In recent years, there has been a growing concern about a number of atmospheric environmental problems, such as the formation of photochemical oxidants, acid deposition, global-scale eff
Trang 1INTRODUCTION
Atmospheric chemistry is a broadly based area of scientific
endeavor It is directed at determining the quantities of
vari-ous chemicals in the atmosphere, the origin of these
chemi-cals, and their role in the chemistry of the atmosphere Many
atmospheric chemists are involved in the development of
techniques for the measurement of trace quantities of
differ-ent chemicals in the atmosphere, in emissions, and in
depo-sitions Other atmospheric chemists study the kinetics and
mechanisms of chemical reactions occurring in the
atmo-sphere Still other atmospheric chemists are involved in the
development of chemical models of the processes occurring
in the atmosphere Atmospheric chemists work closely with
other disciplines: engineers in characterizing anthropogenic
emissions; biologists and geologists in characterizing natural
emissions and in evaluating the effects of air pollution;
physi-cists in dealing with gas-to-particle conversions; and
meteo-rologists, physicists, computer scientists, and mathematicians
in dealing with model development Atmospheric chemistry
plays a key role in maintaining the general well-being of the
atmosphere, which is extremely important for maintaining
the health of the human race
In recent years, there has been a growing concern about a
number of atmospheric environmental problems, such as the
formation of photochemical oxidants, acid deposition,
global-scale effects on stratospheric ozone, the sources and fates of
toxic chemicals in the atmosphere, and urban and regional haze
issues and the presence and effects of fine particulate matter in
the atmosphere These problems are affected by a wide
vari-ety of complex chemical and physical processes Atmospheric
chemistry is the broad subject area that describes the
interrela-tionships between these chemical and physical processes
The principal components of the atmosphere are
nitro-gen and oxynitro-gen These molecules can absorb a portion of the
high-energy solar ultraviolet radiation present in the upper
atmosphere and form atoms These atoms may react with a
variety of other species to form many different radicals and
compounds For example, the short-wavelength
ultravio-let radiation present in the upper atmosphere can photolyze
molecular oxygen to form oxygen atoms These oxygen
atoms may react with molecular oxygen to form ozone
These reactions are only of importance at high altitudes,
where the short-wavelength ultraviolet radiation is present
In the lower regions of the atmosphere, only light of
wave-lengths greater than about 300 nm is present Table 1 lists the
relative concentrations of a number of species present in the atmosphere, near the Earth’s surface The chemistry that is most important at lower altitudes is initiated by a variety of compounds or trace species, which are present in the atmo-sphere at concentrations of much less than 1 ppm
One of the most important reasons to understand atmo-spheric chemistry is related to our need to understand and control air pollution The air-pollution system, shown in Figure 1, starts with the sources that emit a variety of pollut-ants into the atmosphere Those pollutpollut-ants emitted directly
into the atmosphere are called primary pollutants Once these
primary pollutants are in the atmosphere, they are subjected
to meteorological influences, such as transport and dilution,
in addition to chemical and physical transformations to
sec-ondary pollutants Secsec-ondary pollutants are those formed by
reactions in the air The pollutants in the air may be removed
by a variety of processes, such as wet and dry deposition An ambient-air-monitoring program is used to provide detailed information about the compounds present in the atmosphere
TABLE 1 Relative composition of the atmosphere near
the Earth’s surface
Source: Adapted from J Heicklen (1976), Atmospheric Chemistry, Academic Press, New York;
and R.P Wayne (1985), Chemistry of Atmospheres,
Clarendon Press, Oxford.
Trang 2One of the principal goals of air-pollution research is to
obtain and use our detailed knowledge of emissions,
topogra-phy, meteorology, and chemistry to develop a mathematical
model that is capable of predicting concentrations of primary
and secondary pollutants as a function of time at various
loca-tions throughout the modeling domain These model results
would be validated by comparison with
ambient-air-monitor-ing data Model refinement continues until there is acceptable
agreement between the observed and predicted
concentra-tions This type of air-quality model, on an urban scale, is
called an airshed model Airshed models treat the effects of
a set of stationary and mobile sources scattered throughout a
relatively small geographical area (⬃100 km 2 ) These models
are intended to calculate concentrations of pollutants within this geographical area and immediately downwind
It is also necessary to develop a detailed knowledge of the impacts of pollutants on the various important receptors, such
as humans, plants, and materials This impact information
is used to identify the pollutants that need to be controlled
An airshed model can be used to predict the effectiveness
of various proposed control strategies This information can
be passed on to legislative authorities, who can evaluate the costs and benefits of the various strategies and legislate the best control measures
Unfortunately, there are significant gaps in our knowledge
at every step throughout this idealized air-pollution system
Sources
Emissions of Anthropogenic, Biogenic, Geogenic Primary Pollutants e.g.
VOC, NOx, SO2, CO, PM10,2.5, HAP s
Dispersion and Transport
Chemical and Physical Transformations
Scientific Risk Assessment
Effects:
Health and Environmental Exposure
Monitoring
FATES
Wet and Dry Deposition
Transport to Stratosphere
Stratospheric Chemistry, Ozone Depletion
Models Local “Hot-Spot”
Plume, Airshed, Long-range Transport, Global
Risk Management Decisions Air Pollution Control
Impacts on Receptors (Humans, Animals, Agricultural Crops Forest and Aquatic Ecosystems, Visibility, Materials, etc.)
Long-Lived Species e.g CFC, N 2 O
Ambient Air Urban, Suburban, Rural Remote, O3, Acids, Toxics PM10,2.5etc.
FIGURE 1 The atmospheric air-pollution system From Finlayson-Pitts and Pitts (2000) (HAPs—
hazardous air pollutants) With permission.
Trang 3Hence, there is considerable room for continued research
Atmospheric chemistry is involved in several steps through
the air-pollution system First is chemically characterizing
and quantifying the emissions of primary pollutants Second
is understanding the chemical and physical transformations
that these primary pollutants undergo Third is measuring the
quantities of the various pollutants in the ambient air Fourth
is quantifying the deposition processes for the various
pol-lutants Finally, a mathematical formulation of the sources,
chemical and physical transformations, and removal
pro-cesses must be incorporated into the atmospheric model
The chemistry of the formation of secondary pollutants
is extremely complex It requires the identification of all of
the important reactions contributing to the chemical system
There must be a thorough investigation of each specific
reac-tion, which can be achieved only when the reaction-rate
constant has been carefully determined for each elementary
reaction involved in the properly specified reaction
mecha-nism Because of the large number of important reactions
that take place in the atmosphere, the rapid rates of many of
them, and the low concentrations of most of the reactants, the
experimental investigations of these atmospheric chemical
kinetics is an enormously large and complex task
In the United States, a set of National Ambient Air Quality
Standards (NAAQS) have been established, as shown in Table 2
The primary standards are designed to protect the public health
of the most susceptible groups in the population Secondary NAAQS have also been set to protect the public welfare, including damage to plants and materials and aesthetic effects, such as visibility reduction The only secondary standard that currently exists that is different from the primary standard is for
SO 2 , as shown in the table For comparison purposes, Table 3 shows recommended limits for air pollutants set by the World Health Organization and various individual countries
To illustrate the importance and complexity of atmospheric chemistry, a few examples will be presented and discussed: (1) urban photochemical-oxidant problems, (2) secondary organic aerosols, (3) chemistry of acid formation, and (4) stratospheric ozone changes in polar regions These examples also illustrate the differences in the spatial scales that may be important for different types of air-pollution problems Considering urban problems involves dealing with spatial distances of 50 to 100 km and heights up to a few kilo-meters, an urban scale or mesoscale The chemistry related
to acid formation occurs over a much larger, regional scale, extending to distances on the order of 1000 km and altitudes
of up to about 10 km For the stratospheric ozone-depletion problem, the chemistry of importance occurs over a global scale and to altitudes of up to 50 km Secondary organic aero-sol formation can be an urban to regional scale issue
TABLE 2 U.S National Ambient Air Quality Standards
1 Not to be exceeded more than once per year
2 To attain this standard, the expected annual arithmetic mean PM 10 concentration at each monitor within an area must not exceed 50 µ g/m 3
3 To attain this standard, the 3-year average of the annual arithmetic mean PM 2.5 concentrations from single or multiple community-oriented monitors must not exceed 15 µ g/m 3
4 To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area must not exceed 65 µ g/m 3
5 To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm
6 (a) The standard is attained when the expected number of days per calendar year with
maximum hourly average concentrations above 0.12 ppm is 1
(b) The 1-hour NAAQS will no longer apply to an area one year after the effective data of the
designation of that area for the 8-hour ozone NAAQS
Source: Data is from the U.S EPA Web site: http://www.epa.gov/air/criteria.html
Trang 4TABLE 3 Recommended ambient air-quality limits for selected gases throughout the world
0.008 (annual)
0.008 (annual)
0.02 (annual)
0.04 (annual)
0.03 (annual)
0.02 (annual)
Source: Data was collected from Web sites from the individual countries and organizations
Note: Numbers in parentheses represent the averaging time period and number of exceedances allowed
Trang 5URBAN PHOTOCHEMICAL OXIDANTS
The photochemical-oxidant problems exist in a number of
urban areas, but the Los Angeles area is the classic example
of such problems Even more severe air-pollution problems
are occurring in Mexico City The most commonly studied
oxidant is ozone (O 3 ), for which an air-quality standard exists
Ozone is formed from the interaction of organic compounds,
nitrogen oxides, and sunlight Since sunlight is an important
factor in photochemical pollution, ozone is more commonly
a summertime problem Most of the ozone formed in the
troposphere (the lowest 10 to 15 km of the atmosphere) is
formed by the following reactions:
NO 2 hν ( 430 nm) → NO O( 3 P) (1)
O( 3 P) O 2 M → O 3 M (2)
Nitrogen dioxide (NO 2 ) is photolyzed, producing nitric
oxide (NO) and a ground-state oxygen atom (designated as
O( 3 P)) This oxygen atom will then react almost exclusively
with molecular oxygen to form ozone The M in reaction (2)
simply indicates that the role of this reaction depends on the
pressure of the reaction system NO can also react rapidly
with ozone, reforming NO 2 :
These three reactions allow one to derive the photostationary
state or Leighton relationship
[O 3 ] [NO]/[NO 2 ] = k 1 / k 3 or [O 3 ] = k 1 [NO 2 ]/ k 3 [NO]
This relationship shows that the O 3 concentration depends
on the product of the photolysis rate constant for NO 2 ( k 1 )
times the concentration of NO 2 divided by the product of the
rate constant for the NO reaction with O 3 ( k 3 ) times the NO
concentration This photolysis rate constant ( k 1 ) will depend
on the solar zenith angle, and hence will vary during the day,
peaking at solar noon This relationship shows that the
con-centration of ozone can only rise for a fixed photolysis rate
as the [NO 2 ]/[NO] concentration ratio increases Deviations
from this photostationary state relationship exist, because as
we will see shortly, peroxy radicals can also react with NO to
make NO 2
Large concentrations of O 3 and NO cannot coexist, due to
reaction (3) Figure 2 shows the diurnal variation of NO, NO 2 ,
and oxidant measured in Pasadena, California Several
fea-tures are commonly observed in plots of this type Beginning
in the early morning, NO, which is emitted by motor
vehi-cles, rises, peaking at about the time of maximum automobile
traffic NO 2 begins rising toward a maximum value as the NO
disappears Then the O 3 begins growing, reaching its
maxi-mum value after the NO has disappeared and after the NO 2
has reached its maximum value The time of the O 3 maximum
varies depending on where one is monitoring relative to the
urban center Near the urban center, O 3 will peak near noon,
while further downwind of the urban center, it may peak in
the late afternoon or even early evening
Hydrocarbon Photooxidation
The chemistry of O 3 formation described thus far is overly sim-plistic How is NO, the primary pollutant, converted to NO 2 , which can be photolyzed? A clue to answering this question comes from smog-chamber studies A smog chamber is a rela-tively large photochemical-reaction vessel, in which one can simulate the chemistry occurring in the urban environment Figure 3 shows a plot of the experimentally observed loss rate for propene (a low-molecular-weight, reactive hydrocarbon commonly found in the atmosphere) in a reaction system ini-tially containing propene, NO, and a small amount of NO 2 The observed propene-loss rate in this typical chamber run was considerably larger than that calculated due to the known reactions of propene with oxygen atoms and ozone Hence, there must be another important hydrocarbon-loss process Hydroxyl radicals (OH) react rapidly with organics Radicals, or free radicals, are reactive intermediates, such as
an atom or a fragment of a molecule with an unpaired elec-tron Let’s look at a specific sequence of reactions involving propene
The hydroxyl radical reacts rapidly with propene:
OH CH 3 CH=CH 2 → CH 3 CHCH 2 OH (4a)
OH CH 3 CH=CH 2 → CH 3 CHOHCH 2 (4b) These reactions form radicals with an unpaired electron
on the central carbon in (4a) and on the terminal carbon
in (4b) These alkyl types of radicals react with O 2 to form alkylperoxy types of radicals
CH 3 CHCH 2 OH O 2 → CH 3 CH(O 2 )CH 2 OH (5a)
CH 3 CHOHCH 2 O 2 → CH 3 CHOHCH 2 (O 2 ) (5b)
Pasadena, California, on July 25, 1973 From Finlayson-Pitts and Pitts (2000) With permission.
0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36 0.40 0.44 0.48
Time (hours)
Oxidant
NO
NO
2
Trang 615
10
5
Time (min)
O atom rate
O3 rate
Experimentally determined rate
FIGURE 3 Experimentally observed rates of propene loss and
Finlayson-Pitts and Pitts (1986).
an acetaldehyde molecule have been formed, and the hydroxyl radical that initiated the reaction sequence has been re-formed This mechanism shows the importance of the hydroxyl radi-cal in explaining the excess removal rate of propene observed
in smog-chamber studies In addition, it provides a clue about how NO is converted to NO 2 in the atmosphere
Hydroxyl radicals are present in the atmosphere at very low concentrations Since the hydroxyl radical is reformed in the atmospheric photooxidation of hydrocarbons, it effectively acts as a catalyst for the oxidation of hydrocarbons Figure 4 illustrates the role of the hydroxyl radical in initiating a chain
of reactions that oxidize hydrocarbons, forming peroxy radi-cals that can oxidize NO to NO 2 and re-form hydroxyl radicals The NO 2 can photolyze, leading to the formation of ozone
PAN Formation
Acetaldehyde may react with hydroxyl radicals, forming the peroxyacetyl radical (CH 3 C(O)O 2 ) under atmospheric conditions:
CH 3 CHO OH → CH 3 CO H 2 O (10)
CH 3 CO O 2 → CH 3 C(O)O 2 (11) The peroxyacetyl radical may react with NO:
CH 3 C(O)O2 NO → CH 3 C(O)O NO 2 (12)
CH 3 C(O)O O 2 → CH 3 O 2 CO 2 (13) oxidizing NO to NO 2 and producing a methylperoxy radi-cal The methylperoxy radical can oxidize another NO to
NO 2 , forming a HO 2 (hydroperoxy) radical and a molecule
of formaldehyde:
CH 3 O 2 NO → CH 3 O NO 2 (14)
CH 3 O O 2 → HCHO HO 2 (15) Alternatively, the peroxyacetyl radical may react with NO 2
to form peroxyacetyl nitrate (CH 3 C(O)O 2 NO 2 , or PAN):
CH 3 C(O)O 2 NO 2 ↔ CH 3 C(O)O 2 NO 2 (16) Which reaction occurs with the peroxyacetyl radical depends
on the relative concentrations of NO and NO 2 present PAN, like ozone, is a member of the class of compounds known as photochemical oxidants PAN is responsible for much of the plant damage associated with photochemical-oxidant problems, and it is an eye irritant More recent mea-surements of PAN throughout the troposphere have shown that PAN is ubiquitous The only significant removal process for PAN in the lower troposphere is, as a result of its ther-mal decomposition, the reverse of reaction (16) This therther-mal decomposition of PAN is both temperature- and pressure-dependent The lifetime for PAN ranges from about 30 min-utes at 298 K to several months under conditions of the upper troposphere (Seinfeld and Pandis, 1998) In the upper tropo-sphere, PAN is relatively stable and acts as an important res-ervoir for NOx Singh et al (1994) have found that PAN is the
single most abundant reactive nitrogen-containing compound
In both cases the unpaired electron is on the end oxygen in
the peroxy group (in parentheses) These peroxy radicals
react like all other alkylperoxy or hydroperoxy radicals
under atmospheric conditions, to oxidize NO to NO 2 :
CH 3 CH(O 2 )CH 2 OH NO →
CH 3 CHOHCH 2 (O 2 ) NO →
CH 3 CHOHCH 2 (O) NO 2 (6b)
The resulting oxy radicals are then expected to dissociate to
CH 3 CH(O)CH 2 OH→ CH 3 CHO CH 2 OH (7a)
CH 3 CHOHCH 2 (O)→ CH 3 CHOH CH 2 O (7b)
Forming CH 3 CHO (acetaldehyde or ethanal) and a new,
one-carbon radical (7a) and HCHO (formaldehyde or methanal)
and a new, two-carbon radical (7b) These new radicals are
expected to react with O 2 to form the appropriate aldehyde
and a hydroperoxy radical, which can oxidize NO to NO 2
CH 2 OH O 2 → HCHO HO 2 (8a)
CH 3 CHOH O 2 → CH 3 CHO HO 2 (8b)
So far in this hydrocarbon oxidation process, two NO molecules
have been oxidized to two NO 2 molecules, a formaldehyde and
Trang 7in the free troposphere Talukdar et al (1995) have found that
photolysis of PAN can compete with thermal decomposition
for the destruction of PAN at altitudes above about 5 km The
reaction of the hydroxyl radical with PAN is less important
than thermal decomposition and photolysis throughout the
troposphere
The oxidation of hydrocarbons does not stop with the
formation of aldehydes or even the formation of CO It can
proceed all the way to CO 2 and H 2 O CO can also react with
hydroxyl radicals to form CO 2 :
H O 2 M → HO 2 M (18)
The chain of reactions can proceed, oxidizing hydrocarbons,
converting NO to NO 2 , and re-forming hydroxyl radicals
until some chain-terminating reaction occurs The following
are the more important chain-terminating reactions:
HO 2 HO 2 → H 2 O 2 O 2 (19)
RO 2 HO 2 → ROOH O 2 (20)
OH NO 2 M → HNO 3 M (21)
These reactions remove the chain-carrying hydroxyl or
peroxy radicals, forming relatively stable products Thus, the
chain oxidation of the hydrocarbons and conversion of NO to
NO 2 are slowed
Radical Sources
This sequence of hydrocarbon oxidation reactions describes
processes that can lead to the rapid conversion of NO to NO 2
The NO 2 thus formed can react by (1) and (2) to form O 3 In
order for these processes to occur, an initial source of hydroxyl
radicals is required An important source of OH in the nonur-ban atmosphere is the photolysis of O 3 to produce an electroni-cally excited oxygen atom (designated as O( 1 D)):
O 3 h ( 320 nm) → O( 1 D) O 2 (22) The excited oxygen atom can either be quenched to form
a ground-state oxygen atom or react with water vapor (or any other hydrogen-containing compound) to form hydroxyl radicals:
O( 1 D) H 2 O→ 2OH (23) Other possible sources of hydroxyl radicals include the pho-tolysis of nitrous acid (HONO), hydrogen peroxide (H 2 O 2 ), and organic peroxides (ROOH):
HONO h ( 390 nm) → OH NO (24)
H 2 O 2 h ( 360 nm) → 2OH (25) The atmospheric concentration of HONO is sufficiently low and photolysis sufficiently fast that HONO photolysis can only act as a radical source, in the very early morning, from HONO that builds up overnight The photolysis of H 2 O 2 and ROOH can be significant contributors to radical production, depend-ing on the quantities of these species present in the atmosphere Another source of radicals that can form OH radicals includes the photolysis of aldehydes, such as formaldehyde (HCHO):
HCOC h ( 340 nm) → H HCO (26) HCO O 2 → HO 2 CO (27) forming HO 2 radicals in (27) and from H atoms by reac-tion (18) These HO 2 radicals can react with NO by reaction (9) to form OH The relative importance of these different
R´
CO
NO
RO
NO
R´CHO +
FIGURE 4 Schematic diagram illustrating the role of the hydroxyl-radical-initiated oxidation of hydrocarbons in the
Trang 8sources 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 Reactivity
Atmospheric 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 NO2
con-centrations are in units of ppm and VOC concon-centrations 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 other
TABLE 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
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 cm3 (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 VOCs
VOC
MIR a
(grams of O 3 formed per gram of VOC added)
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 NOx 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 NOx 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
Trang 9This concept of changing the VOC mixture is the basis
for the use of reformulated or alternative fuels for the
reduc-tion of ozone producreduc-tion Oxygenated fuel components,
such as methanol, ethanol, and methyl t-butyl ether (MTBE),
generally have smaller incremental reactivities than those
of the larger alkanes, such as n-octane, which are more
characteristic of the fuels used in automobiles The use of
these fuels would be expected to reduce the reactivity of the
evaporative fuel losses from the automobiles, but the more
important question is how they will change the reactivity of
the exhaust emissions of VOCs The data that are currently
available suggests that there should also be a reduction in the
reactivity of the exhaust emissions as well
Ozone Isopleths
Ozone production depends on the initial amounts of VOC
and NOx in an air mass Ozone isopleths, such as those
shown in Figure 5, are contour diagrams that provide a
con-venient means of illustrating the way in which the maximum
ozone concentration reached over a fixed irradiation period
depends on the initial concentrations of NOx and the initial
concentration of VOCs The ozone isopleths shown in Figure
5 represent model results for Atlanta, using the Carbon Bond
4 chemical mechanism (Seinfeld, 1995) The point on the
contour plot represents the initial conditions containing
600 ppbC of anthropogenic controllable VOCs, 38 ppbC
of background uncontrollable VOCs, and 100 ppb of NOx These conditions represent morning center-city conditions The calculations are run for a 14-hour period, as chemistry proceeds and the air mass moves to the suburbs, with associ-ated changes in mixing height and dilution The air above the mixing layer is assumed to have 20 ppbC VOC and 40 ppb
of O 3 The peak ozone concentration reached in the calcula-tion is about 145 ppb, as indicated at the point The isopleths arise from systematically repeating these calculations, vary-ing the initial VOC and initial NO x with all other conditions the same
The base case corresponds to the point, and the horizon-tal line represents a constant initial NOx concentration At a fixed initial NOx, as one goes from the point to a lower initial VOC, the maximum O 3 decreases, while increasing the initial VOC leads to an increase in the maximum O 3 concentration until the ridge line is reached The ridge line represents the VOC-to-NOx ratio that leads to the maximum ozone produc-tion at the lowest concentraproduc-tions of both VOC and NOx The region of the isopleth diagram below the ridge line is referred
to as the NOx-limited region; it has a higher VOC:NO x ratio The region of the diagram above the ridge line is referred to
as the VOC-limited region; it has a lower VOC:NOx ratio In
200
160
120
80
40
Initial VOC, ppbC
180
140
(1995) With permission.
Trang 10the NOx-limited region, there is inadequate NOx present to be
oxidized by all of the peroxy radicals that are being produced
in the oxidation of the VOCs Adding more NOx in this region
increases ozone production The base-case point in Figure 5 is
located in the VOC-limited region of the diagram Increasing
NOx from the base-case point actually leads to a decrease in
the maximum ozone that can be produced
Nighttime Chemistry
At night, the urban atmospheric chemistry is quite different
than during the day The ozone present at night may react
with organics, but no new ozone is formed These ozone
reac-tions with organics are generally slow Ozone can react with
alkanes, producing hydroxyl radicals This hydroxyl-radical
production is more important for somewhat larger alkenes
The significance of this hydroxyl-radical production is limited
by the available ozone Besides reacting with organics, ozone
can react with NO 2 :
O 3 NO 2 → O 2 NO 3 (28)
forming the nitrate radical (NO 3 ) NO 3 radicals can further
react with NO 2 to form dinitrogen pentoxide (N 2 O 5 ), which
can dissociate to reform NO 3 and NO 2 :
NO 3 NO 2 M → N 2 O 5 M (29)
N 2 O 5 → NO 3 NO 2 (30) establishing an equilibrium between NO 3 and N 2 O 5 Under
typical urban conditions, the nighttime N 2 O 5 will be 1 to
100 times the NO 3 concentration These reactions are only
of importance at night, since NO 3 can be photolyzed quite
efficiently during the day
NO 3 can also react quickly with some organics A generic
reaction, which represents reactions with alkanes and
alde-hydes, would be
The reactions of NO 3 with alkenes and aromatics proceed by a
different route, such as adding to the double bond NO 3 reacts
quite rapidly with natural hydrocarbons, such as isoprene and
α -pinene (Table 4), and cresols (Finlayson-Pitts and Pitts,
2000) Not much is known about the chemistry of N 2 O 5 , other
than it is likely to hydrolyze, forming nitric acid:
N 2 O 5 H 2 O→ 2HNO 3 (32)
Summary
The discussion of urban atmospheric chemistry presented
above is greatly simplified Many more hydrocarbon types
are present in the urban atmosphere, but the examples
pre-sented should provide an idea of the types of reactions that
may be of importance In summary, urban atmospheric
ozone is formed as a result of the photolysis of NO 2 NO 2 is
formed by the oxidation of the primary pollutant NO, which accompanies the hydroxyl-radical-initiated chain oxidation
of organics Hydroxyl radicals can be produced by the pho-tolysis of various compounds Ozone formation is clearly a daytime phenomenon, as is the hydroxyl-radical attack of organics
SECONDARY ORGANIC AEROSOLS With the implementation of air-quality standards for fine (or respirable) particulate matter in the atmosphere, there has been increasing interest in the composition and sources of this fine particulate matter It has long been recognized that particles in the atmosphere have both primary (direct emis-sion) and secondary (formed in the atmosphere) sources Among the secondary particulate matter in the atmosphere are salts of the inorganic acids (mostly nitric and sulfuric acids) formed in the atmosphere It has been found that there is a significant contribution of carbonaceous particu-late matter, consisting of elemental and organic carbon Elemental carbon (EC), also known as black carbon or gra-phitic carbon, is emitted directly into the atmosphere during combustion processes Organic carbon (OC) is both emitted directly to the atmosphere (primary OC), or formed in the atmosphere by the condensation of low-volatility products
of the photooxidation of hydrocarbons (secondary OC) The organic component of ambient particles is a complex mixture of hundreds of organic compounds, including: n-alkanes, n-alkanoic acids, n-alkanals, aliphatic dicarbox-ylic acids, diterpenoid acids and retene, aromatic polycar-boxylic acids, polycyclic aromatic hydrocarbons, polycyclic aromatic ketones and quinines, steroids, N-containing com-pounds, regular steranes, pentacyclic triterpanes, and iso- and anteiso-alkanes (Seinfeld and Pandis, 1998)
Secondary organic aerosols (SOAs) are formed by the condensation of low-vapor-pressure oxidation products of organic gases The first step in organic-aerosol production
is the formation of the low-vapor-pressure compound in the gas phase as a result of atmospheric oxidation The second step involves the organic compound partitioning between the gas and particulate phases The first step is controlled by the gas-phase chemical kinetics for the oxi-dation of the original organic compound The partitioning
is a physicochemical process that may involve interactions among the various compounds present in both phases This partitioning process is discussed extensively by Seinfeld and Pandis (1998)
Figure 6 (Seinfeld, 2002) illustrates a generalized mecha-nism for the photooxidation of an n-alkane The compounds shown in boxes are relatively stable oxidation products that might have the potential to partition into the particulate phase Previous studies of SOA formation have found that the aerosol products are often di- or poly-functionally substituted products, including carbonyl groups, carboxylic acid groups, hydroxyl groups, and nitrate groups
A large number of laboratory studies have been done
investigating the formation of SOAs Kleindienst et al (2002)