
Roger Atkinson
125
the substrate dependence of these adsorbed-
phase reactions (see, for example, Behymer
and Hites 1985), no firm conclusions can be
drawn about the importance of these reac-
tions under atmospheric conditions. It does
appear that photolysis and reaction with O3
may be important for certain PAHs and
their derivatives.
Clearly, a comprehensive and systematic
investigation of the gas- and adsorbed-
phase reactions of this class of automotive
emissions is necessary before further risk
assessment studies concerning these com-
pounds can be carried out. This is a difficult
research area because of the partitioning of
many, if not most, of these emissions and
their products between the gas and partic-
ulate phases, and because of the high
potential for analytical artifacts during sam-
pling with the currently available tech-
niques.
· Recommendation 6. The atmospheric
transformation products of PAHs and their
oxygen-, nitrogen-, and sulfur-containing
analogues and homologues require study,
in the gaseous and the adsorbed phases. In
particular, the reaction pathways that lead
to nitro-PAHs need to be quantitatively
established. In addition, the atmospheric
removal processes and resulting products
of these nitro-PAHs should be studied fur-
ther.
Particulate Matter. A variety of other
chemical compounds, including metals
such as Pb, are emitted from automotive
use into the atmosphere in particulate form.
As shown in table 6, particles are removed
from the atmosphere at rates that depend
markedly on the particle size. For particles
of diameter 0.1-1 Em (the size that corre-
sponds to most particles present in the
atmosphere), dry and wet deposition pro-
cesses lead to lifetimes of several days or
more.
Since the metals emitted are expected to
be present mainly in their oxidized form
for example, PbBrC1-chemical reactions
are unlikely and their removal will occur
principally by these physical processes. For
organic chemicals emitted from automo
tive use and present on ambient POM,
reactions may occur during atmospheric
transport. This topic has assumed impor-
tance because of the recent interest in acidic
deposition and the role of aerosols in the
formation of nitric, sulfuric, and organic
acids. The reactions that can occur are
complex and involve aqueous chemistry,
gas-to-particle conversion, and heteroge-
neous reactions (see, for example, Graedel
and Weschler 1981; Chameides and Davis
1982; Graedel and Goldberg 1983; Jacob
and Hoffmann 1983; Chameides 1986;
Graedel et al. 1986~. However, this field is
in a state of rapid change, and further
research is necessary before a full under-
standing can be reached (see Recommenda-
tion 1~.
Analytical Techniques
It has become apparent during the past
several years that a major experimental
. . . . .
lnltlatlVe IS necessary to c .eve op new ana-
lytical techniques to allow the products of
these complex atmospheric reactions to be
identified and quantified. Fourier transform
infrared absorption spectroscopy as well as
gas chromatography/mass spectrometry
(now often used in gas-phase studies) are
subject to significant limitations when the
organic products are complex and because
of the possibility of the formation of arti-
facts. Similarly, for particle-associated
chemicals, studies of their atmospheric
degradation reactions and associated rates
are often complicated by artifacts. Clearly,
there is a need for new in situ analytical
techniques allowing real-time analyses de-
void of artifact product formation prob-
lems. This area is a major research topic
in its own right and should be recognized
as such. If this research area is not ag-
gressively pursued, any advances in our
knowledge about the atmospheric mecha-
nisms and reaction products of automotive
emissions may well become limited by
the available analytical procedures. As re-
search progresses in this area, it is also
apparent that further techniques for study-
ing low-volatility organics in the gas
and the adsorbed phases must be devel-
oped.
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126
Atmospheric Transformations of Automotive Emissions
Recommendation 7. A major research
effort is needed to develop the necessary
analytical techniques for identifying and
quantifying the products of complex atmo-
spheric reactions. Of prime importance is
the development of nondestructive, nonin-
trusive, in situ analytical techniques that
will allow the atmospheric transformations
of gaseous and particulate-associated chem-
ical species to be studied.
Summary
As a result of the last two decades of
laboratory, computer modeling, and ambi-
ent atmospheric experiments, a large body
of data now exists concerning the atmo-
spheric loss processes and transformations
of automotive emissions. However, signif-
icant gaps in our knowledge still remain,
mainly about the products formed under
atmospheric conditions.
As discussed in the sections above, the
physical and chemical processes leading to
the removal of automotive emissions from
the atmosphere include
· wet and dry deposition of gases and
particles;
· chemical reactions of gaseous automotive
emissions with OH, NO3, and HO2 radi-
cals, 03, N2O5, and gaseous HNO3;
· photolysis;
· reaction of particulate-associated organic
compounds with a variety of gas- and
adsorbed-phase species; and
· reactions in the aqueous phase with a vari-
ety of reactive species that are of importance
in clouds, raindrops, and fog droplets, and
lead to the formation of acidic precipita-
tion.
Chemical reactions dominate the removal of
most organic chemicals, with atmospheric
lifetimes ranging from less than 1 min for
highly reactive organic compounds reacting
with the NO3 radical during nighttime
hours, to months or even years for the less
reactive alkalies, haloalkanes, and substituted
benzenes. Inorganic compounds emitted as a
result of automotive use also exhibit a wide
range of atmospheric lifetimes, with NO2,
HNO2, and alkyl nitrites having photolysis
lifetimes measured in minutes. In contrast,
H2O2 and HNO3 are readily removed at
surfaces, and are predominantly removed
from the gas phase by wet and/or dry depo-
sition processes that can take several days or
more.
The chemical transformations of auto-
motive emissions lead to the formation of
a wide variety of products. Many of these
transformation products are unknown, and
the health impacts on humans of those
that are known have not been investigated.
Future research programs must first re-
quire studies to determine the general
chemical classes of products formed from
the atmospheric transformations of auto-
motive emissions. For those products sus-
pected to threaten human health, additional
work will then be necessary to better de-
fine their amounts and formation mecha-
nisms.
Summary of Research Recommendations
Significant gaps still exist in our understanding of the physi-
cal and chemical transformations of automotive emissions that
occur in the atmosphere during transport from source to recep-
tor. The areas requiring further study are ranked in order of
. .
prlorlty.
HIGH PRIORITY
Recommendlation2 Further investigations of the transformations of NOX under
atmospheric conditions are needed. This topic is important for
OCR for page 127
Roger Atkinson
127
indoor environments as well, since certain NOx undergo important
heterogeneous transformations. In particular, laboratory research
has shown that, under conditions representative of certain indoor
environments, NO2 hydrolyzes on surfaces to yield gas-phase
HNO2 at significant rates. The kinetics and mechanism of this
heterogeneous reaction should also be investigated in ambient
nighttime atmospheres as should the reactions of the NO3 radical
(especially with respect to the products formed from its reactions
with organic compounds) and the heterogeneous and/or homoge-
neous reactions of N2O5.
Recommendlation 5 The products arising from the OH radical-initiated reactions of
the aromatic hydrocarbons, a major emission category from auto
motive use, need to be identified. These studies are important
because of the relatively high reactivity of aromatic hydrocarbons,
and will involve the identification and quantification of a plethora
of bifunctional organic compounds, many of which will probably
be present in low yield.
Recommendlation6 The atmospheric transformation products of PAHs and their
oxygen-, nitrogen-, and sulfur-containing analogues and homolo
gues require study in the gaseous and the adsorbed phases. In
particular, the reaction pathways that lead to nitro-PAHs need to
be quantitatively established. In addition, the atmospheric removal
processes and resulting products of these nitro-PAHs should be
studied further. These studies will be difficult to perform because of
the high potential for artifact formation.
MEDIUM PRIORITY
Recommendlation 1 Study is required on the physical removal processes leading to
wet and dry deposition of gases and particles. Investigations of
the processes occurring on and in particulate and aerosol matter
should focus on As-to-particle conversion processes and the chem
ical processes that occur within aerosols (including fogs and
clouds).
Recommendlation 3 The products arising from the OH radical-initiated reactions of
alkanes the major component of automobile emissions require
study. These products are likely to be distributed between the gas
and particulate phases, and data are especially needed for the
alkanes with eight or more carbon atoms.
-
Recommendation4 Investigations, under atmospheric conditions, of the reaction
products for partially oxidized automotive emissions and their
health impacts on humans are needed. This area of research
includes the atmospheric transformations of methanol and ethanol,
formaldehyde and acetaldehyde co-emissions, and any other emis
sions associated with their use as alternative fuels. In addition, the
atmospheric transformation products and associated health impli
cations of aldehydes, ketones, a,,~unsaturated carbonyl com
pounds, carboxylic acids, and other products of incomplete com
bustion should be determined.
OCR for page 128
128
Atmospheric Transformations of Automotive Emissions
Recommendation 7 A major research effort is needed to develop the necessary
analytical techniques for identifying and quantifying the products
of complex atmospheric reactions. Of prime importance is the
development of nondestructive, nonintrusive, in situ analytical
techniques that will allow the atmospheric transformations of
r
gaseous and particulate-associated chemical species to be studied.
This is clearly a long-term ideal, but utterly crucial in order to
advance our current knowledge of the atmospheric transformations
~ . . .
01 automotive emlsslons.
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Representative terms from entire chapter:
atmospheric transformations
