Ž are usually more than one order of magnitude higher than others such as propionic,
. Ž butyric, pyruvic and lactic acids
Andreae et al., 1987; Keene and Galloway, 1988; Yu .
et al., 1991a,b . Second, there are a lot of available experimental results regarding the occurrence, sources and sinks of formic and acetic acids in the gas, aerosol and liquid
Ž .
phases precipitation, cloud and fog . The role of pyruvic and oxalic acids in the formation of organic CCN is analyzed as well.
2. Current state of knowledge of water-soluble organic acids in the troposphere
The role of organic acids as chemical constituents in the troposphere has become an issue of growing interest in the past two decades. An excellent review and discussion of
the occurrence, sources and sinks of organic acids in the troposphere has been presented Ž
. by Chebbi and Carlier 1996 . Organic acids can be directly emitted by vegetation
Ž .
Keene and Galloway, 1988; Yu et al., 1988; Talbot et al., 1990; Kavouras et al., 1998 . Ž
. Natural biomass-burning Andreae et al., 1988b; Talbot et al., 1988a,b , atmospheric
Table 2 Ž
. The physical–chemical properties of several organic and inorganic salts Weast, 1978
3
Ž .
Ž .
Melting point 8C Boiling point 8C
Solubility in gr100 cm Cold water
Hot water Ammonium
Acetate 114
decompose 148
decompose Formate
116 decompose, 180
102 531
Nitrate 169.6
210 118.3
871 Chloride
subl 340 520
29.7 75.8
Sulfate decompose 235
70.6 103.8
Pyruvate –
– –
– Oxalate
– –
2.5 11.8
Sodium Acetate
324 119
170.15 Formate
253 decompose
97.2 160
Nitrate 306.8
decompose, 380 92.1
180 Chloride
801 1413
35.7 39.12
Sulfate 884
s 42.5
Pyruvate –
– –
– Oxalate
3.7 6.33
Potassium Acetate
292 –
253 492
Formate 167.5
decompose 331
657 Nitrate
334 decompose, 400
13.3 247
Chloride 770
subl, 1500 34.4
56.7 Sulfate
558 1689
12 24.1
Pyruvate –
– –
– Oxalate
– –
28.7 83.2
‘‘–’’means that the data are not available.
Table 3 Values of critical radius r and supersaturation S as a function of nucleus mass of different organic salts.
c c
Ž .
Assuming organic salt spheres at a temperature of 293 K 208C
U
Ž . Ž
. Ž
. Ž . Dissolved salts g
nmol r
mm S
S y1
c c
HCOONa 4.00E-17
5.88E-10 0.12
0.65 1.00E-16
1.47E-09 0.18
0.41 4.00E-16
5.88E-09 0.37
0.20 1.00E-15
1.47E-08 0.58
0.13 4.00E-15
5.88E-08 1.16
0.06 1.00E-14
1.47E-07 1.84
0.04 HCOONH
4
4.00E-17 6.35E-10
0.12 0.62
1.00E-16 1.59E-09
0.19 0.39
4.00E-16 6.35E-09
0.38 0.20
1.00E-15 1.59E-08
0.60 0.12
4.00E-15 6.35E-08
1.21 0.06
1.00E-14 1.59E-07
1.91 0.04
CH COONa
3
4.00E-17 4.88E-10
0.11 0.71
1.00E-16 1.22E-09
0.17 0.45
4.00E-16 4.88E-09
0.33 0.22
1.00E-15 1.22E-08
0.53 0.14
4.00E-15 4.88E-08
1.06 0.07
1.00E-14 1.22E-07
1.67 0.04
CH COONH
3 4
4.00E-17 5.19E-10
0.11 0.69
1.00E-16 1.30E-09
0.17 0.44
4.00E-16 5.19E-09
0.35 0.22
1.00E-15 1.30E-08
0.55 0.14
4.00E-15 5.19E-08
1.09 0.07
1.00E-14 1.30E-07
1.73 0.04
CH CO COONa
3
4.00E-17 3.64E-10
0.09 0.82
1.00E-16 9.09E-10
0.14 0.52
4.00E-16 3.64E-09
0.29 0.26
1.00E-15 9.09E-09
0.46 0.16
4.00E-15 3.64E-08
0.91 0.08
1.00E-14 9.09E-08
1.44 0.05
CH CO COONH
3 4
4.00E-17 4.00E-10
0.09 0.80
1.00E-16 1.00E-09
0.15 0.51
4.00E-16 4.00E-09
0.30 0.25
1.00E-15 1.00E-08
0.47 0.16
4.00E-15 4.00E-08
0.93 0.08
1.00E-14 1.00E-07
1.48 0.05
Ž .
Table 3 continued
U
Ž . Ž
. Ž
. Ž . Dissolved salts g
nmol r
mm S
S y1
c c
COO Na
2 2
4.00E-17 2.99E-10
0.10 0.74
1.00E-16 7.46E-10
0.16 0.47
4.00E-16 2.99E-09
0.32 0.23
1.00E-15 7.46E-09
0.51 0.15
4.00E-15 2.99E-08
1.01 0.07
1.00E-14 7.46E-08
1.60 0.05
COO NH
2 4 2
4.00E-17 3.23E-10
0.11 0.71
1.00E-16 8.06E-10
0.17 0.45
4.00E-16 3.23E-09
0.33 0.23
1.00E-15 8.06E-09
0.53 0.14
4.00E-15 3.23E-08
1.05 0.07
1.00E-14 8.06E-08
1.66 0.05
Ž .
oxidation of biogenic isoprene Jacob and Wofsy, 1988 , olefins and hydrocarbons Ž
. Madronich and Calvert, 1990 are thought to be the secondary sources of organic acids
Ž in the atmosphere. Formic acid can be produced by oxidation of HCHO Chameides and
. Davis, 1983 , which is one of the oxidation products of DMS in the marine atmosphere.
Ž .
Saxena and Hildmann 1996 constructed a list of candidate water-soluble organic compounds in atmospheric particles on the basis of the compilation of Graedel et al.
Ž .
1986 . Among organic compounds that are completely miscible with water are mono- Ž
. carboxylic acids and alcohols complete miscibility up to C
or C ; diols and triols
4 5
Ž .
Ž complete miscibility up to C ; and keto-carboxylic acids complete miscibility up to
7
. Ž .
C Saxena and Hildmann, 1996 . On the basis of theoretical analysis of air–water
4
equilibrium, they concluded that C –C monocarboxylic acids, alcohols, carbonyls and
1 6
ethers were too volatile to be distributed in the fine particles, and would be completely partitioned into the gas phase. This result seems consistent with that of the model
Ž .
calculation of Meng et al. 1995 . However, as pointed out by Saxena and Hildmann Ž
. 1996 , there are some discrepancies between their theoretical assessments and observa-
tions. They thought that this might be due to modification of the air–water distribution of these compounds as a result of aqueous phase reactions andror the presence of an
organic film. Dicarboxylic acids, ketoacids and dicarbonyls, which were thought to be Ž
. present in the aerosol phase Saxena and Hildmann, 1996 , constituted only a small
fraction of the total particulate water-soluble organic compounds in the atmosphere. For Ž
. example, Rogge et al. 1993 indicated that water-soluble dicarboxylic acids accounted
for approximately 2 to 4 of total particulate organic mass. Table 1 summarizes the published aerosol and gaseous carboxylic acids in different
parts of the world. Organic acids have been measured over a wide range of environ- ments, e.g., marine and continental air, free atmosphere and surface layer, urban as well
as remote atmospheres. There is a general consensus that formic and acetic acids Ž
. constitute the most abundant carboxylic acids in the troposphere Keene et al., 1995 .
The concentrations of formic and acetic acids in the gas phase ranged from 0.8 to 531 nmol m
y3
and 1.2 to 653 nmol m
y3
, respectively. Unlike the gas phase data, relatively few measurements of aerosol-phase carboxylic acids have been reported. Table 1 shows
that the concentrations of formate and acetate in the aerosols were typically in 0.02–5.3
y3
Ž
y1 5 y13
y3
. nmol m
1.36 = 10 –3.60 = 10
g cm , assuming sodium salts and 0.03–12.4
y3
Ž
y1 5 y12
y3
. nmol m
2.46 = 10 –1.02 = 10
g cm , assuming sodium salts ranges, respec-
Ž .
tively. Table 1 also shows that between 34 to 77 average 58 of formate and Ž
. between 21 to 66 average 45 of acetate were present in the fine fraction. On the
basis of simultaneously available measurements of data on aerosol and gas phase shown in Table 1, only about 2 of total formic acid and about 1 of total acetic acid are
present in the aerosol phase. It indicates that the phase-partitioning is as much a function of the aerosol substrate as it is of the gas equilibrium vapor pressure.
Ž Ž
. .
Ž .
The results for pyruvic acid CH C-O COOH , oxalic acid HOOCCOOH , MSA
3
Ž Ž
. .
Ž .
CH S O OH and propanoic acids
CH CH COOH in the gaseous and aerosol
3 2
3 2
phases are listed in Table 1. The pyruvate and oxalate concentrations in the aerosols
y3
Ž
y1 5 y14
y3
were in the ranges of 0.03–0.6 nmol m 3.3 = 10
–6.6 = 10 g cm
, assuming .
y3
Ž
y1 4 y13
y3
sodium salts and 0.17–1.8 nmol m 2.28 = 10
–2.41 = 10 g cm
, assuming .
sodium salts , respectively. Smog chamber experiments showed that pyruvic acid was Ž
. Ž
. formed by photo-oxidation of o-cresol
Grosjean, 1984 . Jacob and Wofsy 1988
reported that photochemical oxidation of isoprene, which was emitted directly in large Ž
. quantities from trees and other plants
Lamb et al., 1985 , was likely to be the predominant source of pyruvic acid in the atmosphere. Of the dicarboxylic acids, oxalic
Ž .
acid C was found to be the most abundant diacid species in the gaseous and aerosol
2
Ž .
Ž . Ž
phases, followed by malonic acid C or succinic acid C
Sempere and Kawamura,
3 4
. 1994 . Pyruvic and oxalic acids are mostly associated with aerosol particles due to their
low vapor pressures. It is of interest to note that levels of organic acids are lower in the remote marine atmosphere than in the continental atmosphere or highly polluted urban
areas. Organic acids are significantly enriched in the aerosol and gas phases derived from biomass burning plume.
3. The extent to which organic acids act as CCN
