This review would not be complete without a discussion of what is
known about the role of heterogeneous reactions in both clouds and
aerosols in tropospheric chemistry. In the past four years there has been
an explosion of interest in reactions within clouds and aerosols and
their importance in determining the abundance of tropospheric
ozone, peroxides, and the nitrogen oxides as well as their roles in
the cycles of tropospheric chlorine and sulfur. For example, reactions
of N
O5 with sulfuric acid aerosols have been studied mainly for their
role in stratospheric chemistry, but may be important as a loss process
for NO
in the troposphere as well (Fried et al., 1994). Both the
production of peroxyl radicals and peroxides have been identified in
cloud water (Faust and Allen, 1992; Anastasio et al., 1994).
Increased peroxyl radical production in aerosols and clouds could lead
to significant O
destruction because the cloud water environment
is depleted in NO
relative to that in air. The constituents responsible
for such production are not known and further work is needed to
quantify their importance, especially relative to proposed decreases
in peroxy radicals that result from transition metal catalysis (Ross
and Noone, 1991). Increased peroxide production could alter the rates
of conversion of SO
to H
SO
. Because the gas phase
concentration of H
O
is less than that of SO
in air
masses polluted by SO
sources, and because gas to liquid partitioning
has been thought of as the main source of liquid water H
O
,
it has been argued that H
O
is the limiting reagent in the
aqueous phase conversion of SO
to sulfate, with a consequent
non-linear response of acid deposition to reduced SO
emissions.
The proposed source of cloud water H
O
could therefore
have important consequences both for acid deposition and for
our understanding of the rate of production of sulfate aerosols which
affect climate forcing.
Atomic chlorine may be an important oxidant for
non-methane hydrocarbons and dimethysulfide in the marine environment.
It may also provide a source or a sink for O
, depending on NO
levels. Measurements indicate a significant concentration of an
inorganic gas-phase chlorine component (probably HOCl or Cl
) that is
not HCl (Pszenny et al., 1993). Its concentration accumulates during
the night and decreases after sunrise. However, the source for this
chlorine is not known. Observations of Cl- deficits in sea salt aerosol
have lead to postulations that heterogeneous reactions within the
aerosol might explain the source (Keene et al., 1993) and a variety
of mechanisms have been suggested, including reaction of N
O5 with sea
salt aerosol to produce gaseous ClNO
and CL
(Ganske et al., 1992) as
well as oxidation of chloride by O
and free radicals. Current
models, however, are not able to predict the rates of oxidation needed
to explain the observed deficits (Chamiedes and Stelson, 1992; 1993).
A final area where heterogeneous reactions may play a significant
role is in the rapid depletion of boundary layer O
in the Arctic in
spring. These depletion events are anti-correlated with
filterable (presumably particulate) bromide. Further, high concentrations
of bromoform (CHBr3), which are thought to originate from marine algae,
are present in the Arctic in late winter and spring. Sturges et al.
(1993) demonstrated that ambient Arctic air, when exposed to
solar irradiation, rapidly formed particulate bromide. They also
found evidence of organic bromides in the particulate phase. These
findings indicate some role for heterogeneous reactions, but the
specific reactions have not yet been identified. The reaction of HOBr
with HBr to form Br
and H
O and the reaction of HOBr with HCl
to form BrCl and H
O have both been shown to occur readily on ice
surfaces (Abbot, 1994). Br
and BrCl would both photolyze rapidly
to produce Br, initiating a catalytic cycle of O
destruction. HOBr,
HBr, and BrNO
can also convert to Br and BrO through aqueous phase
reactions on sulfuric acid aerosols (Fan and Jacob, 1992).