**2.5. Composition of atmospheric aerosol**

326 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

with only the accumulation mode present.

**2.3. The study of atmospheric aerosols** 

mechanics, biology and medicine.

research (Adam et al., 1999).

monitored (MOSTI, 2000).

**2.4. Aerosols and effect on quality of life** 

is volcanic injection. Due to their long lifetime, these aerosols are distributed relatively homogeneously throughout the whole stratosphere and the size distribution is unimodal

Atmospheric aerosol particles are a ubiquitous part of earth's atmosphere, present in very lungful of air breathed. They are produced in vast numbers by both human activity (anthropogenic) and natural sources and subsequently modified by a multitude processes. They are known to be crucially important in many issues that directly affect everyday life which include respiratory health, visibility, clouds, rainfall, atmospheric chemistry and global regional climate but they are also one of the more poorly understood aspects of the atmosphere. These shortcomings in understanding are partly due to their small size, which is typically of the order of microns or less, making them difficult to study and also the fact that the processes involved are complex. The description of the organic chemistry in atmospheric aerosol is by no means straightforward, but the addition of the solubility variables, aerosol thermodynamic, hygroscopic properties, deliquescence behaviour makes understanding the atmosphere and its effect is even more challenging, requiring the application of wide spectrum of scientific disciplines including chemistry, physics,

The effects of aerosols on the atmosphere, climate and public health are among the central topics in current environmental research. Urban areas have always been known to be a major source of particulate pollution (Finlayson-Pitss, 2000) which is expected to continue to increase due to world population growth and increasing industrialization and energy use, especially in developing countries (Fenger, 1999). The most obvious effects are the contributions to unsightly smogs and visible deterioration of the building materials (Grossi, 2002). In addition, the fact that urban particulate pollution impact directly on human health has been known for centuries (Brimblecombe, 1987) and has been the subject of much

In an attempt to reduce the health burden of atmospheric particulate pollution, regulatory authorities have attempted to place controls on the emission and the magnitude of pollution episodes within conurbations. The monitoring of particulate air pollution has traditionally focused on particles of less than 10 μm in aerodynamic diameter (the PM10 standard), as these are more likely to pass the throat when inhaled (DEFRA, 2005; Larrsen, 1999) but it has become apparent that the smaller particles are more significant, as these particles will penetrate deeper into the lungs and potentially cause more physiological distress or damage. This has lead to the use of the PM2.5 standard in countries such Malaysia, where the total mass of particulate matter less than 2.5 μm in diameter is The atmospheric aerosol consists of a complex mixture of organic and inorganic compounds (Cruz, 1998). The typical composition of fine continental aerosol will usually contain various sulphates (mostly ammonium and calcium), nitrates (mostly ammonium), chlorides (mostly sodium), elemental carbon (EC) and organic carbon (OC), especially traffic-related soot, biological materials and other organic compounds, iron compounds, trace metals, and mineral derived from rocks, soil and various human activities. Aerosol composition also can be influenced by local geology, geographic location and climate (Moreno et al., 2003).

## *2.5.1. Organic and elemental carbon of aerosol*

Several studies have shown that over 30% of aerosol is organic carbon, and carbon containg matter can account for as much as 50%. Typically, two classes of carbonaceous aerosol are commonly present in ambient air: organic carbon (OC) and elemental carbon (EC), which are the largest contributors to the fine particle burden in urban atmospheres and heavily industrialised areas (Cachier et al., 1989).

Field measurements also shown a significant mass fraction of atmospheric aerosol consist of organic compounds (Rogge et al., 1991). Around 5 to 10% of the known fraction is often limited to low molecular weight species, which are identified by standard analytical techniques, using gas chromatography coupled with mass spectrometry. A significant fraction of the organic mass in tropospheric aerosol, is comprised of high molecular weight, oxygenated species which remain unidentified (Decesari et al., 2002).

Organic compounds are emitted into the atmosphere from various anthropopgenic and biogenic sources. These include primary emission, mainly from combustion and biogenic sources and secondary organic aerosol resulting from the reaction of primary volatile organic compounds in the atmosphere (Fisseha et al., 2004). In urban areas, a number of emission sources are responsible for the presence of organic aerosol in the atmosphere among which are road traffic, industrial processes, waste incineration, wastewater treatment processes and domestic heating. Some of these are pure organic aerosols, which may be formed by primary particle emissions (primary organic carbon) or produced from atmospheric reactions involving gaseous organic precursors (secondary OC)(Cruz and Pandis, 1998).

Organic material is important in controlling the aerosol physico-chemical properties (Cornell et al., 2003). They also found that the uptake of liquid water in aerosol was enhanced by the presence of organic carbon compounds. Organic carbon is also an effective light scatter and may contribute significantly to both visibility degradation and direct aerosol climate forcing (Heintzenberg., 1989). Elemental carbon (often named black carbon or soot) may be the second most important elemental in global warming in terms of direct forcing, after CO2 due to specific surface properties. Elemental carbon provides a good adsorbtion site for many semi-volatile compounds such as poly-aromatic hydrocarbon (PAH) and offers a large specific surface area for interactions with reactive trace gases such as ozone. Annually, about 13 Tg black carbons are emitted into the atmosphere, mainly through fossil fuel combustion and biomass burning (Jacob, 1999).

As for other aerosols, the removal of particulate carbon is likely to occur via two main scavenging processes: the in-cloud process, whereby particles are directly incorporated into cloud droplets; and the below-cloud process, where particles are washed out by precipitation itself. The physico-chemical atmospheric processes which transform young combustion particles, expected to be hydrophobic, into a water soluble aerosol phase remains a major unknown. The atmospheric behaviour of the carbonaceous particles is likely to be dictated by the chemical nature of their surfaces (Cachier et al., 1989). If the surface is hydrophobic, the particle remains inactive. However, if it is coated with hygroscopic substances, it may be activated enough to be incorporated into water droplets (Charlson and Heintzenberg, 1995).

The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols 329

·). Among these species, it is very likely that OH· is the

were found to account on average for 87% of total fine aerosol WSOC (Decesari et al., 2000). The most frequently determined WSOC are the low molecular weight (LMW) carboxylic and dicarboxylic acids (Yu, 2000). Most of carboxylic acids compound are a secondary oxidation products of atmospheric organic compounds and also found in remote marine as well as continental rural and urban areas (Simoneit and Mazurek, 1982). Among these dicarboxylic acids (DCA's), oxalic acid is the most abundant, followed by succinic and

In the aqueous phase, organic oxidation also can be initiated by various radical anions in the

most efficient iniating organic oxidation (Dutot et al., 2003). The DCA's are the late products in the photochemistry of aliphatic and aromatic hydrocarbons, and due to the low vapour pressure, it is almost entirely partitioned to the particulate phase. They also constitute an important fraction of the water soluble part of particulate organic matter (POM) in

During the past decade, much attention has been paid to the low molecular weight dicarboxylic acids and related polar compounds which are ubiquitous water-soluble organic compounds that have been detected in a variety of environmental samples including atmospheric aerosols, rainwaters, snow packs, ice cores, meteorites, marine sediments, hypersaline brines and freshwaters (Kawamura and Ikushima, 1993; Tedetti et al., 2006). In the atmosphere, dicarboxylic acids originate from incomplete combustion of fossil fuels (Kawamura and Ikushima, 1993; Kawamura and Kaplan, 1987), biomass burning (Narukawa et al., 1999), direct biogenic emission and ozonolysis and photo-oxidation of

Low molecular weight (LMW) dicarboxylic acids have also been identified in cloud water samples collected at a high mountain range in central europe (Puxbaum and Limbeck, 2000), in the condensed phase at a semi-urban site in the northeastern US (Khwaja, 1995) and in Arctic aerosol (Kawamura et al., 1996). As a result of their hygroscopic properties, dicarboxylic acids can act as cloud condensation nuclei and have an impact on the radiative forcing at earth's surface (Kerminen et al., 2000). Dicarboxylic acids also participate in many biological processes. They are important intermediates in the tricarboxylic acid and glyoxylate cycles and the catabolism and anabolism of amino acids

Photochemical reactions are also an important source of atmospheric dicarboxylic acids. For example, glutaric acids photooxidation is likely the dominant pathway formation, as measured atmospheric concentrations of dicarboxylic acids in Los Angeles far surpasses contributions from direct emissions and seasonal trends suggest that dicarboxylic acids are largely produced in photochemical smog (Puxbaum and Limbeck, 2000; Rogge et al.,

atmospheric aerosol particles at remote and urban areas (Rohrl and Lammel, 2001).

malonic in atmospheric aerosol especially during summer season.

3·,SO24·,Cl-

·,NO-

organic compound (Sempere and Kawamura, 2003).

atmosphere (e.g. OH-

**3. Dicarboxylic acids** 

(Tedetti et al., 2006).

1993).

## *2.5.2. Water soluble organic compounds*

A significant fraction of the particulate organic carbon is water soluble, ranging from 20% to 70% of the total soluble mass, thus making it important to various aerosol-cloud interactions (Decesari et al., 2000; Facchini et al., 2000). Water soluble organic compounds (WSOC) contribute to the ability of the particles to act as cloud condensation nuclei (CCN) (Novokov and Penner, 1993).

WSOC have been postulated to be partially responsible for the water uptake of airbone particulate matter, which can substantially affect the physical and chemical properties of atmospheric aerosols (Yu et al., 2005). Decesari et al. (2001) have suggested that WSOC are composed of higly oxidised species with residual aromatic nuclei and aliphatic chains. The current understanding of atmospheric particles describes their WSOC fraction as a complex mixture of very soluble organic compounds, slightly soluble organic compounds, and some undetermined macromolecular compounds (MMCs)(Saxena and Hildemann, 1996).

The composition of WSOC varies among sampling regions. It was found to constitute between 20 and 67% of the total organic carbon present in aerosol samples collected in Tokyo (Sempere and Kawamura, 1994). The percentage is ranged from 65 to 75% in aerosol samples collected in Hungary, Italy and Sweeden (Zappoli et al., 1999). The study also found that the percentage of WSOC species with respect to the total soluble mass was much higher at the background site (Aspvreten, Central Sweeden) (c.a. 50%) compared to the polluted site (San Pietro Copofiume, Po Valley, Italy) (c.a. 25%). A very high fraction (over 70%) of organic compounds in the aerosol consisted of polar species. A study by Wang et al. (2002) showed that most water soluble carbon is total organic carbon (TOC) and range between 20.53 to 35.58 μg m-3 in PM10 and PM 2.5. A further study by (Narukawa et al., 1999) concluded that individual haze particles over Kalimantan of Indonesia were mainly composed of water soluble organic materials and inorganic salt such as ammonium sulphate.

The ionic organic compounds (including carboxylic, dicarboxylic and ketoacids) were distributed between both sub-micron and super micron mode, indicating origins in both gas-to-particle conversion and heterogeneous reaction on pre-existing particles. WSOC in atmospheric aerosols and droplets can be divided by their functional groups into three classes which are neutral, mono- and dicarboxylic acid and also polycarboxylic acid, which were found to account on average for 87% of total fine aerosol WSOC (Decesari et al., 2000). The most frequently determined WSOC are the low molecular weight (LMW) carboxylic and dicarboxylic acids (Yu, 2000). Most of carboxylic acids compound are a secondary oxidation products of atmospheric organic compounds and also found in remote marine as well as continental rural and urban areas (Simoneit and Mazurek, 1982). Among these dicarboxylic acids (DCA's), oxalic acid is the most abundant, followed by succinic and malonic in atmospheric aerosol especially during summer season.

In the aqueous phase, organic oxidation also can be initiated by various radical anions in the atmosphere (e.g. OH- ·,NO-3·,SO24·,Cl- ·). Among these species, it is very likely that OH· is the most efficient iniating organic oxidation (Dutot et al., 2003). The DCA's are the late products in the photochemistry of aliphatic and aromatic hydrocarbons, and due to the low vapour pressure, it is almost entirely partitioned to the particulate phase. They also constitute an important fraction of the water soluble part of particulate organic matter (POM) in atmospheric aerosol particles at remote and urban areas (Rohrl and Lammel, 2001).
