-------
1-35
In summary, our status of knowledge of the formation of
H2S04 by dissolved oxidants is:
1. The oxidation rates are known for NO and N03~ and
are too low to be important.
2. The oxidation rate is known for N02~, but the
tropospheric concentration of HN02 is probably too low for
this reaction to be important.
3. The oxidation rate is known for 03, but it is usually
expected to be unimportant.
4. The oxidation rate for ^2Q2 *s known and appears to
be potentially a highly effective reaction for formation of
H2S04 in the troposphere. This rate could possibly be enhanced
by metal ions, but no studies have been reported.
5. Organic oxidizers may be important, but no studies
have been reported.
2.3.4.5 The Influence of NHj— It has been recognized for
several decades that NH3 may influence the formation rate
of H2S04 in aqueous particles, mist, fog, and rain. Hegg
and Hobbs (1978) have reviewed the studies of the NH3 influence.
They have called attention to a misunderstanding in the
literature. NH3 is commonly reported incorrectly to be a "positive
catalyst" for the oxidation of dissolved 803. In the strict
sense of the definition of "catalyst," the term cannot be
applied to the role of NH3. The observed enhancement by NH3
of the oxidation rates of the auto-oxidation, metal -ion
oxidation, and the 03 oxidation is due to its action to raise
and maintain a high pH. The following process occurs to
-------
1-36
raise and maintain a higher pH through the conversion of HN3
to NH4+:
1. Ambient gaseous NH3(g) dissolves in the water,
NH3(g) -» HN3(aq)
2. The dissolved NH3^a_j reacts with H+, which raises
the pH NH3(aq) + H+ -» NH4+.
4—
Therefore, the ambient pathways of auto-oxidation, Mn(II)-
and Fe(III)- catalyzed oxidation, and 03 oxidation would have
their rates enhanced by absorption of NH3« However, the
ambient pathways of H202 and HNO2 would have their rates
retarded by NH3 absorption. The rate for soot would not be
influenced.
NH3 can play other important roles. Reinders and Vies
(1925) observed qualitatively that Cu(II) was complexed by
NH3 and rendered non-catalytic. At high pH's (>9) such that
NH3(aq) is the dominant form, NH3 is explained in terms of
its influence on the pH of the water system; NH3 is not a catalyst,
2.3.5 Surface Chemical Reactions
Industrial emissions of solid particles (e.g., fly ash)
and fugitive dust (e.g., windblown soil and minerals) provide
a solid-surface that may chemisorb S02 and yield sulfate ions.
The work reviewed in this section will include investigations
of the S02 oxidation on the surfaces of: metal oxides, fly ash,
charcoal, and soot. Although reaction kinetics have not been
identified, two general types of processes have been: a
capacity-limited reaction for SC>2 removal and a catalytic SC>2
-------
1-37
oxidation process. The inital contact of S02 with the solid
produces a rapid loss of S02 from the gas phase; the reaction
rate decreases with time, for the capacity-limited reaction,
the rate approaches zero; for the catalytic process, the rate
levels off for a time and them approaches zero. The latter
phenomenon is attributed to a pH decrease caused by sulfuric
acid formation.
Urone et al. (1968) and Smith et al. (1969) found a
number of solids to be effective in removing S02« In Urone's
studies, SC>2 was admitted to a flask containing a powder that
was allowed to react with no mixing, and the product and
remaining SC>2 were determined. Only the average reaction
rates can be calculated from these experiments; more importantly,
with this experimental procedure the rates may be diffusion-
limited. The highest rate determined was for SC>2 with ferric
oxide; the value was >75 percent per minute. Other materials
found to be slightly less reactive than ferric oxide were
magnetite, lead oxide, lead dioxide, calcium oxide, and
aluminum oxide. The rate for the ferric oxide experiment was
for 20 mg of ferric oxide in a 2-liter flask, the ferric
oxide concentration would thus be 10? ug/m^. Assuming a direct
proportionality between rate and particle concentration, the
S02 removal rate in the atmosphere would be calculated to be
0.04 percent per hour for 100 ug/m^ of particles with the same
reactivity as ferric oxide. However, since the mass transfer
characteristics of the reactor were not reported, these
results cannot be considered reliable for estimating rates.
-------
1-38
Smith et al. (1969) did not focus on sulfate formation
kinetics; instead, they illustrated through a novel experiment
the ability of solid particles to adsorb SC>2 and to release SC>2
during passage through a tube with a wall that adsorbes SC>2.
They measured the number of S02 monolayers absorbed on suspended
Fe304 as function of S02 partial pressure. (The monolayer
coverage data reported in their Table I are in error by a
factor of 100 too large; e.g., the number of monolayers at
1.13 ppm should be 0.38 x 10"2.)
Chun and Quon (1973) measured the reactivity of ferric
oxide to SC>2, using a flow system involving a filter containing
suspended particles. They determined a removal rate constant
of 9.4 x 10~3 ppm""1 min"1 [-d(lnp )dt], where p is the fraction
of surface sites available for reaction. Extrapolating this
to an atmospheric particle concentration of 100 ug/M3 with an
equivalent reactivity and an SC>2 concentration of 0.1 ppm,
the data project an atmospheric removal rate of 0.1 percent
per hour.
Stevens et. al. (1978) report total iron concentrations
in six U. S. cities ranging between 0.5 and 1.3 ug/m3. Other
species such as manganese, copper, or vanadium had total con-
centrations usually below 0.1 ug/m3. Thus actual ambient air
concentrations are a factor of approximately 50 times less
than that assumed by the authors in the above papers. A
reactive particle concentration of 2 ug/m3 would yield a
predicted SC>2 removal rate of no more than 0.002 percent
-------
1-39
per hour. Therefore, surface reactions are probably not
not important except in sources prior to or immediately after
emission.
The most comprehensive study to date on SC>2 removal by
pure solids was made by Judeikis (1974) and Judeikis et al.
(1978), who used a tubular flow reactor in which solids were
supported on an axial cylinder to measure reactivities of MgO,
Fe2°3' Al2°3' MnO2, PbO, NaCI, charcoal, and fly ash. They
found that the rates of S02 removal diminished with exposure
until the solids completely lost ability to react with SO2»
The relative humidity was important in determining the total
capacity for S02 removal, but not the initial rate of uptake;
total capacity increased as relative humidity increased. The
capacity for SC>2 could be extended by exposure to NH3« This
type of behavior is consistent with the formation of H2S04
on the surfaces.
Because of the ubiquitous nature of carbonaceous matter
in ambient air particulate samples, various workers have studied
the SC>2 removal rate by carbon. A comparison of the results
is rather difficult because of the varieties of carbon available
for study, such as activated charcoal, graphite, acetylene flame
products, and combustion products of diesel oil and heating oil.
We cite here a few investigations that deal with the gas-solid
reaction of S02 with carbon.
Novakov et al. (1974) performed laboratory experiments
that showed that graphite and soot particles oxidize SC>2 in air.
-------
1-40
_
The. soot exposed to humidified air produced more SO^ than
that exposed only to dry air. They also observed for downtown
Los Angeles a strong correlation between the concentration of
o
ambient carbon and 804 formation. (See discussion in
Section 2.3.4.3.)
Tartarelli et al. (1978) studied the interaction of S02
with carbonaceous particles collected from the flue ducts of
oil burning power stations. They concluded that the amount of
adsorption is increased by the presence of oxygen and water in
the gas stream. Reaction rates were not determined in this
study.
Liberti et al. (1978) studied the adsorption and oxida-
tion of S02 on various particles, including soot from an oil
furnace and various atmospheric particulate samples. They
concluded that the main interaction between the S02 and parti-
culate matter is adsorption/ with most catalytic reactions
occurring at high temperatures/ near the combustion source.
Their experiments with atmospheric particulate samples lead
them to the conclusion that any heterogeneous nonphotochemical
sulfate formation is strongly dependent on the reactivity of
the particle surface/ and hence the history (aged/ freshly
emitted)/ of the aerosol.
In summary/ the status of our knowledge of surface
reactions is:
1. The reactions are capacity-limited. Those that
involve catalysis in liquid films can be extended by the
absorption of
-------
1-41
/
2. The intial rates may be large/ but quickly approach
zero.
3. Except for the carbon (soot) reaction, solid surface
reactions do not appear to be effective pathways for H2S04
formation in the troposphere.
2.3.6 Estimates of 862 Oxidation
It is of interest at this point to compare the rates of
SC>2 oxidation by the more important reactions identified in
the previous sections of Chapter 2. The important reactions
for gas-phase and aqueous-phase oxidation are listed in Table
2-11, and rates of SO2 oxidation for an assumed set of
conditions are present. These calculations ignore the non-
homogeneous nature of the troposphere and assume that all of
the reactants are well-mixed. (The more general case is
treated in Section 2.3.7).
-------
1-42
Table 2-11. Estimates of S02 Oxidation Rates in a Well-Mixed
Troposphere
Discussion
Reaction Rate, % h-1 Section Comments
I. Gas Phase
HO radical
H02 radical
CH302 radical
II. Aqueous Phase pH=
Mn(II) catalysis
Fe(III) catalysis
C (soot) catalysis
03 (40 ppb)
03 (120 ppb)
H202 (1 ppb)
H202 (10 ppb)
0.3
0.4
0.3
1
1E-1
5E-5
3E+1
2E-8
6E-8
2E-2
2E-1
- 1.3
- 2.0
- 1.5
2
1E+1
5E-1
3E+1
2E-6
6E-6
3E-2
2E-1
3
1E+3
5E+3
3E+1
2E-4
6E-4
3E-2
3E-1
2.
2.
2.
2.
2.
2.
2.
2.
2.
2.
3
3
3
3
3
3
3
3
3
3
.3
.3
.3
.4
.4
.4
.4
.4
.4
.4
.2
.2
.2
.2
.2
.3
.4
.4
.4
.4
1
1,
1,
2,
3,
6
3,
3,
3,
3,
,2
,2
,3,4
,5
,7
,7
,8
,8
NOTE: "E" denotes "exponential to 10th power;" e.g., 3E-1 =
3 x 10'1
1. Typical range for daytime at northern midlatitudes
during the summer.
2. This reaction rate is not well established; see
discussion section.
3. Assumed that liquid water volume of aerosol
= 50 x 10~12m3/m3, [S02]g = 10 ppb (or 27 ug/m3).
4. Assumed that Mn(II) mass concentration = 20 ng/m3;
also , the Mn(II) is assumed to be uniformly
dissolved in the liquid water of the aerosol
[Mn(II)] = 8.9 x 103M). Rate calculation used the
expression of Neytzell-de Wilde and Taverner (1958);
see Table 2-7.
-------
1-43
5. Assumed that Fe(III) mass concentration = 2
also, the Fe(III) is assumed to be uniformly dissolved
in the liquid water of the aerosol ([Fe(III]) = 0.9 M).
Rate calculation used the expression of Neytzell-de
Wilde and Taverner (1958); see Table 2-9.
6. Assumed that C mass concentration = 10 ug/m3 and behaves
as the soots studied by Chang et al. (1979), whose
expression was used for this calculation (Equation
2-32).
7. Rate calculation was based on Equation 2-35.
8. Rate calculation was based on Equation 2-39.
For this comparison, it has been assumed that the SC>2
concentration is 10 ppb for all of the reactions, and that
the liquid water content of the aerosol is 50 x 10"12 m3/m3.
The gas-phase rates have been calculated based on the
discussion material presented in Section 2.3.3.2. The aqueous-
phase rates have been calculated based on the discussion
material presented in Sections 2.3.4.2-4. Several of the
assumptions made do not have any basis, namely:
1. The ambient mass concentration of 20 ng/m3 for Mn is
reasonable, but: (a) it is not known if the predominant form
is Mn(II), and (b) it is unlikely that Mn is uniformly
distributed and dissolved.
2. Likewise, the ambient concentration of 2 ug/m3 for
Fe is reasonable, but: (a) it is not known if Fe(III) is the
predominant form, and (b) it is unlikely that Fe is uniformly
distributed and dissolved.
-------
1-44
3. There is no basis to assume that the rate equation
observed for laboratory-generated carbon (soot) applies to
atmospheric carbon.
4. The rates for the HC>2 and CH3C>2 reactions recommended
by Calvert et al. (1978) are not well established.
It is very likely that the rates estimated for Mn(II)
catalysis, Fe(III) catalysis, and C (soot) catalysis are
gross over-estimates. Also, the H02 and CH302 rates may be
too high.
Uncritical acceptance of all of the rates, at a pH = 3,
and [H202] = 10 ppb, would lead to the SO2 conversion rate
exceeding 40% h~l. However, if only the well-established rates
are considered, the S02 conversion rate becomes -^-1.1% h"1.
2.3.7 Field Measurements on the Rate of 502 Oxidation
The majority of SC>2 oxidation studies in the atmosphere
have been only carried out in recent years and of those, most
have involved power plant plumes. One reason for the late
start in this research area has been the lack of adequate
measurement technology for particulate sulfur, but recent
developments Huntzicker et al. (1978), Cobourn et al. (1978)
seem to have alleviated this problem. A summary of S02
oxidation rates based on field measurements in power plant,
smelter and urban plume studies carried out from 1975 to the
present is given in Table 16-1. The rates of S02 oxidation
in industrial plant plumes consistently range from 0 to 10%/h,
with urban plumes showing only a slightly greater maximum
rate of 13%/h. The pre-1975 studies, Gartrell et al. (1963),
-------
1-45
Dennis et al. (1969), Weber (1970) and Stephens and McCalden
(1971), which observed conversion rates an order of magnitude
larger than more recent observations, must be considered
suspect due to possible artifact formations in the sulfate
analysis technique and limitations in the analytical methods
in general.
Newman (1980) recently reviewed the majority of the
power plant and smelter plume studies presented in Table 2-12
and arrived at the following conclusions.
1) The diurnal average oxidation rate of sulfur dioxide
to sulfate is probably less than 1% per hour.
2) Little or no oxidation of sulfur dioxide occurs from
early evening through to early morning.
3) Maximum oxidation rates of sulfur dioxide to sulfate
of 3% per hour can occur under midday conditions.
4) The contribution of homogeneous and heteorogeneous
mechanisms to sulfur dioxide oxidation in plumes can-
not be elucidated from the present studies.
It should be noted that the reported S02 oxidation rates
are estimates based on analyses of measured physical and
chemical parameters and in many instances have incorporated
within them certain simplifying assumptions which are not
totally substantiated. Typically uncertainties in reported
values are 50%, but may be greater if inappropriate assumptions
have been used. Even with these uncertainties in mind, the
overall consistency in the observed range of S02 oxidation
rates is gratifying.
-------
1-46
TABLE 2-12. Field Measurements on the Rates of SC>2 Oxidation
in Plumes*
Plume Type
Location
S02 Oxidation
Rate (%h~1
Method
Reference
Power Plant
Keystone 0-10
(Pennyslvania)
Labadie 0.41-4.9
(Missouri)
Four Corners 0.27-0.84
(New Mexico)
Labadie and
Portage des
Sioux (Missouri )
Muscle Shoals 0-5
(Alabama)
Kyger Creek
(Ohio)
Labadie 0-3
(Missouri)
Four Corners 2-8
(New Mexico)
Labadie 0-4
(Missouri)
Cumberland 0-7
(Tennessee)
Great Canadian 0-3
Oil Sands
(Alberta/ Canada)
Keystone 0-5
(Pennyslvania)
32S/34S ratio,
change with
oxidation
Total change in
particle volume
Sub-micron
suflate and S02 •
change of ration
with time
Newman et al. (1975)
Cantrell and Whitby
(1978)
Ursenbach et at.
(1977)
Particulate sulfur Forrest and Newman
to total sulfur (1977a)
ratio
Particulate sulfur Gillani et al. (1978)
to total sulfur
ratio
CCN production (CCN Pueschel and Van Valin
to S02 ratios) (1978)
Particulate sulfur Husar et al. (1978)
to total sulfur '
ratio
Particulate sulfur Meagher et al. (1978)
to total sulfur
ratio
Particulate sulfur Lusis et al. (1978)
to total sulfur
ratio
Particulate sulfur Dittenhoefer and de-
to total sulfur Pena (1978)
ratio
-------
1-47
Table 2-12 (Continued)
Plume Type
Location
S02 Oxidation
Rate (%h~1
Method
Reference
Central 0-6
(Washington)
Four Corners
(New Mexico)
Four Corners
(New Mexico)
0.15-0.5
Leland-Olds
(North Dakota)
Sherburne County 0-5.7
Minnesota
Big Brown
(Texas)
Smelter
INCO Nickel 0-7
(Copper Cliff,
Canada)
INCO Nickel 1.2-5.2
(Copper Cliff,
Canada)
Mt Isa Mines 0.25t
(MT. ISA,
Australia)
Urban
Los Angeles 1.2-13
(California)
St. Louis 7-12.5
(Missouri)
St. Louis 3.6-4.2
(Missouri)
Total change in
particle volume
CCN production (CCN
to S02 ratios)
Total change in
particle volume
Hobbs et al. (1979)
Particulate sulfur
to total sulfur
ratio
Particulate sulfur
to total sulfur
rates
Particulate sulfur
to total sulfur
ratio
Particulate sulfur
to total sulfur
ratio
Particulate sulfur
to total sulfur
ratio
Particulate sulfur
to total sulfur
ratio
Mamane and Pueschel
(1980)
Hebb and Hobbs (1980)
Lusis and Wiebe (1976)
Forrest and Newman
(1977b)
Roberts and Williams
(1979)
Roberts and Friedlander
(1975)
Alkezweeny and Powell
(1977)
Chang (1979)
,*Adapted in part from Hegg and Hobbs (1980)
tdiurnal average rate
-------
1-48
2.4 Summary and Conclusions
Present understanding of the homogeneous gas phase
reactions of SC>2 indicates that the rate of SC>2 oxidation
in the atmosphere is dominated by free radical reaction
processes. The free radical species identified as important
contributors to the SC>2 oxidation process are hydroxyl (HO),
methylperoxyl (CH302) and other organic peroxyl species (R02/
R'C>2/ etc.). The concentration of these radicals in the
atmosphere are dependent on many factors, the more important
of which are the concentration of volatile organic compounds
and nitrogen oxides (NO and N02) in the atmosphere, tempera-
ture and solar intensity. Theoretical estimates have shown
that maximum SO2 oxidation rates of 4.0% h are possible
in polluted atmospheres. However, recent experimental rate
constant determinations for the H02 and CH3O2 reactions
with S02 indicate that these processes may not be as important
as previously thought and that the maximum possible homo-
geneous S02 oxidation rate under optimum atmospheric conditions
may only be of the order of 1.5% h"~l. This rate is result
of SO2 reaction with hydroxyl radical only.
Present knowledge of heterogenous pathways to S02
oxidation in the atmosphere indicates that the liquid phase
, o
catalyzed oxidation of S02 by Mn * ion and carbon are potentially
important processes, as is oxidation by hydrogen peroxide. Theo-
retical estimates of atmospheric S02 oxidation rates via these
-------
1-49
processes are of the order of 10% h"1. Unfortunately/ a great
deal of uncertainty surrounds the actual availability of
these catalyzing substances in ambient fine particulate
matter. The quantitative determination of rates of SC>2
oxidation via these processes has never been demonstrated
under actual atmospheric conditions.
Organic and nitrate particulate matter forming processes
are presently thought to be dominated by homogeneous gas
phase reactions. In the case of atmospheric nitrates/ a
particularly significant production pathway is through reac-
tion between hydroxyl free radical and nitrogen dioxide
resulting in nitric acid (HON02) formation. The fate of
nitric acid in the atmosphere is not well understood/ though
a portion of gaseous nitric acid is known to enter into an
equilibrium with ammonia (NH3) to form particulate ammonium
nitrate (NH4NC>3). Present knowledge provides little support
for liquid phase oxidation as an important pathway to NOX
transformation.
-------
I-R1
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6.1 CHEMISTRY OF THE OXIDES OF NITROGEN IN THE LOWER ATMOSPHERE
Solar radiation triggers a series of reactions in the
atmosphere between gaseous organic molecules and nitrogen
oxides, producing a wide variety of secondary pollutants.
The totality of primary and secondary pollutants involved in
these photochemical reactions is known as photochemical smog.
To understand the chemistry of the oxides of nitrogen in the
lower atmosphere, it is necessary to consider the interactions
that take place between the oxides of nitrogen and organic
constituents. Several reviews of atmospheric chemistry are
available, 1~3 as are detailed discussions of reaction
mechanisms ^-8 and rate constants.^ In this section the
chemistry of the oxides of nitrogen in the lower atmosphere
is briefly reviewed. The above-cited references should be
consulted for more detail.
Most of the chemistry that occurs in a sunlight-irradiated
urban atmosphere involves the interaction of a variety of
unstable, excited molecules and molecular fragments that have
only a transitory existence. These species include: the
unexcited and first excited electronic states of the oxygen
atom, triplet-P oxygen atoms [O(-^P)], and singlet-D oxygen
atoms [O(-*-D)]; ozone (0^); symmetrical nitrogen trioxide
(N03); dinitrogen pentoxide (^05); hydroxyl radicals (HO);
alkylperoxyl radicals (R02); acylperoxyl radicals RC(0)02;
and less important species. In the formulas, R represents a
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1-51
methyl (CH3), ethyl (C2H5), or another, more complex hydrocarbon
radical. The paths by which these intermediates are formed
and destroyed are important keys in explaining the chemical
changes that occur in the polluted atmosphere.
6.1.1. fiejac.tAffl^lJJJWPX^
The major portion of the total oxides of nitrogen emitted
by combustion sources is nitric oxide (NO). The rate at
which NO is converted to nitrogen dioxide (N02) through
oxidation by molecular oxygen in air:
2NO + 02 -»2NO2 (6-1)
is proportional to the square of the nitric oxide concentration
since two molecules of NO are required for the oxidation; it
is, therefore/ very sensitive to changes in nitric oxide con-
centration. Reaction 6-1 can be important in the vicinity of
sources in converting up to 25 percent of the total NOX to NO2
during the initial stages of dilution with air when the
concentration of NO is still quite high. Reaction 6-1 is
much too slow, however, to account for the high conversion
rates of nitric oxide to nitrogen dioxide observed in the
atmosphere under typical ambient concentrations.
Since sunlight triggers the phenomenon of photochemical
smog formulation, it is important to recognize those constituents
that will absorb light energy. In some cases, these constituents
decompose or become activated for reaction. Nitrogen dioxide,
a dominant sunlight absorber in the urban atmosphere,
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1-52
photodissociates upon absorbing wavelengths of light <430 nm.
This photolytic reaction results in the formation of the
ground state/ triplet-P oxygen atom, and a nitric oxide
molecule. The efficiency of this process is wavelength-
dependent:
N02 + sunlight (290-430 nm) -»0(3P) + NO (6-2)
The highly reactive triplet-P oxygen atom predominantly
reacts with oxygen molecules in the air/ resulting in the
formation of ozone.
0(3P) + 02 + M —>03 + M (6-3)
M in this equation represents a nitrogen/ oxygen/ or other
third molecule that absorbs the excess vibrational energy
released/ thereby stabilizing the ozone produced. For most
concentration conditions common in polluted atmospheres/
ozone molecules regenerate nitrogen dioxide by reaction with
nitric oxide:
03 + NO -»N02 + 62 (6-4)
To a much lesser extent/ ozone can react with nitrogen dioxide
to form the transient species, nitrogen trioxide:
03 + N02 -»N03 + 02 (6-5)
The nitrogen trioxide can further react with nitrogen dioxide
to form dinitrogen pentoxide, the reactive anhydride of
nitric acid.
NO3 + N02 -»N205 (6-6)
Dinitrogen pentoxide which is in equilibrium with
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1-53
nitrogen trioxide and nitrogen dioxide can dissociate or
react with water to form nitric acid (HONC>2):
N2°5 -*N03 + N02 (6-7)
N2°5 + H20 -»2HON02 (6-8)
Additional reactive pathways which can take place between
oxygen atoms and N02 and NO include:
NO2 + 0(3P) -»NO -i- 02 (6-9)
N02 + 0(3P) + M -» N03 + M (6-10)
NO + 0(3P) + M -> N02 + M (6-11)
Also, NO and N©3 can react to regenerate N02:
N03 + NO -» 2N02 (6-12)
Nitrous acid is produced by:
NO + N02 + H20 -» 2HONO (6-13)
and may react bimolecularly to regenerate the original reactants:
HONO + HONO — > NO + N02 + H20 (6-14)
The unexcited and first excited electronic state of the oxygen
atom are produced by ozone photolysis in sunlight:
(290-350 nm)-»02 -t-0(1D) or 0(3P) (6-15a)
03 + sunlight
(450-700 nm)-»02 + 0(3P) (6-15b)
The singlet-D oxygen [0(^0)] atom is much more reactive
than the ground state triplet-P oxygen [0(3P)] atom. For
example, it reacts efficiently during collision with a water
molecule to form an important transient species in the
atmosphere, the hydroxyl radical:
O^D) + H2O -» 2HO (6-16)
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1-54
This radical is also formed through the photodecomposition of
nitrous acid (HONO):
HONO + sunlight (290-400 nm) ->HO + NO (6-17)
The hydroxyl radical can react with nitric oxide to give
back nitrous acid:
HO -I- NO + M -»HONO + M (6-18)
or form nitric acid by reacting with nitrogen dioxide:
HO + N02 + M -HJON02 + M (6-19)
Demerjian et al.106 have shown through computer simulations
of the reaction sequence 6-1 through 6-19 that these reactions
cannot explain the rapid conversion of NO to N02 observed in
the ambient atmosphere. In fact, if these reactions alone
occurred, the original supply of nitrogen dioxide in our atmo-
sphere would be slightly depleted under irradiation with
sunlight, and a small and near constant level of ozone would
be created in a few minutes. The key to the observed nitric
oxide to nitrogen dioxide conversion lies in a sequence of
reactions between the transient species present and other
reactive molecules such as the hydrocarbons and aldehydes
present in the polluted atmosphere.
In the presence of hydrocarbons the number of reactions
greatly increases. Thus, the hydroxyl radicals produced by
reactions 6-16 and 6-17 can react with a hydrocarbon (paraffin,
olefin, aromatic, or any compound having C-H bonds):
OH + Hydrocarbon -» R + H20 (6-20)
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1-55
Reaction 6-20 produces an alkyl radical (R ) which
contains a free electron. This radical quickly picks up an
oxygen molecule from the air to form a peroxyl radical R02 :
M
R + 02 -» R02 (6-21)
Typically, the next reaction in the series converts NO to N02
and produces an oxyl radical, RO :
R02 + NO -»RO + N02 (6-22)
A hydrogen abstraction by molecular oxygen may then produce a
hydroperoxyl radical, H02 • The rest of such an RO radical
typical forms a carbonyl compound, OHC:
RO + 02 -»OHC + H02 (6-23)
Finally, the hydroperoxyl radical (H02) can react with a second
NO to form N02 to complete the cycle:
H02 + NO -»OH + N02 „ , (6-24)
Although this description is very simplified, these series of
reactions contain the essential features of NO to N02 oxidation
and subsequent ozone formation.
The initial source of radicals is very important; although
the rate and yield of oxidant formation depend on many other
factors, the length of the induction period before accumulation
of oxidant depends strongly on the initial concentration of
radicals. (The length of the induction period is important
primarily in constant light intensity smog chambers. Diurnally
varying radiation tends to lessen the importance considerably.)
In smog chambers and possibly in the ambient atmosphere, the
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1-56
photolysis of nitrous acid, reaction 6-17, may be the most
important initial source of radicals. Nitrous acid has been
detected in smog chambers in concentrations sufficient to explain
the observed induction time for smog chemistry, but the concen-
trations necessary to initiate smog chemistry in the atmosphere
are below the limits measured by most modern instruments.
Another possible source of radicals in the atmosphere is
the photolysis of aldehydes:
RCHO + hv-*HCO + R (6-25)
Aldehydes are emitted from many sources, including automobiles.
They are also formed in smog.
During the course of the overall smog formation process,
the free radical pool is maintained by several sources, but
the dominant one appears to be photolysis of the aldehydes
formed from the initial hydrocarbons. Since the reactions of
free radicals with NO form a cyclic process, any additional
source of radicals will add to the pool and increase the
cycle rate. Conversely, any reaction that removes free
radicals will slow the cycle rate. For example, a primary
radical sink and a. primary sink for oxides of nitrogen is
reaction 6-18 to form nitric acid.
The hydrocarbon classes important in the chemistry of
the polluted troposphere are alkanes, olefins, and aromatics.
In addition, the oxygenated hydrocarbons, such as aldehydes,
ketones, esters, ethers, and alcohol are also important. A
great variety of chemical reactions take place among these
-------
1-57
organic species and the free radicals cited above. The
reactions of typical hydrocarbon species are now discussed
briefly. Throughout the discussion references to more
extensive coverages are given.
The most important atmospheric reaction involving alkanes
is with the HO radical. For n-butane, for example, the
reaction is
CH3CH2CH2CH3 + HO -»CH3CH2CH2CH2 + H20 (6-26a)
and
-»CH3CH2CHCH3 + H2O (6-26b)
The alkyl radicals will rapidly add 02 to form the corresponding
peroxyalkyl radicals, e.g.
CH3CH2CH2CH2 + 02 + M -»CH3CH2CH2CH202 + M (6-27)
(subsequently the third body M will not be indicated). A
reaction of substantially lesser importance is with oxygen atoms,
3 °2
CH3CH2CH2CH3 + 0(JP) -*HO + CH3CH2CH2CH202 (6-28a)
and
02 00
-»HO + CH3CH2CHCH3 (6-28b)
The importance of both the HO and 0(3p) reactions with alkanes
is the generation of the peroxyalkyl radical R02 , which
plays a substantial role in the conversion of NO to N02.
Rate constants for alkane reations are summarized by Baulch
et al.9
The atmospheric chemical reactions involving olefins
have been widely studied.4'5,10 Tne most important reactions
-------
1-58
in which olefins participate are with HO radicals, ozone, and
atomic oxygen, in that order. The reaction of HO with an
olefin, such as propylene, may proceed by addition of OH to
the double bond or by abstraction of a H-atom from the
olefin. For propylene, for example, the reaction paths with
HO are:
CH3CH = CH2 + HO -»CH3CHCH2OH (6-29a)
qp
-*CH3CHCH2 (6-29b)
-»CH2CH = CH2 + H20 (6-29c)
In each case the free radical product will quickly react with
02 to produce a peroxyalkyl radical that is capable of
converting NO to N02«
Ozone-olefin reactions are a source of free radicals and
stable products in air pollution chemistry. The initial
attack of 03 on an olefin produces an unstable intermediate,
which may decompose by several pathways. 1°'H
For propylene, for example, the initial step in the reaction
with 03 is believed to be:
0 •
/\
0 0.
0 ' '
CH3CH-CH2
0 0
i i
CH3CH = CH2 + 03 ->CH3CH-CH2\ *0 (6-30)
0 0
i i
'CH3CH-CH2
Subsequent de-composition of the products leads to a variety of
free radicals and stable products. 12,13 i>ne mechanisms of
ozone-olefin reactions are still under considerable study,
-------
1-59
although most of the potential paths have been delineated.
The reaction of olefins with atomic oxygen plays a minor
role in olefin consumption and radical and product formation.
Again/ for propylene the reaction is:
CH3CH = CH2 + 0(3P)-»CH3CH2 + HCO (6-31a)
or
->CH3CO + CHa (6-31b)
or
-»CH3CH2CHO (6-31c)
The mechanism of photooxidation of aromatic species in
the atmosphere is perhaps the area of greatest uncertainty in
atmospheric hydrocarbon chemistry. The principal reaction of
aromatics is with the hydroxyl radical.14,15 por aromatic-OH
reactions, the initial step can be either addition to or
abstraction from the aromatic ring.103 The free radical
addition products may then react, most likely with either 02
or N02/ leading to the cresols or nitrotoluences, respectively.
The abstraction route probably leads to benzaldehyde. The
mechanism of aromatic-hydroxyl radical reactions is yet to
be clarified.
Aldehydes, both aliphatic and aromatic, occur as primary
and secondary pollutants and are direct precursors of free
radicals in the atmosphere.16 Consequently, aldehyde chemistry
represents an important subject area in atmospheric chemistry.
Although aldehydes are the main oxygenated hydrocarbons
generally considered with respect to their role in atmospheric
chemistry, other classes of oxygenated hydrocarbons, such as
ketones, esters, ethers and alcohols, are present and participate
to a somewhat lesser extent. Major secondary sources of
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1-60
aldehydes include the reactions of ozone and OH radicals with
hydrocarbons, and radical decomposition products. In addition,
aromatic aldehydes can be formed as an ultimate consequence
of the reaction of OH with aromatics/ e.g. benzaldehyde. The
aldehydic hydrogen-carbon bond in aldehydes is relatively
weak (CH bond strength is 86 kcal/mol"^). Consequently, this
hydrogen atom will be susceptible under atmospheric conditions
to attack by radical species, such as 0(3P), 0(^0), HO, and
H02- Of these HO is by far the most dominant. Hydroxyl
radicals are generally thought to abstract a H-atom from
aldehydes — chiefly the aldehydic H-atoms, i.e.
HO + RCHO -»H20 + RCO (6-32)
If one assumes an atmospheric concentration of 10^ radicals
cm"3, the rates of decay of HCHO and CH3CHO by reaction with
OH are approximately 4.2 percent and 5.8 percent per hour,
respectively. -^
The photodissociation of aldehydes is an important
radical generation mechanism in the formation of photochemical
air pollution. The reactions that are most significant can
be generalized in terms of a radical and a molecular route:
RCHO + hv -»R + HCO (6-33a)
and
-*RH + CO (6-33b)
(Reaction 6-33a was previously given as reaction 6-25.) The
radical route is the more important one from the point of view
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1-61
of atmospheric chemistry. Considerable attention has been
given to formaldehyde photolysis in recent years. There
appears to be general agreement that the primary paths are:
HCHO + hv -»H + HCO (6-34a)
and
+ CO (6-34b)
In order to compare the rates of photolysis with the depletion
of formaldehyde by HO reaction, one can calculate a photolysis
rate of approximately 13 percent per hour for a solar zenith
angle of 20° using the value of the photodissociation rate
given by Horowitz and Calvert.17
The interaction with NO and N02 of the organic free
radicals produced by hydrocarbon oxidation represents an
extremely important aspect of the chemistry of the oxides of
nitrogen in the polluted atmosphere. The radicals can be
classed according to:
R alkyl 0
n
RO alkoxyl RC acyl
ROO peroxyalkyl 0
N
RCO acylate
0
ii
RCCO peroxyacyl
In air it can be assumed that combination with 02 is the sole
fate of alkyl (R ) and acyl (RCO ) radicals and that the
reaction is essentially instantaneous. Consequently, in
reactions with alkyl or acyl radicals as products, these
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1-62
products are often written as the corresponding peroxy radicals.
Also/ acylate radicals will decompose rapidly to give an alkyl
radical and C02- Therefore, only alkoxyl, peroxyalkyl, and
peroxyacyl radicals need to be considered explicitly in terms
of NOX chemistry. Table 6-1 shows th^& various raction combina-
tions that are important between these radicals and NO and N02.
The reactions of HO with N02 and NO are reasonable well
understood and have been previously listed as reactions 6-18
and 6-19. Rate constants for these two reactions are
available.9
The rate constant for the reaction of H02 and NO has
recently been determined by direct means and is substantially
larger than previously calculated indirectly. 9 The H02~NO
reaction, as noted earlier, is a key reaction in the atmospheric
conversion of NO to NO2.
The reaction of H02 and NO2 has the following two possible
mechanisms.^O Reaction 6-35b is not considered to be important
in atmospheric chemistry:
H02 + N02-» H02N02 (6-35a)
and
H02 + N02 ~»HONO + 02 (6-35b)
In addition, the peroxynitric acid formed in reaction 6-35a
thermally decomposes as follows:^1
H02N02^H02 + N02 (6-36)
At the present time it appears that, at the temperatures
prevalent in summer smog episodes (>20°C), peroxynitric acid
-------
Table 6-1. REACTIONS OF ALKOXYL, ALKYLPEROXYL AND ACYLPEROXYL RADICALS WITH NO AND
NO
•'ree Radical Reaction Reference
OH OH 4- NO— *HONO Hampson and
Garvin, 19789
H02 H02 + NO— *-NO2 4- OH Howard and
Evenson, 197719
RO RO + NO-^RONO Batt et al.23
(RONO + hv-^RO + NO)
RO2 R02 + NO-*N02 4- RO
•—•RON02
N02
Reaction
OH + N02— »-HONO2
HO2 4- NO2->-HONO + O2
-^HO2NO2
(HO2NO2-^-H02 4- N02)
-»RCHO 4- HONO
(RO2N02-*.R02 4- N02)
Reference
Tsang
1977"
Howard
Graham
Wiebe
et al. ,
, 197720
et al. /
et al.24
RCO-
RCO3 4- NO-*N02 4- RC02 Cox and Roffey
Hendry and
Kenley26
25
+ NO2-^ RC03NO2 Cox and Roffey
(RCO3NO2-^RCO3 + NO2)
25
Hendry and
Kenley26
-------
1-64
does not represent an appreciable sink for N02 because of the
rapid thermal decomposition reaction 6-36. At lower temperatures
H02NO2 will achieve higher concentrations and its importance as
a sink for NC>2 increases.
The reactions of RO, RC>2 and RCC>3 with NO and NC>2
represent key reactions in the conversion of NO to N02 and
the formation of organic nitrites and nitrates.
The main alkoxyl radical reactions with NO and N02 are:
RO + NO-*RONO (6-37a)
or
-»RCHO + HNO (6-37b)
and
RO + N02 -»RONO2 (6-38a)
or
-»RCHO + HONO (6-38b)
The reaction of alkylperoxyl radicals with NO is generally
assumed to proceed by the oxidation of NO to N02 with formation
of an alkoxyl radical:
RO2 + NO -»N02 + RO (6-22)
Reaction 6-22 is believed to be an important route for the
oxidation of NO to N02 in the atmosphere (the alkoxyl radical
may react further to produce H02, which also converts NO to N02).
It has been postulated that longer chain peroxyalkyl
radicals (n>4) from alkane photooxidation will add to NO to
form an excited complex that can be stablized to produce an
-------
1-65
alkyl nitrate:22
R02 + NO-»RON02 (6-39)
The peroxyalkyl-NC>2 reaction proceeds principally by
R02 + N02 -»R02N02 (6-40)
The peroxynitrate may thermally decompose according to
R02N02 -»R02 + N02 (6-41)
Measured rate constants for the RO2-N02 reaction and the RO2NO2
decomposition are not currently available.
Peroxyacyl nitrates have been recognized as important
components of photochemical air pollution.2^ Peroxyacetyl
nitrate (PAN) exists in equilibrium with the peroxyacyl
radical and N02:
0 0
w _^ "
CH3COO + N02<- CH3COONO2 (6-42)
There exists a competition between NO and N02 for the
peroxyacyl radical through:
0 0
ii H
CH3COO + NO-» CHaCO + N02 (6-43)
The acetyl radical will rapidly decompose as follows:
0
H
CH3CO -»CH3 + C02 (6-44)
followed by:
CH3 + 02 -»CH302 (6-21)
CH302 + NO -»CH30 + N02 (6-22)
CH3O -I- 02 -»HCHO + H02 (6-23)
H02 -l- NO-HJH + N02 (6-24)
-------
1-66
Thus, PAN chemistry is intimately interwoven in the NO to N02
conversion process. Rate constants for reactions 6-42 and 6-
43 have recently been reported by two groups of investigators.25'26
The chemistry of the oxides of nitrogen in a hydrocarbon-
containing atmosphere can be summarized as follows: the major
observed phenomenon in the system is conversion of NO to N02
and formation of a variety of nitrogen-containing species, such
as nitrites and nitrates. The conversion of NO to N02 is
accompanied by accumulation of 03. NO2 serves as both as
initiator and terminator of the chain reactions that result
in conversion of NO to N02 and buildup of 03. Termination of
the chain reactions leads to nitric acid and organic nitrates.
The nature of the system can be explained by considering its
behavior as a function of the initial concentrations of NOX
and hydrocarbon in the irradiation of a static system, as
well as the ratio of two reactants, i.e., the [HC]/[NOX] ratio.
At low [HC]/[NOX] ratios (usually ratios of less than
about 1 to 2:1) the rate at which NO is converted to N02 is
influenced by the availability of organic compounds. Therefore,
the effects of reducing organic compounds are to slow the
conversion of NO to NO2, thereby lowering the NO2/NO ratio.
When this occurs, a larger proportion of the NO that is
converted to N02 occurs through the destruction of ozone. This
then has the overall effect of reducing the rate of ozone
formation. If the oxidation of NO by organics is delayed
-------
1-67
sufficiently so that the sun has passed its zenith before
significant amounts of N02 are created, photodissociation of N02
will be diminished and less ozone will accumulate on that
date. At moderately high [HC]/[NOX] ratios (usually greater
than about 5 to 8:1), the greater availability of organic
radicals means that all of these radicals are not consumed
as rapidly in reactions with NO, and more reactions between
the radicals and NC>2 are able to occur. Thus, the amount of
ozone formed and accumulated begins to become limited by the
availability of NOX, and becomes less sensitive to additional
organic precursors. At very high [HC]/[NOX] ratios (greater
than about 20 to 30:1), ozone cannot accumulate because either
the ozone is consumed by reaction with hydrocarbons or radical-
radical termination reactions occur which reduce oxygen atom
and, hence, ultimate ozone concentration.
Identification of the nitrogen-containing products in
atmospheric reactions has been under investigation for a
number of years.28-30 jn general, the most important gaseous
nitrogen-containing products in the N0x-organic system are
nitric acid and PAN. As noted, reactions of NO and N02 with
free radicals produce, in addition to nitrous,%nitric, and
peroxynitric acids, a variety of organic nitrogen-containing
species (Table 6-1). There currently exist important areas
of uncertainty with regard to the formation of nitrogen-
containing products in atmospheric reactions. The extent of
-------
1-68
formation and decomposition of peroxynitrates, RO2N02, is
unknown, and rate constants for the key reactions in the
series, RO2 + NO, are yet to be determined.
6 • 1 • 2 '
In the previous section, the nature of chemical
reactions involving oxides of nitrogen and hydrocarbons in
the atmosphere was discussed. These reactions have traditionally
been studied experimentally in laboratory vessels called smog
chambers. These chambers characteristically employ radiation
sources that closely approximate the UV portion of the solar
spectrum as observed at the earth's surface and clean,
chemically inert interior surfaces. It is believed that the
chemical processes that take place in smog chambers are
S
similar to those that take place in the atmosphere.
The presence of surfaces in a smog chamber may, however,
be a source of difficulty in interpreting chamber results
because of possible surface-catalyzed reactions or absorption
of species on the walls. In addition, most chamber experiments
have been conducted by initially injecting fixed amounts of
reactants rather than simulating the continuous time-varying
injection and dilution of reactants that characterize the
ambient situation. Nevertheless, the behavior of irradiated
mixtures of oxides of nitrogen and hydrocarbons in smog
chambers has served as the foundation for our understanding
of atmospheric chemical mechanisms.
-------
1-69
Considerable effort has been devoted to the development
of chemical reaction mechanisms that are capable of describing
the processes observed in smog chambers.4-8 Smog chambers have
been used extensively to determine how concentrations of NOX
and other photochemical products respond to changes in the
initial composition of nitrogen oxides and organics. A
previous Criteria Document2? discusses smog chamber evidence
concerning the relationship between ozone/oxidant and the
photochemical precursors. This section focuses on how N02
concentrations respond to changes in the input levels of
organics and nitrogen oxides.
Several researchers have used smog chambers to investigate
the dependence of nitrogen dioxide concentrations on the
levels of precursor inputs:
0 The University of North Carolina (UNC) study using
an 11,000 cubic-foot (311 m3) outdoor Teflon chamber,
a simulated urban hydrocarbon mix, and twelve-hour
irradiations32
0 The Bureau of Mines study, using a 100 cubic-foot
(2.8 m3) aluminum-glass chamber, auto-exhaust hydro-
carbons, and six-hour irradiations 33,34
0 The General Motors study, using a 300 cubic-foot
(8.5 m3) stainless steel-glass chamber, a simulated
Los Angeles hydrocarbon mix, and six-hour irradiations3^
0 The Health, Education and Welfare (HEW) study using a
-------
1-70
335 cubic-foot (9.5 m3) chamber, auto-exhaust hydro-
carbons, and up to ten-hour irradiation timers and
0 The HEW study using a 335 cubic-foot (9.5 m3) chamber,
toluene and m-xylene, and 6-hour irradiations.37
Trijonis38'3^ has recently reviewed the results of these
studies, as summarized in Table 6-2. As indicated in Table 6-2,
the various chamber studies basically agree concerning the
dependence of maximum N02 and average N02 on NOX input. With
other factors held constant, maximum NC>2 and average NC>2 tend
to be proportional to initial NOX. The minor deviations away
from proportionality that sometimes occur tend to be in the
direction of a slightly less than proportional relationship,
i.e., a 50 percent reduction in NOX input sometimes produces
slightly less than a 50 percent reduction in NC>2.
There is less agreement among the chamber studies con-
cerning the dependence of N02 on initial hydrocarbon concen-
trations. With respect to maximum NO2, the Bureau of Mines
study indicates essentially no dependence on hydrocarbons.
However, two other studies suggest that hydrocarbon reductions
decrease maximum NC>2 concentrations. The UNC results indicate
that 50 percent hydrocarbon control tends to decrease maximum
N02 by about 10 percent to 20 percent. The General Motors
studies imply that 50 percent hydrocarbon control reduces
maximum N02 by about 25 percent.
-------
Table 6-2. Summary of Conclusions from Smog Chamber Experiments
CHAMBER STUDY
MAXIMAL N02
Dependence Dependence
on NO,
on HC
AVERAGE NO2
Dependence Dependence
on NO,
on HC
University of North
Carolina3^
Bureau of Mines33'3^
General Motors3
HEW, Auto Exhaust36
Proportional
or slightly
less than
proportional
Proportional
Slightly less
than propor-
tional
50% HC reduc-
tion reduces
maximal N02
by 10% to 20%
No effect
50% HC reduc-
tion reduces
maximal NO2
by 25%
Proportional
or slightly
less than
proportional
Proportional
Proportional
to slightly
less than
proportional
Proportional
Uncertain, 50% HC
reduction may de-
crease average
NO2 by 20% or may
increase average
NO2
50% HC reduction
increases average
N02 by 10% to 30%
No effect
No consistent
effect
HEW, Toluene37
Proportional
No effect
-------
1-72
With respect to average NC>2, the Bureau of Mines study
indicates that hydrocarbon reductions would tend to increase
NC>2 dosage. This result is consistent with the theoretical
argument of Stephens,4° who hypothesized that hydrocarbon
reduction would increase average N02 because these reductions
would delay and suppress the chemical reactions that consume
NO2 after it reaches a peak. However, the General Motors
chamber study and the two HEW studies indicate that hydrocarbons
produce no consistent effects on average NO2 concentrations.
The UNC experiments imply that a 50 percent reduction in
hydrocarbons produces about a 20 percent decrease in average
N02« There is some question about the UNC conclusion, however,
because the UNC chamber runs were of a 10-hour duration and the
NC>2 levels at the end of the experiments were greater when
hydrocarbons were reduced. The extra N02 remaining after the
10-hour period could cause an increase in 24-hour average NO2,
even though average NC>2 was reduced during the first 10 hours.
Considering the results of all the chamber studies,
Trijonis suggested a consensus based on existing chamber
results which would appear to be as follows: fifty percent
hydrocarbon reduction would have little effect on average
N02 concentrations (a change of _+ 10 percent) but would yield
moderate decreases in maximal NC>2 (a reduction of about 10 to
20 percent). It should be noted that these conclusions are
meant to apply to one basic type of ambient situation — the
situation of well-mixed urban air.
-------
1-73
Some additional support for these conclusions was provided
recently by studies of actual ambient data on NOX and hydro-
carbon levels from a number of cities in the U.S. Using
empirical modeling and historical trend analysis, Trijonis38'39
concluded that the ambient data were generally consistent
with the consensus of chamber results. The exact form of the
NO2/precursor relationship, however, was found to vary some-
what from one location to another, presumably depending on
local hydrocarbon/NOx ratios, on the details of the hydro-
carbon mix, and on specific meteorological conditions.
Reference is made also to another body of data due to
ltts et al.55 (collected for a different purpose) which
also contains potential information on the relationship
between NOX and its precursors. However, the data have not
been analyzed to date for its pertinence to the NOx/precursor
question.
6.1.3 .Npx. .Ch,em;Lg.t.ry,;m' Pl.upi.es
The atmospheric chemistry involving oxides of nitrogen
in plumes from major fuel burning installations is essentially
that described earlier. However, the relatively high concen-
trations of NO and NC>2 in such plumes compared with those
in the ambient urban atmosphere leads to certain chemical
phenomena particularly characteristic of plumes. For example,
ambient ozone is quickly scavenged in the plume by the large
of NO through reaction 6-4. Because the rate of
-------
1-75
6.1.4 Computer Simulation of Atmospheric Chemistry
A key problem underlying the development and evaluation
of kinetic mechanisms for atmospheric chemistry is determining
the sensitivity of the concentration predictions to those
uncertain aspects of the reaction scheme. Such a determina-
tion can serve as a valuable guide for future experimental
studies and for identifying those parameters that, when
varied within accepted bounds, will be most influential on
the predictions of the mechanism.
Although the qualitative aspects of the chemistry of the
polluted troposphere appear to be reasonably well understood,
there are many important details that still need to be investi-
gated before a complete quantitative understanding of the photo-
chemical smog system is possible. Several groups 4-8 have
formulated chemical reaction mechanisms for polluted tropos- "
pheric chemistry. Some of these are based on specific surro-
gate hydrocarbon chemistries; in others, attempts have been
made to simulate the complex ambient atmospheric system by
representing the general features of the hydrocarbon chemistry.
All mechanisms contain aspects of uncertainty, whether in
unknown rate constants, in the importance of competing reaction
paths, or in the manner of representing the reaction of a
generalized species. The measure of the accuracy of a mechanism
is usually based on the extent of agreement between predicted
concentration profiles and those generated experimentally in
smog chambers. But here also, the inherent uncertainty of the
-------
1-76
experimental data base must be taken into consideration when
comparing predictions to observations.
The chemistry of the inorganic portion of the photochemical
smog mechanism is, by and large, well understood. Uncertainties
remaining include:
0 photolysis rates
p alkane-HO product distributions
0 olefin-HO and olefin-C>3 product distributions
0 aromatic chemistry
0 alkoxyl radical reactions
0 ROX/NOX reactions
Uncertainty in the model prediction can result from inappro-
priate specification of photolysis rate constants. In the case
of computer simulations of smog chamber data, photolysis rate
constants relative to the reported value for NC>2 are frequently
used. While under real atmospheric conditions theoretical
estimates are used corrected relative to empirical measurements,
if available.
Kj = f <5j(X)$j(X)KX)dX (6-45)
where
KJ = photolysis rate constant for species j
6j()0 = absorption cross section of species j
-------
1-77
Peterson indicates that for species such as NO2/ HONO, H^O^,
03, and formaldehyde, extensive experimental determinations
of absorption cross sections and quantum yields are fairly
reliable. However, since cross section and quantum yield
data for higher aldehydes, alkyl nitrites and other carbonyl
compounds are much less well characterized, these photolysis
rate constants are subject to a large uncertainty. Of course,
even if absorption cross sections and quantum yields could be
determined accurately for all photo-sensitive species,
uncertainties in atmospheric photolysis rate constants would
still exist, as meteorological conditions, clouds, dust, and
aerosols cause unknown variances in actinic irradiance.
Whereas rate constants in the inorganic portion of the
mechanism are known fairly well, many more uncertainties,
both in reaction rate constants and products, are associated
with the organic reaction steps. Still to be determined
are product distributions and reaction rate constants for the
initial steps of the reactions of OH and hydrocarbon species,
the largest uncertainties lying in the routes of the various
radical species produced. For example, although rate constants
for alkane-OH reactions are well established, the ratio of
internal to external abstraction for all alkanes is not known.
Addition to 02 to form peroxyalkyl (R02) radicals can be con-
sidered as the sole fate of the alkyl radicals first produced
-------
1-78
in alkane-OH reactions, but after the formation of alkoxyl
radicals through the conversion of NO to N02, the reaction
mechanism becomes uncertain. Alkoxyl radicals can decompose/
react with 02/ isomerize, or react with NO or N02/ with the
importance and rate of each reaction path depending on the
nature of the alkoxyl group. Even for the most studied of
the alkane-hydroxyl radical reactions, the relative rates
between decomposition, isomerization, and reaction with 62,
NO, and N02 for alkoxyl radicals have not been measured,
but must be estimated.6
Less well understood than alkane reaction mechanisms
are olefin oxidation processes, primarily by HO. Olefin-
hydroxyl radical reactions may proceed by addition or abstra-
tion. For smaller olefins, the addition path predominates.
However, the abstraction fraction increases with the size
of the olefin. Along the addition path for terminally bonded
olefins, there is uncertainty as to the ratio of internal to
external addition. Similar to alkyl radicals, the hydroxy-alkyl
radicals formed in the initial HO addition to olefins are
thought to immediately add 02 to form hydroxy-peroxyalkyl
radicals and thereafter react with NO to give N02 and hydroxy-
alkoxyl species. The fate of the hydroxy-alkoxyl radicals
is subject to speculation, although the analogous alkoxyl
reaction paths of decomposition, isomerization, and reaction
with NO, N02 and 02 are most likely possibilities.
-------
1-79
The inherent uncertainty of the decomposition, reaction
with 02, and isomerization of the alkoxyl and hydroxy-alkoxyl
radicals class can be presented by the generalized reaction
step:
RO -»<*H02 + (1-«OR02 +/SHCHO + tfRCHO (6-46)
From the earlier discussions of alkoxyl radical behavior, RO
always gives rise to either H02 or R02 in any of the decom-
position, isomerization, or 02 reaction pathways. Hence, the
stoichiometric coefficients representing the fraction of H02
and RO2 found in the lumped RO reaction should sum to one.
Since the RO lumped species represents a large class of
different-sized radicals and because splits between reaction
paths for even specific radicals are not known, «<• can have a
value in the range of 0 to 1. Many RO reaction routes pro-
duce aldehydes. Thus, 0 .< fl .< 1 and 0 X ^ .< !• Since the
composition of the RO radical pool is continually changing
during the course of a photooxidation, the actual values of
**/$./ and }f are functions of time. Thus, the selection of
constant values of these coefficients introduces uncertainty.
A comprehensive sensitivity/uncertainty analysis of
photochemical smog mechanisms has been carried out by Falls
et al.^l in this study the effects of rate constant and
mechanistic uncertainties on predicted concentrations are
illustrated.
-------
1-80
6.2 NITRITE AND NITRATE FORMATION
The oxides of nitrogeo are converted eventually to
nitrites and nitrates by the reactions given in Section 6.1.
In particular, the following gaseous nitrites and nitrates
have been idenified:
HONO nitrous acid
HONO2 nitric acid
H02N02 peroxynitric acid
RONO alkyl nitrite
RON02 alkyl nitrate
0
ti
RCOON02 peroxyacylnitrate (PAN)
R02N02 peroxyalkyl nitrate
In addition to these gaseous species, particulate nitrites
and nitrates may be formed. The 'object of this section is
to present estimates of the importance of the various nitrites
and nitrates. In most cases, estimates are necessary because
ambient measurements of the concentration level of all but
a very few of the species are lacking.
Typical ambient concentration levels of the gaseous
nitrogen-containing species listed above can be estimated
from simulations of smog chamber experiments using chemical
mechanisms representing the hydrocarbon-NOx chemistry.
Table 6-3 lists calculated concentrations of HONO, HON02,
-------
1-81
f RONO, RONO2/ RC(0)OON02f and R02NC>2 for smog chamber
experiment EC-237 carried out at the Statewide Air Pollution
Research Center of the University of California/ Riverside/
using the chemical mechanism of Falls and Seinfeld.8 The
conditions of the experiment are given in the footnote of
Table 6-3. The simulated and predicted concentrations of the
major measured species/ such as NO/ N02 03, PAN/ and hydro-
carbons/ agreed well.
The concentrations of HONO/ H02N02/ and RONO are pre-
dicted to be small relative to those of NO and N02. Each of
these species has decomposition reactions/
HONO + hv -» OH* + NO (6-17)
H02N02 -» H02. + NO2 (6-36)
RONO + hv -» RO' + NO (6-47)
that/ at the temperatures and solar intensities prevalent in
the experiment and in the summer atmosphere/ are fast enough
to insure that the concentrations of each of the three species
are low. At lower solar intensities than those in the experi-
ment/ HONO and RONO can be expected to reach higher concentra-
tions/ and at lower temperatures/ such as those in the stratos-
phere/ H02N02 may accumulate.
-------
1-82
Table 6-3. Predicted Nitrite and Nitrate Concentrations in
Simuluation of Experiment EC-237 of the Statewide
Air Pollution Research Center of the University
of California, Riverside, Using the Chemical
Mechanism of Falls and Seinfeld8
Concentration, ppm
HONO
HON02
HC-2N02 '
RONO
RON02
0
RCOON02
R02NC-2
60 min.
0.0061
0.067
0.00083
0.0030
0.0041
0.025
0.034
180 min.
0.00040
0.22
0.0019
0.00054
0.0070
0.089
0.075
300 min.
0.00036
0.29
0.0025
0.000080
0.0072
0.13
0.098
Conditions of the experiment: T = 303°K, k2 = 0.3 min ,
[N02]o = 0.106, [N0]o = 0.377, [H20] = 2.4 x 104, [CO]2 =
0.96, [Aldehydes]o= 0.0012, [Alkanesjo = 1.488, [Non-ethylene
Olefins]o= 0.15, [C2H4]o = 0.875, [Aromaticslo = 0.177, [HONOJo
(assumed) =0.1 (All concentrations in ppm). Dilution rate =
2.93 x 10"4 min"1.
-------
1-83
Under daytime conditions the reactions that govern the
concentration of HONO are 6-17 and 6-18. At night/ however,
the only apparent destruction route for HONO is reaction 6-14.
Depending on the relative importance of reactions 6-18, 6-13,
and 6-14 HONO may reach substantial concentrations under night-
time conditions. A lower limit on the nighttime concentration
of HONO can be estimated from the equilibrium HONO concentra-
tion based on reactions 6-13 and 6-14.
k!3
(6-48)
k14
At [NO] = [N02] =0.1 ppm, [H20] = 2.4 x 104 ppm (50 percent
relative humidity), the equilibrium HONO concentration calcu-
lated from equation 6-48 is 1.9 x 10"^ ppm.
Like HONO, H02NO2 and RONO, PAN undergoes both formation
and decomposition steps (reactions 6-42a,b). Unlike these
former species, however, the balance between the formation
and decomposition reactions is such that PAN may achieve
appreciable concentration levels relative to those of NO and
NO2. Because the decomposition reaction for PAN is strongly
temperature dependent, the steady state PAN concentration is
highly dependent on the temperature. As temperature increases
the role of PAN as an N02 sink decreases markedly; at low
temperatures, on the other hand, steady state PAN concentra-
-------
1-84
tions can reach rather substantial levels.
Little is known about the existence and importance of
peroxynitrates other than HO2N02 and PAN. It is presumed in
the mechanism on which the results of Table 6-3 are based
that R02N02 thermally decomposes at a rate between those for
H02N02 and PAN. Assessment of the importance of R02N02 as
a sink for NOX will depend on measurement of the rates of
reactions 6-40 and 6-41.
In contrast to the other species of Table 6-3, nitric
acid and alkyl nitrates apparently do not undergo appreciable
decomposition reactions. Thus/ these two species potentially
serve as important atmospheric sinks for N02» Both nitric
acid and alkyl nitrates may remain in the gas phase or react
with other atmospheric constituents, such as ammonia, to
produce low vapor pressure species that have a tendency to
condense on existing particles or homogeneously nucleate to
form particles.
Figure 6-1 depicts the potential paths by which particu-
late nitrate species may be formed from NO and NC>2. Path 1
involves the formation of gaseous nitric acid by reactions
6-8 and 6-19. Nitric acid concentrations resulting from
these two reactions for the simulated smog chamber experiment
have been given in Table 6-3. Comparisons of the individual
rates of reactions 6-8 and 6-19 indicate that reaction 6-19
-------
1-85
Figure 6-1. Paths of nitrate formation in thed atmosphere.
Source: Orel and Seinfeld (1977)
-------
1-86
is the predominant route for gas-phase nitric acid formation
under typical daytime conditions. Nitric acid vapor, once
formed, may then react with NH3, a ubiquitous atmospheric
constituent with both natural and anthropogenic sources, to
produce ammonium nitrate, NH4N03 (path 2), which at standard
temperature and pressure, exists as a solid. Alternatively,
the nitric acid vapor may be absorbed directly onto a particle
(path 3), although thermodynamic and kinetic considerations
favor reaction with NH3 to form NH4N03 as the path of conversion
of gaseous nitric acid to nitrate in particulate form.48"50
Path 4 involves the direct absorption of NO and N02 into an
atmospheric particle, a route that is likely for certain aqueous
particles, particularly when accompanied by the absorption of
ammonia (path 5). 51 Path 6 depicts the formation of organic
nitrates through reactions such as 6-38a, followed by absorp-
tion of these nitrates into particles. At present little is
known about the existence or importance of mechanisms such as
that depicted by path 6.
There have been a limited number of measurements of nitric
acid and particulate nitrate concentrations in ambient air, and
of these measurements many have identified the particulate
nitrate as NH4N03, suggesting that the aerosol may consist of
+
solid NH4N03 or NH4 and N03 in solution in approximate stoichio-
metric balance. It is difficult to estimate the relative
importance of the paths in Figure 6-1 for several reasons.
First, the rate of reaction of nitric acid and ammonia is
-------
1-87
not well known/ although the forward reaction is probably
rapid and, in fact, can be presumed to be in equilibrium
with the dissociation of solid ammonium nitrate.48"50
NH3(g) + HON02(g) -» NH4N03(S) (6-49)
Second, the rate of absorption of NO and N02 into existing
particles depends on the composition and size of each particle
and cannot generally be predicted a priori. In either case
it is apparent that the presence of NH3 is required, either
to form NH4N03 or to neutalize the acidity of a liquid drop-
let in which NO and N02 dissolve.
The current state of understanding of atmospheric
inorganic nitrate formation can be summarized as follows.
The principal gas-phase nitrate forming reaction is reaction
6-19. The nitric acid vapor formed in reaction 6-19 probably
reacts rapidly with ammonia to form small particles of solid
ammonium nitrate such that the equilibrium of reaction 6-49
is established. In competition with the nitric acid/ammonium
nitrate path is the path consisting of direct absorption of
NO and N02 into aqueous droplets. The relative rates of
these two paths cannot be determined in general. Although
measurements of particulate organic nitrate levels have been
reported,-" the mechanisms of formation of organic aerosol
nitrates have not been fully identified.
-------
I-R12
6.6 REFERENCES
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Inc., New York, 1977.
2. Seinfeld, J.H. Air Pollution: Physical and Chemical Fun-
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4. Demerjian, J., J.A. Kerr, and J.G. Calvert. The
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5. Carter, W.P.L., A.C. Lloyd, J.L. Sprung, and J.N.
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6. Baldwin, A.C., .R. Barker, D.M. Golden, and D.G. Hendry.
Photochemical smog. Rate parameter estimates and com-
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7. Whitten, G.Z., and H. Hogo. Mathematical Modeling of
Simulated Photochemical Smog. EPA-600/3-77-011. U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, January 1977.
8. Falls, A.H., and J.H. Seinfeld. Continued development
of a kinetic mechanism for photohemical smog. Environ.
Sci. Technol., 12_: 1398, 1978.
9. Baulch, D.L., R.A. Cox, R.F. Hampson Jr., J.A. Kerr,
J. Troe and R.T. Watson. Evaluated Kinetic and Phto-
chemical Data for Atmospheric Chemistry. J. Phys.
Chem. Ref. Data, 9_:295, 1980.
10. Niki, H. Reactions of ozone and hydroxyl radicals with
olefins. Workshop on Chemical Kinetic Data Needs for
Modeling the Lower Troposphere. U.S. Environmental
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Reston, Virginia, May 15-17, 1978.
-------
I-R13
11. O'Neal, H.E., and C. Blurastein. A new mechanism for
gas phase ozone olefin reactions. Inter. J. Chem. Kine-
tics j>: 397, 1973.
12. Herron, J.T., and R.E. Huie. J. Am. Chem. Soc. 99:
5430, 1977.
13. Niki, H., P.D. Maker, C.M. Savage, and L.P. Breitenbach.
Chem. Phys. Lett. ^: 327, 1977.
14. Hendry, D.G. Reactions of aromatic hydrocarbons in the
atmosphere. Workshop on Chemical Kinetic Data Needs
for Modeling the Lower Troposphere, U.S. Environmental
Protection Agency and the National Bureau of Standards,
Reston, Virginia, May 15-17, 1978.
15. Perry, R.A., R. Atkinson, and J.N. Pitts, Jr. Kinetics
and mechanism of the gas phase reaction of OH radicals
with aromatic hydrocarbons over the temperature range
296-472K. J. Phys. Chem. 81_: 296, 1977.
16. Lloyd, A.C. Tropospheric chemistry of aldehydes. Work-
shop on Chemical Kinetic Data Needs for Modeling the
Lower Troposphere. U.S. Environmental Protection Agency
and the National Bureau of Standards, Reston, Virginia,
May 15-17, 1978.
17. Horowitz, A., and J.G. Calvert. Wavelength Dependence of
the Quantum Efficiencies of the Primary Processes in
Formaldehyde Photolysis at 25°C. Inter. J. Chem. Kine-
tics, _10, 805-819, 1978.
18. Tsang, W., D. Garvin, and R.L. Brown. NBS Chemical
Kinetics Data Survey — The Formation of Nitric Acid
from Hydroxyl and Nitrogen Dioxide, 1977.
19. Howard, C.J., and K.M. Evenson. Kinetics of the reaction
of H02 with NO. Geophys. Res. Lett. £: 437, 1977.
20. Howard, C.J. Kinetics of the reaction of H02 with N02«
J. Chem. Phys., 67:5258 (1977).
21. Graham, R.A., A.M. Winer, and J.N. Pitts, Jr. Tempera-
ture dependence of the uni-molecular decomposition of
pernitric acid and its atmospheric implications. Chem.
Phys. Lett. _5_1: 215, 1977.
22. Darnall, K.R., W.P.L. Carter, A.M. Winer, A.C. Lloyd,
and J.N. Pitts, Jr. Importance of R02 + NO in alkyl
nitrate formation from C4 - Cg alkane photooxidations
under simulated atmospheric conditions, J. Phys. Chem.
80: 1948, 1976.
-------
I-R14
23. Batt, L., R.D. McCulloch, and R.T. Milne. Thermochemical
and kinetic studies of alkyl nitrates (RONO) - D(RO-NO).
The reactions between RO and NO and the decomposition of
RO. Inter J. Chem. Kinetics Symp. 1.: 441, 1977.
24. Weibe, H.A., A. Villa, T.M. Hellman, and J. Heicklen.
Photolysis of methyl nitrite in the presence of nitric
oxide, nitrogen dioxide and oxygen. J. Amer. Chem. Soc.
.95: 7f 1973.
25. Cox, R.A., and M.J. Roffey. Thermal decomposition of
peroxyacetyl nitrate in the presence of nitric oxide.
Environ. Sci. Technol. 11; 900, 1977.
26. Hendry, D.G., and R.A. Kenley. Generation of peroxy
radicals from peroxy nitrate (RC^NC^) decomposition of
peroxyacyl nitrates. J. Am. Chem. Soc. 99; 3198, 1977.
27. Air Quality Criteria for Ozone and Other Photochemical
Oxidants. EPA-600/8-78-004. Office of Research and
Development, U.S. Environmental Protection Agency,
Washington, D.C., 1978.
28. Gay, B.W., and J.J. Bufalini. Nitric acid and the
nitrogen balance of irradiated hydrocarbons in the
presence of oxides of nitrogen. Environ. Sci. Technol.
_5: 422, 1971.
29. Spicer, C.W., and D.F. Miller. Nitrogen balance in
smog chamber studies. J. Air Pollut. Control Assoc.
^6: 45, 1976.
30. Pitts, J.N., Jr. Mechanism of Photochemical Reactions
in Urban Air. Vol. I. Chemistry Studies. EPA-600/
3-77-014a. U.S. Environmental Protection Agency, 1977.
31. Falls, A.H., G.J. McRae, and J.H. Seinfeld, Sensitivity
and uncertainty of reaction mechanisms for photochemical
air pollution. Int. J. Chem. Kinetics Ll:1137, 1979.
32. Jeffries, H., D. Fox, and R. Kamens. Outdoor Smog
Chamber Studies: Effect of Hydrocarbon Reduction on
Nitrogen Dioxide. EPA-650/3-75-011. U.S. Environmental
Protection Agency, 1975.
33. Dimitriades, B. Effects of hydrocarbons and nitrogen
oxides on photochemical smog formation. Environ. Sci.
Technol. 6: 253, 1972.
-------
I-R15
34. Dimitriades, B. Oxidant control strategis. Part I.
Urban control strategy derived from existing smog
chamber data. Environ. Sci. Technol. 11; 80/ 1977.
35. Heuss, J.M. Smog Chamber Simulation of the Los Angeles
Atmosphere. General Motors Research Publication
GMR-1082, 1975.
36. Korth, M.W., A.H. Rose/ and R.C. Stahman. Effects of
hydrocarbons to oxides of nitrogen ratio on irradiated
auto exhaust. J. Air Pollut. Control Assoc. 14: 168,
1964.
37. Altshuller, A.P., S.L. Kopczynski, W.A. Lonneman, F.D.
Sutterfield, and D.L. Wilson. Photochemical reactivities
of aromatic hydrocarbon-nitrogen oxide and related systems,
Enviro. Sci. Technol. £: 44, 1970.
38. Trijonis, J. Empirical Relationships Between Atmospheric
Nitrogen Dioxide and its Precursors. EPA-600/3-78-018.
U.S. Environmental Protection Agency, 1978.
39. Trijonis, J. Dependence of ambient NC>2 on precursor
control. Session on Secondary Pollutant Control.
National AIChE Meeting, Philadelphia, June 1978.
40. Stephens, E.R. Proceedings of the Conference on Health
Effects of Air Pollution. U.S. Senate Committee on
Public Works. U.S. Government Printing Office Stock
No. 5270-02105, 1973.
41. Kewley, D.J. Atmospheric dispersion of a chemically
reacting plume. Atmos. Environ. 12; 1895, 1978.
42. Shu, W.R., R.C. Lamb, and J.H. Seinfeld. A model of
second-order chemical reactions in turbulent fluid —
Part II. Application to atmospheric plumes. Atmos.
Environ. 12; 1695, 1978.
43. Hegg, D.P.V. Hobbs, L.F. Radke, and H. Harrison. Ozone
and nitrogen oxides in power plant plumes. Paper 5-2.
Proc. Int. Conf. on Photochemical Oxidant Pollution
and its Control. EPA-600/3-77-001a. U.S. Environmental
Protection Agency, 1976.
44. White, W.H. NOX - 03 photochemistry in power plant plumes;
comparions of theory with observation. Environ. Sci.
Technol. 11; 995, 1977.
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I-R16
45. White, W.H., J.A. Anderson, D.L. Blumenthal, R.B. Husar,
N.V. Gillani, J.D. Husar, and W.E. Wilson, Jr. Formation
and transport of secondary air pollutants: ozone and
aerosols in the St. Louis urban plume. Science 1.9.4; 187,
1976.
46. Davis, D.D., G. Smith, and K. Klauber. Trace gas analysis
of power plant plumes via aircraft measurement: 03, NOX,
and SC>2 chemistry. Science J..8.6; 733-736, 1974.
48. Bradner, J.D., N.M. Junk, J.W. Lawrence, and J. Robins.
Vapor pressure of ammonium nitrate. J. Chem. Eng. Data
7: 227, 1962.
49. Morris, E.D., and H. Niki. Mass spectrophotometric study
of the reactions of nitric acid with O atoms and H atoms.
J. Phys. Chem. ,7J>: 3193, 1971.
50. Stelson, A.W., S.K. Friedlander, and J.H. Seinfeld. Note
on the equilibrium relationship between gaseous nitric
acid and ammonia and particulate ammonium nitrate.
Atmos. Environ. X3:367, 1979.
51. Orel, A.E., and J.H. Seinfeld. Nitrate formation in
atmoshpheric aerosols. Environ. Sci. Technol. 11; 1000,
1977.
52. Kenley, R.A., J.E. Davenport, and D.G. Hendry. Hydroxyl
radical reaction in the gas phase. Products and pathways
for the reaction of OH with toluene. J. Phys. Chem. 8/!:
1095, 1978.
53. Grosjean, D. Formation of organic aerosols form cyclic
olefins and diolefins. Chapter 19 in the Character and
Orgins of Smog Aerosol, Adv. in Environ. Sci. Technol.,
Wiley, New York, 1979.
54. Demerjian, K.L., K.L. Schere and J.T. Peterson. Theoretical
Estimates of Active (Spherically Integrated) Flux and
Photolytic Rate Constants of Atmospheric Species in the
Lower Troposphere. Advan. Environ. Sci. Technol. Vol. 10,
Wiley, New York, pp 369-459, 1980.
55. Pitts, J.N., Jr., K.R. Darnell, A.M. Winer, and J.M. McAfee.
Mechanisms of Photochemical Reactions in Urban Air. VI.
Chamber Studies. EPA-600/3-77-014b. U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina,
February 1977.
-------
SECTION II
TRENDS IN PRECIPITATION COMPOSITION
AND DEPOSITION
by
John M. Miller
Air Resources Laboratory
National Oceanic and Atmospheric Administration
Silver Spring, Maryland 20910
-------
Il-i
TABLE OF CONTENTS Paqe
1. Introduction II-l
2. Data Bases in North America II-2
3. Data Evaluation Criteria II-6
4. Trends in Precipitation Chemistry Data II-8
5. Conclusions 11-16
-------
n-ii
LIST OF FIGURES
Page
II-9
11-10
11-12
Figure 1. SOX Emissions Trends for the U.S.,
1940-2000.
Figure 2. NOX Emissions Trends for the U.S.,
1940-2000.
Figure 3. Deduced Weighted Annual Average pH
of Precipitation in the Eastern
United States in 1955-1956 and 1975-
1976.
Figure 4. Wet Hydrogen Ion Deposition (equivalents/ 11-13
hectare) for 1965-1966.
Figure 5. Wet Hydrogen Deposition (equivalents/ 11-14
hectare) for 1975-1976.
Figure 6. Hydrogen Ion Concentrations at the Nine 11-15
USGS Network Sites in New York and
Pennsylvania for the Period 1966 to 1980
-------
LIST OF TABLES
Page
Table 1. Agricultural Data Bases (1875-1955) II-4
Table 2. Atmospheric Data Bases (1955-1975) II-5
Table 3. Ecological Data Bases (1975-Present) II-7
-------
II-l
1. Introduction
The establishment of trends in atmospheric chemistry
parameters has been one of the major thrusts in evaluating
man's influence on his environment. Trends in urban ozone
concentration, regional sulfate values and global carbon
dioxide levels are examples of measurements that show secular
changes of possible anthropogenic origin. Scientists working in
the precipitation chemistry field have also looked for possible
trends in the chemical composition of precipitation. Unfor-
tunately, no one consistent set of network data exists over a
long enough period of time (>10 years) that can be used to give
trends in wet depositions over an area. Because of this, workers
have used a mix of data from networks that did not use comparable
collection and analysis methods. Though definite changes could
be seen in the period from 1955 to present, no clear cut trend
can be established because of before mentioned data inconsist-
encies. In contrast to the past spotty performance of North Amer-
ican monitoring, a strong effort has been mounted in recent years
that should go a long way to establish trends in the coming
decade.
This paper reviews the historical data available, the
new networks that have been established and the status of
trend analysis in North America.
-------
II-2
2. Data Bases in North America
The history of precipitation chemistry measurements in
North America have been extremely erratic. National, regional/
and local networks have been established and disbanded without
thought of long-term considerations. In order to organize
these efforts into a historical perspective, three major
chronological thrusts can be recognized—agricultural (1875-
1955), atmospheric (1955-1975) and ecological (1975-present).
Though no one period can be identified by a single theme,
they formed the major impetus in the given time period. To
explain in more detail, each period is discussed in the
following:
Agriculture (1875-1955)
Before the advent of commercial fertilizers, agricultural
scientists both in Europe and North America were interested
in the amount of substances such as nitrate and sulfate which
were brought to the soils by rain. Throughout the western
world, collection networks were established generally at agri-
cultural research sites. The studies were generally limited
to the measurement of only one or two ions. The method to
measure pH was established generally after these studies
were completed. Thus a full chemical balance could not be
established. Also questions on collection methods, analysis
techniques, site criteria and other important factors are
not always clear from the references. It might be useful to
-------
II-3
look at this period in more detail, however no definitive
results can be expected. A summary of the studies taken from
Erikssen (1952) is given in Table 1.
Atmospheric (1955-1975)
In the late forties, atmospheric chemistry became a
recognized branch of atmospheric studies. As a part of this
interest in chemical cycles in the atmosphere, measurements
of precipitation chemistry were begun in Europe under
Professor Rossby. This interest spread to North America
where the first continent-wide network was established by
C. Junge. This network only lasted one year; other networks
were established but soon were discontinued mainly because of
the lack of support. Table 2 summarizes the work done during
this period. Some of these efforts have continued through
the present time. Initially precipitation chemistry had
been studied mainly from the atmospheric viewpoint but as
ecologists in the early seventies began to document the
effects of acid deposition, the thrust of precipitation
chemistry work began to change. This concern culminated in
the first major international meeting on acid deposition in
North America (Dochinger and Seliga 1976).
Ecological (1975-Presentj
Though several of the networks mentioned in the previous
section were aimed at evaluation of ecological effects, a
major thrust in this area was begun with the establishment
of the National Atmospheric Deposition Program (NADP). Though
-------
II-4
Table 1. Agricultural Data Bases (1875-1955)
PERIOD
1875 - 1895
1895 - 1915
1915 - 1935
1935 - 1955
NUMBER OF STUDIES
3
7
8
LOCATIONS
MISSOURI, KANSAS,
UTAH
OTTAWA, IOWA,
TENNESSEE, WISCONSIN,
ILLINOIS, NEW YORK,
KANSAS
KENTUCKY, OKLAHOMA
NEW YORK, ILLINOIS,
TEXAS, VIRGINIA,
TENNESSEE
ALABAMA, GEORGIA,
INDIANA, MINNESOTA,
MISSISSIPPI, TENNESSEE,
MASSACHUSETTS
SOURCE: ERIKSSON, E., 1952
-------
Table 2. Atmospheric Data Bases (1955-1975)
NETWORK
PERIOD
ATIONAL AFCRC (JONES) 1955-1956
PHS-NCAR (NASN) 1956-1966
WMO/EPA/NOAA
OCAL
SOURCE FOR ORIGINAL DATA
ILL. STATE WATER SURVEY
EPA/NOAA THROUGH 1964
1964-66 NOT AVAILABLE
CANADIAN WMO/
CANSAP
EGIONAL USGS EASTERN
1972-PRESENT WMO PUBLICATION/EPA
ARCHIVE NADP
1973-PRESENT CANSAP PUBLICATIONS
EPA ARCHIVE
1962-PRESENT USGS PUBLICATIONS
CANADIAN CENTRE 1969-PRESENT CCIW PUBLICATIONS
FOR INLAND
WATERS (CCIW)
TVA
1971-PRESENT TVA
HUBBARD BROOK 1963-PRESENT NOT AVAILABLE
WASHINGTON, DC 1975-PRESENT NOAA
ONTARIO HYDRO 1975-PRESENT EPA ARCHIVE
SUMMARIES & PUBLICATIONS
JUNGE'S PUBLICATIONS AND
BOOK
LODGE, ET AL, REPORT
LAZRUS, ET AL, UNPUBLISHED
EPA/NOAA REPORTS AND
PUBLICATIONS
ATMOSPHERIC ENVIRONMENT
(M.STILL)
PUBLICATIONS & REPORTS
(FISHER, ET AL)
PUBLICATIONS & REPORTS
(CHAN)
PUBLICATIONS & REPORTS
(PARKHURST)
SEVERAL PUBLICATIONS (LIKENS)
TECHNICAL MEMORANDUM (MILLER)
PUBLICATIONS & REPORTS
I
tn
-------
II-6
effects research gained nationwide recognition/ event collec-
tion of precipitation under the MAP3S, EPRI, and SPN networks
focused on providing data for atmospheric models though these
data could also be used for ecological studies. Plans are
now being made to coordinate networks on both a national and
international level. A summary of these networks is given
in Table 3. Reviews of precipitation chemistry activities
have been documented in Miller and Wisniewski, 1976; Niemann
et al., 1979; and Wisniewski and Kinsman, 1981.
3. Data Evaluation Criteria
Because different network objectives and monitoring
techniques, it is difficult to evaluate one set of data with
another. Networks with similar protocol such as EPRI and
MAP3S have been shown to be compatible (Pack, 1980). How
measurements of precipitation chemistry from diverse networks
can be used to show trends and areal distributions is a
difficult question. Some of the factors that must be
taken into account are the following:
Collection Methods
type of collectors, i.e., bulk or wet only
- materials used in collector
- aerodynamics of collector
- period of collection, i.e., event, daily,
weekly, monthly
reliability of collector
-------
Table 3: Ecological Data Bases (1975 - PRESENT)
NATIONAL
REGIONAL
NETWORK
NADP
MAP3S
APN
DOE/EML
EPRI
MEXICO CITY
PERIOD
1978-PRESENT
1976-PRESENT
1970-PRESENT
1977-PRESENT
1978-PRESENT
1980-PRESENT
SOURCE FOR ORIGINAL DATA
NADP REPORTS
MAP3S REPORTS
APN REPORTS
EML REPORTS
EPRI DATA
NOT AVAILABLE
SUMMARIES AND PUBLICATIONS
NADP REPORTS AND
PUBLICATIONS (GIBSON)
NUMEROUS PUBLICATIONS AND
REPORTS (HALES, ET AL.)
PUBLICATIONS (BARRIE)
EML REPORTS (FEELY)
PUBLICATIONS (PERHAC)
PRELIMINARY REPORTS
(BRAVO)
NUMEROUS REGIONAL AND LOCAL STUDIES (>60)
-------
II-8
Sitting Criteria
- purpose of site, i.e./ atmospheric or ecological
- influence of local sources
- meteorological parameters availability
- gas and aerosol measurements
Chemical analysis methods
- analysis techniques and their limitation
intercalibration
priority and analysis
Shipment and storage
time of storage and shipment
- method of shipping
storage procedures
Quality assurance
- well define program
- outside reviewers
All of the above impinge to some degree on the final
data a network produces.
4. Trends in Precipitation Chemistry Data
The obvious question that is asked in connection with the
acid rain problem is whether we can see increasing (or decreasing)
trends in precipitation acidity. Especially one would expect an
increase of acidity with the corresponding increase in emissions
of SC>2 and N02 (Figures 1 and 2). To evaluate the severity of the
problem, researchers constructed areal acidity distributions from
the historical data (Cogbill and Likens, 1979, Likens et al.,
-------
II-9
SOX EMISSIONS TRENDS
CO
c
o
O
CO
O
20
ui
x
O
CO
10
T
T
T
ALL SOURCES
RESIDENTIAL
COMMERCIAL
1940
Figure 1.
50
60
t
\ /'*-*. *'
\' --.. ^*
W ^^ ^
V *»^-^
PROCESS SOURCES
INDUSTRIAL
^UTILITIES-OIL--
UTILITIES-COAL
70
YEAR
80
90 2000
SOX Emissions Trends for the U.S.(1940-2000)
Source: U.S. EPA (1978).
-------
11-10
NOX EMISSIONS TRENDS
ALL
SOURCES
1940 50
TRANSPORTATION
60 70 80
YEAR
90 2000
Figure 2. NOX Emissions Trends for the U.S. (1940-2000).
Source: U.S. EPA (1978).
-------
11-11
1979). The periods 1955-56, and 1975-76 were compared (Figure
3). Because of the uncertainties mentioned in Section 3, the
implication of increasing acidity from the areal comparison has
been criticized. Recent analysis by Butler and Likens (personal
communication) has shown that the deposition of H ion has
increased from the sixties to the seventies (Figure 4 and 5),
at least when the two periods in the mid-sixties and mid-seventies
are compared. However, the differences in amount of precipitation
between the two periods was also significant and, of course, fig-
ures in the deposition calculations. The differences in precip-
tation amounts was, in some areas of the Northeast, over twenty
inches(500 mm) between the dry sixties and the wet seventies.
This trend cannot then be completely resolved.
Another set of data tht has been used to show either there
has been a trend (Likens) or there has not been a trend (Perhac)
is the USGC network in New York. Likens has reworked these
data (Figure 6). However, because of the low priority given
this network in the early years, the data may have some problems.
Two detailed reviews have been completed which discuss
the question of trends in the historical data (Hansen et al.,
1981; Wisniewski and Kinsman, 1981). A summary of their
findings is as follows:
o It is difficult to justify a trend in acidity from
1955 to present because of the inconsistent data sets, questions
of data validity and other factors cited in this paper.
-------
11-12
1955-56
1975-76
Figure 3. Deduced Weighted Annual Average pH of Precipitation
in the Eastern United States in 1955-1956 and 1975-
1976 (after Cogbill and Likens, 1974; Likens et al.,
1979).
-------
11-13
1965-66
Wet H* Deposition
equivalents /hectare
x wet only
• bulk
Total Deposition: 27 x to3
metric tons H*
Figure 4. Wet Hydrogen Ion Deposition (equivalents/hectare)
for 1965-1966.
-------
11-14
364
500
1975-76
Wet H* Deposition
equivalents/ hectare
x wet only
. bulk
Total Deposition = 116 x 103
metric tons H*
Figure 5. Wet Hydrogen Ion Deposition (equivalents/hectare)
for 1975-1976.
-------
Figure 6.
cr
o>
rt.
c
o
c
0)
o
c
o
o
Hydrogen Ion Concentrations at the Nine USGS Network Sites
in New York and Pennsylvania for the Period 1966 to 1980
100r Rock Hill
, (rural)
80
60
40
20
a •
e
120
100
80
60
40
\- Mays Point
(rural)
20
100- Albany
. (urban)
66 68 "70 72 74 76 78 80
o
1
80r Athens
(rural)
60'
20
0
120h Allegheney
(rural)
Upton
(residential)
140h
120
100
80
60
40
80|- Canton
(rural)
60 f
40
20
0
••
120
\- Hinckley
100
80
60
40'
20
(rural)
66 68 70 72 74 76 78 80
Time (yrs)
66 68 70 72 74 76 78 80
-------
11-16
o The areal spread of acidity west and south seems to
be fairly well substantiated from the existing data.
o Because of the uncertainty of the data set, direct
reliability between emissions and precipitation chemistry
data would not be fruitful.
o The only reliable trend data over North America are
those collected from the late seventies.
5. Conclusions
From the data available at single stations or short-term
networks over the last twenty-five years, it is difficult to
establish trends in acidity over the northeastern United States,
However, there is evidence of areal spreading to the west and
south from data from different sites and networks in the same
general area.
-------
II-R1
REFERENCES
COGBILL, C.V. and G. E. LIKENS, 1974: Acid Precipitation in
the "Northeastern United States, Wat. Resources Res. 10:1133-1137.
DOCHINGER, L.S. and T.A. SELIGA, 1976: Preceeding of the First
International Symposium on Acid Precipitation and Forest
Ecosystems: USDA Forest Service, Technical Report NE-23, 1079 pp,
ERIKSSEN, E., 1952: Composition of Atmosperhic Precipitation;
Part I and II, Tellus 4: 215-232; 280-303.
HANSEN, D.A., G.M. HIDY, and G.J. STENSLAND, 1981: Examination
of the Bias for Trend Interpretation of Historical Rain
Chemistry in Eastern United States. ERT Document No. P-A097
(Draft Report).
LIKENS, G.E., R.F. WRIGHT, J.N. GALLOWAY and T.J. BUTLER 1979:
Acid Rain. Sci. Amer., 241:43-51.
MILLER, J.M. and J. WISNIEWSKI, 1976: A Survey of Precipita-
tion Chemistry Studies - North America and Adjacent Areas.
Unpublished NOAA document.
NIEMANN, B.L., J. ROOT, N. VAN ZWALENBURG, and A.L. MAHAN, 1979:
An Integrated Monitoring Network for Acid Deposition: A Pro-
posed Strategy. Interim Report R-023-EPA-79, 236 pp.
PACK, D.H., 1980: Precipitation Chemistry Patterns: A Two-
Network Data Set. Science, 208, 1143-1145.
U.S. ENVIRONMENTAL PROTECTION AGENCY: National Air Pollution
Emission Estimates, 1940-1976. EPA-450/1-78-003, 1978.
WISNIEWSKI, J. and J.D. KINSMAN, 1981: An Overview of Acid
Rain Monitoring Activities in North America. IR&T unpublished
report, 83 pp.
-------
SECTION III
THE SEASONAL DEPENDENCE OF
ATMOSPHERIC DEPOSITION AND
CHEMICAL TRANSFORMATION RATES
FOR SULFUR AND NITROGEN COMPOUNDS
by
M.A. Lusis and L. Shenfeld
Air Research Branch
Ontario Ministry of the Environment
Toronto, Ontario M5S1Z8
-------
Ill-i
TABLE OF CONTENTS
Paqe
1. Introduction III-l
2. Atmospheric Deposition III-2
2.1 Wet Deposition II1-2
2.2 Dry Deposition 111-17
3. Atmospheric Chemical Transformations II1-28
3.1 General Considerations 111-28
3.2 Experimental Results 111-32
3.3 Summary 111-36
4. Nitrogen Oxides Deposition and Chemistry II1-42
5. Conclusions 111-51
References III-R1
-------
Hl-ii
LIST OF FIGURES
Figure 1. Average Annual Distribution of Snowfall
in North America(cm) from Findlay(1980)
Figure 2. Dependence of the Washout Ratio of SC>2
on pH and Temperature for Equilibrium
Scavenging Conditions from Barrie(1981)
Figure 3. The Percentage Frequency of Hourly Temp-
erature for North America After Rayner
(1961)
Figure 4. Deposition Velocities of S02 for June-
August
Figure 5. Deposition Velocities of S02 for December-
February
Page
111-14
111-15
111-16
111-26
111-27
-------
Ill-iii
LIST OF TABLES
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Field Measurements of Rain Scavenging
Coefficients of Particles Expected to
be Mainly in the Submicron Size Range
Deposition Velocities of Sulfur Dioxide
to Snow
Deposition Velocities for Submicron
Particles
Effects of Month and Latitude on Photo-
chemical Oxidation of Sulfur Dioxide in
Clean Air
Conversion of Sulfur Dioxide in Power
Plant and Smelter Plumes
Conversion Rates of Sulfur Dioxide in
Urban Plumes
Deposition and Chemical Transformation
Rates of Nitrogen Compounds
Summary of Deposition and Chemical Trans-
Page
111-13
111-24
111-25
111-38
111-39
111-41
111-50
111-56
formation Rates for Sulfur Compounds
-------
III-l
1. Introduction
The objective of this report is to compare wintertime
deposition and chemical transformation rates for sulfur and
nitrogen compounds with the corresponding summertime values,
in order to determine if, on the basis of current theoretical
and experimentally determined information, there are significant
differences between the summer and wintertime parameters commonly
used in mathematical long-range transport models - differences
which should be taken into account by the modelers. The objec-
tive was not to recommend values of deposition/transformation
rate parameters to be used in current models: indeed, in some
of the relevant areas (e.g. dry deposition of particulate
sulfur and nitrogen, or incloud processes) our lack of under-
standing seems to preclude any such recommendations at present.
Due to time limitations, an exhaustive and critical review of
the literature was not possible. However, it is hoped that
the most relevant scientific investigations have been included
in this report.
Chapter 2 deals with wet and dry deposition of sulfur
dioxide and sulfates, both from the viewpoint of general
theoretical consideration and the results of relevant field
and laboratory studies. Chapter 3 takes the same approach
with regard to chemical transformation rates. Since infor-
mation on nitrates is so scarce, all the relevant data on
these compounds are summarized in Chapter 4.
The conclusions of this investigation are given in Chapter
5, where some brief recommendations are made for modellers
concerning the seasonal dependence of deposition/transformation
rates, and areas needing further study are also given.
-------
III-2
2. Atmospheric Deposition
2.1 Wet Deposition
The atmospheric wet deposition of particulate matter and
gases has usually been parameterized either in terms of a
scavenging coefficient -A. or a washout ratio W (e.g. Dana, 1979)
The former results from the assumption that wet deposition is
an exponential decay process (since particles or gases are
repeatedly exposed to cloud or precipitation elements, with
some chance of collection for each exposure) obeying the
equation:
Ct = C0 exp (-At) (1)
where
C^- = atmospheric concentration at time t
Co = atmospheric concentration at time zero
7\. = scavenging coefficient (in units of time "^).
The washout ratio is defined as the concentration of
contaminant in precipitation divided by its concentration in
air (usually at the suface level), i.e.,
W = X (2)
C
where
X = concentration of contaminant in precipitation
C = concentration of contaminant in unscavenged air
W = washout ratio (dimensionless)
-------
III-3
Most of the discussion to follow will be in terms of the
scavenging coefficient, but the same considerations apply to
the washout ratio/ since A. and W can be related. For example,
for the simple case of pollutant washout from a column of air
having a uniform concentration over height h, one obtains:
-A_ = WJ (3)
h
where
J = the precipitation intensity.
2.1.1 General Considerations
When comparing wet deposition rates by rain and snow, it
is convenient to express the scavenging coefficient as follows
(Engelmann, 1968; Slinn, 1977):
-A- » I FEAdD (4)
O
where
A = the cross-sectional area of drops of diameter D
F = the flux density of drops with diameter between D
and D+dD (drops/area x time x diameter interval)
E = the collection efficiency (i.e. the fraction of
particles or gas molecules in the path of the drop
which are washed out by it). For particulates the
the collection efficiency depends not only on the
size of the droplet/snowflake, but also on that of
the particle under consideration.
The following approximations to Equation (4) have been
suggested for rain and snow scavenging (Slinn, 1981):
-------
III-4
For rain,
<* JE(Dm) (5)
A. & 1
Dm
where
ot = a constant
Dm = mean raindrop size
E(Dm) = collection efficiency of drops with diameter Dm
For snow,
— • Ds
where
= a constant
X,DS = characteristic snowflake dimensions which depend on
crystal type (see Slinn, 1981)
E(X) = collection efficiency of snowflakes having characte-
ristic dimension X •
A consideration of Equations (5) and (6) suggests a number
of factors which could lead to differences between summer and
wintertime scavenging coefficients.
(i) The scavenging coefficient depends on the precipi-
tation intensity, which can have seasonal variations at
any given location.
(ii) It is also a function of the raindrop/snowflakes
size distribution and effective scavenging area. In
this connection it should especially be noted that
compared to rain, snow has a larger surface area-to
-------
III-5
mass ratio, and thus is potentially a much more effective
scavenger of aerosol particles than is rain (Knutson and
Stockham, 1977). However, there is also a strong depen-
dence on the type of snow. Plane dendrites, for example,
are predicted to be much more effective as scavengers
than graupel (Slinn, 1977).
(iii) For aerosol scavenging, the value of E depends on
••••••••^KMHMIMHB j
the size and chemical compositon of the particles, because
several processes which depend upon these factors are
involved during within and beneath cloud scavenging (rain-
out and washout respectively), including Brownian motion,
phoretic attachment, inertial impaction and nucleation
(see, for example, Slinn, 1977; Scott, 1978). Actually,
for an atmospheric aerosol consisting of a mixture of
different sizes, the right-hand side of Equations (4) to
(6) should be integrated over all aerosol sizes, and
even relatively simple theoretical considerations indicate
a resulting strong dependence of -A. on aerosol size distri-
bution (Hales, 1978). Hence, the scavenging coefficient
for particles could have a strong seasonal variation if
the particle size distribution and chemical properities
vary due to seasonal changes in emission characteristics,
meteorological factors such as temperature and relative
humidity, or atmospheric chemical reaction mechanisms,
for example.
-------
III-6
(iv) The collection efficiency of gases in Equation (4)
depends on the solubility and chemical reactivity of the
gas in water, and a number of other factors related to
interfacial transfer between raindrop/snowflake and the
ambient air - see, for example, Hales (1978) and Barrie
(1978) for more detailed discussions for the case of
sulfur dioxide scavenging. With reference to seasonal
variations in the gas scavenging rate, snow scavenging
is usually negligible compared to rain scavenging because
of low gas absorption rates on ice crystals (Engelman,
1968; Slinn, 1981). However, the type of snow should
again be considered, since wet snow can have a much higher
gas scavenging efficiency than dry snow (Summers, 1977;
Hales et al., 1971). Also, it should be noted that for
sulfur dioxide, due to the nature of its solubility
characteristics in water, the scavenging coefficient is
expected to depend strongly on seasonal variations in
the ambient temperature and precipitation pH (Barrie,
1981).
(v) There is one more factor, which is implicit in the
above discussion, but perhaps deserves elaboration - viz.,
the importance of seasonal variations in storm type at a
given geographical location. This is especially important
with regard to incloud (rainout) processes, because the
mechanisms whereby particles and gases are removed by such
-------
III-7
processes can depend strongly on the storm type. For
example, Scott (1978) has discussed the dependence of
sulfate rainout upon the mechanism of precipitation
formation ( - it should be noted that rainout is thought
to be the dominant process responsible for particulate
sulfur removal on a regional scale - see, for example,
Scott (1978) and Garland (1978)), and has predicted
scavenging rates to differ by a factor of 10 or more
depending upon whether or not the precipitation origi-
nated as snow. Similarly, Summers (1977) has shown
striking differences in removal rates of sulfur dioxide
by summertime convective storms and snowstorms. These
results will be discussed in more detail in the following
section.
2.1.2 Experimental Results
For comparing wet deposition rates for sulfur dioxide
under winter and summer like conditions, one of the more
relevant data sets is that of Summers (1977). For the rain-
out and snowout of sulfur dioxide, he obtained the following
equations:
-/V rainout = 3 x 10~5 j sec -1 (7)
-A-(dry) snowout = 10~6 J sec -1 (8)
Taking into account representative rainfall rate data for
various types of precipitation, and using Equations (7) and
(8), Summers estimated the following range of values for -A_ :
-------
III-8
light, moderate and heavy snow -J = l()7,so-A- =1,5 and 25 respec-
tively; moderate rain or showers, heavy rain or thunderstorms
-J = 104,so-A. = 3 and 7.5 respectively. Clearly, for regions where
the wintertime precipitation largely consists of dry snow
( - as for much of Central Canada), the above results indicate
a seasonal change in the wet scavenging rate of two or three
orders of magnitude, which cannot be ignored in air quality
modeling work. It is interesting to note - see Figure 1 -
that the snowfall drops sharply south of the states of
Pennsylvania, Ohio, Indiana and Illinois.
Other areas, such as the east and west coasts of Canada
and much of the United States, receive appreciable amounts
of wet snow and cold rain during the winter months. Fortu-
nately, our understanding of the physical and chemical
processes involved in sulfur dioxide wet deposition is
sufficiently complete to allow estimates of the resulting
seasonal deposition changes. Summers (1977) has argued that,
because of the large sweepout area associated with wet snow
(which should have a collection efficiency E similar to that
of rain), its scavenging coefficient should be somewhat greater
than for rain at an equivalent rainfall rate ( - he recommends
a value approximately twice as great). On the other hand, for
sulfur dioxide removal by rain droplets, several relevant
theoretical investigations are available, which are based on
the acceptable assumption that, for purposes of regional
-------
III-9
transport modeling, the sulfur dioxide in the air is in equili-
brium with that in the rain ( - see, for examples, Hales and
Dana, 1979a; Barrie, 1981). Figure 2, for example, taken
from Barrie (1980), shows the predicted functional dependence
of the washout ratio W, for sulfur dioxide, on rainwater
pH and temperature. In going form 25 to 0°C, for example,
W increases roughly by a factor of four. It should be
noted that in much of North Eastern America precipitation
tends to be somewhat more acidic in summer than winter (e.g.,
Pack and Pack, 1979), so from Figure 2 the expected difference
between the winter and summertime values of W for rain would
be even greater than predicted on the basis of temperature
alone. For example, for a unit increase in pH and a change
in temperature from 25 to 0°C, which might be representative
of the seasonal variation at some of the stations in the
northeastern U.S. (Pack and Pack, 1979), the predicted increase
in W is roughly one order of magnitude. Limited experimental
measurements of sulfur dioxide in precipitation support the
above arguments (Hales and Dana, 1979). As a matter of
interest, Figure 3 gives the N. American percentage frequency
of hourly temperatures below 0° C in January and 10°C in
July, and above 10°C in January and 21°C in July, from which
the importance of the above temperature effect can be estimated,
-------
111-10
For the case of particulate sulfur, very few data are
available on which to base conclusions about winter-summertime
differences in wet deposition, and our understanding of the
details of rain and snow interactions with particles is not
nearly as complete as for the case of sulfur dioxide. The
existing theories are of little help if one wants to go beyond
order-of-magnitude washout rate estimates. The only work
where a direct experimental comparison of rain and snow
scavenging coefficients has been made, for particles compar-
able in size to sulfates, is that of Graedel and Franey (1975).
They found below-cloud -A- values for snow to be twenty times
or more than those for rain. However, Graedel and Franey's
interpretation of their data has been questioned by Slinn (1976).
Knutson and Stockham (1977) have developed equations for
the scavenging coefficient of snow from laboratory study
results with single snowflakes, which usually agreed with
available field data to within a factor of three. For
particles of about 0.5 microns in diameter, and a precipi-
tation intensity of 1 mm h~l, their model predicts scavenging
coefficients in the range of 0.5 - 1.0 x 10~5 sec ~1, depending
on the type of snow. These values may be compared to some
data on rain scavenging coefficients for particles of similar
size compiled by McMahon and Denison (1979). Table 1 is
abstracted from their Table 9. No attempt was made at a
critical evaluation of the methods used to arrive at the
scavenging coefficients in this Table. However, the last
-------
III-ll
value shown (for 0.3 - 0.5 micron particles) was obtained by
Graedel and Franey (1977) ( - McMahon and Denison, in their
Table 9, have incorrectly labelled Graedel and Franey's data
as being obtained from snowfalls), and if we accept Slinn's
criticism of their methodology (see above) and reject their
data point, all we can say is that, for a particle size which
might be representative of sulfates, most of the available
data suggest that below-cloud rain and snow scavenging coeffi-
cients should be about the same order-of-magnitude for com-
parable rainfall rates. The scanty relevant data on washout
ratios support the above conclusion (see McMahon and Denison,
1979) although in this case both within and below cloud
processes are involved. Scott (1981) has recently presented
data on washout ratios of sulfates in snowstorms, which fall
within about the same range of values as has been observed
for a number of other substances in rain. However, he found
that the washout ratio is strongly dependent on the micro-
physical processes of precipitation growth, W values being
10 to 50 times higher when abundant cloud water was available
(and snowflake growth was primarily through collection of
supercooled droplets) than when negligible liquid water was
detected in the clouds (and growth was primarily by vapour
deposition on snowflakes).
-------
111-12
2.1.3 Summary
To summarize the above discussion on wet deposition of
sulfur compounds: it would seem that for sulfur dioxide, the
washout coefficient for both within and below cloud processes
can show a strong seasonal dependence (order-of-magnitude or
more), even after seasonal variations in precipitation rate
are taken into account, because of temperature and pH effects
on the solubility, and the low uptake of the gas by ice
crystals.
The available data on particulate sulfur wet scavenging
are too meager to allow any definite conclusions about seasonal
difference. All one can say is that below cloud scavenging
coefficients for rain and snow seem to be of the same order-
of-magnitude. However, rates of incloud processes depend
strongly on the specific mechanisms involved (Scott, 1978;
1981), and seasonal variations in storm type could result
in corresponding changes in scavenging rates of an order-of-
magnitude or more, but more data are needed to substantiate
or disprove this speculation.
-------
111-13
Table 1. Field Measurements of Rain Scavenging Coefficients
of Particles Expected to be Mainly in the Submicron
Size Range.
Source of Data
Kalkstein et al. (1959)
Makhonko (1964)
Bakulin et al. (1970)
Burtsev et al. (1970)
Greadel and Franey (1977)
2
1
3
15 J 0<5
18
Comments
804 washout
Fission products washout
Pb washout from thunderstorm
0.2 urn particle washout
0.3-0.5 urn particle washout
-------
111-14
80
280
Figure 1. Average Annual Distribution of Snowfall in
North America (cm) from Findlay (1980).
-------
111-15
10
PH
Figure 2. Dependence ot the Washout Ratio of SO2 on pH and
Temperature for Equilibrium Scavenging Conditions
from Barrie (1981).
-------
111-16
Figure 3. The Percentage Frequency of Hourly Temperature for
North America ( A-January, less than 0°C; B-January,
greater than 10°C; C-July, less than 10°C; D-July,
greater than 21°C) After Rayner (1961).
-------
111-17
2.2 Dry Deposition
The dry deposition rate of gases and particles to the
earth's surface has usually been parametrized using a depos-
tion velocity v, defined by the equation
v = Me"1
where M = the flux of material (9)
C = the ambient concentration at a particular
height
and v (which is a function of height) refers to the same
level as the concentration measurement.
2.2.1 General Consideration
To better understand the role of different factors which
affect the deposition velocity, it is convenient to work in
terms of resistances to transport. .The total resistance rt,
defined as the reciprocal of v,
rt = v"1 (10)
can be broken down into an aerodynamic resistance ra and a
surface resistance rs:
rt = ra + rs (11)
The surface resistance can be further split into a resistance
due to a (usually thin) stagnant air film adjacent to the
absorbing surface, r^,, and an actually resistance to transport
within the surface (for plants, this is largely the canopy
stomatal resistance), rc (Wesely and Hicks, 1977; Chamberlain,
1979):
rs = rb + rc (12)
-------
111-18
The magnitude of the aerodynamic resistance ra is largely
governed by atmospheric turbulence in the boundary layer,
which in turn depends on factors such as wind speed, atmos-
pheric stability, and characteristics of the surface (i.e.
the surface roughness). There are well-known micrometeoro-
logical formulate relating ra to meteorological and surface
parameters (see e.g., Garland, 1978; Wesely and Hicks, 1977).
The value of ra decreases as the wind speed and surface
roughness increase, and also depends to a certain extent on
the atmospheric stability, although the latter dependence
only seems to become marked when the atmosphere is very stable -
e.g., for Pasquill stability category F (Sheih et al., 1979).
Because of the dominant influence of turbulent diffusion, ra
should be the same for both particles and gases, except where
the particles are so large that gravitational settling plays
an important role in the deposition process (e.g., Sehmel,
1980). In view of the above considerations, seasonal varia-
tions in meteorology and surface characteristics will obviously
change the aerodynamic resistance to transport of both particles
and gases.
The stagnant film resistance, r^, also depends to a cer-
tain extent on the same factors as ra, since surface drag forces
and the intensity of atmospheric turbulence will determine the
average thickness of the film and also the intrusion of turbulent
eddies into the film. However, because of the importance within
-------
111-19
the film of molecular diffusion for gases/ and Brownian
diffusion and impaction for aerosols, r^ is also strongly
dependent on the characteristics of the material under con-
sideration - in particular, the diffusivity of gases and the
particle size distrubtion of aerosols (see, for example,
Wesely and Hicks, 1977: Sehmel, 1980). As with ra, it can
be seen that seasonal variations in meteorological and
surface factors can lead to corresponding variations in rfc>.
In addition, if for particles there are appreciable seasonal
changes in the size distribution, potentially large changes
in rb could result (e.g. Sehmel, 1980; Ibrahim et al., 1980).
As implied by its definition, for a given substance rc
depends only on the characteristics of the surface. For
particles, probably for lack of reliable information to the
contrary, it has sometimes been assumed that rc is equal
to zero (e.g. Ibrahim et al., 1980; Sievering, 1979 a). For
gases, rc depends on the properties of the gas ( - mainly
the solubility in water (Hill, 1971) and molecular diffusi-
vity (Wesely and Hicks, 1977)) and the absorbing surface.
For surfaces covered by vegetation, rc depends on the nature,
and state of growth and condition, of the plant in question,
the latter being influenced by the water availability, solar
radiation level, and air temperature, among other things.
Obviously, seasonal changes in the surface coverage will
lead to corresponding changes in rc.
-------
111-20
For a more detailed discussion of the resistances which
determine the overall magnitude of the deposition velocity,
and the factors affecting them, see Wesley and Hicks (1977),
Shieh et al. (1979), Chamberlain (1979) and Sehmel (1980).
2.2.2 Experimental Results
Several authors have recently presented compilations of
experimental data on the deposition velocities of gases and
particulates, which are of value in assessing the effects of
seasonal variation on v (see Garland, 1978; McMahon and
Denison, 1979; Chamberlain, 1979; Sehmel, 1980).
For the case of sulfur dioxide, a reasonably consistent
picture emerges. For a wide range of surfaces, including
vegetation, water, and soils, the deposition velocity has
been found to be typically in the range 0.4 to 0.8 cm s~l.
Garland (1978) rationalizes the above result by pointing
out that in general, there is a counter-balance between the
aerodynamic and surface resistance terms (ra and rs in
Equation (11)). For relatively smooth surfaces, rs has
been found to be rather small, and uptake is largely deter-
mined by ra. On the other hand, surfaces with taller
vegetation, such as wheat and forest, exhibit a higher rs,
which counteracts the reduced ra. Of course, there are
exceptions to the above generalizations: for example, much
higher v values than indicated above may apply to wet or
snow covered forests (Chamberlain, 1979). However, it
would seem that such exceptions have only marginal relevance
-------
111-21
when seasonal variations are being considered (unless, for
example, the percentage of the time that a forest is wet
changes significantly, due to seasonal changes in precipi-
tation) .
It is interesting to compare measured deposition velo-
cities to snow with the 0.4 to 0.8 cm s~l range in v noted
above, which is assumed to be representative of surfaces for
most of the year other than winter. Table 2 lists the
available data for sulfur dioxide deposition to snow surfaces.
The values measured generally tend to be lower than 0.4,
although a "representative" v of 0.1 cm s"1 for snow, as
suggested by Husar et al. (1978b), would seem to be too low,
except for very stable atmospheric conditions.
The above results suggest that seasonal variations in
sulfur doxide dry deposition velocities should be relatively
modest - certainly less than the order-of-magnitude changes
expected for wet deposition. It seems that for the winter-
time, at least in Canada and the northern United States, dry
deposition velocities of sulfur dioxide should be somewhat
lower than in the summer, due to the larger proportion of the
surface covered by snow and dead vegetation. In this connec-
tion, it is interesting to compare some estimates of v for
the summer (June - August) and winter (December - February)
months in Ontario, made using the methods of Shieh et al. (1979).
Surfaces characterized according to their classification,
-------
111-22
and values of surface roughnesses and resistances of Pasquill
stability classes at 22 stations in Ontario was provided by
the Atmospheric Environment Service. Calculated seasonal
average deposition velocities are shown in Figure 4 and 5.
A comparison of these figures supports the expectation that,
for sulfur dioxide, seasonal changes in v should not be large.
The picture for sulfate particulates is much more con-
fusing, because of the current controversy regarding depos-
tion velocities of particles in the 0.1 - 1.0 micron size
range (where most of the sulfate mass is expected to occur).
Theoretical predictions (see, for example, Sehmel and Hodgson,
1978; Ibrahim et al., 1980) indicate that, for this size
range, (ra + r^) should be generally larger than 10 s cm~l
(i.e., "v should be no greater than 0.1 cm s'1). Available
experimental data on deposition velocities of sulfates, or
other substances such as lead (which are expected to be in
a similar size range) are shown in Table 3. It would seen
that for relatively smooth surfaces, such as snow, water, or
filter paper, the deposition velocity is about 0.1 - 0.2 cm s~
or in some cases so small that the removal of particles is
countered by surface emissions (i.e., the effective deposition
velocity is negative). For rougher surfaces and vegetation
there is little data, but values ranging from less than 0.1
to about 1.0 cm s~^ have been reported, including a number of
negative values. For such surfaces, Sheih et al. (1979) have
-------
111-23
proposed sulfate deposition velocities comparable to those
for sulfur dioxide. However, at present, because of the
large uncertainty in v for particulate sulfate, little can
be said about seasonal variations in this parameter.
2.2.3 Summary
To summarize: there are several theoretical reasons
why dry deposition velocities for both gaseous and parti-
culate sulfur could have a significant seasonal variation.
However, for sulfur dioxide, this variation is probably
rather small, as far as long-range transport modeling is
concerned. Summertime values have been typically observed
to be in the range 0.4 to 0.8 cm s~l. Wintertime observa-
tions (Table 2) indicate a "representative" range of 0.1 to
0.4 cm s~l. The few available data suggest that this latter
range is also representative of the deposition of particulate
sulfates to smooth surfaces. However, for rougher surfaces
and vegetation, the picture is so confusing at present that
little can be said concerning seasonal variations in the
particulate dry deposition velocity.
-------
111-24
Table 2. Deposition Velocities of Sulfur Dioxide to Snow
Reference
v cm s
-1
Comments
Whelpdale and
Shaw (1974)
Garland (1976)
Dovland and
Eliasen (1976)
Barrie and
Walmsley (1978)
1.6
0.52
0.05
0.10 - 0.17
0.1
0.25 + 0.20
0.3 - 0.4
Lapse
Neutral
Stable
Ice-laboratory
experiments; rs
5 s cm~l
Low wind speed/
stable atmosphere;
rg 5 s cm'1
(Garland/ 1976)
Determined from
simulaneous depo-
sition and concen-
tration measure-
ments during a
pollution episode
Determined from
sulfur accumulation
by snow and ambient
concentrations calcu-
lated with a disper-
sion model.
-------
111-25
Table 3. Deposition Velocities for Submicron Particles
Reference
Cawse (1976)
Dovland and
Eliasen (1976)
Garland (1978)
Droppo (1979)
Sievering (1979)
v cm s
-1
0.2 - <0.7
- <0.6
0.16
0.1, 0.27
Negative
(surface source)
<0.5
Sievering et al. 0.2 +_ 0.16
(1979)
Wesley and
Hicks (1979)
Everett et al.
(1979)
Ibrahim et al.
(1980)
Negative
(surface source)
Negative
1.4
0.1 - 0.2
Comments
Vanadium on filter paper
at 1.5m.
Arsenic on filter paper
Lead on snow, stable
atmosphere
Atmospheric sulfate over
grass
Sulfates over arid vege-
tation
Sulfates, daytime, non-
arid vegetation
Estimated annual average
over Lake Michigan, sub-
micron atmospheric aerosols
Atmospheric sulfate over Lake
Michigan, stable atmosphere
Submicron particles to grass
Night-time, pine plantation,
senescent maize
Snow, and cold, wet, bare soil
Atmospheric particulate sulfur
over slightly rolling grassland
Bimodal ammonium sulfate aerosol
over snow. (These results are
complicated by the fact that,
although over 90% of the total
mass was in the 0.7 micron mode,
most of the material actually
deposited came from the 10
micron mode).
-------
111-26
0.41 - 0.6
0.61 - 0.8
0.81 and up
scale
25 0 50 100
i • • ' ^ miles
Figure 4. Deposition Velocities of S02 for June-August,
-------
111-27
0.2 - 0.4
0.41 - 0.6
0.61 - 0.8
0.81 and up
25 0 50 100
scale
mi 1 es
Figure 5. Deposition Velocities of SC>2 for December-February.
-------
111-28
3. Atmospheric Chemical Transformations
3.1 General Considerations
Both laboratory studies and computer simulation of
atmospheric chemical reactions have shown that gas-phase
homogeneous, as well as heterogeneous, reactions are poten-
tially important for oxidation of sulfur dioxide to sulfates
in polluted air - see, for example, Calvert et al. (1978)
and Beilke and Gravenhurst (1978).
Homogeneous reactions can lead to conversion rates as
high as 4% h~l in the summertime, largely due to attack of
sulfur dioxide by hydroxyl, hydroperoxyl and alkylperoxyl
radicals (Calvert et al., 1978). The seasonal dependance
of homogeneous reactions has been discussed by a number of
authors, and is reasonably well understood. Thus Altshuller
(1979), on the basis of computer modeling studies, found
that while seasonal effects are small at lower latitudes, at
middle and higher latitudes the ratio of summer-winter noon-
time and average diurnal photochemical rates can be large:
for example, at 35°N, summer noontime and averge diurnal
rates are 4-5 times greater than the corresponding winter
values. At 55°N, the summertime values are 16-20 times
greater. In fact, Altshuller finds that during the winter
months, homogeneous oxidation reactions cannot account for
any significant conversion of SC>2 to sulfate at latitudes
greater than 45°N. Bottenheim et al. (1977), in their study
-------
111-29
of seasonal effects on photochemical air pollution at 60°N/
have come to similar conclusions. These results are largely
due to changes in available ultraviolet solar intensity
(Nieboer et al., 1976; Bottenheim et al., 1977; Altshuller,
1979), although temperature effects may also play a role
(Carter et al., 1979). Table 1, taken from Altshuller (1979),
illustrates the expected effects of season and latitude on
noontime sulfur dioxide homogeneous oxidation rates in clean
air. In polluted air, the corresponding rates could be a
factor-of-two or so higher (Calvert et al., 1978; Altshuller,
1979).
Heterogeneous sulfur dioxide oxidation processes are less
well understood than their homogeneous counterparts. Oxidation
can take place in droplets, liquid films surrounding particles
at high relative humidity, or on dry particle surfaces. At
present, the most important heterogeneous reactions which take
place in the aqueous phase are thought to be (Beilke and
Gravenhurst, 1978): (1) SC>2 oxidation by 02 in the presence
of catalysts (e.g., dissolved transition metals); and (2) S02
oxidation by strongly oxidizing agents (e.g., ozone and hydrogen
peroxide). The reaction with hydrogen peroxide in particular
has recently excited the interest of the scientific community,
due to its potential atmospheric importance (- see, for example,
Penkett et al., 1979; Dasgupta, 1980). To date, there has
been no clear demonstration that S02 - dry particle reactions
-------
111-30
are of atmospheric significance, except possibly in smoke-
stack plumes close to the chimney (where concentrations are
relatively high). For example, Britton and Clark (1980)
have recently reviewed past work on heterogeneous S02 reac-
tions on dry particles, and have presented the results of
some of their own studies with soot and S02/N02 mixtures.
In their conclusions, they state: "From the point of view
of the overall SC>2 oxidation process in the atmosphere the
heterogeneous reactions on soot or other particles play a
negligible part compared to either photochemical or droplet
phase mechanisms...". Therefore, such processes will not be
considered further here.
It was noted above that, for homogeneous processes,
large seasonal changes may be theoretically expected in the
oxidation rate, depending on the latitude. The picture is
not as clear for heterogeneous reactions. It has been
theorized (e.g., Foster, 1969; Freiberg, 1974) that the
oxidation rate involving trace metal catalysts should be
increased at high ambient ammonia concentrations and relative
humidities, both of these factors leading to lower solution
acidity and hence higher sulfite ion concentrations ( - sulfite
ions are thought to be the species actually oxidized to sulfate
by dissolved 02). Lower temperatures may also promote oxidation
by hydrogen peroxide or trace metal catalysts (Penkett et al.,
1979; Freiberg, 1974). Thus, seasonal variations in all these
-------
111-31
factors - e.g., lower wintertime temperatures and ambient
ammonia concentrations - could potentially have an effect on
the conversion rate. These effects are difficult to predict.
For example, Middleton et al. (1980) have carried out a
modeling study into the relative importance of various urban
sulfate aerosol production mechanisms, including photochemical
reactions followed by sulfuric acid vapor condensation, and
catalytic as well as non-catalytic heterogeneous oxidation.
They conclude that the dominant mechanism for sulfate produc-
tion can depend on the season and time of day. During the
daytime in summer, photochemical reactions/sulfuric acid
vapor condensation are the most important, followed by liquid
phase oxidation by hydrogen peroxide. At night, however,
catalytic and non-catalytic oxidation mechanisms on the
wetted aerosol surface become important. In the winter,
most of the daytime S02 oxidation is predicted to take place
by reactions with hydrogen peroxide in droplets, at overall
rates comparable to those in the summer. However, the above
conclusions are based on a large number of assumptions about
reaction mechanisms and model input conditions, and should
not be regarded as final. For a better assessment of winter/
summertime difference in atmospheric chemical transformation
rates, the results of actual field studies were consulted
(see below).
-------
111-32
3.2 Experimental Results
A large number of field investigations have been carried
out into the oxidation of sulfur dioxide in urban, industrial
and power plant plumes. Since, on the basis of our present
understanding of atmospheric chemistry, there is a fundamental
difference between urban plumes and those from strong point
sources ( - at least, before the latter have been well-mixed
into the "background" air), the two will be considered sepa-
rately. Also, in view of the suggested importance of latitude
when considering winter and summertime difference in photo-
chemical processes (see above), the geographic location of
each experiment will be noted whenever possible in the
following discussion.
Newman (1980) has recently summarized the available
results on atmospheric oxidation of S02 from power plant and
smelter plume studies. Table 5 is largely based on the most
relevant references taken from his paper. In preparing this
table, we have concurred with Newman in that some of the early,
and often-cited, studies of plume oxidation, such as that by
Gartrell et al. (1963) and Stephens and McCaldin (1971), are
probably flawed, and the results are unreliable. Rather we
have emphasized those particular more recent studies which
have a direct bearing on seasonal effects on the plume
oxidation rate.
-------
111-33
An inspection of Table 5, and the references cited therein,
leads to the following general conclusions about the sulfur
dioxide oxidation rate in power plant and smelter plumes:
1. The "dry" oxidation rate is usually no more than a
few percent per hour.
2. Although there is still a debate about the relative
importance of homogeneous gas-phase reactions/ and
heterogeneous processes involving catalytic plume
aerosols, most of the evidence suggests the former
are important, and can lead to S02 transformation
rates of up to about 4% h~l. Heterogeneous reac-
tions may dominate near the point of emission where
the particle concentration is high, or under high
humidity conditions.
3. Under conditions where homogeneous photochemical
reactions are not expected to be significant (e.g.,
during the night or early morning, or during the
winter at far northern locations), conversion rates
have been observed to be small - generally, well
below 1% h"1.
4. Temperature or relative humidity may or may not have
an effect on the conversion rate (compare Lusis et
al., Forrest et al., Chan et al., Garber et al.,
with Eatough et al., Dittenhoefer and DePena in Table
5). However, the degree of plume dilution due to
atmospheric mixing is certainly important (Wilson and
Gillani, 1980).
-------
111-34
It must be emphasized that the above summary represents
a considerable simplification of what happens during atmospheric
S02 oxidation in chimney plumes, and a more rigorous treatment
would require separate consideration of each point source,
including the composition of the emissions as well as that of
the background air into which the plume is mixing.
Before drawing any conclusions on the seasonal dependence
of the SO2 transformation rate from the above data, consider
the corresponding data obtained from studies of urban plumes.
Although urban plume data are more relevant to long-range
transport modeling than chimney plume data, which generally
have represented conditions within the first hour or two of
emission, they are also more difficult to interpret. To extract
the oxidation rate from the data, assumptions have to be made
about other atmospheric processes such as air parcel dilution
and dry deposition (see, for example, Alkezweeny and Powell,
1977). This should be kept in mind when evaluating the urban
plume results.
Alkezweeny (1980) has summarized the available data on
S02 oxidation in urban plumes, and the results in Table 6 are
largely taken from his paper. An inspection of this table
suggests that:
1. "Dry" oxidation rates in urban plumes are somewhat
higher than those in chimney plumes ( - at least,
before the latter have been mixed into the background
air), possibly by a factor-of-two or more.
-------
111-35
2. Although homogeneous photochemical processes are
important (Alkezweeny/ 1980), heterogeneous
mechanisms may play a greater role in urban than in
power plant and smelter plumes (close to the source)
Thus, whereas power plant and smelter plume data
suggested very low oxidation rates under conditions
where homogeneous processes are not expected to be
significant, the urban plume data in Table 6 indi-
cate that quite high oxidation rates have been
observed in the wintertime at latitudes greater
than 45°N { - see, for example, the results of
Benarie et al., Elshout et al., Prahm et al., and
Smith and Jeffrey). Alkezweeny (1980) also has
noted in his own data that high oxidation rates
seem to be associated with high aerosol loadings.
3. There is some indication that temperature may have
an effect on the conversion rate, although, as
with chimney plume studies, the information is
conflicting (compare Benaire et al. with Meszaros
et al.). There is also confusion on the correla-
tion of conversion rate and relative humidity
(compare Benaire et al. with Smith and Jeffrey).
-------
111-36
3.3 Summary
The above experimental results make it difficult to draw
any firm conclusions on the seasonal variation of SC>2 oxidation
rate. Perhaps this much can be said: during the summer months,
photochemical processes are important, and both point source
and urban plume data indicate daytime "dry" rates of several
percent per hour (a summertime diurnal average of 1-2% h~l
might be suitable for long-range transport modeling, although
some of the urban plume data suggest rates several times
greater than this). In the winter, it is not clear what one
should expect. The chimney plume data, most of which may be
of questionable relevance to long-range transport modeling
because estimates were determined at short distances from
the chimneys, indicate oxidation rates generally considerably
less than 0.5% h~l at latitudes where photochemical reactions
should not be contributing appreciably. On the other hand,
the limited urban plume data suggest a greater role of hetero-
geneous processes, and therefore appreciable oxidation
rates, but at the same time they have been derived with more
assumptions about the history of the air parcel, and are
therefore more open to criticism ( - see, for example, Forrest
et al., 1979b). It must be concluded that at present the
available data is too conflicting to draw any firm conclusions
about the seasonal dependence of the S02 oxidation rate.
-------
111-37
There is one more factor which makes any conclusions
difficult at the present time - viz., the possibly important
role of clouds (both precipitating and fair weather) in con-
verting S02 to sulfates. Recently, there has been a consider-
able amount of speculation that in-cloud processes contribute
significantly to S02 oxidation (see, for example, Hegg and
Hobbs, 1978; Hales and Dana, 1979b; Newman, 1979; McNaughton
and Scott, 1980; Scott, 1980). If this can be proven experi-
mentally (work is currently in progress by several groups of
researchers), then expected seasonal variations in S02 trans-
formation could increase considerably, especially in areas
where clouds consist largely of dry ice crystals in the winter
(and hence have very low chemical transformation potential,
as compared to summertime conditions).
-------
111-38
Table 4. Effects of Month and Latitude on Photochemical
Homogeneous Oxidation of Sulfur Dioxide in Clean
Air (Noontime rates in % h"1 from Altshuller,1979)
Latitude (°N]
Month
January
February
March
April
May
June
July
August
September
October
November
December
_5
1.81
1.81
2.06
2.01
1.82
1.81
1.78
1.86
1.91
1.79
1.56
1.58
L5
1.10
1.21
1.55
0.79
1.68
1.72
1.80
1.78
1.68
1.54
1.33
1.16
2!
0.61
0.74
1.01
1.36
1.48
1.56
1.58
1.61
1.50
1.29
0.98
0.74
11
0.26
0.32
0.48
0.73
1.01
1.22
1.30
1.31
1.15
0.84
0.48
0.33
1
11
0.08
0.13
0.21
0.35
0.58
0.74
0.88
0.82
0.76
0.39
0.19
0.11
^5
0.01
0.04
0.09
0.17
0.30
0.42
0.52
0.45
0.34
0.17
0.06
0.02
11
0.01
0.01
0.03
0.07
0.16
0.25
0.31
0.26
0.17
0.06
0.02
0.01
-------
Table 5. Conversion of SO2 in Power Plant and Smelter Plumes
Source
Forrest and
Newman (1977)
SO2 Oxidation Rate
(% h"1)
Husar et al.
(1978a)
Lusis et al.
(1978)
Dittenhoefer
and de Pena
(1979)
Forrest et al,
(1979a)
Forrest et al.
(1980)
1-4 (noontime)
<0.5 (night)
1-3 (June/ noon
and p.m.)
<0.5(winter, or
summer early a.m.)
0 «65% RH)
1 (65 - 90%
2-6 (90% RH)
<2
RH)
0.1 - 0.8 (night,
early a.m.)
1-4 (late a.m.
and afternoon)
Comments
-four coal-fired power plants
(30° - 40°N)
-no correlation could be found
between conversion and tempera-
ture (10 - 25°C), humidity or
time of day.
-St. Louis (38°N)
-power plant
-photochemistry may be the
dominate mechanism.
-Fort McMurray (57°N)
-power plant
-evidence of photochemical
activity during relative high
conversion rates
-temperature varied from -13
to 23°C.
-Pennsylvania (41°N)
-power plant
-evidence that both gas phase
and aqueous phase oxidation
are important.
-Tarpon Springs, Florida (28°N)
-oil-fired power plant
-no correlation was found
between individual meteoro-
logical parameters and extent
of oxidation, although higher
conversions were observed in
August than in February.
-Cumberland coal-fired power
plant.(35°N) . fc , ...
-reactions were correlated with
solar radiation.
i
Ul
vo
-------
Table 5 (continued)
Source
SC>2 Oxidation Rate
(% h-1)
Comments
Garber et al.
(1980)
<1
Hegg and Hobbs
(1980)
0. - 5.7
Gillani et al.
(1980)
Chan et al.
(1980)
Eatough et al,
(1980)
rate = 0.03 R.H.O3
R = solar radition
H = mixing height
03 = background ozone
<0.5
<0.5 to 6
-Northport oil-fired power plant
-a wide range of meteorological
conditions were examined. The
data suggest a weak positive
correlation of conversion rate
with temperature, water partial
pressure and insolation.
-five coal-fired power plants,
W. and Midwest U.S.A.
-various times of year
-evidence of photochemical
reactions. Conversion depended
on u.v. light intensity.
-plumes from Labadie, Cumberland
and Johnsonville power plants
-for dry conditions only.
-Sudbury smelter plume (47°N)
-no correlation of rate with
temperature, relative humidity.
-Western U.S. smelter and power
plant plumes
•positive temperature dependence
of oxidation rate; data are
consistent with a homogeneous
mechanism.
*>.
o
-------
111-41
Table 6: Conversion Rates of S02 in Urban Plumes
Source
Benarie et al.
(1972)
S02 Oxidation Rate
(% h"1)
6 to 25
Robert and
Friedlander
(1975)
Mezaros et al.
(1977)
Alkezweeny and
Powell (1977)
Alkezweeny
(1978)
•eeding et al
(1976)
Alkezweeny
(1980)
Forrest et al.
(1979b)
Chang (1979)
Alkezweeny
(1980)
Elshout et al.
(1978)
Prahm et al.
(1976)
Smith and
Jeffrey (1975)
ivery et al.
(1979)
1.2 to 13
30
10 to 14
8 to 12
5 to 32
1 to 5
0, 6.8
0.6 to 4.4
1.1
0 to 3
0.7
Comments
-Rouen (49°N)
•wintertime results
-aerosol catalysis important;
acid formation increases as
temperature decreases.
•Los Angeles (34°N)
-July, October.
-Budapest (48°N)
-mainly June to September.
-St. Louis (38°N)
-August.
-St. Louis (38°N)
•summertime
-St. Louis (38°N)
-October, April
-Chicago, Gary (42°N)
-summertime
-conversion dominated by photo-
chemical reactions.
-St. Louis (38°N)
-Flight Da Vinci II, June.
-Milwaukee (44°N)
-measurements on two days in
August.
-Arnheim, Amsterdam (52°N)
-January to March.
-Faroe Island and British Isles
(50 to 60°N)
-February/
-British Isles (52 to 56°N)
-mainly September to November
-relative humidity is important,
-Ohio Valley (40°N)
-August.
-------
111-42
4. Nitrogen Oxides Deposition and Chemistry
The atmospheric chemistry and deposition of sulfur oxides
has in the past received considerably more attention than that
of nitrogen oxides. However, the recognition that the latter
are important precursors of acid deposition, and that emissions
of nitrogen oxides will grow considerably more than those of
sulfur oxides in the coming two decades (U.S.-Canada Research
Consultation Group on LRTAP, 1979), has recently led to a
substantial increase in the level of research devoted to the
atmospheric nitrogen compounds. Nevertheless, at present
relatively little has been published on deposition and chemical
transformation rates of nitrogen oxides. Most of the infor-
mation we are aware of is summarized in the present chapter.
Since many of the general physical and chemical conside-
rations regarding deposition and transformation (outlined in
Sections 2.1.1, 2.2.1 and 3.1) also apply to nitrogen oxides,
some speculation is possible in the absence of field data
regarding their transformation and deposition rates and the
seasonal dependence of these rates, but this has been kept to
a minimum in the discussion below. The situation is somewhat
more complicated than for the sulfur oxides, because the
behavior of more species must be considered. Thus, although
anthropogenic emissions are primarily in the form of nitric
oxide (NO), this compound is converted relatively rapidly to
-------
111-43
nitrogen dioxide (N02), nitric acid (HN03) and particulate
nitrates, and peroxyacetyl nitrate (PAN) ( - see, e.g. Spicer,
et al., 1980). The chemical reactions and atmospheric deposi-
tion of each of these species will be considered separately
below.
Nitric oxide has a very low solubility in water (Sher-
wood and Pigford, 1952). Since the rate of wet deposition,
especially unde the "equilibrium scavenging" conditions of
relevance to long-range transport (Hales, 1978), is directly
dependent on the solubility of the gas under consideration,
the wet scavenging coefficient of NO is also expected to be
low. As far as dry deposition is concerned, a number of
investigators, working with different types of surfaces (see
Table 7), have found the deposition velocity of NO to be very
small - about one-tenth that for S02 over the same surface.
Moreover, NO is oxidized to products fairly rapidly (in the
order of a few hours) in the summertime, so the short lifetime
and low wet and dry scavenging rates of this gas suggest that
its contribution to deposition is small. Under wintertime
conditions, at latitudes where photochemical reactions are of
relatively little importance (see Section 3.1), the atmospheric
lifetime of NO could be increased considerably, and the amount
deposited by wet and dry removal processes could become impor-
tant. However, no information is available on scavenging
coefficients and deposition velocities of NO under these con-
ditions, especially to snow surfaces.
-------
111-44
Nitrogen dioxide is formed in the atmosphere mainly from
the oxidation of NO by ozone and various free radicals (see,
for example, Bottenheim et al., 1977; Calvert et al., 1978;
Altshuller, 1979). It dissolves in water according to a
fairly complex chemical mechanism (Sherwood and Pigford, 1952),
the overall reaction be
2N02 + H20 ^—'** HNO3 + HN02.
As can be seen from the above reaction, the solubility of to02
will, like that for S02, depend on the acidity of the rain or
cloud droplet, and laboratory experiments (Beilke, 1970) have
shown the scavenging coefficient of N02 to be roughly one-
quarter that for S02. Experiments on dry deposition rates of
N02 to a number of surfaces have shown the deposition velocity
of N02 to be roughly one-half that for SO2 under comparable
conditions (see Table 7). No wet and dry deposition data for
NO2 under wintertime conditions are available, especially to
snow, but it seems reasonable that qualitatively the same
differences in winter and summertime deposition behavior will
obtain as for S02.
As far as atmospheric chemistry is concerned, N02 is
formed so quickly from NO under summertime conditions that,
in mathematical modeling of long-range transport (where time
steps of the order of several hours are involved in the
calculations), as a first approximation emissions can probably
-------
111-45
be assumed to consist of N02 (i.e./ the role of NO as an
independent precursor species of nitrates and PAN can be
neglected). This may not be true under nightime conditions,
or in the winter at northerly latitudes, but the scope of
this report does not allow for more detailed speculation on
this point.
Nitric acid is now thought to be one of the major
products of NO emissions (e.g., see Spicer, 1979; Spicer
et al., 1980). Under typical atmospheric conditions, it
exists mainly in the vapor form. However, it is a highly
soluble vapor (e.g., see Okita and Ohta, 1979), and poten-
tially has a higher scavenging coefficient than that of S02
(no experimental data are available to verify this). There
are also reports of unpublished experimental results which
suggest that, unlike 803, HN03 is scavenged very efficiently
by snow. Thus, even though no experimental data are avail-
able on wet removal rates of HNO3, one can speculate that
the seasonal dependence of j\. for this substance is probably
smaller than that for S02 (and its overall magnitude is larger)
Similarly, because of the apparently high reactivity of HN03
with all types of surfaces, it would seem reasonable to assume
that the total resistance to mass transport is approximately
equal to the aerodynamic resistance (i.e. rs « ra in Equation
(11)), and to use values calulated from micro-meteorological
theories (see, for example, Table 1 in Garland, 1978). This
-------
111-46
leads us to expect fairly small seasonal variations of the
deposition velocity for HN03, probably smaller than those for
S02.
Nitric acid can be formed by homogeneous reaction path-
ways, mainly by reaction with hydroxyl radicals/ or in the
droplet phase (Orel and Seinfeld, 1977; Middleton and Kiang,
1979). Homogeneous processes are relatively well understood
compared to the heterogeneious ones, and are probably more
important during the daytime in summer, and display a similar
seasonal dependence to those for S02 oxidation (see Section 3.1
and also Bottenheim and Strausz, 1980). Nitrogen oxides trans-
formation rates of over 20% h"1 have been observed in urban
plumes, most likely due to homogeneous reactions (Spicer, 1979),
and power plant plume studies have also yielded nitrate forma-
tion rates several-fold those of sulfates (e.g., Forrest et al.,
1980) which are consistent with a homogeneous reaction mechanism.
Heterogeneous processes may be an important source of nitrates
at night and in the winter (Middleton and Kiang, 1979), but too
little reliable information is available on the chemistry
involved to speculate on seasonal trends in the HN03 formation
rate.
Particulate nitrates are probably formed by heterogeneous
processes, and consist of a number of salts, with ammonium
nitrate being one of the most important (Orel and Seinfeld,
1977; Middleton and Kiang, 1979). During the summertime,
-------
111-47
particulate nitrate concentrations are apparently relatively
small compared to those of vapor phase nitric acid (e.g., see
Spicer, 1979; Spicer et al., 1980; see also, Tang, 1980).
During the winter, especially at northern latitudes, the
reverse could be true, but due to difficulties involved in
sampling HN03 and particulate nitrates separately, insufficient
data are available to answer this question. Little can be
said about the wet and dry deposition of nitrates, and its
seasonal dependence. In the absence of experimental data,
one might postulate a behavior similar to that of sulfates.
As far as chemical formation rates are concerned, some theo-
retical estimates suggest that wintertime rates for particu-
late nitrate may be considerably greater than those in the
summer (Middleton and Kiang, 1979), but again, this result
requires experimental verification.
The other major nitrate product to be considered is
peroxyacetyl nitrate. This substance is stated by Hill (1971)
as being "relatively insoluble", and Garland and Penkett
(1976) found negligible PAN uptake rates by water in a wind
tunnel. This suggests a relatively low value of the scavenging
coefficient. The dependence of PAN solubility on the tempera-
ture, and the nature of its interactions with snow, are
unknown; therefore, we will not speculate on the seasonal
dependence of its wet scavenging rate. Some data are available
on the dry deposition rate of PAN over a number of surfaces
-------
111-48
(Table 7), and/ with the exception of water/ the PAN deposi-
tion velocity seems to be about one-third that for 803• Again
due to lack of supporting data/ speculation on the seasonal
dependence of the PAN dry deposition rate is not possible.
As for as chemical formation rates are concerned/ PAN is
the product of photochemical reactions involving peroxyacetyl
radicals and N02:
CH3CO(02) + N02^H^ PAN.
The proportion of PAN and HN03 appearing in the irradiated/
polluted air mixture depends on the level of hydrocarbons
present in the mixture (Spicer et al./ 1980)/ but limited
field measurements suggest that the two products can exist in
comparable amounts (e.g./ Spicer/ 1979). Thus/ PAN formation
rates are expected to be several percent per hour under con-
ditions where photochemical reactions are important. As far
as the seasonal dependence of the PAN formation rate is con-
cerned/ .one would expect it to be qualitatively similar to
that resulting from the photochemical components of the
sulfuric and nitric acid formation rates (see also Bottenheim
et al., 1977), but it must be noted that the PAN formation
reaction (see above) is reversible. The stability of PAN is
a strong function of the temperature, and its atmospheric half-
life at 275°K is about two orders of magnitude greater than
that at 305°K (Hendry and Kenley, 1979). This will offset
-------
111-49
some of the seasonal dependence in net PAN formation rate due
to photochemical reactions alone, but quantitative conclusions
are not possible at present.
Table 7 summarizes the published data on wet and dry
deposition rates that we are aware of. Also shown are some of
the chemical transformation rates that have been measured in
field experiments.
-------
Table 7: Deposition and Chemical Transformation Rates for Nitrogen Compounds
Compound Parameter Rate Reference
N02
NO
PAN
HNO3
Conversion rate
from NOX
HNO-j and parti- Conversion
culates nitrates from NOX
N2 were in the range
0. 3-2.5 cms with most
values around 1 cms.
Alfalfa canopy/ calculated
from experiments of Hill
(1971). For the same
canopy, deposition velocity
for SO2 was 2.7 cms" .
Soil, cement surfaces.
See comments for NO2 above.
Alfalfa canopy. See
comments for NO2 above.
Alfalfa canopy. See
comments for NO2 above.
Water. Grass and soil
surfaces.
Daytime measurements,
Navajo generating station
plume (Arizona); June-July
and December.
Cumberland coal-fired
generating station, August.
NOX conversion rate was 2
to 4 times S02 rate.
Boston urban plume,
August.
Annual average,
Los Angeles.
in
o
-------
111-51
5. CONCLUSIONS
A literature survey has been carried out into the seasonal
variations of the wet and dry deposition rate, as well as the
chemical transformation rate, of sulfur and nitrogen oxides,
with particular reference to deposition and transformation
parameters of relevance to long-range transport mathematical
models. Both relevant theoretical and experimental results
have been considered although a critical evaluation of the
references has not been attempted.
As indicated in Chapters 2, 3 and 4, from a theoretical
viewpoint the deposition and transformation rates of sulfur
and nitrogen compounds could potentially have a substantial
seasonal variation. However, it is difficult to draw conclu-
sions about the magnitude of this variation with any degree
of confidence from the current theories, with the possible
exception of the wet and dry deposition of sulfur dioxide and
the photochemical component of its chemical transformation
rate. Therefore, the available field data were also con-
sidered, although these were often too scanty to be of much
assistance.
An attempt is made in Table 8 to summarize the available
information on the seasonal variation of transformation/depo-
sition rates for the sulfur compounds. It is not intended to
recommend the values shown in this Table for use by the
-------
111-52
long-range transport modeler - much more experimental and
theoretical work is needed before this will be possible - but
rather, to indicate whether seasonal changes in the parameter
of interest are expected to be greater or less than an order
of magnitude (at present/ little more than this can be done).
The following conclusions can be made on the basis of Table 8
and the discussions in Chapters 2 to 4:
1. The scanty available data suggest that the washout
rates of sulfates (and probably nitrates) should be
comparable in summer and winter. The rainout rates
could be strongly dependent on storm type, and hence
the time of year, because of the different mechanisms
whereby particles can be incorporated into precipitation
( - some data suggest variations of an order-of-magni-
tude or more).
2. Experimental results and theoretical considerations
suggest a seasonal variation of the wet scavenging
coefficient for sulfur dioxide which can be up to several
orders of magnitude, depending on the latitude, being
most pronounced in the northern parts of America which
receive appreciable amounts of snow in the winter.
Probably the same conclusions also apply to nitrogen
dioxide. Nitric acid vapor, being highly reactive with
all kinds of surfaces, is expected to show a smaller
seasonal dependence of the scavenging coefficient.
-------
111-53
3. The situation is too confusing at present to draw
any conclusions about the seasonal dependence of the
dry deposition rate for sulfates (or nitrates). In
the winter/ deposition velocities would seem to be
0.2 cms'1 or less, but values reported for summertime
conditions range over an order of magnitude, including
negative numbers.
4. The dry deposition velocity of sulfur dioxide is
expected, from available experimental and theoretical
results, to show only a modest seasonal variation -
generally, less than a factor-of-two or so in any given
area. The same is probably true of nitrogen dioxide
and nitric acid vapor.
5. The gas-phase homogeneous component of sulfuric and
nitric acid formation rates is relatively well understood,
and has a strong seasonal variability, especially at the
northern latitudes. However, our knowledge of the hetero-
geneous component, including in-cloud processes, is too
poor at present to allow any conclusions regarding the
seasonal dependence of the overall chemical transforma-
tion rate of sulfur and nitrogen oxides.
6. For many of the parameters under consideration,
during the winter months, rates are strongly dependent
on latitude - e.g., photochemical conversion rates of
sulfur and nitrogen oxides above 45°N become negligible,
-------
111-54
as do also wet deposition rates of gases such as sulfur
dioxide (because precipitation is largely in the form
of dry snow). This indicates that not only the seasonal,
but also the spatial variability of deposition and trans-
formation rates should be taken into account in long-
range transport models. Although it may be too early to
speculate, the following approach does not seem unreason-
able: during the summer months, one might assume, as a
first approximation, the same values for deposition/trans-
formation parameters irregardless of location, for each
species of interest. During the winter months, while
rates at the southerly latitudes might stay roughly the
same as those in the summer, the models would include a
dependence of deposition/transformation on latitude,
which could be quite pronounced for some of the para-
meters (such as wet deposition of sulfur dioxide).
7. For the sulfur compounds, more experimental data
are badly needed, both under summer and winter-time
conditions, particularly on wet and dry deposition rates
of particulates and chemical transformation rates in
regional scale air masses (as opposed to chimney plumes).
Very little is also known about in-cloud transformation
and deposition processes. For the nitrogen compounds,
data are required in almost every area of interest, and
-------
111-55
immediate support for laboratory and field investigations
into deposition and transformation rates of the major
species (NO, N02, HN03, nitrates and PAN) is strongly
recommended.
-------
Table 8: Summary of Deposition-and Chemical Transformation Rates for Sulfur Compounds
Representative Range (Observed)
Parameter
Summer
Conditions
j\. (S"1) Sulfates
Winter
Conditions
~ 10-5
Suggested
(Winter/Summer)
Ratio
10-1
7V(S~1)S02
(3-8)xlO~4
(l-25)xlO
-7
v(cms~l) Sulfates
SO4
transformation
rate h"1
0.4 - 0.8
1-4
(chimney plumes)
1-30
(urban plumes)
<0.2
0.1 - 0.4
(chimney plumes)
1-25
(urban plumes)
1/2
Comments
-washout: based on very few
data.
-rainout: order-of-magnitude
seasonal variation could be
possible, depending on storm
types (e.g., Scott, 1981).
-rainout« based on data
of Summers (1977); applies
to areas where wintertime
precipitation is largely in
the form of dry snow.
-theoretically predicted possi-
bility for areas which largely
receive cold rain or wet snow
during the winter.
-very large uncertainty about
deposition velocity for sulfates
-seasonal changes expected to
be modest (i.e., not order-of-
magnitude).
-gas-phase homogeneous pro-
cesses should be smaller at
latitudes greater than 45°N
in the winter.
However, there is too much
uncertainty about the magni-
tude of heterogeneous processes,
including in-cloud S02 conver-
sion, to allow conclusions about
seasonal variations.
I
tn
a\
-------
III-R1
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Deposition Parameters - A Survey," Atm. Env., 13, 571-585.
McNAUGHTON, D.J. and B.C. SCOTT (1980), "Modeling Evidence of
In-Cloud Transformation of Sulfur Dioxide to Sulfate," J. Air
Poll. Control Assoc., 30, 272-273.
MAKHONKO, K.P. (1964), "Determination of Capture Coefficient
of Radioactive Dust by Rain," Isv. AN SSSR Ser. Geophys., II,
1709.
MESZAROS, E., MOORE, D.J. and J.P. LODGE JR. (1977), "Sulfur
Dioxide - Sulfate Relationships in Budapest," Atm. Env., II,
345-349.
MIDDLETON, P., KIANG, C.S. and V.A. MOHNEN (1980), "Theoretical
Estimates of the Relative Importance of Various Urban Sulfate
Aerosol Production Mechanisms," Atm. Env., 14, 465-472.
MIDDLETON, P. and C.S. KIANGE (1979), "Relative Importance of
Nitrate and Sulfate Aerosol Production Mechanisms in Urban
Atmospheres," in Nitrogenous Air Pollutants, D. GROSJEAN Ed.,
Ann Arbor Science, Ann Arbor, pp. 269-288.
NEWMAN, L. (1979), "General Considerations on How Rainwater
Must Obtain Sulfate, Nitrate and Acid," Internat. Symp. on
Sulfur Emissions and the Environment, London, May 8-10.
NEWMAN, L. (1980), "Atmospheric Oxidation of Sulfur Dioxide as
Viewed from Power Plant and Smelter Studies," Symp. on Plumes
and Visibility, Grand Canyon, Nov. 10-14. To be published in
Atm. Env.
NIEBOER, H., CARTER. W.P. L., LLOYD, A.C., and J.N. PITTS JR.
(1976), "The Effect of Latitude on the Potential for Formation
of Photochemical Smog," Atm. Env., 10, 731-734.
OKITA, T. and S. OHTA (1979), "Measurements of Nitrogenous and
Other Compounds in the Atmosphere and in Cloudwater: A Study of
the Mechanism of Formation of Acid Precipitation," in Nitro-
genous Air Pollutants, D. GROSJEAN Ed., Ann Arbor Science, Ann
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OREL, A.E. and J.H. SEINFELD (1977), "Nitrate Formation in
Atmospheric Aerosols," Environ. Sci. Technol., II, 1000-1007.
-------
III-R7
PACK, D.H. and D.W. PACK (1979), "Seasonal and Annual Behavior
of Different Ions in Precipitation," proc. of the W.M.O. Tech.
Conf. on Regional and Global Observations of Atmospheric
Pollution Relative to Climate, Aug. 20-24, Boulder, Colorado.
PENKETT, S.A., JONES, B.M.R., BRICE, K.A., and A.E.J. EGGLETON
(1979), "The Importance of Atmospheric Ozone and Hydrogen
Peroxide in Oxidizing Sulfur Dioxide in Cloud and Rainwater,"
Atm. Env., 13, 123-137.
PRAHM, L., TROP, V., and R.M. STEIN (1976), "Deposition and
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(1980), "The Chemistry, Aerosol Physics and Optical Properties
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SCOTT, B.C. (1978), "Parameterization of Sulfate Removal by
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III-R8
SHEIH, C.M., WESELY, M.L., and B.B. HICKS (1979), " A Guide
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"Deposition of Sulfate During Stable Atmospheric Transport
Over Lake Michigan", Atm. Env., 13, 1717-1718.
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Chamber Studies of NOX Chemistry in Power Plant Plumes,"
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Urban Atmospheres," EPA Workshop on the Formation and Fate
of Atmospheric Nitrates, October 22.
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Power Station Plumes as Determined by Instrumented Aircraft,"
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Precipitation Type," in Precipitation Scavenging, R.G. SEMONIN
and R.W. BEADLE Eds., Tech. Information Centre, ERDA, pp. 88-94,
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III-R9
TANG, I.N. (1980), "On the Equilibrium Partial Pressures of
Nitric Acid and Ammonia in the Atmosphere," Atm. Env., 14,
819-828.
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"The LRTAP Problem in North America: A Preliminary Overview".
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the Deposition Rates of Sulfur Dioxide and Similar Gases on
Vegetation," Jl. Air Pollut. Control Assoc., 27, 1110-1116.
WESLEY, M.L. and B.B. HICKS (1979), "Dry Deposition and
Emission of Small Particles at the Surface of the Earth,"
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by Turbulent Transfer over Grass, Snow and Water Surfaces,"
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Transport: A State of the Art Survey of the Conversion of
S02 to Sulfate," Symp. on Plumes and Visibility, Grand Canyon,
Nov. 10-14. To be published in Atm. Env.
-------
SECTION IV
THE GLOBAL DISTRIBUTION OF ACIDIC PRECIPITATION
AND THE IMPLICATIONS FOR EASTERN NORTH AMERICA
by
Peter W. Summers
Air Quality Research Branch
Atmospheric Environment Service
Environment Canada
Downsview, Ontario M3H5T4
-------
IV-i
TABLE OF CONTENTS
Page
1. Introduction IV-1
2. Global Distribution of Emissions IV-2
2.1 Sulfur Emissions IV-2
2.2 Nitrogen Emissions IV-4
2.3 Emissions of Alkaline Material IV-5
3. Influences on Precipitation Chemistry and pH IV-7
3.1 Global Background _ IV-7
3.2 Regional SC>2 and-SC<4 Levels IV-12
3.3 Local S02 and 804 Levels IV-12
3.4 Alkaline or Neutralizing Substances IV-12
4. The Factors Controlling pH IV-14
4.1 General Considerations IV-14
4.2 Examples Using Data from the CANSAP Network IV-16
4.3 Observed Variation of pH with Height IV-19
4.4 Observations at Remote Locations IV-20
5. Conclusions IV-25
6. Recommendations for Further Monitoring IV-27
and Research
7. References IV-Rl
-------
IV-ii
LIST OF FIGURES
Figure 1. Natural and Anthropogenic Sources of
Background Concentrations/Depositions
for State/ Province to Global Scales
Figure 2. Schematic Representation of Behavior of
Emissions and 804 Aerosol Concentrations
Around the Northern Hemisphere in the Mid-
Latitude (30° - 60° N) Westerlies Belt.
Figure 3. Factors Controlling the pH of Precipitation
Page
IV-8
IV-11
IV-15
-------
IV-iii
LIST OF TABLES
Table 1. Man-made and Natural Emissions of
Sulfur (Tg S per year) in 1976
Table 2. Emissions of S02 from Stationary Sources
in the ECE Region (1973)
Table 3. Distribution of Estimated Natural Emissions
of S Among Land and Ocean by Hemispheres
(Tg S per year)
Table 4. Estimated Emissions of NOX Expressed as
N02 (Tg/year)
Table 5. Examples from the Canadian Network for
Sampling Precipitation (CANSAP) Illustrating
the Role of S04 and Ca"1"*" in Determining
Precipitation pH.
Table 6. The pH of Precipitation in Some Remote
Areas.
Page
IV-2
IV-3
IV-4
IV-4
IV-17
IV-20
-------
IV-1
1. INTRODUCTION
The question is occasionally asked "how do we know that the
low pH measured in precipitation in the eastern part of North
America is due to regional anthropogenic sources when measurements
in remote locations (in the sense of being well moved from popu-
lation centres and industrial areas) also indicate comparably
low pH values at times?" Alternatively the question could be
rephrased as follows" how much of the acidity measured in precipi-
tation at a given location is due to anthropogenic sources and
how much to natural sources and for either case, can we distinguish
between regional sources and world-wide background?"
Clearly, if one is to fully understand the patterns of pH
over eastern North America, and especially if one is to use these
as a basis for developing an emissions control strategy and inter-
national agreements, one must consider these patterns in the
total North American and global context.
In this paper, the available data* on precipitation pH around
the world will be reviewed and discussed in terms of the large-scale
processes taking place that could explain the observed pH values.
At the start it must be cautioned that the data-base is very
sparse and generally for a short time period, and that the expla-
nations, are therefore, preliminary and somewhat speculative.
The purpose of this chapter is to stimulate discussion and indicate
the further observations and research that will be required to
improve our understanding of the issue.
* Much of this is only now becoming available through draft
reports and has not yet appeared in the scientific literature.
-------
IV-2
2. GLOBAL DISTRIBUTION OF EMISSIONS
2.1 Sulfur Emissions
The most recent estimates of sulphur emissions (Cullis and
Hirschler, 1980) have been summarized in Table 1 by hemisphere
for man-made and natural sources.
TABLE 1 - Man-made and Natural Emissions of
Sulfur (Tg S per year)* in 1976
Man-made
Natural
TOTAL
Northern
Hemisphere
98
76
174
Southern
Hemisphere
6
71
77
Global
Total
104
147
251
On a global basis, natural emissions exceed man-made at the pre-
sent time by about 40%. This gap is slowly narrowing and, depend-
ing on the general rate of worldwide industrial growth, the rate
of increase of coal use for electric power and effectiveness
of SO2 emission controls, then man-made emissions could become
equal to natural emissions early in the next century.
The situation in each of the hemispheres is quite different.
In the sparsely populated and generally less industralized
southern hemisphere, natural far outweight man-made emissions
by a factor 12:1. In the northern hemisphere man-made
emissions already exceed natural by about 30%.
An important feature of the northern hemisphere distribution
is the concentration of most of these emissions into a few
heavily industralized regions (Barnes, 1979) as summarized in
Table 2.
* 1 Tg = 1012 g = 109 kg = 106 metric tons
-------
IV-3
TABLE 2 - Emissions of S02 from Stationary Sources
in the ECE Region (1973)
Reg ion
Emissions Tg S / Year
Western Europe (ECE)
Eastern Europe
U.S.S.R.
U.S.A.
Canada
Total
11.2
6.0
12.0
15.0
3.5
47.7
Note that the total in Table 2 is considerably less than the
northern hemisphere total in Table 1. The discrepancy is due
to the increases in emissions that have taken place since 1973,
and the fact that emissions from other regions/ especially
Japan and China, are not included in Table 2 and the different
methods of estimates. However, Table 2 does show that the
man-made emissions in the northern hemisphere are concentrated
into a few regions in each of which the man-made emission
strength far exceeds the natural.
The natural emissions in both hemispheres are widely
distributed with the oceans being the major source as shown
in Table 3. (Cullis and Hirschler, 1980).
-------
IV-4
TABLE 3 - Distribution of Estimated Natural Emissions of S
among Land and Ocean by Hemispheres (Tg S per year)
Land (biogenic &
volcanoes)
Oceans (sea-spray
& biogenic
TOTAL
Northern
Hemisphere
35
41
76
Southern
Hemisphere
18
53
71
Global
Total
53
94
147
2.2 Nitrogen Emissions
The other major contribution to the acidity of precipita-
tion is nitrate originating from emissions of oxides of nitrogen
(NOX). Unfortunately data on these emissions is less complete
than for SO2/ but in very general terms the current estimates
are given in Table 4.
TABLE 4 - Estimated Emissions of NOX Expressed as N02
(Tg/year)
Global Total (1978)
USA
ECE
Man-made
(Tg N02/year)
Natural
75
30
11
53
-------
IV-5
In summary, Table 4 shows that natural emissions of NOX
are about one-sixth those of SC>2 (294 Tg/year) and man-made
emissions about one-third of those for S02« This is reflected
in the rainfall chemistry since significant amounts of nitrate
are found only in precipitation in areas within, and downwind
of, industrialized regions. The most concentrated emissions
of NOX are in the eastern United States and the European
Economic Community (EEC). Little data is available from
elsewhere around the world.
2.3 Emissions of Alkaline Material
As will be discussed in more detail later, the pH of
precipitation is determined by the balance (or more correctly
the H+ ion imbalance) between the acidic and aklaline materials.
Sulfates and nitrates are the major acidic components and
calcium and ammonium are the major alkaline or neutralizing
components. Unfortunately, only very crude estimates are
available for natural (or man-made emissions) of either of
the latter two. In general, natural far outweigh the man-
made emissions. The main sources of calcium are fly-ash
(now largely removed from stack emissions) and windblown
soil materials. The former, where they still exist, occur in
polluted regions in association with generally larger amounts
of acid forming pollutants.
The windblown materials tend to be in highest concentration
in the arid and semi-arid areas of the world. These are regions
-------
IV-6
of low population and industrial activity and, therefore, are
not associated with high acidic loadings. Also, relatively
dry agricultural areas (such as the Canadian Prairies and the
U.S. high plains) with low industrial activity tend to have
high calcuira loadings. These loadings are heavily dependent
on frequency of precipitation and are highest in drought years.
There is a seasonal variation with maxima in spring and fall
when seeding and plowing are going on. In the summer the crops
themselves and irrigation reduce the atmospheric loadings
and in the winter frost and snow cover inhibit the release
of material from the surface.
-------
IV-7
3. INFLUENCES ON PRECIPITATION CHEMISTRY AND pH
The chemical composition of precipitation and its pH
when it reaches the ground is the end product of a complex
series of processes.
The composition is made up of material incorporated into
the cloud water through the initial nucleation processes,
material incorporated into the growing cloud droplets ("rain-
out") and material scavenged out by the falling precipitation
between cloud-base and the ground ("washout"). These processes
obviously take place at different heights in the atmosphere/
and since nucleation efficiency and scavenging efficiency
depends on the composition and size distribution of the par-
ticles/ the source of the incorporated materials could be
quite different in each case. The final chemical composition
will/ in turn, depend on the proportion of incorporated
material from each of these sources.
Conceptually one can separate the source of the materials
into three general categories - global background, regional,
and local - although the exact cut-off in terms of time and
space scales separating one from the other is somewhat
arbitrary (see Figure 1).
3.1 Global Background
In the past/ the global background level of atmospheric
particles has been of great interest to cloud physicists
searching for the origins of the cloud condensation nuclei
-------
IV-8
SCALE
GLOBAL
CONTINENT
MODELING
DOMAIN
NATURAL & ANTHROPOGENIC SOURCES
LUMPED, BUT NOT USED
SEPARATED, BUT NOT USED
SEPARATED AND USED
STATE/PROVINCE SEPARATED AND USED, BUT SMOOTHED
Figure 1: Natural and Anthropogenic Sources of Background Concentra-
tions/Depositions for State/Province to Global Scales As
Defined and Used by the Modelers
-------
IV-9
and freezing nuclei so essential to the processes of preci-
pitation formation. Now we are also concerned about the
impacts such particles have on the chemical composition of
'precipitation. One of the most ubiquitions of the substances
in the global background is sulphur in the form of ammonium
sulfate or sulphuric acid aerosols. Both of these, being
highly hygroscopic, act as efficient condensation nuclei
and are therefore readily removed from the atmosphere by
precipitating clouds. The size of these particles is <0.2u
and they/ therefore, have very large residence times in the
atmosphere and can be transported over long distances and
be mixed to considerable heights in the troposphere.
Some measurements of background levels of such particles
have been made in remote areas and these indicate levels of
= o
S04 between 0.1 and 0.5/ag S m"~J (Granat et al.,~197fr).
Over the North Atlantic (which may be considered as in the
"plume" from North American) levels of 0.3 to 1.0 yag S m~3
have been observed (Meszarios, 1978).
-------
IV-10
The concept of the residence time of particles in the
atmosphere can be used in a schematic sense, as shown in
Figure 2, to describe subjectively the background levels.
Considering the northern hemisphere, most of the man-made
pollutants are emitted in the mid-latitude belt 30° to 60°N
where the westerly winds transport the material, on average,
around the globe from west to east. Around the-belt there
will be an almost uniform background emission of sulphur and
a background concentration of 804 attributable to this. The
three major man-made emissions areas are the U.K.-Europe-
U.S.S.R., Japan and North America. In these regions, 804=
levels will be elevated to values at least one order of
magnitude above background and reaching two orders in severe
pollution episodes. Since the removal processes are in
general linearly related -to concentration, the concentration
will fall off exponentially with time (or downwind distance)
as shown in Figure 2. It is clear that nowhere will the
concentrations of 864 reach zero, and on the west coast of
North America or Europe one would expect small quantities of
residual man-made emissions to be present. The size of this
residual cannot be estimated at the present time since it
requires an in-depth review of existing data and an attempt
to quantify this qualitative concept. The latter could be done
by a simple box model approach to ultra LRTAP on a hemispheric
scale using large emission blocks, mean transport time across
the oceans etc.
-------
Figure 2: Schematic Representation of Behavior of Emissions and 804
Aerosol Concentrations around the Northern Hemisphere in
the Mid-Latitude (30°-60°) Westerlies Belt.
/\
\
\
\
CONCENTRATION OF
V SULFUR EMISSIONS
// U.K
// EUROPE
V, USSR
JAPAN
BACKGROUND X
SULFUR EMISSIONS
/ N
/ 1
\
o
30
60
go
120
150
180
150
EAST
LONGITUDE
-------
IV-12
3.2 Regional SO2 and 804 Levels
The regional levels of 864 have been studied in detail in
Western Europe and North America and appear to be the main
source of acidic materials leading to regional acid precipitation
problems. The concentrations are elevated by one to two orders
of magnitude above background and are well mixed in the lowest
levels of the atmosphere up to and and sometimes above the typical
cloud base. These materials are therefore present in updrafts
or general ascent feeding cloud systems, as well as in the sub-
cloud layer through which precipitation would be falling.
3.3 Local S02 and 804 Levels
These are highly variable in space and time but are
obviously highest in point source plumes and over and down-
wind of urban areas (the so-called "urban plume"). Whether
such material is incorporated into precipitation is clearly
dependent on the local spatial distribution of the precipi-
tation pattern (especially for convective showers) in rela-
tion to the plume geometry. This tends to be an "on-off"
situation - either the precipitation falls through the plume
or it doesn't. This in turn leads to very variable precipi-
tation chemistry, in terms of both substances and concentra-
tions, at samplers located near to (<50 km) from large sources
(c.f. Metromex).
3.4 Alkaline or Neutralizing Substances
The previous three sections have indicated the general
-------
IV-13
sources of acidic material found in precipitation. However,
the final pH will be determined by the amount of alkaline
material also present. The main alkaline component in the
atmosphere is calcium which originates from fly ash (now
largely removed from plumes by precipitators) and wind blown
dust. Alkaline particles tend to be much larger (>10u) than
acidic particles and, therefore, have much shorter residence
times and are not mixed as high through the atmosphere. Thus,
such particles remain much closer to the source and exhibit
very marked regional patterns over land - high concentrations
are found over arid areas, agricultural areas and as component
of the pollution in the near downwind of major sources. Over
the oceans, well removed from sources, concentrations of
calcium are near zero and there is no global background in
the same sense as there is for 804. Because of their larger
size and shorter residence time, calcium particles are not
found in significant quantities much above the surface
boundary layer.
-------
IV-14
4. THE FACTORS CONTROLLING pH
4.1 General Considerations
The discussion in the previous four sections suggests the
following general sequence of events in determining the pH
of the precipitation, as illustrated schematically in Figure 3.
(a) Formation of cloud droplets will tend to take
place around an acidic nucleus and thus the pH
will be less than 5.6 (the value for clean liquid
water in equilibrium with atmospheric C02).
(b) Rain formation in the cloud will collect other
small particles which could be both acid or alka-
line, but with a tendency toward the former. Thus,
in cloud rain-drops will tend to remain acidic.
(c) Falling rain will scavenge material in the sub-
cloud layers "which will be in larger concentrations
than in the cloud. The final pH at ground impact
will thus be strongly controlled by the character
of the sub-cloud atmospheric loading. There are four
possible general cases for the modulation imposed by
the sub-cloud layer as follows:
i) in remote marine areas the sub-cloud layer
will be dominated by the marine aerosol pro-
duced by breaking waves i.e. sodium chloride
which is a neutral salt. Thus the pH is mainly
determined by the background acid aerosol and
values will be less than 5.6.
-------
IV-15
BACKGROUND ft
SOME REGIONAL
POLLUTION
MODULATION BY
MIXED LAYER
COMPOSITION
FALLING PRECIB
., (WASHOUT)/
GENERALLY
ACIDIC
FOUR CASES
1. OCEANIC
2. ARID ZONES
3. REGIONALLY
POLLUTED
4. LOCALLY
POLLUTED
(PLUMES)
FINAL CHEMICAL COMPOSITION
AND pH IS END PRODUCT OF
LONG CHAIN OF COMPLEX
PROCESSES
FINAL
PRODUCT
NEUTRAL
BASIC
ACIDIC p> BASIC
— ACIDIC3: BASIC
ACIDIC
BASIC
HIGHLY ACIDIC
ACIDIC OR BASIC
Figure 3: Factors Controlling the pH of Precipitation
-------
IV-16
ii) over arid land, or agricultural areas (especially
in spring and fall) the wind blown dust will
usually neutralize the acidic components and
produce rain with a pH greater than 5.6 and
as high as 6.0 or 7.0.
iii) over regionally polluted areas both acidic and
alkaline materials will be scavenged, but with
the former usually being dominant and resulting
in acidic precipitation.
iv) in falling through a plume the precipitation
will again scavenge both acidic and alkaline
materials producing a wide variation of resul-
tant final pH.
In the"later two cases c(iii) and c(iv) the background
aerosol is clearly a very small component of the final chemical
composition and, therefore, has little impact of the final pH.
4.2 Examples Using Data from the Canadian Network for Sampling
Precipitation (CANSAP)
The CANSAP data show a very wide variation in annual
average pH ranging from 4.0 to 7.0 during the three years 1977-
1979. In order to show the role of S04= and Ca++ (the major
anions and cations found in Canadian precipitation) on the
average pH, the data from four regions are shown in Table
5. The groupings have been arranged to minimize the within-class
variation of the two chemical constituents and to maximize the
between-class differences.
-------
Table 5: Example from the Canadian Network for Sampling Precipitation (CANSAP)
Illustrating the Role of SO4 and Ca*"*" in Determining Precipitation pH.
(Averages are for the three year period 1977 to 1979.)
Region
Symbol
A
Fp
Ap
F
Regional Characteristics
Dry, prairie agricultural
area with no regional
pollution.
Moderately wet, native
mixed forest on
perphery of regionally
polluted region
Moderately wet,
agricultural area
with high regional
pollution levels
Relatively dry northern
forested area with
no regional pollution
Station
Lethb ridge, AB
Kindersley, SK
Wynyard, SK
Gander, NF
Truro, NS
Seven Isles, PQ
Mt. Forest, ON
Peterborough, 6N
Simcoe, ON
St. Hubert, PQ
Quebec City, PQ
Fort Chimo, PQ
Cree Lake, SK
Fort Reliance, NT
Concentration ir
804 (mg I"1)
2.6
2.1
2.6
2.3
2.6
2.5
6.1
6.7
6.3
7.7
6.8
1.2
1.2
0.9
i Precipitation
Ca++ (mg l"1)
3.7
3.2
3.1
0.2
0.3
0.1
1.3
1.4
0.8
2.2
1.2
0.2
0.4
0.2
pH
6.6
6.8
6.4
4.6
4.5
4.4
4.4
4.1
4.1
4.2
4.2
5.1
5.1
5.3
f
Note; Only stations sufficiently far from the coast to neglect the oceanic
influence and with more than 15 months of record are included in the
table.
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IV-18
The lowest concentrations of both S04 and Ca++ occur in the
northern forest region (F), but there is sufficient excess of
acidic over alkaline material to give an average pH of 5.1 to
5.3.
In the prairie agricultural region (A), 804 is somewhat
higher but large amounts of Ca++ originating from wind-blown
soil more than neutralize the acids and the result is a pH far
in excess of 5.6. These pH values of between 6.0 and 7.0 are
the highest in Canada.
In the wetter forest regions on the periphery of the
regionally polluted region (Fp) in eastern Canada, 804 concen-
trations are about the same as in region A, but Ca++ amounts
are very low and the pH is about 4.5 representing an acidity
value about 10 times that in clean rain.
Finally/ the agricultural region in southern Ontario and
the St. Lawrence lowlands with high regional pollution values
(Ap) has by far the highest 804 concentrations. Even though
there are moderate concentrations of Ca++ originating from
the calcareous soils in the region, the resultant precipitation
pH is still the lowest in Canada with average annual values
between 4.1 and 4.4.
The role of 864 and Ca++ concentrations in determining the
pH of precipitation in Canada is clearly evident from the
above analysis. 864 plays the dominant role in regionally
polluted regions and Ca++ plays the dominant role elsewhere
in determining pH.
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IV-19
Note that nowhere does the 864 concentration fall much
below 1.0 mg 1~1 even in remote arctic areas and this could
thus be interpreted as the typical "background" contribution.
With no other alkaline components, such a concentration would
result in a pH of about 5.0 which could, therefore, be con-
sidered the background value of pH. Departures from this
value, rather than from pH 5.6, should be considered in
discussing regional and local influences.
4.3 Observed Variation of pH with Height
The ideal way to verify the sequence suggested in section
4.1 would be detailed airborne observations collecting cloud
droplets, in-cloud precipitation and falling precipitation
from the same cloud system. This has not been done in any
systematic way, although there are isolated airborne measure-
ments that are not inconsistent with the hypothesis.
Two sets of ground level measurements on mountains are
available from Mauna Loa, Hawaii in an essentially remote,
largely unpolluted oceanic environment and from Whiteface
Mountain, N.Y. immediately downwind of a heavily polluted
area.
Mauna Loa, Hawaii
Data collected from June 1975 to June 1980 at several
sites ranging from sea-level to the Mauna Loa observatory at
3400 m MSL have been reported by Miller and Toshinaga (1981)
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IV-20
There is wide variability at all elevations with the rainfall
pH of daily and bi-weekly samples ranging between extreme
values of 3.5 and 6.2. However, both the median and mean
values showed a steady decline with increasing altitude. The
mean value at sea-level was 5.2 decreasing to 4.3 at elevations
above 2500 m MSL. The corresponding concentrations of sulfate
ranged from 0.3 to 8.0 mg 1~1, but most of the values were
<1.0 mg 1~1 and this is down by a factor 5 from the annual
average in the region of maximum concentration in eastern
North America (see Region Ap in Table 5).
Whiteface Mountain, N.Y.
Observations at the observatory atop Whiteface Mountain
(elevation 1483m) have been compared with data from the
nearest low-elevation MAP3S station at Ithaca. These show
the pH of rain to be generally lower atop the Mountain. In
addition, the Whiteface data shows*the mean pH to be lower
in non-precipitating clouds (i.e., small droplets) by about
0.4 to 0.5 units than in precipitating clouds (larger drops).
4.4 Observations at Remote Locations
In recent years sampling stations have been set up at
remote locatons by several agencies. The World Meteorological
Organization (WMO) have instituted a global monitoring network
to establish global trends in C02 and other atmospheric
chemistry parameters. NOAA have also set up a less widespread
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IV-21
network. The stations can be separated into two broad cate-
gories - oceanic and continental (but in regions of low popu-
lation usually coinciding with arid areas). A third category
of locations which is useful for studying background precipi-
tation chemistry consists of stations on the west coast
of the major continents in the westerlies belt where most of
the precipitation occurs with on-shore air-mass trajectories.
Finally, data from the polar regions represents the most remote
locations. Data on precipitation pH in each of these categories
are summarized on Table 6.
Remote oceanic stations
These locations indicate considerable varability with
monthly average pH values of precipitation ranging over at
least 1.5 units. In all cases, the median value is less
than pH 5.6 with lowest values about 4.6 to 4.7. This represents
an increase in acidity by a factor of 10 above that expected
in "clean" rain.
West-coast stations
Except for the heavily populated areas around Los Angeles,
San Francisco and Seattle-Vancover, pH values on the west-coast
of North America average about 5.0.
On the west-coast of Norway (well removed from the pol-
luted region in the south) the pH of the rain is averaging
over 6-month periods between 4.6 and 4.9. The sulfate content
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IV-2 2
TABLE 6. The pH of Precipitation in Some Remote Areas
OCEANIC
SAMOA (1)
MAUNA LOA (1)
HILO, HAWAII (2)
AMSTERDAM Island (2)
BERMUDA (2)
WEST COAST
CAPE GRIM, TASMANIA
(1)
INDIA (4 STNS) (1)
IRELAND (3)
NORWAY (3)
•
NORTH AMERICA (3)
CONTINENTIAL
: INDIA (9 STNS) (1)
GRAND JCT (COLO) (1)
; ALAMOS A (COLO) (1)
SAN ANGELO (TEXAS)
(1)
; POLAR
• CANADIAN ARCTIC*
INLAND ANTARCTIC*
Monthly Average Precipitation pH
Median
5.3
5.0
4.6
4.7
5,1
6.3
6.3
4.8 -» 5.6
4.8 -» 5.2
— 5.0
7.5
6.5
6.8
6.6
— 5.0
4.6 -» 5.6
Minimum
4.5
3.3
3.7
3.8
4.0
5.8
5.8
5.8
5.4
5.0
5.2
Maximum
6.0
6.7
5.4
5.4
5.6
7.2
6.6
8.9
8.0
8.2
7.3
Length of
Record or
Number of
Events
36
47
100's
17
33
12
13
1968 - 1974
Oct. 1977 -
Sept. 1979
Variable
51
40
73
43
Few
Few
*'ESTIMATEDD FROM ION BALANCE (see text)
SOURCES: (1) Acid Rain - An Assessment based on Acid-Base Considerations
of Factual Data, R. Sequira, 1981 (In Press).
(2) Progress Report - Global Precipitation Chemistry Network,
Jan. 1981, J. Galloway, W.C. Keene, G. E. Likens
(Draft Report to NOAA).
(3) See Text.
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IV-23
is low at between 0.1 and 0.4 ug S m~^ with the lowest values
with winds in the sector W through NE (Jorranges et al., 1980).
A study of rain chemistry over Ireland (Fisher, 1980)
shows pH on the exposed west-coast stations between 5.1 and
5.6. It is estimated that about 50% of the measured sulfate
in the rain is attributable to natural background.
Continental remote
Except for a few individual months, the pH is greater
than 5.6 and occasionally alkaline (pH >7). The main cause
of this is the wind blown dust in arid and semi-arid areas
which is calcium rich. There is more than required to
neutralize any background acidic components and thus the
remainder produces rain more basic than pH 5.6.
Remote Polar regions
A joint study by Canada, the U.S. and Denmark has been
monitoring air chemistry on sites in the Arctic. These show
that the winter months sufficient acid particulate material
reaches high latitudes to reduce visibility producing arctic
haze. Concentrations of sulfates measured at Mould Bay and
Igloolik range between 1 and 3 ug/m^ in the winter months
(Barrie, 1980) and in the absence of any significant alkaline
content would be sufficient to produce snow with a pH of
about 5.0.
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IV-2 4
In Antarctica the chemistry of snow has been investigated
by Delmas et al., (1979), by sampling freshly fallen snow on
transects from the coast inland for 800 km. The concentra-
tions of sodium and chloride fell off rapidly and the main
influence had disappeared 200 km inland. The sulfate concen-
trations remained essentially constant at between 60 and
100 x 10~9 g g~l (0.1 ppm). Although the pH was not
measured directly, the authors estimated that the excess of
acid over basic components would be sufficient to reduce the
pH by up to 1.0 unit below the C02 equilibrium value of 5.6.
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IV-2 5
5.0 CONCLUSIONS
A review of the world-wide data on precipitation pH
in remote and exposed mid-latitude west-coast areas indicates
that all precipitation contains at least small quantities of
acid materials. In the absence of any neutralizing basic
components this is sufficient to reduce the pH to a value of
about 5.0 and in some cases less. Nowhere, though, are pH
values in remote aras as low as those found in the most
acidic precipitation areas of the northeastern U.S. and western
Europe. The minimum median value is pH 4.6 at a few isolated
locations. In contrast to this, there is a large contiguous
area in eastern North America containing about 90 monitoring
stations reporting annual average pH less than 4.6 and as low as
4.1. The latter value represents an increase in acidity by a
factor 30 above that in "clean" rain, and clearly shows the
regional influence of the concentrated man-made emission sources.
Several authors are now suggesting that the reference level
of 5.6 (the pH of rainwater in equilibrium with atmospheric
C02) is not appropriate and that departures from a value of
near 5.0 would indicate the regional and local modulations
to the influence of "global background".
While pH is a useful single number that characterizes
the precipitation, it is the total deposition of acidity
(H+ ions) that is important in assessing the effects on
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IV-2 6
ecosystems. The deposition is the product of the concentration
and the rainfall amount. Thus in considering the relevance
of the low pH values in remote areas, this must be considered.
Sensitivity, in the form of the buffering capacity of the
receptor surfaces is also important in defining the seriousness
of impact. Most remote areas, especially arid regions, are well
buffered and so the impact of any acidic deposition is minimized,
In contrast, the regions with lowest pH and highest depositions
of H"1" ion in the northeastern U.S., eastern Canada and
southern Scandinavia cover large areas of poorly buffered
lakes and soils and thus have a major impact on the receptors
there.
While there is considerable varability in the background
pH values, they are in general consistent with the concepts
proposed in sections 3 and 4. The limited vertical profiles
available are also supportive of the hypothesis that most
precipitation starts off as acidic cloud droplets. The
higher the elevation the more important is the global background
of chemical components relative to those generated locally.
It must also be pointed out that some of the observations
cannot be readily explained and clearly, more analysis of
existing data bases (for example trajectory analyses to
identify whether observed background levels are due to natural
sources or far downwind residuals from man-made sources) are
required to refine the ideas presented here.
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IV-2 7
6.0 RECOMMENDATIONS FOR FURTHER MONITORING AND RESEARCH
The number of remote stations that have been established
in recent years is now beginning to generate substantial data
relevant to the issue of establishing and understanding the
background levels of air and precipitation chemistry. Rather,
than establishing many more such stations the priority should
be to analyse and interpret the existing data base.
Some specific recommendations are as follows:
— wherever possible at precipitation chemistry
stations sampling should be done on an event
or at least on a weekly basis.
the precipitation chemistry data are much more
valuable and can be interpreted more readily
if concurrent basic air chemistry measurements
are made such as filter-pack sampling.
— more observations of the vertical distribution
of precipitation chemistry (and where possible
air chemistry) are needed. This can be done
in two ways
a) at mountain sites
b) with instrumented aircraft
continuing efforts are required to refine the
estimates of natural emissions of acid compo-
nents into the atmosphere (they are presently
less accurate than estimates of man-made
emissions yet are equally important on the
global scale).
— estimates (however approximate) are required
for emissions of the most important alkaline
materials into the atmosphere; at present
none exist.
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IV-R1
7.0 REFERENCES
Barnes, R.A., 1979: The Long-Range Transport of Air Pollution;
A Review of European Experience. J. Air Poll. Cont. Assoc.,
_29, 1219-1235.
Barrie, L. A., R.M. Hoff and S.M. Daggupaty, 1981: The
influence of mid-latitudinal pollution sources on haze in
the Canadian Arctic. Atmospheric Environment, 15, (in press).
Cullis, C.F. and M.M. Hirschler, 1980: Atmospheric Sulfur:
National and Man-Made Sources. Atmospheric Environment, 14,
1263-1278.
Delmas, R. , M. Briat and M. Legrand, 1979: Chemistry of
South Polar Snow. Presented at XVIII IUGG Meeting, Carberra,
Australia, Dec. 1979. (in press in J. Geophys. Res.)
Fisher, B.E.A., 1980: Deposition of Sulfur and the Acidity of
Precipitation over Ireland. Lab Note RD/L/N 152/80, CERL,
Leatherhead U.K.
Granat, L. , Rodhe, H. and Hallberg, R.O. 1976: The global -
sulfur cycle. In Nitrogen, phosphorus and sulfur global
cycles. Edited by B. H. Svensson and R. Soderlund. SCOPE
Report 7, pp. 89-134.
Joranges, E.J. Schang and A. Semb, 1980: Deposition of Air
Pollutants in Norway. Proc. Int. Conf. Ecological Impact of
Acid Precipitation, Norway.
Meszarios, E. 1978: Concentration of sulfur compounds in remote
continental and oceanic areas. Proceedings of the International
Symposium on Sulfur in the Atmosphere, September 7-14, 1977,
Dubrovnik, Yugoslavia, Atmospheric Environment 12, 699-705.
Miller, J. and A.M. Yoshinaga, 1981: The pH of Hawaiian Precipi-
tation, A Preliminary Report. (In press)
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