Cap+4+Tropospheric+Chemistry+precipitation

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5.1 TH E COMPOSITION OF RAIN 101
chapter we will discuss the chemical and physical behaviour of dry species in the
atmosphere. In this chapter, the emphasis is on wet deposition.
When water condenses to form clouds, various chemical species are incol:porated into
the cloud droplets and sometimes fürther transformed there. During a precipitation event,
the accumulated species are carried to the Earth's surface. When chemicals accumulate
and are removed â'om the atmosphere in this way, it is referred to as raÍnout;, while washout
is the term used when chemicals are taken up by water droplets as they pass though air
below the clouds.
CHAPTER 5
Tropospheric chemistry--
precipitation 5.1 The composition of ram
If rainwater is in equihbrium with gaseous species in the air, it contains soluble forma of
the atmospheric gases with concentrations determined by Henry's law. Among the gases is
carbon dioxide whose atmospheric mixing ratio in the Northern Hemisphere averaged
approximately 378 ppmv in 2004. Dissolved carbon dioxide is a weak acid and an aqueous
solution in equilibüum with the unpolluted atmosphere has a pH of 5.7. The significance of
this vague will become clear when we cany out calculations ofgas solubilities in water and
the effêct on pH in Chapter 11. Rainwater containing dissolved carbon dioxide is therefore
slightly acidic, with a hydronium ion concentration approximately 20 tomes greater than
that of pare water. Since much of the carbon dioxide is of 'natural' origin, it is nequently
suggested that the pH ofunpolluted ram is 5.7.
In fact, other natural acid-producing species, including organic acids and sulfür
compounds that are produced by microbiological processes õ'om hving and dead biomass,
can be a source of enhanced acidity in ram in áreas remote from human influence. Still
other 'natural' chemicals such as dust containing calcium carbonate, when incorporated in
ram, can result in a mildly alkahne solution. Depending on the setting there6ore, a variety of
anionic and cationic species accumulate in ram and the distinction between natural and
anthropogenic origin is not always clear. As a consequence of the combined factors, the
pH ofunpolluted ram might range Rom below 4.5 to 8 or higher. In any case, ram and snow
are not pure water and their chemistry is detemiined by a range of complex, interrelated
Eactors. 'lbe concentration of major species in ram taken at a variety of locations is given
inTable5.1.
The game dissolved species as those listed in the table are algo important componente
of precipitation occurring at other locations around the globe. VVhile individual species
cannot be assigned with certainty to a specific origin, sodium and chloride are, to a large
extent, derived õ'om the oceano jas, for example, in the St. Georges, Bermuda rainfalll.
Accompanylng sodium and chlodde are somewhat smaller concentrations ofother marine
ions including potassium, calcium, magnesium, and sulfate. In lesa common situations,
elevated concentrations of sodium and/ or calcium and chloride in the atmospheric aerosol
originate from salts used to melt ice and snow on highways during the winter in countries
like Russia and Canada. From dust, calcium and magnesium are incorporated inca water
droplets and, consequently, high leveis of these elemento are usually associated with a
terrestrial origin. An example is the Pune, Índia sample. Ammonium ion originates õ'om
biological processes involving nitrogen â'om plant and animal residues and â'om inorganic
fertilizers. As well as the principal species, smaller concentrations of other elements are
incolporated into ram, especially in heavily industriahzed regions.
Topicsto be covered
Precipitation chemistiy--sources, properties, controls
H Rainfall composition
H Generation of acidity in precipitation
H Ram, snow, and fog
H Global variation in composition
H Emission contrai technologies
One ofthe first environmental issues to generate widespread publicity and public attention
was the problem of acid ram. Emissions õ'om energy and smelting industries were shown
to be a source of sulfüric and nitric acid in raid and, at various locations, precipitation pH
values of 4 or even lower were measured. Studies showed that this low pH rainfall, espe-
cially in northern Europe and eastern North America, was causing acidification of lakes,
aífecting growth in forests, and causing damage to stone buildings and other structures.
Tais was a much discussed subject ofthe 1960s and 1970s and üie research, publicita, and
political pressure led to positive action on the part of many industries. While the problem
is now much lesa severe, it has not been totally solved. Furüermore, in terms ofthe envir-
onment, there is more to precipitation than its acidity. It is for tais reason that we emphas-
ize that the subject of this chapter is predpítation chemistty, not only acid ram. Importantly,
also, there are other causei of environmental acidification beyond precipitation. This is
a subject that will be discussed in Chapter 18.
Ram and snow are the forms of precipitation that are most familiar to all of us. In an
environmental context, however, the term precipitation has a broader deíinition and can
be subdivided unto two categories. Precipitation by wet depositíon refere to deposition on the
Earth's surface of water-based particles in liquid or solid form--ram, snow, and various
topes of ice such as sleet and hall. In its wider meaning, precipitation algo includes surface
deposition of dty species, including gases such as sulíür dioxide and solid particles such as
various components of dust. Without specincally using the term d?y deposition, in the next
102 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.1 THE COMPOSITION OF RAIN 103
Table 5.1 The composition of ram from severas locatlonsa. In this case there is a 232 199 = 33 »mol L'i or approximately 15% difference in the total concentra-
tions of positive and negative charme. Given that this calculation is based on mne separate analytical measure-
rnents all at quite low concentrations, the difference is probably not signifícant. Furthermore. the deficit is on the
positive ion side; there are no environmentally common cations that would be likely to account for the difference.
In other cases where this kind of calculation is done. however. a major discrepancy in charge balance can indi-
cate either a significant analytical erros or that some important species in the sample has not been measured.
Concentration/ Hmol L'l in ram samples froi
UrbanGuiyang. Birkenes,
Guizhou, PRCb southern No1wayc
112(PH 5) 57(PH
58
Katherine,
Northern Territories.
Australiad
Pune. Maharashtra
State.Indiae
St. Georges,
Bermudad
H+
cl-
NO;
soã
Ca2+
Mg2+
Na+
:+ 4 0.9 36 4.3
Ja+ 56 70 150 147
a.2+ 12 0n 2 2
JO; I0.3 38 4.3 18 5.5
;oã 222 68 6.3 ll 36.3
la2+ 128 9 2.5 55 9.7
;1 58 11.8 155 175
16.6(pH - 4.8)
11.8
0.04(pH - 74)
155
16.2(pH - 4.8)
175 lince the sodium, potassium, calcium, and magnesium cations and the chloride anion are
aU common, ofnatural origtn, and generally benign species at üie p,mol L'l concentrations
üound in precipitation, their presence has a relatively minar environmental signiíicance. It is
the sulfür and nitrogen species that generate acidity and are there6ore ofparticular concern.
To a large extent these species derive ú'om anthropogenic sources and togedier they create
the phenomenon known as acid precipitat:ion. Table 5.2 lists sources and estimates of
mixing rabos and residence tomes for sulftlr and nitrogen species present in the atmosphere.
As with other compounds, distinctions between natural and anthropogenic sources are
not clear, but there is no doubt that a major contribution of both sulftlr and nitrogen
atmospheric species arises â'om combustion and a variety ofindustrial processes. It is these
l0.3
222
128
38
68
9
13
4.3
6.3
2.5
2.0
18
11
55
35
150
36
28
5.5
36.3
9.7
34.5
147
4.3
3.8
56
4
38
zo
0.9
2.4
K+
NH; 57
a Thesites in China and Norway are considered to contam anthropogenic-source chemical species. The others are
influenced to a smaller or negligible extent by human activity.
b Díanwu, Z. and X. Jillng, Acidification in southwestern China. In ,4c/d7#caf/0/7 ü frop/ca/ counfrfes(ed. Rodhe. H. and
R. Herrera), John Wiley and Sons, Chichester; 1988. No data are provided for CI, Mg, Na. and K
c Overrein, LN. H.M. Seip, and A. Tollan, .4c/dprec@/faf/o/v--e#ecfs 0/7 áoresf a/7d/bó, Final repare of the SNSF project
1972-1980,0slo;1980.
d Legge, A.H. and S.V. l<rupa. .4cld depôs/f/on, su/Óur and n/fraga/v oxdes. Lewis Publishers, Chelsea, Michigan; 1990.
e Khemani. LT. G.A. Momin, M.S. Naik. PW. Prakasa Rao, P.D. Safam. and A.S.R. Murty. Influence of alkaline particulates
on pH of cloud and ram water in Índia. ,4fmos. EnPüo/v., 24 (1987), 1137-45.
Main point 5.1 The various forms of liquid and solid water in the atmosphere ínclude ram.
snow, and fog. 'pese and other atmospheric forms of precipitatíon always contam various
dissolved species derived from both natural and anthropogenic processes. Of these
substances, acids produced from the oxides of nitrogen and sulfur are of widespread
concern. üeir presence in excessive amounts can cause the rainfall pH to be depressed to
values as low as 4 or even leis.
Example 5.1 Charge balance in a rainwater sample
It can be useful, as a check on the analytical results, to calculate the charge balance in a water sample such as
those given for the ram simples ín Table 5.1.
For example. determine the magnitude of positive and negatíve charge associated with the major elements
in the ram sample obtained at Bírkenes, Norway.
Table 5.2 Nitrogen and sulfur species present in the atmosphere
Approximate Approximate
atmospheric residence
mixing ratio/ ppbVa time / days Source
Positive(cationic)charme
H+
Ca2+
Mg2+
Na+
Concentration of ion / Kmol L'l
57
Charge / Fenol L-t
57
Nitrogen specíes
Nitrogen oxides, NOx 1->10 (urban)
0.1 1 (remove)
0.2 (urban, summer) to
lO (remote. wínter)
Fóssil fuel, biomass,
combustion; lightning;
microbiological release
Animal excreta. fertílizers
microbiologícal release
9
13
56
4
38
18
26
56
4
38
199
Ammonia. NH3 o.l-l 2-70
K+
NH;
Total
Sulfurspecies
Sulfur dioxide, SO2 0.01-0.3 3-5 Fóssil fuel, biomass combustion
sulfide ore smelting
Submerged sons, wetlands
Submerged sons, wetlands
Oceans
Oceano. sons
Oceans,sons
Oceans, sons
Hydrogen sulfide.H2S 0.05 0.3
Carbon disulfide.CS2 0.02-0.5
Dimethyl sulfide.(CH3)ZS 0.01-0.07
Carbonyl sulfide. COS 0.3-0.5
Methyl mercaptan. CH3SH
Dimethyl dísulfide, CH3SSCH3
a Mixina fatias are far unpolluted áreas unless otherwise
1 2
-50
-1
200-2500
Negative(anionic)charge
CI
Concentration of ion / Fmol L'l
58
Charge / Fmol L-i
58
NO;'
soã'
Total
38
68
38
136
232 noted. Data are from various sources
104 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.2 ATMOSPHERIC PRODUCTION OF NITRIC ACID 105
emissions that ultimately contübute to enhanced production of the strong acids found in
wet and dry precipitation.
The radical can take part in a number of reactions. To some extent, it is destroyed by
reactions with the NO* compounds.
'N03+.N02-->.NO +.NOz+ 02 IS.41
5.2 Atmospheric production of nitric acid
The principal reaction sequence contríbuting to production of nitric acid is relatively
straightforward and has actually been discussed in Chapters 3 and 4. It begins with nitric
oxide emissions occurring primarily during combustion processes. For high-temperature
combustion, most of the nitrogen originates õ'om the atmosphere, but some can also
be derived hom organic nitrogen compounds in füels such as wood. Smaller amounts of
nitric oxide are released as a byproduct of microbial nitriíication in soil, a process that
is enhanced in the high-temperature t:ropical environment. Lightning, which is also most
õ'equent in the tropics, adds a filrther small input to the global nitric oxide budget.
'N03 + .NO -) 2NOz (5.5)
In the same manner as hydroxyl, nitrate radical is able to add to the double band ofolefins.
'N03 + CnH2n -+ 'CnH2nNO3
Because the product is also a radical, it is susceptible to further reactions. Typically, addition
ofthe nitrate radical is 6ollowed by rapid addition of oxygen.
Also like hydroxyl, nitrate radical initiates reaction sequences by íirst abstracting a
hydrogen. In these cases, nitric acid acid is formed.
Daytime nitrogen oxide chemístry with aldehydes .NOs+RCH0-+RC0+ HN0: IS.71
Nitric oxide is oxidized by O2, O3, or R00. jwhere R is an alkyl groupl. For example with alkanes.NO3 + RH -+ .R + HN03 l5.8)
In both reactions 5.7 and 5.8, R is an alkyl group. The alkyl radicais that are produced take
part in fürther reactions such as the addition of dioxygen.
A kinetically rapid paio of reactions involving the nitrate radical and nitrogen dioxide
results in the íormation of diniü'ogen pentoxide and then nitric acid.
NO + 03 --> .NOz + 02 IS.ll
The nitrogen dioxide produced in this way subsequendy contHbutes to ozone and hydroxyl
radical production and therefore is responsible(in parti for the initiation of a photochemical
smog sequente. In the process, nitric oxide is regenerated and is ülerefore available to once
again contribute to ad(htional ozone and smog production. Fortunately, the NO. compounds
have only a limited atmospheric lifetime, or otherwise smog events would persist and
increase. But, unfortunately, the principal mechanism for their removal âom the atmosphere
is through conversion to nitiic acid via oxidation ofnitrogen dioxide by the hydroxyl radical
N03+'NOz;eN20s IS.9)
N20s + Hz0 -) 2HN03 IS.lol
N0: + .0H + M -+ HN03 + M IS.21
where M is a third body. The pseudo secondorder rate constant for the reaction has a value
of 1.2 x 10 n(T / 298)'lõ cm3 molecule-t s-l. Because reaction 5.2 envolves starting materi-
ais fomaed in part through photochemical processes, it is largely a daytime reaction.
In many environmental situations, most of the NO* present in the atmosphere at
night-time proceeds through reactions 5.3, 5.9, and 5.10 and this is an additional means of
removing these smog-precursor species by producing nitric acid. A smaller ú'action of NO*
is íinally converted to l:AN by the reaction
RC(0100. +.NOz -+ RC002NOz IP:AN) IS.ll)
Example 5.2 Conversion rate for nítrogen dioxide to nitric acid
Assuming a constant hydroxyl radical concentration of 2 x IOÓ molecules cm 3 and temperature of 20 'C,
what ís the half-life of NOZ according to this reaction?
Rate = kzIN02]EOH], where k2 is the pseudo second-ordem rate constant
= klENO2], where ki = k2EOH] = 1.2 X 10 n(293 / 298) ió X 2 X 10ó
=2.5 x 10 's l
Removal of nitric acid
Nitric acid is removed â'om the atmosphere by either wet or dry deposition, and is one of
Uie main contributors to precipitation acidity. To some degree it also reacts with ammonia,
whose principal source is volatilization Rom nitrogen fertilizers, urea in animal urine, and
other organic reduced nitrogen sources.
NH3+ HN03-->NH4N03 IS.121
Half-life 0.693/(2.5 x 10 5)= 2.8x104s= 78 h The ammonium nitrate can act as a condensation nucleus for the formation of a water
droplet or it is deposited as part ofthe solid aerosol. Tais issue will be described more fülly
in Chapter 6.
Night-time chemistry
Aíter sunset, an altemative sequence becomes more important for producing nitric acid. It
envolves the niü:ate radical, which is 6ormed during both day and night but accumulates only
at night-time because it is destroyed by photolysis. The reaction for nitrate radical 6ormation is
'NOz + 03 -) .N03 + 02 (5.3)
Main point 5.2 Nitric acid is one of the important acidifying components of precipitation
It is produced from the nitrogen oxides that, to a large degree, originate from combustion
reactions.
106 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.3 ATMOSPHERIC PRODUCTION OF SULFURIC ACID 107
5.3 Atmospheric production of sulfuric acid
Oxidation of reduced sulfur speciesBy means ofthese processem, any reduced sulfür compounds are able to be oxidized, with
one ofthe principal products being sulftlr dioxide. The sulíür dioxide is ultimately converted
to sulfilric acid. In áreas of the world remate õ'om human activity the emitted reduced
sulítlr compounds, dimethyl sulüde in particulu, are a major source of sulfür dioxide that
produces excess acidity in atmospheric aerosols and precipitation.
Sulfür dioxide is also released in large quantities directly unto the atmosphere õ'om sulâde
ore sme]ting and fossi] füe] combustion. It is this additional anthropogenic source that gives
pise to even higher acidity in some continental regions of the globe that are influenced by
intense industüal activity.
The production of sulíilric acid is a more complex process üian that of nitric acid. For one
thing, üie reaction sequence can begin with a wide range of reduced as well as partially
oxidized sulfilr compounds, mostly of natural origin. Hydrogen sulíide, carbonyl sulíide, car-
bon disulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulíide all contam sulhr in
the lowest(--21 oxidation state. 'lhese compounds ue released Rom the oceano and Rom sons
under reducing conditions as a consequence ofvarious microbiological processes. Some fea-
tures oftheir production will be discussed in Chapter 15. High temperatures favour microbial
activity and se the release ofreduced sulfür compounds is especially significant in the tropics.
Once in the atmosphere, a sequence of reactions begins. Hydrogen sulfide, carbon
disulnde, and carbonyl sulâde are oxidized via hydroxyl giving the thionyl radical l.SHI as
aninitialproduct.
Oxidation of sulfur dioxide by homogeneous reactions
Sulftlric acid production ftom sulfür dioxide takes place by at least two distinct sets of
processes. Tbe íirst sequence occurs homogeneously in the gas phase and most õ'equently
begins with reaction 5.21 as the rate-determining step.
H2S+.0H-+H20 +.SH IS.13) SO, + .0H + M --> HOS02 + M IS.211
CS2 + '0H --> COS + .SH IS.14) The reaction as shown is third-ordem but, in the lower troposphere, where the concentration
ofdie 'third body' M--mostly diniü'ogen and dioxygen--is large, it becomes pseudo second-
order. The second-order rate constant üierefore would decrease with decreasing pressure
moving to higher altitudes in the troposphere and stratosphere. However, a second factor
must be considered in deter)ining the rate constant. As with many other radical-radical
and ion-molecule reactions involving simple species, the activation energy 6or this
association reaction is negative. Therefore, the decrease in temperature with altitude in üie
troposphere by itself would result in a corresponding increase in the rate constant for the
reaction. Combining both temperature and pressure factors, best estimates of the second-
order rate constant are shown in Fig. 5.1.
The HOSO, radical can then undergo a number of relatively rapid reactions, some of
which result in sulftlric acid production. The simplest and most important acid-producing
processos
cos + .on --, coz + 'sH (5.15)
Of the three sulíide compounds, hydrogen sulfide and carbon disulíide react quickly but
carbonyl sulíide ECOS), released directly ú'om the oceans or produced by oxidation ofcmbon
disulnde. is kinetically relatively stable with respect to the oxidation reaction(5.151 shown.
Furljier oxidation of thionyl leads to production of sulfür dioxide.
-SH + 02-)SO +.0H l5.161
SH + 03-)SHO.+ 02 IS.t7)
l5.18)SHO.+ 02-)SO + H00.
0,
SO + O3 -) SO2 + other products
Phytoplankton living in suiface waters of the oceans produce a large amount of dimethyl
sulíide--in quantity, one ofthe most important reduced sulíür compounds that are released
unto the atmosphere. T.he hydroxyl radical reacts with dimethyl sulíide by hydrogen abstrac-
tion or by addítion.
NOz
l5.19)
HOS02 + 0z + M -+ H00' + S03 + M
This is followed by dissolution in water to form sulfilric acid.
IS.221
S03 + H20 -+ H2S04 l5.23)
'rhe hydroperoxyl radical produced in üie sequence also reacts with nitric oxide
NO +HOO.-->NO,+.OH
and the nitrogen dioxide and hydroxyl radical take part in the nitric acid-generating
sequence
IS.24)
--> ICH3SCH2 + H20
ICH3j2S + '0H ---> fürther oxidatíon products described in the previous section. A portion ofthe hydroxyl radicais reacts with additional
sulfür dioxide and se the set of reactions 5.21-5.24 is a self-acceleratíng sequence.
Starting again with sulfür dioxide, there are other quantitatively lesa important homogen-
eous reaction series that produce sulfüric acid, including direct reaction with atomic
oxygen. The rate constant for the reaction between sulfür dioxide and atomic oxygen is
similar to that for the reaction with hydroxyl radical, but the atomic oxygen tropospheric
mixing ratio is approximately two ordens of magnitude below that of hydroxyl
.N02 + .0H + M -) HN03 + M
-> ICH3gjOH)CH3
IS.20l
The fürther oxidation products include dimethylsulfoxide and methane sulfonic acid, bota
ofwhich have been found in the marine atmospheric aerosol. Sulfilr dioxide is algo formed;
the pathways and mechanisms for the complex reaction sequences have not been clearly
mapped out.
108 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.3 ATMOSPHERIC PRODUCTION OF SULFURIC ACID 109
10
5
1000 atmospheric mixing ratio is around l or 2 ppbv and which is readily soluble in water
IKU - 7.0 x 10 1 mol L't Pa i).
0
X
a)
0
E
HSO; (aq) + H2O2 (aql ;e H00SO2 (aq) + H2O
k.
2
l5.281
l 100 Peroxymonosulfite, H00SO;, has a structure
0
/ S .. ,,Oa-o' 'o'0
0
Q.
0.1 and rapidly rearranges to form hydrogen sulfate, HSO4 , whose structure is
o\K #o
S
/ \
OH O'
=
g3
g
g
E0
b-
0.01
300
250
200
0
In protonated form, hydrogen sulfate is sulfüric acid. The combined rearrangement and
protonation reaction is therefore
H00SO; jaql + H3O+ (aq) --{ H2SO4 jaq) + H2O IS.29)
k
In üie steady-state condition, the rate ofproduction of sulfüric acid by hydrogen peroxide
oxidation of sulfür dioxide is calculated in the following way. Based on reaction 5.29, the
rate ofproduction of sulftlric acid is given by
l HOOSO;jjH:O'''l
Assuming a steady-state concentration of HSO; ,
10 20 30 40
Altitude / km
50 60
Fig. 5.1 Varíation in the second-ordem rate constant for the production of HOSO2 as a function of
altitude in the troposphere and stratosphere. (Redrawn from Calvert, J.G. and W.R. Stockwell.
Mechanisms and iates of the gas-phase oxidations of sulfur dioxide and nítrogen oxídes in the
atmosphere, in ,4c/c#ó depôs/f/on. su/p/7ur and /7/fraga/7 0x/des(ed. A.H. Legge and S.V. Krupa),
Lewis Publishers Inc. Chelsea. Michígan; 1990.).
IS.30)
gi!!Çll?!9i! = O = kljHSO;jjH2O21 KbjH00SO;l k3jH00SO;jjHaO ''' l
In eqn 5.31, tl! = k2jH2OI is a pseudo íirst-ordem rate constant since IH2OI >>jH00SO;l.
IS.311
Oxidation of sulfur dioxide by heterogeneous reactions
In Chapter 3, we encountered a sequence ofheterogeneous reacüons that leads to large-scale
losses of suatospheric ozone during the polar spring. Sulfüric acid can also be produced in
a heterogeneous process when the required reactants are available in cloud droplets.
Beginning again with sulfilr dioxide, the following reactions occur.
SO,(gl # SO,jaql KH = 1.81 x 10's moIL':Pa': is.2sl
hjHS0;jjHz021 lnooso; l(kl; + k,ln:o"l) IS.32)
[HOOSO;]
k.IHS0; jjHz021
Kl: + k3jH30'''l
IS.331
Substituting eqn 5.33 in 5.30,
. djH2S04] . klk3jHS0; jjHzOzjjH30*l
dt k;+ k3jH30+lSO2jaql +2H2O ;e HSO;jaq) +H:O' jaql K,: = 1.72 x IO''mol L': l5.261
HSO;(aql +H,O ;e SO:' jaq) +H,O+ (aq) K,z = 6.43 x 10'' moIL': is.271
Like that ofcarbon dioxide, the aqueous solubility ofsulfür dioxide is pH-dependent, but
it is much larger throughout the pH range. Ifthe atmospheric mixing ratio ofsulfür dioxide
is 10 ppbv at P', the solubility is 2.2 x IO'6 mol L't when the aerosol has a pH of 4.0,
and is 2.2 X 10'3 mol L't when the pH is 7.0. The solubility is one ofthe factors aHecting
the rate of the heterogeneous reaction.Oxidation of sulfür species takes place within
the water droplets. The most important oxidant is hydrogen peroxide, a chemical whose
IS.341
The values ofthe rate constante atei
h = 5.2 x 10Õ Lmol't s'i
kl! / k3 = lO l
l Martin, L.R, Kineüç studíes QfsuHte oxÍdatÍotl {n aqueous solutíon, {tl SOZ, NO and NO2 oxídatíon mechanisms
atmosphedc c07üderaüons(ed. J.G. Calvert), Butterworth Publishers, Boston; 1984.
 
' l l l l l
110 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.3 ATMOSPHERIC PRODUCTION OF SULFURIC ACID 111
VVhen the pH is greater than 2, k3 IH3O+l << kl! and the rate is given by
rate = KaK3jHSO; jjH2OzjjH3O" l
2
is an important component of the stratospheric aerosol. Volcanic activity can be a means
by which sulfür dioxide is injected directly into the stratosphere increasing the
concentration of the aerosol there and leading to significant depression of the global
temperature by physically blocking solar radiation. The sulftlr compounds in the
stratosphere also take part in other chemical processem having diverse environmental con-
sequences For example, consider the case ofthe Mount Pinatubo volcanic eruption.
IS.3s)
Using üie equations for Km (see Chapter 111 and Kal
IH3O+jjHSO;l - KUKatPso, l5.361
IS.37)
l5.38)
rate = lhk3KHK. IH,O,Ipso,
= k'jHzOzjPso;
Volcanoes--the 1991 eruption of Mount Pinatubo
Mount Pinatubo is a vo]canic mountain ]ocated about 100]<m norüi-west of Manila in the
Phijippines. After being quiescent for 635 years, it erupted dramaticaUy inJune 1991, widi peak
releases on 14 and 15 June. Approximately 7 kin3 of magma was expelled as lava, and as ash
unto the atmosphere. ']bere was heavy ashíãU up to 40 ]<m ítom the volcano, and the typhoon
Yunya that occurred shordy after the volcanic eruption carried solids, deposiüng them as far
away as Thailand and Singapore. 'lhe ash--a cale-alkaline pumice--contained phenocrysts of
anhydrite(CaSO41, indicating that there were Hgh concentrations ofsulhr in the magma.
Besides ash, there was a release of large quantities of gases with composition in the
carbon-oxygen-hydrogen-sulfür family. Principal gases included water vapour and carbon
dioxide along with about 20 Mt of sulfilr dioxide IThis amount(--10 Mt expressed as sulfür) is
about one-tenth ofdle annual global anthropogenic release of sulfür dioxide; see Table s.s.l
'lbe gases were injected unto the stratosphere at an altitude of 20 to 30 km, and the sulfür
dioxide over time was converted to a sulfuric acid aqueous aerosol The aerosol doud driRed
to the north-west and could be observed as far away as the Greenland-lceland área in euly
January 1992. Vãrious processes caused the cloud to dissipate over a 1-3 year period.
There have been a number of environmental eHects attributed to the aerosol. The
dispersed cloud particles blocked solar radiation and a measurable, but not uniform,
average global cooling was observed in the 2 years following the eruption. There is also
evidence that ozone loss was accelerated, especially within the polar vortexes. This is attrib-
uted to enhanced conversion of dinitrogen pentoxide to nitric acid (reaction 5.10), a
reaction that readily occurs on üie suiface ofthe sulftlric acid-ice ciystals. By removing NO*
õ'om the stratosphere there was leis tenden(y for nitrogen dioxide to react with chlorine
monoxide(reacüon 3.52). This, in turn, augmented the chlorine catalytic cycle for ozone
destruction and caused a reduction of ozone concentrations.
The amount ofsulftJr dioxide released õ'om Pinatubo was too luge to have occurred only by
exsolution(release out of solution) úom the magma at üie time of the eruption; it is believed
that pre-eruption releases of large amounts of vapour also occurred. Such emissions occur
regularly at other terrestüal and marine sites, and are not always associated with catastrophic
volcanic eruptions. As we noted at the beginning of the book, this tape of release of gases of
has occurred throughout the entire expanse ofEarth histoiy. 'llüs led to the üoiTnation ofour
planet's unique atmosphere. Much, ifnot all, ofthe water on Earth was derived in ülis way.
Tais rate law apphes between pH approximately 2 and 5 where hydrogen sulíite IHSO;) is
the principal aqueous sulfür jlV) species. Within tliis range, die oxidation of sulfur dioxide
by hydrogen peroxide is the dominant mechanism and the reaction rate is approximately
independent of pH. Below pH --2 the rate decreases as reflected by an increase in the
denoininator of eqn 5.34. At higher pH values, sulíite(SO:) becomes the dominant sulfür
species and, because it does not react with hydrogen peroxide, the rate of oxidation vÍa this
oxidant again decreases.
A second heterogeneous pathway envolves ozone as the oxidant. In this case both hydro-
gen sulíite and sulfite itens are oxidizable.
HSO; (aq) + O3 -+ SO:' (aq) + H;O+ (aq) + Oz (5.391
SO: (aql + O, --} SOÍ' (aq) + O, IS.40)
The rates of the ozone-based reactions are such that, taken together with the previous
processem, below pH 5.5, hydrogen peroxide is the primary oxidant while, above that value,
ozone becomes more important. Depending on availability of oxidants, the heterogeneous
reactions can make a greater contribution to sulfür dioxide oxidatíon than do the gas-phase
processes.
Catalytic enhancement of oxidation of sulfur dioxide
By itselC dioxygen is able to oxidize sulíite only very slowly in aqueous solution. However,
the presence of small amounts of some metal itens catalyses üie reaction. Metais ülat have
been shows to increase the rate of the reaction include iron jli) and (111), manganese (11),
copper jll), and cobalt jlll). In acidified water, small concentrations ofthese metais are soluble
and the soluble species are the catalytic agents. However, even in higher pH situations
where metais suco as iron jlll) are very insoluble, surface-based catalysis still occurs. Other
sohds including carbon particles have also been found to increase the rate of oxidation by
molecular oxygen. Even when catalysed, however, oxidation of sulfür dioxide by dioxygen
in aqueous solution probably makes a relatively small contribution compared to other oxida-
tion routes. Metal ions may also enhance the rate of heterogeneous reaction of sulflir
dioxide by hydrogen peroxide and ozone.
Alternative fates of atmospheric sulfur compounds
We noted earher that carbonyl sulíide is quite stable to oxidation by the hydroxyl radical.
lts atmospheric lifetime is diíficult to determine and has been estimated to be between 0.5
and 7 years. As a consequente, difhlsion ofdiis particular sulfur-containing species isto the
stratosphere is an important removal process. In the stratosphere, it can undergo
photochemical oxidation to produce sulftlr dioxide and ultimately sulfate anion, which
Main point 5.3 The sulfuric acid in precipitation originates from a number of chemícal
precursora íncluding sulfur dioxide emitted directly into the atmosphere. usually from
anthropogenic sources, and natural-origin reduced sulfur compounds. The latter are
oxidized to sulfur dioxide with hydroxyl radical as the primary oxidizing agent. The sulfur
dioxide dissolves in water droplets and is then further oxidized to sulfuric acid P/a several
homogeneous and heterogeneous processes.
r112 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.5 RAIN, SNOW, AND FOG CHEMISTR\t-SIMILARITIES AND DIFFERENCES 113
5.4 Acidifying agents in precipitation For sulfur dioxide the concentration is
25 p,gm'3 = 25 x 10 ógm' x 6.02 x 102s molecmol'i x 10 6 m3cm'3
64 g mol'l
[SO2] = 2.4 x 10ii mo]eccm 3
For nitrogen dioxide, the rate of oxidation is
-=:gli:9Ü . k..:IN0211'0H]
= 1.2 x 10'zi cmSmolec l s'i x 6.5 x 10ii moleccm 3 x 1.7 x 10ó moleccm'3
= 1.3 x 107 moleccm 3 s
= 4.8 x 10:' moleccm 3hour l
l
The major tons present in precipitation are well-known and are listed in Table 5.1. As
noted, sodium, potassium, calcium, and magnesium itens are cations of strong bases, and
chloride, nitrate, and sulfate are anions of strong acids. As such,all these species are them-
selves neutral and therefore the only major itens that perturb the acid-base balance ofthe
water are ammonium and hydronium ion itself. The activity of hydronium ion, of course,
directly detemiines the solution pH. Since ammonium is a very weak acid(pK: - 9.25), in
the presence of even a small excess of hydronium ion, it is unable to donate protons and
has a negligible eífect on the precipitation pH. 'l'his is not to say that ammonium lacks
acid-producing capability. On üle contrary, when deposited in soil or water, under aerobic
conditions, microbial oxidation ofammonium to produce nitrate generates two hydronium
ions for each ammonium molecule. This last figure represents a loas rate of approximately 7% of the original concentration of nitrogen dioxide in
l hour.
for sulfur díoxide. a similar calculation is as follows:
rate of oxidation = -:gi?o2] . kso,[SOzJ['OH]
= 1.2 x 10 i2 cm3molec'i s l x 2.4 x IOii moleccm 3 x 1.7 x 10ó moleccm
= 4.9 x IOs moleccm ss l
= 1.8 x 109 molec cm 3hour'i
3
NH4+ jaq) + 202 + H2O c'oo:ganisms . NOã jaq)+ 2H3O+ jaql IS.41)
In this indirect way, ammonium in precipitation is a potent contributor to acidification.
More will be said about this vezy important react:ion called nitriâcation in Chapter 18 when
we deal with the chemistry of soil processes.
We have indicated that the hydronium ion in ram is associated with either nitric or
sulfüric acids produced by mechanisms described above. If these two componente were
the only sources, there should be a good correlation between hydronium ion concenüation
and that of sulfate and/ or nitrate. Of the numerous studies designed to examine the
issue, many do show excellent correlations(correlation coefíicient > 0.81 for one or other of
the relaüons. But there are a number ofexceptions indicating that additional factors, includ-
ing meteorological anomalies, other emissions, and proximity to source, overrule any
simpliíied relation. A good example is in the prairie regions of western North America
where hydronium ion is best conelated--in an inverte fashion--to calcium ion, indicating
terrestrial control of precipitation acid/ base balance through alkaline soil minerais.
The relative importance of nitric and sulfüric acids depends on distance â'om source
because the rate of conversion of NO* to nitric acid, and its deposition velocity is greater
than corresponding iates for sulfüric acid. This can be shown by the following example
that considers the rate-determining step for the two principal homogeneous reaction
processem.
The initial loas rate of sulfur dioxide is therefore approximately 0.7% of the original concentration in l hour.
Note that we have only considered the homogeneous oxidation processem in this calculatíon
Taking unto account these and other oxidation pathways, it is found that, when both
sulh.r and nitrogen oxides are emitted â'om an industrial region and the emissions along
with transformation products move downwind together, the molar ratio of sulfate to
nitrate increases away õ'om the source. For example, the ratio is approximately 1: 1 to 1.5 : 1
in the Netherlands, a levei indicative ofthe industrial heartland ofWestern Europe. Moving
norte-east with prevailing winds, the ratio becomes 2 : 1 in southern Scandinavia and as
high as 5 : 1 in northern Scandinavia.
Main point 5.4 Nitric and sulfuric acid, origínating largely from combustion sources
in industrialized áreas, are the principal causei of unnaturally low pH in precipitation at
various places around the globe.
Example 5.3 Rates of oxidation of NO2 and SO2
Consider a situation where the atmospheríc concentrations of nitrogen dioxide and sulfur dioxide are. respect-
lvely, 50 and 25 p,g m'3. These are typical concentrations observed in heavily industrialized áreas such as in
Western Europe and Eastern North America. A reasonable average 24 hour value for the concentration of
hydroxyl radical during the summer months is 1.7 x 10Ó moleccm-3. The relevant pseudo second-order rate
constants at the Earth's surface for reactions 5.2 and 5.21 are 1.2 x 10 Ucm3 molec-is-i and
1.2 x 10 i2 cm3 molec-l s i, both estimates made at P' and 25 'C.
The atmospheric concentration of nítrogen dioxide is
50 p,gm'3 = !9->< 10 ógm x 6.02 x IOZ3molecmol l x 10'óm3cm'3
46 g mol
[NOp] = 6.5 X 10n mo]ec cm 3
5.5 Ram, snow, and fog chemistry--similarities and diHerences
Ram
T.he chemical composition of ram is highly variable depending on the geographic location
and the infLuence of natural and anthropogenic chemical processes on the atmosphere in
that region. We have emphasized the role ofnitrogen and sulfür compounds in determining
the acidity of ram and other precipitation forms. Table 5.1 reported the major element
composition of ram at severas sites. Addiüonal elements(including metalsl are present in
ram in trace amounts, again depending on location. To some extent the trace components
114 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.5 RAIN, SNOW, AND FOG CHEMISTRY-SIMILARITIES AND DIFFERENCES 115
are derived õ'om soil and other dust particles that act as nuclei around which water
condenses to form cloud droplets. The solubility of metais â'om such sources dependa on
the nature of the metal and Uie original form in which it was present. Metais associated
with a sihcate mineral matrix are almost completely insoluble. Where insoluble iron jlll)
and aluminium hydrous oxide particles falso commonly of terrestrial origina are found in
water droplets, they act as scavengers by adsorbing various chemical species on their
surface. Elevated amounts of these solids therefore can suppress the solubility of other
metais. T.he rainwater pH is another factor that controls metal solubility, wiüi most metais
becoming more soluble under more acid conditions. a 'He Antarctic results are from 14 simples of surface show taken along a transect moving inland from 100 to 430 km in
Berre Adelie. From Legrand M. and R.J. Delmas. Spatial and temporal variations of snow chemistry in berre Adelie (East
Antárctica), ,4/7/7. G/ac/o/., 7(1985), 20-5.
b The limited data from Scotland are based on 15 simples taken along a 700 m transect. From Brimblecombe P.
M.Iranter. P.W. Abrahams, 1. Blackwood. T.D. Davies. and C.E. Vincent, Relocation and preferential elution of acidic solute
through the snowpack of a small, remate. high-altitude Scottish catchment. ,4/7/7. G/ac/a/., 7(1985). 141-7
As in ram, water droplets in fog and mist algo contam chemical species accumulated ítom
the atmosphere. The composition is similar to that of ram, but concentrations tend to be
higher in fog because of its location near the Earth's surface where leveis ofcontaminating
gases and other species are usually greater. One ofthe foggiest áreas in the world is the Bay
ofFundy on the Atlantic coast of eastern Canada between Nova Scotia and New Brunswick.
During the summer season Rom April to October, warm air is drawn lato the bay üom the
south. Passing over the com ocean water, it is chilled, causing condensation and â'equent
heavy fog conditions. During this season, áreas adjacent to the Bay of Fundy are subjected
to fog for 12-30% ofthe time, sometimes for 3 t0 5 days continuously at a stretch.
There is concern that birch trees growing in the forests adjacent to the bay would be
adversely aífected by prolonged exposure to the acid-containing fog. In one study, analysis
offog composition along a 37.5 km transect inland õ'om the coast has been canied out. The
volume-weighted mean concentrations(over the 1987 growing season) ofmajor constituents
at all õve sites are given in Table 5.3.
Fog
is subject to ftlrther inputs õ'om the atmosphere by wet and dry deposition processes.
'rherefore we must also consider the chemistry ofthe snowpack as an accumulated deposit.
Table 5.4 shows concentrations ofionic species in freshly fallen snow in the Antarctic and
in Scotland. The extremely low values found in the Antarctic samples are indicative of a
location remote õ'om anthropogenicsources of these tons. In both cases, for samples
reZatíveZy dose to one another, considerable variabilily in concentrations was observed in
spite ofthere being no obvious local iníluences that should aífect üie results. The inhomo-
geneity indicates diíFerent iates of atmospheric scavenging of ions over space and time
and / or the eHects oflateral movement ofsnow due to wind action.
At other locations around the Earth,l there is even more variability in the chemical
composition of snow simples.
Snow remaining on üie ground over the winter season is subject to chemical alteration
due to inputs from dry and wet deposition influenced by urban or industrial sources as well
as natural organic debris--the latter especially important in forested áreas. 'lbe additional
deposits contübute to spatial variability in physical and chemical composition and, perhaps
more importantly, can influence the melting processes and the nature ofthe meltwater.
Aside õ'om the additional deposits, during the winter season, snow undergoes metamorpho-
sb with individual particles coalescing and recrystallizing unto larger grains.2 As part of
this process, the solute tons are partially excluded h'om üie ice crystal latüce and tend to
migrate to the crystal surfaces. In winter it is also normal that üiere be peüods of partial
melting of the snowpack due to higher temperatures and/ or to exposure to intense
sunlight. During these periods, the early melt â'actions encounter the surface impurities and
dissolve them in the meltwater, leaving reduced concentrations in the remaining snow.
Such events may occur several tomes be6ore complete melting in the spdng mn-(2B Tbe total
amount of solute available for dissolution declines as winter proceeds but, during each
event, an initial ílux ofhigh-concentration solution is produced. For the major precipitation
anions, preferential elution occurs in the ordem: sulfate > nitrate > chloride. Tais means that
early meltwater is enriched in sulfate. The snowpack is generally depleted of ions but in
relative terras is eniiched in chloride--which will then be canied away in the final meltwater.
2 Jeíftíes, D.S. Snowpack storage ofpollutants, release duüng melting, and impact on receiving
waters, in Addíc p'ec@ítatíon. Vo1. 4(ed. Norton, S.A. S.E. Lindberg, and A.L Page). Springer-Verlag,
NewYork; 1989.
a volume-weíghted mean values from ave sites, taken between April and October, 198Z(Cox, R.M. J. Spavold-'nms, and
R.N. Hughes. Acid fog and ozone: their possible role in birch deterioration around the Bay of Fundy, Canada. Mafec ,4#
Soir Poflution. 48 (1 989). 263-76).
These average concentrations are greater than those that have been measured in many
rainfall simples(Table 5.1). By following the trend of values beginning at the coast and
moving inland, it was found that concentrations of most of the dissolved itens increased
steadily and this was attributed to evaporation of the aqueous solvent. Increases were
largest for hydrogen ion and for sulfate, probably an indication of an additional cause--
rapid heterogeneous oxidation of dissolved sulfür dioxide in the aqueous aerosol during
the time taken üor üie fog to draft away üom the ocean. The oxidant may have been ozone
or alkyl peroxides, both produced by reactions involving the hydrocarbon emissions â'om
the forest.
Snow
Tbe chemistry of snow must be considered in two aspecto. First is the nature ofthe snox4áaZI
as precipitation--i.e. its composition at the time it is deposited on the surface of the Earth.
Secondly, because snow õ'equently remains on the ground for extended periods oftime, it
iaDie s.'i çomposltion OT two Tresn snow sampies (bracketed values are standard deviations) 
Concentration / »mol L'i 
Lacation HSO+ Na+ K+,Ca2+ NH; Mg2+ cl' NO; soÍ'
Antárctica 1.52(0.60) 0.64 Not 0.11 0.073 0.84 0.82 0.26
surfacesnowa pH,5.82(0.26) detected(0.04)(0.030)(0.31)(0.35) (0.09)
Ciste Mhearad 279 (31) 13 23
Scotlandb pH,3.55 (5) (g) : ,
Table 5.3 Concentraüons OT major consutuents OT Tog near the uay OT t"unay, çanaGa' 
Species H+ Na+ K+ CaZ+MgZ+' NHl; CI' NO; soã
Conc/p,maIL'l 330(pH=3.5) 78 31 13 ll 50 61 160 245
116 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.6 THE GLOBAL PICTURE SOURCES AND SINKS 117
The combination of additions of organic and inorganic species to the snowpack by wet
and dry deposition, and removal of chemical species by mid-winter melting means that
snowpack chemistry inevítably changes over the season. For any particular species,
depending on location and winter chmate, the snow concentration may show an increase
or a decrease.
In later parts ofthe book IChapters ll and 18), we will discuss the ability ofwater and soil
to neutralize inputs of acid. Where acid is provided continously and slowly over extended
periods of time, depending on the nature of the soil, it may be neutralized. However, a
sudden release ofacids unto the aqueous or tenestüal envüonment can lead to a phenomenon
]<nown as add stock. Ttlis temi is used to reter to üie large flux ofwater that passes over and
through the soir when the final spring thaw occurs. Because there is limited üme üor contact
with the soil, a large proportion ofthe dissolved tons remains unreacted and ends up directly
in sMace or groundwater. Although meltwater released at that time may not have as high a
concentration of some species as was present in earlier releases, the great volume ofwater
ensures that the soil, rivers, and lakes that receive the water me also receiving large amounts
ofcations jincluding hydronium ion), anions, and organic species. Tbe sudden influx can have
a major eKect on water, soil, and bioma.
a Nitrogen data based on Jaffe. D.A.. The nitrogen cycle. In G/oóa/ b/ogeocóem/ca/ qJ/c/es,(eds. Butcher. S.S. R.J. Charlson
G.H. Orians, and G. V Wolfe), Academic Press, Londonl 1991
b Sulfur data based on Scriven, R. What are the sources of acid ram? in Scottish Wildlife Trust,/?epo# offóe ac/d ra/n
#7qaliry. Edinburgh; 1985.
condensation nuclei for water and play an important role in cloud and fog formation. For
the most part, however, the suUate is a chemically unreactive species.
Ofüie compounds that are precursors ofsulfüric and nitric acids, about halfare emitted as
a result ofhuman activities, mostly related to energy production, while the other halfarise
&om naturally occurring geochemical and biogenic processem. There are two major anthro-
pogenic sulfür dioxide sources--emissions üom the smelting of sulfide-based odes and the
combustion of fóssil fuels. The formei sources include production of copper, nickel, lead,
and zinc, which are â'equently found as metal sulfide minerais. One well-known case is
that of the high-grade nickel-copper ores at Sudbury, Ontario, Canada. In üie nineteenth
century and the early part of the twentieth century, refining of the ore was done in open
roasting beds, using much ofüie ümber â'om forests in the local region, releasing huge quan-
tities of sulfür dioxide at ground levei. The ambient sulfür dioxide and the acid generated
hom it destroyed much ofthe remaining vegetation in the Sudbury área and the barren, shal-
low sons were eroded õ'om üie underlying bedrock. In recent years, stringent controla have
reduced the emissions substantiany. At present, there are smaller leveis ofemissions and üie
reduced amounts of sulfür dioxide are released through a 'superstack' 400 m above ground
levei Of course, while the emissions that are now released cave minimal eHect in the local
área, they still contribute to üie regional and global budget of sulfiJr dioxide.
All fossil füels contam some sulftlr. Coal isconsidered to be the major source, wiüi sulfür
contents ranging õ'om fractions of a per cent to 10% in some cases. Smaller amounts are
present in hquid füels.Gasoline may contam 10 to 500 ppm sulfür depending on its origin
and the reíining process. Concentrations in diesel fiel typically rali between 1000 and
5000 ppm. There can be large amounts of sulíür compounds in some natural gas(methanelsupplies, but üiese are substantially removed in the reíining process.
You will recall that combustion-produced nitrogen oxides always originate õ'om
atmospheric dinitrogen whenever the burning temperature is very high, as in internal
combustion enganes or in large-scale industrial units that bum fóssil füels. When biomass is
burned as in forest ares or for domestic heating and cooking purposes, however, the
temperature is usually too low to oxidize substantial amounts of atmospheric nitrogen, yet
nitric oxide is süll released. In these cases, the nitrogen oxides are derived almost exclusively
Main point 5.5 Various chemical species are present in small concentrations in ram, snow,
and fog. Most of these species are derived from natural sources; however, anthropogenic
nfluences in certain local environments may add to the occurrence of similar species.
5.6 The global picture--sources and sinks
It is vezy diííicult to make quantitative estimates ofthe sources and sinks of nitrogen and
sulfür compounds found in the atmosphere, and amoupts measured and calculated by
diHerent researchers show large variability. The amounts from various sources reported in
Table 5.5 are recent estimates for the Earth as a whole and give some pense of the major
processes aífecting atmospheric concentrations. The ultimate sinks for these compounds
are through deposition onto water and soil.
Sources and sinks of tropospheric gases
Sources: Prímary sources--the chemical is released directly into the
troposphere.
Secondary sources--atmospheric reactions involving primary emission
products produce the chemical of interest.
Sinks: Chemicals can be removed from the troposphere by deposition on to
land or water or by leakíng into the stratosphere. They may algo be
'removed' by reactions to form other specíes.
We have noted that there are a number of reduced sulfür species emitted õ'om the
oceano that contribute to aerosol and precipitation acidity. Sea salt particles algo contam
sulfür that is in the form of alkah and alkaline earth metal sulfates. These particles act as
Table S.S ivialor sources OT nltrogen ana sulTur compounas in tne atmospnere 
Nitrogen compounds; N/ g x IO lz y l Sulfur compoundsb S/gx 10 t2y l
NH3 Solid species mostly SOZ' 
Bíogenic volatilization 122 Sea salt 44
N0*Dust 20
From stratosphere l Reduced sulfur 
Atmospheríc oxidatíon of NH3 l Biogenic(oceans and land) 98
Lightníng 5 Partially oxidized sulfur 
Biogenic 8 Volcanoes(average) 5
Biomass combustion 12 Fossíl fuel combustíon/ smelting 104
fóssil fuel combustion 20 
118 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.6 THE GLOBAL PiCTURE SOURCES AND SINKS 119
â'om the fiel itself. The amount released is then equal to the amount of nitrogen in the
biomass multiplied by the conversion efnciency. Tais latter term is open around 10%
and dependa on combustíon conditions; it can only be determined experimentally for each
situation.
L
=
B
B
C
0
g
=
C
=
Ba
g
g
ã
}
.q
c:
8
à
8
'q
c:
a)
a)
0
E
'
0
a)
c:
=
3
a
E0
C
a)
g
C
C
=
g
g
Ba
C
B
E
g
a
C
Z
E
C0
a)
a)
g
=0
>
C
=0
=
g
=
=
B
a)
0
=
=:
=
E0
E
g
a)
X0
=
a)
0
C
0
=0
E
a)
N
=C
C
Q
In
a)
=
g
N = mass ofbiomass x õ'action ofN in biomass x conversion eíficiency (5.42)
Table 5.6 gives values estimated for annual nitrogen oxide emissions associated with
agrícultural practices in various regions in the tropics. The total nitrogen oxides produced
are calculated to be between 3.2 and 6.1 Tg y 1, a significant part ofthe 12 Tg y'l attributed
to biomass combustion ITable 5.5).
Most atmospheric ammonia is derived 6'om biogenic sources jsee Chapter 6l; nevertheless
some ofthese sources have oügins closely related to humans. Tbe caule industry--both in
termo ofmilk and meat production--has grown with the human population and ammonia
produced from manure makes a large contribution to the atmospheric budget. A small
portion of the volatilized ammonia is oxidized in the atmosphere and the rest (about
8Tmol y i) is neutralized by atmospheric nitric and sulftlric acids of which 4 and
5.5 Tlrnol y 1, respecüvely, are potentially produced ú'om the atmospheric nitrogen
and sulfür compounds. These acids are sufficient to completely neutralize 4 jüa nitric
acidl + 2 x 5.5 (via sulfüric acidl = 15 Tmol y-l of ammonia, and thereüore, taking the
Earth as a whole, there is excess acid that contübutes to precipitation acidity.
If all anthropogenic releases of acidic and basic substances were eliminated, natural
production ofboth might be in closer balance and the global average pH could be near the
5.7 value.
The release ofnitrogen and sulftlr compounds varies â'om country to country around the
Earth. About 60 or 70% of the anthropogenic emissions of both elements come üom
sources in Europe and North Amelica. As a consequence, rainfall chemistry also varies and
some ofthe most severe acid precipitation problems occur in these continents. 'lbe average
pH in parts of the highly industüalized world is as low AS 4.0, and individual events con-
taining even more acidic precipitation have been observed. At the turn of the century,
growing amounts ofrelease are occurTing in China and Índia leading to oüier áreas experi-
encing low-pH ram. Figure 5.2 shows mean rainfall pH values in regions around the globe,
-H ; ,--+ l -l
>1
Z
.?
'B Q
= -1
0 '-x
0 ; (N : cq
(\l : (\J : r--l
l : l ; l
0 : CN : 0
--ll -l l -l
.:
cn : 0 : 0
--H l -l l -'l
8
:
z ã
ü.o l..o
-i l C) 1 0
a)..4
n l
o : r\ : r-''l
o l cxi l -'+
E
Q
g
N
g
60 N
30 N
EQ
30S
60 S
0- l -"i l.o
tâ ; (N ; r<
-l l cu ! -{
=
g
g
C
a)
€
C0
a)
>C0
m
mo\
a)
a)
C0
'
a)
8
a
g
C
@
=
3
<:
180W 120W 60W 0 60E 120E 180E
Fig. 5.2 Annual average pH of precipitation. (Reproduced from Rodhe. H.. FI Dentener. M. Schulz. The
global distribution of acídifying wet deposition. Enp/ron. Sci necó/70/., 36 (2002), 4382-8.).
120 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION 5.7 CONTROL OF ANTHROPOGENIC NITROGEN AND SULFUR EMISSIONS 121
Fermi question
If China were to double its current production of electricity using coal-fired power plants,
how would this affect the global budget of sulfur dioxide that is released annually to the
atmosphere?
Exhaust
some individual values for less well-characterized locations, and places where problems
associated with acidic depositions have been reported. Such maps provide a simple means
of displaying pemlrbations in precipitation chemistry but it is important to be aware that
pH is not the only precipitation property of concern. Other cations and especially anions
can also adversely aHect the properties ofthe receiving water and soil. Some ofthese eHects
will be discussed in later chapters.
Process
coolant coilsCoaland
limestone""s-.
Fluidized
bed
Main point 5.6 Áreas where acid precipitation is endemíc are usually in regions of intense
ndustrialization--Europe and North America being prime examples. Anthropogenic
sources of both nitrogen and sulfur oxíde emíssions are associated with combustion
processem. Sulfur dioxide is also released during the smeltíng of sulfíde minerais.
Fig. 5.3 A fluidized-bed combustion unit with cyclone for removal of particulate material in the flue gases.
is a reaction between sulfür dioxide and lime (calcium oxide) to produce solid calcium
sulfate.
CACO:jsl --> CaO(sl + CO2jgl (5.431
5.7 Control of anthropogenic nitrogen and sulfur emissions
CaOjsl + SO,(gl + âOzjgl -.> CASO.(sl IS.«l
We have seen that the anthropogenic sources of nitrogen oxides and gaseous sulftJr
compounds centre around energy-related activities. Four approaches for reducing these
emissions are possible--decreasing energy use by various eíliciency measures, producing
energy vÍa non-combustion processem, preventing emissions of the problem gases, or
removing the gases ater they have been generated. All of the approaches are technically
possible and there are philosophical, political, and economic arguments to be made for and
against eachone. In Chapter 4, we considered catalytic methods of reducing nitric oxide
emissions â'om vehicles. Here, using coal combustion as an example, we will look brieíly
at technology related to methods of minimizing emissions of both nitrogen and sulfiJr
compounds. Simultaneously, other environmental eHects ofthe modified technology must
be considered.
At the high bed temperature, unbumed coal particles are mechanicany separated â'om the
blue gas by a centriítlgal cyclone device and Fed back into the combustion bed. Heavy ash,
including the calcium sulfate, set:nes through a güd under the fluidized bed and íiner
particles, carried upward in the blue gases, are trapped by an electrostatic precipitator or a
fabric íilter. Although conventional precipitators are capable of removing well over 99%
jby massa of the airbome particulates emitted during coam combusüon, the eíficiency may be
much less jabout 30%l when calculated on the bases of number of particles. In particular,
these systems are much less eHective in cona:olling particles with diameters less than approx-
imately 5 p,m and it is these colloids that are potentiaUy most hazardous to human health.
More will be said about atlnospheric particulates and their control in the nexo chapter.
As a means ofremoving sulíür dioxide, tais is a highly efficient process, eHecting around
90% recovery. Nitrogen oxides emission is reduced by about 50% due to the controlled com-
bustion conditions, which also enhance conversion of carbon monoxide to carbon dioxide.
Fluidized-bed combustion
New topes of combustion chambers have been designed se as to enhance the eíficiency of
coal combustion and of heat transfer and therefore to minimize fiel use. Fluidized-bed
combustion IFBC) for burning coal is one such technique(Fig. s.31.
In the FBC combustion chamber, preheated air is forced upward through a bed of
powdered coal. The passage ofair, as well as convection generated by the hot gases from
the burning ninely divided particles of coal, creates a fluid-like suspension. In this
configuration, uniform and complete combustion occurs with reduced emissions of car-
bon monoxide. The combustion temperature is somewhat lower than that in a static
bed, thus reducing the amount of nitric oxide produced. A modiâlcation of the method
allows for the simultaneous injection of powdered limestone unto the bed se that there
Retrofitted flue gas desulfurízation
Existing conventional coal-íired plants can be retroíitted with devices that remove sulftlr
dioxide from the blue gases. Among the many processes used the most comman are lime
and limestone slurry scrubbers in which the combustion gas passes through an aqueous
slurry where react:itens to form calcium sulíite take place:
With hydrated leme sluny
Ca(OHj2 + SO2jgl --> CaSO3jgl + H2O (5.451
With limestone slurry
CaC03 + SOzjg) -+ CaSO3js) + CO2(gl IS.4õ)
122 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION ADDITIONAL READING 123
The insoluble calcium sulfite may be oxidized downstream to produce CaSOs '2H,O,
which is the mineral gypsum.
CASO,lsl + 102jgl + 2H2O -) CASO. .2H2Ojsl is.47)
which settles in disposal pondo or may be recovered for use in applications such as manu-
facturing plaster or plasterboard.
This technology eíficiently desulfüüzes the gas stream (--90%) but vast quantities of
water and leme jor limestonel are required and the resulting wastes are correspondingly
large. For a 1000 MW coal-íired electricity-generating plant supplied with coal containing
10% ash and 2% sulhr, about 10 000 t of coal are burned daily. The solid wastes include
1000 t ofash. The daily volume ofwater required for the scrubber slurry is around 7000 m3
and the limestone requirement is about 600 t. Over 900 t of gypsum are produced in the
process 6or each day of operation.
An obvious altemative to sulfür removal by these processes is the use of low sulfür coam.
The decision about reduction of sulfür emissions then becomes one of economics--
whether it is leis expensive to transport low sulfür coam to the plant site or to equip the
facility with the requisite control system.
Exhaustgas
Slurry containg
additives
Electrostatic
Airheater precipitator
(
Burner
Airin
To disposal
Fig. 5.4 The SONOX process for removal of nitrogen and sulfur oxides from stack gases
The SONOX process for removal of both sulfur and nitrogen precursora
A recently developed process for contrai of acid gas emission ú'om power planta is called
SONOX. This process, developed in Canada by Ontario Hydro at its smal1 640 MJh l
Combustion Research Facility, is soon to be tested on filll-sized boilers.
'l'he control system involves in-fümace injection of an aqueous sluny of a calcium-based
sorbent, usually powdered limestone, and a nitrogen-containing additive, usually urea, at
temperatures ranging between 900 and 1350 'C. The following reactions occur in Uie high-
temperature atmosphere ofthe fürnace reactor
CACO,is) --!!SgL-' CaOjs) + COZjg)(5.43)
gases is augmented ú'om a value of I0-25 ppmv jwithout addiüve) to 50-150 ppmv. More
studies may result in methods for reducing these concentrations.
Tbe solid waste obtained ater sluny injection is about double in mass ofthat that results
â'om combustion ofcoal without emission contrai. IThe actual amount depends on the ash
content ofthe coal as well as the amount of sulfur dioxide sorbent used. Thís latter amount
depends, in tum, on the sulfür content of the coal.l The additional material in the solid
waste consists of unreacted calcium oxide and calcium suUate and consideration must be
given to disposal of these mixed residues.
Conversion of coal to gaseous and liquid forms
Finally, the environmental consequences of using coam to manufacture synthetic gaseous
and liquid füels, open called WIÜels, must be considered. The conversion ofcoal to gaseous
or liquid forms is carüed out in ordem to create energy commodities that can be transported
via pipelines, are readily stored in containers, are clean, and are suitable for use in small-
scale facilities, particularly in vehicular enganes. Conversion, especially ofthe poorer grades
of coal, to liquid or gaseous forms algo allows for the upgrading of energy content of the
fiel. The ftlel components ofcoal are principany carbon and hydrogen, and the basic prin-
cipie for conversion is to increase the relative proportion of hydrogen compared to carbon.
We will look at these technologies in a later section.
CaO(s) + SO2jg) + ãO,Igl --> CASO.(sl
NH2CONH2jsl + 2N0(gl + {0,Igl -+ 2N:jgl + CO,(gl + 2H,0 (5.481
Spraylng the additives into the fürnace vía a high-pressure nebulizer ensures rapid
evaporation ofthe solvent and eíhcient 'cracking' ofthe calcium carbonate and urea to pro-
duce calcium oxide and the reactive amidogen radical(NH2CO&HI respectively. A schematic
ofthe process is shown in Fig. 5.4.
Capture eíficiency of sulftlr dioxide and nitrogen oxides depends on the nature of the
additive, the rate of addition, the spray characteristics, the reactor temperature, and
the sulfür content ofthe coal. Optimum condidons include:
ftlrnace temperature --1150 'C;
Main point 5.7 lechnologies for control of nitrogen and sulfur oxides in industrial emissions
have been developed. Economics of the control processem and problems associated with
disposal of the waste materiais are two factors limiting applicatíon of these technologies.concurrent injection;
mean droplet diameter --6.6 p,m.
The sulfür dioxide sorbent is --90% porous limestone and 10% dolomite or hydrated lime,
and the solids are present at a concentration of 40% in an aqueous sluny concentration,
calcium:sulfilr ratio 2.5 to 3 :1. The nitrogen oxides sorbent is urea or ammonium cubonate
in aqueous solut:ion at a stoichiometric molar ratio, additive:nitrogen oxide = 1.7 to 2.0 :1.
Under these conditions up to 85% sulh.r dioxide and 85-95% nitrogen oxides removal can
be eífected. When urea is used as an additive, the concentration ofnitrous oxide in the blue
ADDITIONALREADING
1. Ca\verá. J.G.(ed.). SOZ, NO and NO2 oxidation mechanismsl atmospheric consideratíons. Butterworth
Publishers, Boston; 1984.
2. Legge. A.H. and S.V. Krupa(eds.), .4c/c#c cepos/f/0/7: su/p/7ur a/7d n/froge/v ox/des, Lewis Publishers Inc.
Chelsea, Michigan; 1990.
r'
124 CHAPTER 5 TROPOSPHERIC CHEMISTRY PRECIPITATION
3. Lindberg, S.E. A.L Page, and S.A. Norton(eds.), ,4c/c#c prece/faf/on. Volume 3: Sot/rce$ danos/f/on. a/zd
ca/70py #zferact/ons. Springer-Verlag, New York; 1990.
4. Rodhe, H. and R.Herrera (eds.). 4c/d7/7caf/o/z /n frop/ca/ cot//vedes, John Wiley and Sons, Chichester; 1988.
PROBLEMS CHAPTER 6
l
2
The tropospheric processes discussed in this chapter cannot be considered as independent reactions.
Discuss the relationship between the productíon of nítrogen oxide species and the formation of 'acid ram'
and the role played by carbon monoxide, methane, and the hydroxyl radical.
In a particular 3000 km2 region of southern Sweden, the annual rainfall averages 850 mm, its mean pH ís
4.27. and 66% of the hydrogen íon ís associated wíth sulfuric acid with the remaining 34% derived from
nitric acíd. Calculate whether sons of this region are subject to excessive sulfate loading if the only source
of sulfate is rainfall and if the recommended maximum is set at 20 kg SOã ha-i.
The mean monthly pH values of rainfall in Guiyang cíty ín Guizhou province in southern China in 1984
were as follows:
Jan 3.9. 4.0, 3.8, 4.1, 4.0, 4.5, 4.5, 4.1, 3.7. 3.8. 3.7. 3.4 Dec
Atmospheric aerosols
Topicsto be covered
3. Aerosol particles
H What are aerosols and why are we concerned?
Sources ofaerosols, natural and anthropogenic
Chemistry of condensation aerosols
Concentration, lifetime, and properties
H Aerosol control technology
For the some year. the measurements at Luizhang, an adjacent rural área were:
Jan 4.3. 4.4. 4.4, 4.2, 4.5, 4.9, 4.9, 4.6. 4.8. 4.3, 5.4. 5.4 Dec
(a) Calculate the mean monthly pH of the ram at the two iocations.
(b) What is the ratio of the mean hydrogen ion activity at the two sítes?
(c) What is likely to be the most important anion ín the raínfall at the urban site?
Refez to Dianwu, Z. and X. Jiling. Acidification in southwestern China. In ,4c/dPcaf/on /n frop/ca/ cou/7fdes
(ed. Rodhe. H. and R. Herrera), John Wiley and Sons, Chichester; 1988.
The ionic composition (in uníts of mg m-3) of an atmospheric aerosol in a tropical ram forest ís
soã , 207; NO;, 18; NH4'', 385; K '. 180; Na''', 247;
4.
An aerosol is a suspension of particles in a gas and an atmospheric aerosol consists of
particles that remam aloÊ in the air.
When we reter to particles in an aerosol, in the definition we are including both solids
and liquids. Particles are distinguished from smaller gas molecules or molecular clusters by
their ability to cause incoherent scattering of visible light and therefore to interfere with
light transmission. As a consequence, the presence of a high concentration of aerosol is
indicated by a hazy appearance in the atmosphere. To scatter visible light, particles musa
have dimensions comparable with or larger dian the wavelengths ofthat light, say, within
an order of magnitude(for example, at least one-tenth of 400 nm, that is 40 nm or
0.04 p,m).
There are a number of factors that determine how long particles can stay suspended in
the air and we will examine these in detail later in this chapter. Large particles readily
settle out. Except for those ofvery low density, most parücles with dimensions greater than
10 p,m require strong air currents to keep them aloft. On the other hand, veiy small particles
have limited lifetimes as independent entities, because they come together and coagulate
to form larger ones. Particles within the size range O.Ol-l p,m are most likely to remam
suspended for long periods oftime, sometimes up to a month or even longer, thus allowing
them to move with the air mass.
Aeroso[ partic[es are](nown by a vaüety ofnames, depending on the source and nature of
the particles. Many of the nomes are common; we are all familiar with termo like dust,
smoke, fly ash, and pollen jsolids in gas) and cloud, mist, fog, and smog jliquids in gas).
Figure 6.1 documenta some properties ofvarious topes ofatmospheric aerosols.
The pH of the aerosol is 5.22.
Use these data to calculate the total positive and negatíve charge 'concentration' (mol m 3) in the aerosol
and suggest reasons that míght account for any discrepancy in anionic and cationic charge.
The tropospheric mixing rabos of carbon monoxide are higher in the Northern Hemisphere than ín the
Southern Hemisphere. However. it has been observed that there was a general global decline in carbon
monoxide concentration everywhere in the 1990s. Two reasons cave been suggested for this--one is the
eruption of Mount Pínatubo and the other is the occurrence of several relatively dry years in the tropics.
Comment on these two possibilíties in termo of tropospheric and stratospheric processes.
The estímated atmospheric carbon dioxide concentration in the Northern Hemisphere in 1950 was
310 ppmv. It may be predicted with some certainty that the concentration in 2010 will be 390 ppmv.
Calculate the pH ofpure ram that would be in equílibrium with the carbon dioxide ín each of the 2 years
cited, and comment on the contríbution that carbon dioxide makes towards precipitation acidity.
Using the SONOX process, how much limestone would be required each year in ordem to effect quantitative
removal of sulfur dioxide from a power plant whose daily consumption of coal (1.5% sulfur) is 6000 t?
The following two problems use concepts that are developed in Chapter ll.
Assuming an atmospheric pressure of 83 kPa, an atmospheríc mixing ratio of 1.5 ppbv for hydrogen
peroxide, and a vague of KH(H2OZ) = 70 x 10-i mol L'i Pa i, calculate its solubility in the cloud water
droplets. Will thís concentration depend on pH over the range 5 to 8?
Using the values for constants provided in reactions 5.25-5.27. calculate the solubilíty of sulfur dioxide ín
water at pH = 9.0
5.
6.
7
8.
9.