Abstract
Abstract. Atmospheric aerosols have both a direct and an indirect cooling effect that influences the radiative balance at the Earth's surface. It has been estimated that the degree of cooling is large enough to weaken the warming effect of carbon dioxide. Among the cooling factors, secondary organic aerosols (SOA) play an important role in the solar radiation balance in the troposphere as SOA can act as cloud condensation nuclei (CCN) and extend the lifespan of clouds because of their high hygroscopic and water soluble nature. Oxalic acid is an important component of SOA, and is produced via several formation pathways in the atmosphere. However, it is not certain whether oxalic acid exists as free oxalic acid or as metal oxalate complexes in aerosols, although there is a marked difference in their solubility in water and their hygroscopicity. We employed X-ray absorption fine structure spectroscopy to characterize the calcium (Ca) and zinc (Zn) in aerosols collected at Tsukuba in Japan. Size-fractionated aerosol samples were collected for this purpose using an impactor aerosol sampler. It was shown that 10–60% and 20–100% of the total Ca and Zn in the finer particles (<2.1 μm) were present as Ca and Zn oxalate complexes, respectively. Oxalic acid is hygroscopic and can thus increase the CCN activity of aerosol particles, while complexes with various polyvalent metal ions such as Ca and Zn are not hygroscopic, which cannot contribute to the increase of the CCN activity of aerosols. Based on the concentrations of noncomplexed and metal-complexed oxalate species, we found that most of the oxalic acid is present as metal oxalate complexes in the aerosols, suggesting that oxalic acid does not always increase the hygroscopicity of aerosols in the atmosphere. Similar results are expected for other dicarboxylic acids, such as malonic and succinic acids. Thus, it is advisable that the cooling effect of organic aerosols should be estimated by including the information on metal oxalate complexes and metal complexes with other dicarboxylic acids in aerosols.
Highlights
Some anthropogenic aerosols, such as organic aerosols and sulfate aerosols, have a direct cooling effect by scattering solar radiation, and an indirect cooling effect by acting as cloud condensation nuclei (CCN) because of their hygroscopic properties (Novakov and Penner, 1993; Claeys et al, 2004; Kanakidou et al, 2005; Intergovernmental Panel on Climate Change (IPCC), 2007; Hallquist et al, 2009)
We focused on oxalic acid as a representative component of low molecular weight dicarboxylic acids (DCA) in the atmosphere, and our results can be extended to other DCA, such as malonic and succinic acids
Suggested that the air mass in winter in Tsukuba was transported through northeastern China, Korea, and Japan which was possibly subject to the influences of (i) anthropogenic aerosols from the mega cities in the three countries, (ii) mineral aerosols from arid area in northeast part of China (Mikami et al, 2006), and (iii) sea salts from Japan Sea (Fig. S1)
Summary
Some anthropogenic aerosols, such as organic aerosols and sulfate aerosols, have a direct cooling effect by scattering solar radiation, and an indirect cooling effect by acting as cloud condensation nuclei (CCN) because of their hygroscopic properties (Novakov and Penner, 1993; Claeys et al, 2004; Kanakidou et al, 2005; IPCC, 2007; Hallquist et al, 2009). A number of studies have been performed on sulfate aerosols and on organic aerosols because of their complex nature in terms of composition and chemical transformation in the atmosphere, and because of their importance in the global CCN budget (Novakov and Penner, 1993; Claeys et al, 2004; Kanakidou et al, 2005; IPCC, 2007; Hallquist et al, 2009). We can assume that dicarboxylic acids (DCA) contribute to the CCN activity, considering that DCA, which are major constituents of WSOCs, have hygroscopic
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