Review
Role of organic acids (formic, acetic, pyruvic and oxalic) in the formation of cloud condensation nuclei (CCN): a review

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Abstract

Although it is believed that organic aerosols play a key role in cloud nucleation and make an important contribution to the cloud condensation nuclei (CCN) population, their specific species remain poorly characterized. This paper reviews the current knowledge of organic acids (mainly formic, acetic, pyruvic and oxalic acids). Without specification, organic acids in this paper refer to these four organic acids in the gas and aerosol phases. This paper analyzes the extent to which organic acids act as CCN and compares the physical and chemical properties of organic acids with those of CCN. The results show that aerosol formate and acetate concentrations range from 0.02 to 5.3 nmol m−3 and from 0.03 to 12.4 nmol m−3, respectively, and that between 34 to 77% of formate and between 21 to 66% of acetate are present in the fine fraction of aerosols. It was found that although most (98–99%) of these volatile organic acids are present in the gas phase, their concentrations in the aerosol particles are sufficient to make them a good candidate for CCN. The results also show that organic acids may make an important contribution to the formation of CCN in some special sources such as vegetation emissions and biomass-burning. Organic acids are expected to contribute significantly to the estimates of indirect (cloud-mediated) forcing due to aerosols.

Introduction

Aerosol nucleation properties, i.e., the propensity of particles to form water droplets at atmospheric supersaturation levels, are determined by the particle size, chemical composition, and surface characteristics Mason, 1971, Twomey, 1977, Andreae et al., 1995. The nucleation properties of water-soluble inorganic aerosols, such as sulfate and chloride, are well documented. However, until recently, the nucleation properties of organic aerosols have not received much attention. This is partly because chemical composition of particulate organic matter itself is poorly understood. In the continental US, White (1990) reported that organic compounds typically accounted for 20–50% of total fine particle mass concentration. Cadle and Groblicki (1982) found that on average about 40% of total organic mass was water soluble for samples collected at Denver, CO. The recent study of Zappoli et al. (1999) shows that organic compounds account for ca. 50% of the total water soluble fraction of the fine aerosol (<1.5 μm, aerodynamic diameter) and that ca. 80% of the total organic compounds is water soluble at a background site (Aspvreten, Central Sweden). Zappoli et al. (1999) also found that over 70% of the organic compounds in the fine aerosol are of polar nature. Among the organic compounds, identified in urban environments using gas chromatography/mass spectroscopy (GC/MS), are low-molecular weight aliphatic dicarboxylic acids, and high-molecular-weight alkanes (Rogge et al., 1993). Typically, however, only about 10% of total organic mass was identified, and approximately 50% of organic mass either did not extract in organic solvents or did not elute in the GC column used, and the remaining 40% could not be resolved into specific molecular entities by GC/MS (Rogge et al., 1993). On the other hand, ion chromatography (IC) analysis shows that the most frequently observed organic species in the condensed phase in the atmosphere are low molecular weight monocarboxylic and dicarboxylic acids, with monocarboxylic acids much more abundant than dicarboxylic acids. Of monocarboxylic acids, formic and acetic acids are the dominant species, which have most frequently been observed in precipitation Keene and Galloway, 1988, Yu et al., 1991a, Yu et al., 1991b, Yu et al., 1992, Yu et al., 1998, cloudwater and fogwater Weathers et al., 1986, Yu et al., 1991a, Yu et al., 1991b, gaseous phase Dawson et al., 1980, Yu et al., 1990, and in aerosols Andreae et al., 1988a, Andreae et al., 1988b.

The origin and nature of cloud condensation nuclei (CCN) recently have become one of the central issues in climate change because CCN strongly modulate cloud microstructure and hence the radiative properties of clouds Charlson et al., 1987, Yu, 1996. Observations by Desalmand et al. (1982) showed that vegetation could produce CCN. Novakov and Penner (1993) indicated that ∼37% of CCN number concentration measured at a marine site could be accounted for by total sulfates and the remaining 63% by organic aerosols. Measurements by Hegg et al. (1995b) also showed that most CCN over the northeastern Atlantic coast of the United States were not sulfates. Saxena et al. (1995) found that organics could enhance water absorption by inorganics at a nonurburn location (Grand Canyon). It is in order here to ask: what are the chemical components of CCN produced by vegetation and the chemical components of organic aerosols that act as CCN and alter hygroscopic behavior of atmospheric particles? Presently this question cannot be answered.

The purposes of this study are to review the current state of knowledge of water-soluble organic acids in the atmosphere, analyze the extent to which organic acids actually act as CCN, and identify the major unknowns in this field. We will focus principally on formic and acetic acids for two reasons. First, formic and acetic acids are found to be ubiquitous trace gases in the troposphere (Keene et al., 1995), and their concentrations are usually more than one order of magnitude higher than others (such as propionic, butyric, pyruvic and lactic acids) Andreae et al., 1987, Keene and Galloway, 1988, Yu et al., 1991a, Yu et al., 1991b. 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 phases (precipitation, cloud and fog). The role of pyruvic and oxalic acids in the formation of organic CCN is analyzed as well.

Section snippets

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 by Chebbi and Carlier (1996). Organic acids can be directly emitted by vegetation Keene and Galloway, 1988, Yu et al., 1988, Talbot et al., 1990, Kavouras et al., 1998. Natural biomass-burning Andreae et al., 1988b, Talbot et al., 1988a,

Aerosol organic salts as CCN

Theoretical considerations of water vapor condensation (Dufour and Defay, 1963) shows that there is a well-determined relationship between physical and chemical properties of the particles and their activities in the condensation process. Larger and more hygroscopic particles have lower critical supersaturations than smaller and less water-soluble ones.

Table 2 lists the comparison of chemical–physical properties of common organic and inorganic salts. It is of interest to note that formate and

Natural emissions of CCN

According to the analysis of the averaged weekly CCN concentrations, Desalmand et al. (1982) reported that vegetation could produce CCN, and smaller CCN originating from vegetation were released every day into the atmosphere after sunrise in the Ivory Coast of West Africa. In the DEAFE 88 experiment in the African equatorial rainforest, Helas et al. (1992) showed that concentrations of organic acids at the surface had a distinct diurnal behavior and rose after sunrise. They thought that this

Conclusions

In so far as organic acids or salts have the same activity as inorganic acids or salts, they will have the same impact on CCN activity and can be intrinsically effective CCN. However, direct experiments to prove this have so far been lacking. Organic acids in the gas phase can partition into aerosol particles with the aid of foreign nuclei such as alkaline mineral particles and elemental carbon, and by condensation and co-condensation with NH3. The surprisingly high concentrations of CCN in the

Acknowledgements

This study was supported through the Southeast Regional Center of the National Institute for Global Environmental Change by the US Department of Energy under cooperative agreement No. DE-FC03-90ER61010 and by the US NOAA Office of Global Program under grant No. 3439503. The author wishes to thank Drs. V.K. Saxena and P. Kasibhatla for help and support, and also to thank S. Menon, J.S. Schafer and B.N. Wenny of NCSU for their friendly help. The author is indebted to Dr. Austin W. Hogan for his

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