Clouds, although only occupying a relatively small fraction of the troposphere volume, can have a substantial impact on the chemistry of the troposphere. In newly formed clouds, or in clouds with air rapidly flowing through, the chemistry is expected to be far more active than in aged clouds with stagnant air. Thus, frequent cycling of air through shortlived clouds, i.e. cumulus clouds, is likely to be a much more efficient media for altering the composition of the atmosphere than an extensive cloud cover i.e. frontal cloud systems. The impact of clouds is tested out in a 2-D channel model encircling the globe in a latitudinal belt from 30 to 60 deg N. The model contains a detailed gas phase chemistry. In addition physiochemical interactions between the gas and aqueous phases are included. For species as H2O2, CH2O, O3, and SO2, Henry's law equilibria are assumed, whereas HNO3 and H2SO4 are regarded as completed dissolved in the aqueous phase. Absorption of HO2 and OH is assumed to be mass-transport limited. The chemistry of the aqueous phase is characterized by rapid cycling of odd hydrogen, (H2O2, HO2, and OH). O2(-) (produced through dissociation of HO2) reacting with dissolved O3 is a major source of OH in the aqueous phase. This reaction can be a significant sink for O3 in the troposphere. In the interstitial cloud air, odd hydrogen is depleted, whereas NO(x) remains in the gas phase, thus reducing ozone production due to the reaction between NO and HO2. Our calculations give markedly lower ozone levels when cloud interactions are included. This may in part explain the overpredictions of ozone levels often experienced in models neglecting cloud chemical interactions. In the present study, the existence of clouds, cloud types, and their lifetimes are modeled as pseudo random variables. Such pseudo random sequences are in reality deterministic and may, given the same starting values, be reproduced. The effects of cloud interactions on the overall chemistry of the troposphere are discussed. In particular, tests are performed to determine the sensitivity of cloud frequencies and cloud types

Isaksen, Ivar S. A.

Jonson, Jan E.

English

NASA Technical Reports Server

N95. 10616
THE SENSITIVITY OF TROPOSPHERIC CHEMISTRY TO CLOUD INTERACTIONS.
Jan E. Jonson and 1vat S. A. Isaksen
Institute of Geophysics,
University of Oslo,
P.O.Box 1022, Blindern,
N-0315 Oslo 3, Norway
ABSTRACT
Clouds, although only occupying a relatively small frac-
tion of the tropospheric volume can have a substantial im-
pact on the chemistry of the troposphere. In newly formed
clouds, or in clouds with air rapidly flowing through, the
chemistry is expected to be far more active than in aged
clouds with stagnant air. Thus frequent cychng of air
through shorthved clouds, i.e. cumulus clouds, is likely
to be a much more efficient media for altering the compo-
sition of the atmosphere than an extensive cloud cover i.e.
frontal cloud systems.
The impact of clouds is tested out in a 2-D channel
model encirchng the globe in a latitudinal belt form 30
to 60°N. The model contains a detailed gas phase chem-
istry. In addition physiochemical interactions between the
gas and aqueous phases are included. For species as H202,
CH20, O3 and SO2, Henry's law equihbria are assumed,
whereas HNOa and H2SO4 are regarded as completely dis-
solved in the aqueous phase. Absorption of HO2 and OH
is assumed to be mass-transport hmited.
The chemistry of the aqueous phase is characterized by
rapid cychng of odd hydrogen (H202, HO2 and OH). O_
(produced through dissociation of HO2) reacting with dis-
solved Oa is a major source of OH in the aqueous phase.
This reaction can be a significant sink for O3 in the tro-
posphere. In the interstitial cloud air, odd hydrogen is de-
pleted, whereas NO_ remains in the gas phase, thus reduc-
ing ozone production due to the reaction between NO and
HO2. Our calculations give markedly lower ozone levels
when cloud interactions are included. This may in part ex-
plain the overpredictions of ozone levels often experienced
in models neglecting cloud chemical interactions.
In the present study the existence of clouds, cloud types
and their lifetimes are modelled as pseud<, ran!l<mi vari
ables. Such pseudo random sequences are in reality <l,q,.r-
ministic and may, given the same starlin_z value.,. I,e i'__
produced. The effects of cloud interaclic,ns on the overall
chemistry of the troposphere will 1)e di,cussed. In parlic-
ular, tests are performed to delerlnill, th," sensitivilv ,,t
cloud frequencies and cloud types.
1. INTRODUCTION.
Measurements indicates that since the turn of the cen-
tury there has been a marked increase in tropospheric ozone
levels particulary over northern mid latitudes (Bojkov, 1986;
Kley et at., 1988). The increase is a result of man made
emissions of hydrocarbons and NO_ (NO and NO2) com-
pounds (Crutzen, 1988). Tropospheric ozone is an active
greenhouse gas, and thus increased ozone levels, particu-
lacy in the Northern Hemisphere could contribute to global
warming.
Several past model studies of tropospheric ozone have
had a tendency to overpredict ozone (i.e. Isaksen et at.,
1989; Jonson and Isaksen, 1992a). Lelieveld and Crutzen
(1990, 1991) proposed that clouds may act as an efficient
sink for ozone, predominantly through the aqueous phase
reaction with O_. In fact, they found this reaction to
be so fast that without a fixed ozone value, ozone levels
would have become unreahstically low. Jonson and Isaksen
(1992b) calculate a much smaller, but nonetheless, signif-
icant impact of clouds on ozone levels. They found that
about half of the ozone reductions could be attributed to
indirect effects on the gas phase chemistry, the other half
to the aqueous phase reaction with O_-.
Leheveld and Crutzen (1990, 1991) and Jonson and
Isaksen (1992b) used crude parameterizations for clouds.
Lelieveld and Crutzen (1990. 1991) assumed average cy-
cling times in and out of clouds based on lagrangian statis-
tics. Jonson and Isaksen (1992b) assumed stationary clouds
with the air flowing through. The present study apphes the
same model, but a more flexible cloud parameterization is
used. Based on statistics for the fraciionai cloud cover,
clouds are distributed according to a pseudo random func-
tion.
Several species will reach concenlralions in the aqueou_
phase several orders of magnitude higher than in the ga_
phase. Provided fast aqueous phase reaclions occur, sev-
eral soluble species are rapidly removed in clouds. Thus.
important aqueous phase processes are likely 1o become
far less important with increasing residence time for air in
clouds.
109
https://ntrs.nasa.gov/search.jsp?R=19950004204 2020-06-16T11:28:08+00:00Z
2. DESCRIPTION.
The model covers the latitudinalbelt from 30 - 60°N.
The horizontalresolutionis 10° longitude and the vertical
resolution500 m from ground levelto 3 km and i km from 3
to 16 kin,the upper boundary of the model. The timestep
is 1 hour, and the model is run for 30 days. Mid-april
conditions are simulated. The zonal distributionsof both
natural and anthropogenic emissions of NO_ (NO+NO2),
non-methane hydrocarbons and CO are taken from Isaksen
et at. (1989), while emissions of SO2 are from Jonson and
Isaksen (1992a). For the advection, seasonallyaveraged
wind fieldsare used (Oort, 1983). The advcction equation
is solvedby an upwind scheme with correctionsfor numer-
icaldispersion(Smolarkiewicz, 1983). In the vertical,the
eddy diffusionis separated into a slow diffusivetransport,
and a rapid cloud transport.The cloud transportisa com-
bination ofdirecttransportfrom cloud base to any layerbe-
tween cloud base and cloud top,and a slowercompensating
subsidence to the layerbelow. This method was firstde-
scribed by Chatfieldand Crutzen (1984). Cloud transport
takes placeat every longitude,regardlessof whether cloudy
or cloud freeconditions,as described below, are assumed.
However, in clouds, the cloud transport is multiplied by
1.6,outside clouds by 0.2,empphasizing cloud advection
where the bulk of the clouds are assumed to be concen-
trated. The advcction,eddy diffusionand cloud transport
are described in more detailby Solberg (1989).
Whether a grid rectanglewillbe in clouds (cloudmode)
or not is determined by a pseudo random function. Each
grid rectangle is assigned a probabilityfor cloud based
on statisticsforthe fractionalcloud cover (Gordon et at.,
1984). Whenever the cloud mode isselectedby the random
function,the model has a choiceof 4 cloud categories.Each
cloud category isassigned a probabilityand a correspond-
ing residencetime. Probabilitiesforthe 4 cloud categories,
theirresidencetimes and theirheight intervalsand optical
thicknessare listedin Table I. Ifa clearsky situationis
selected,a choice ismade between 4 residencetimes. The
probabilityfor a residencetime of 31 hours is 0.1,a resi-
dence time of 6 hours, 0.2,a residencetime of 2 hours,0.4,
and of I hour, 0.3. The overallprobabilityfor the indi-
vidual cloud categoriesand for the length of the clearsky
periods are determined by the product of the probabilities
and their residence times. When selected the average resi-
dence times for clear sky and cloud events are equeal. The
overall probabilities for clouds are thus determined by the
cloud fractions. This pseudo random sequence can be re-
produced given the same starting values. Different sets of
dissociation rates are calculated for the 4 cloud categories
and for clear sky conditions (Jonson and Isaksen, 1992b).
The chemistry (gas- and aqueous- phase) has been de-
scribed by Jonson and Isaksen (1992b). In clouds, we as-
sume that H2SO4 and HNOa are completely absorbed by"
the droplets. For H202, SO2, On, HCHO and HCOOH,
equilibrium between the gas- and liquid- phase, determined
by Henry's law constants and equilibrium constants, are
applied.The fractionof the total(gas and liquid)concen-
tration dissolvedin the aqueous phase is definedas
1
f=
1 + (LH, HRT) -1
where L is the volume ratio between gas and liquid wa-
ter, H,_,/is the efficient Henry's constant (taking into ac-
count the effect of dissociation), R is the universal gas
constant and T the temperature. In clouds, the aqueous
phase concentration is then calculated as f multiplied by
the total (gas and liquid) concentration, and the gas phase
concentration as (l-f) multiplied by the total concentra-
tion. The mass-transfer of OH and HO2 is assumed to be
mass-transport limited, and is calculated as described by
Schwartz (1986). In the previous study (Jonson and Isak-
Cat. Prob. Res.
1 0.1 3
2
0.2 I0
3 0.2 8
4 0.5 3
Height (km) Opt.
0 - 0.5 see caption
7- 8(9) S
5- 7 15
0.5(2)- 4 20
2.5 - 5 30
0.5(2)-6(7) 30
Table h When cloud mode is selected, Prob. denotes the
probability for a cloud category, Res. the residence time in
hours of the cloud event, Height, denotes the height inter-
val(s) for the cloud layer(s) and Opt., the optical thickness
of the cloud. For category 1, J-values for clear sky are
mulhplied by a factor 1.5 in the lowest layer. Numbers in
brackets denote cloud-base and cloud-top heights over land.
sen, 1992b), we showed that aqueous phase chemistry has
a substantial impact on the composition of the gas phase.
The processes were studied with clouds fixed in space and
time. This simple approach has the advantage of provid-
ing stable conditions where the individual reactions can
be studied in more detail. Two processes, resulting in re-
duced ozone levels compared to gas-phase chemistry only,
were identified:
The first process is the aqueous phase loss thr.ugh the
reaction:
03+0_ * H _ _ OH _ 202 (t)
Although ozone has a low solubility in water, the reaction
is nonetheless a significant sink for ozone. This reaction
is also a major source of Ott in lhe aqueous phase. The
sources of O 2 _HO2 are absorption from the gas phase
and recycling of hydrogen from Ott through the aqueous
phase reactions:
Ott+ H20,. _ HOz+ H20 (2)
OH + CIt2(OH)2 -4- (), -- H('OOH + H_O _ IfO, (3)
I I(/
shortcycling
Long. fc P Q0 I Q_q
150°w 0.63 1.8E+6 1.1E-6 7.5E-7
80°W 0.74 6.8E+6 5.0E-7 2.3E-7
30°W 0.7 2.5E+6 1.4E-7 3.3E-7
20°E 0.48 6.8E+6 5.0E-7 2.1E-7
mean 0.38 4.3E+6 8.7E-7 4.4E-7
long cycling
fc P Qg Q.q
0.7 2.0E+6 1.2E-6 6.2E-7
0.69 6.5E+6 5.4E-7 2.2E-7
0.72 2.4E+6 1.5E-6 2.4E-7
0.41 6.7E+6 6.0E-7 2.1E-7
0.39 4.7E+6 9.9E-7 3.4E-7
Table 2: Ozone production and loss terms for selected longitudes and mean values for the whole model volume, fc is
the fractional time in the cloud mode. P is the production o/ozone in molecules cm-3s -1, and Qg and Q,q the gas-
and aqueous- phase loss of ozone in s-1 averaged over a 30 day period for the lowest 6750 meters of the troposphere.
Formic acid partially evaporates, but it also contributes to
the cycling of hydrogen:
OH + HCO0- + Os _ COs + OH- + H02 (4)
OH + HCOOH + 02 --_ COs + H20 + HOs (5)
The second process which leads to reduced ozone levels is
due to reduced gas phase production through the reaction
sequence:
NO + H02 --+ NOs + OH (6)
In sunlight NO2 is photolyzed:
NO2 + hv ---*NO + 0 (7)
O rapidly combines with O2 giving ozone. Whereas most of
the odd hydrogen (OH, HO2 and H202), and thereby HO2,
resides in the aqueous phase, NO_ has a low solubility, and
will remain in the gas phase. The effect is reduced ozone
nrnduction.
12.5" _ .(3
10.0 -_----_/_ -- _^ l(30.OU __4
7.5
/
.....
4'¢'t'_""-'m. V\ _. 52.00 _//F_ ?'.00... f-_
tt I I IIll'IIIIlt rw_Ir l_Wlll It l_I lill t _
-150 -lOO -50 o 50 loo 15o
LONGITUDE(DEC.W - E)
Figure 1: Diurnally averaged ozone concentratwns for the
Northern mid- latitudes as a function of latitude and alti-
tude. Concentrations are given in parts per billion (ppb).
In the gas phase, ozone is destroyed by a number of
species, including HO2.
03 + H02 _ 0H + 202 (8)
12,5-
10.0, __-_----- -2.0
7.5' __ _-__6.0_
...... o
c ,
.... ,,,,,,,%,,,,.... ,.... ,,,,,,,
-150 -lO0 -50 0 50 too 150
LONG4TUDE(DEG.W - E)
Figure 2: Percentage difference in ozone concentrations be-
tween model runs without cloud chemistry and with cloud
chemistry as a function of longitude and altitude.
3. COMPUTATIONAL RESULTS.
In this study we are interested in quantifying the effects
of different cloud representations rather than to study the
individual reactions in detail. In Figure 1 ozone concen-
trations are shown with random clouds as described above.
Figure 2 shows the percentage difference between concen-
trations calculated with no cloud chemistry (but with iden-
tical dissociation rates) and calculations with cloud chem-
istry. With cloud chemistry included, concentrations are
lower throughout the troposphere. Ozone produced over
polluted continents (North America, Europe and East Asia)
is transported eastwards. During transport ozone is de-
stroyed in clouds. In particular, large differences are found
at the eastern rim of the Atlantic and Pacific oceans, down-
wind from Noth America and East Asia respectively.
Figure 3 shows the percentage difference in ozone con-
centrations between a model run with residence times dou-
bled, and a run where residence times have been approx-
imately halved. Effects on ozone are generally small, and
are slightly negative over polluted continents, most likely
due to changes in the NO= distribution. In Table 2, ozone
production and loss rates are shown. The tabulated values
are averages for the whole period studied, with both cloudy
and cloud free periods included. The effect of long and
111
,251 L/
-150 -100 -50 0 50 I00 150
LONGITUDE(DEG.W - E)
Figure 1: Percentage difference in ozone concentrations be-
tween model runs with long and short residence times for
clouds.
short cloud cycling times are compared. Production rates
(reaction 6) and loss rates for ozone reacting with HO2
in both gas and aqueous phase (reaction 8 and 2 respec-
tively) are tabulated. Over the polluted continents (80°W
and 20°E) production rates are about a factor of 10 higher
than over remote oceanic sites (150°W and 30°W). With
short residence times the aqueous phase loss (reaction 2) is
about 30% higher averaged over the whole model volume.
This is compensated for by lower loss rates in the gas phase
(reaction 8). However, Figure 3 show that ozone concen-
trations generally are lower in calculations with short cloud
cycling times. These cha.nges are caused by indirect effects
on the NOz chemistry and distribution rather than changes
in aqueous phase destruction.
4. CONCLUSIONS.
Aqueous phase chemistry can reduce ozone concentra-
tions in the troposphere significantly. Two mechanisms are
identified by which ozone is reduced. Ozone absorbed in
the droplets are consumed by a reaction with 02. Sec-
ondly HOz and NO are separated in the clouds, as HO2 is
absorbed by the droplets whereas NO is virtually insolu-
ble, thus slowing ozone production. Variations in residence
times for cloud events does not alter the ozone concentra-
tions by more than a few percent. With such a low sensitiv-
ity to cloud cycling parameterization, it is justified to apply
the proposed mechanism even in models with low resolu-
tion where individual clouds, or even entire cloud systems
are not properly resolved.
Acknowledgement: This work is sponsered by the Nor-
wegian Research Council for Science and Humanities under
grant 447.90/016b.
5. REFERENCES.
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