Environ. Sci. Technol. 2005, 39, 1555-1562
Hygroscopicity of Water-Soluble Organic Compounds in Atmospheric Aerosols: Amino Acids and Biomass Burning Derived Organic Species MAN NIN CHAN,† MAN YEE CHOI,‡ N G A L E E N G , ‡,§ A N D C H A K K . C H A N * ,‡ Environmental Engineering Program, School of Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, and Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Amino acids and organic species derived from biomass burning can potentially affect the hygroscopicity and cloud condensation activities of aerosols. The hygroscopicity of seven amino acids (glycine, alanine, serine, glutamine, threonine, arginine, and asparagine) and three organic species most commonly detected in biomass burning aerosols (levoglucosan, mannosan, and galactosan) were measured using an electrodynamic balance. Crystallization was observed in the glycine, alanine, serine, glutamine, and threonine particles upon evaporation of water, while no phase transition was observed in the arginine and asparagine particles even at 5% relative humidity (RH). Water activity data from these aqueous amino acid particles, except arginine and asparagine, was used to revise the interaction parameters in UNIQUAC functional group activity coefficients to give predictions to within 15% of the measurements. Levoglucosan, mannosan, and galactosan particles did not crystallize nor did they deliquesce. They existed as highly concentrated liquid droplets at low RH, suggesting that biomass burning aerosols retain water at low RH. In addition, these particles follow a very similar pattern in hygroscopic growth. A generalized growth law (Gf ) (1 - RH/100)-0.095) is proposed for levoglucosan, mannosan, and galactosan particles.
Introduction Atmospheric aerosols play an important role in climatic change through radiative forcing. Organic compounds contribute to a substantial portion of the atmospheric aerosol mass, especially to the submicrometer-sized aerosols. Considerable attention has recently been given to water-soluble organic compounds (WSOC) in terms of their substantial concentrations in atmospheric aerosols and their hygroscopicity. Understanding the hygroscopicity of WSOC in atmospheric aerosols is vital for improving current understanding * Corresponding author phone: (852) 2358-7124; fax: (852) 23580054; e-mail:
[email protected]. † Environmental Engineering Program, School of Engineering. ‡ Department of Chemical Engineering. § Current address: Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91125. 10.1021/es049584l CCC: $30.25 Published on Web 02/04/2005
2005 American Chemical Society
of radiative forcing by atmospheric aerosols. In recent studies, low molecular weight C2-C5 dicarboxylic acids are usually the first class of WSOC detected in atmospheric aerosols, contributing to less than 10% of WSOC (1,2). Recently, watersoluble organic nitrogen (WSON) has been identified as an important portion of aerosols and rainwater (3,4). WSON contributes about 18% to the total fine aerosol mass (5). In WSON, combined and free amino compounds account for an average of 13% of the dissolved organic carbon in fog droplets and about 10% of the WSOC in fine aerosols (6). A major portion of amino compounds is comprised of amino acids, which have been detected in atmospheric aerosols (6-9) and fog droplets (10, 11). In particular, glycine, threonine, serine, and alanine are the most abundant free amino acids detected (6). While the concentrations of the combined amino compounds (e.g., proteins and peptides) have been found to be larger than those of the free amino compounds (amino acids and alkylamine), they are reported in terms of free amino acids because the combined amino compounds are first hydrolyzed and then analyzed (6). Thus, the composition of the combined amino compounds is not known. Furthermore, these combined amino compounds are only two to three times more than the free amino compounds in total and there may be a large number of these combined amino compounds. Because of the complexity of the composition and the many possible sources of the combined amino compounds (e.g., pollen and bacteria), they are much less characterized in atmospheric aerosols. Biomass burning generates a substantial amount of aerosols, referred to as smoke particles or biomass burning aerosols, which contain a considerable amount of WSOC (12-13). Levoglucosan, mannosan, and galactosan are formed during the pyrolysis of cellulose and are emitted to the atmosphere during wood combustion (14). These organic species have been found in wood smoke particles and in aerosols collected from urban and rural areas (15). In particular, levoglucosan has been identified as the most abundant carbohydrate (16) and accounts for 2-8% of WSOC in biomass burning aerosols over Amazonia (13). It has been suggested to be as a model compound for stimulating the thermodynamic properties of WSOC (17). Biomass burning has also been suggested to be as a source of WSON in atmospheric aerosols (8). Hence, because of their atmospheric abundance, WSON and biomass burning derived organic species can potentially affect aerosol hygroscopicity, cloud activation activity, the global climate, and other atmospheric phenomena (18,19). Laboratory and field hygroscopic measurements have revealed that organic species can change the hygroscopic growth, deliquescence, and crystallization characteristics of particles of inorganic species and are responsible for a sizable water uptake of atmospheric aerosols, especially at low relative humidity (RH) (20-23). The empirical power law expression in the form of the growth diameter factor, Gf ) (1 - RH/100)-γ, has been commonly used to model the dependence of the hygroscopic growth of atmospheric aerosols on RH (24-25). Gf is the ratio of particle diameter at a given RH to the particle diameter at a reference RH (usually
