**10. References**

	- IPCC, Climate Change (2001). Working Group I, *Contribution to the Intergovernmental Panel on Climate Change. 3rd Assessment Report Climate Change 2001: the PhysicalSciense Basis*  (Cambridge Univ., UK, New York);

**Chapter 2** 

© 2012 Hoshiko et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Hoshiko et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Temporal Variation of Particle Size Distribution** 

**of Polycyclic Aromatic Hydrocarbons at Different** 

**Roadside Air Environments in Bangkok, Thailand** 

Polycyclic aromatic hydrocarbons (PAHs) have been drawing attention as a major hazardous air pollutant due to their potential carcinogenicity and mutagenicity [1-2]. Polycyclic aromatic hydrocarbons are formed during the incomplete combustion of oil, coal, gas, wood and other organic substances. PAHs are initially generated in the gas phase, and they are adsorbed on pre-existing particles undergoing condensation during further cooling of the emission. Thus, most atmospheric PAHs exist in the particulate phase, while some higher volatility PAHs or low molecular weight PAHs remain partly in the gas phase (e.g., [3]). There are basically five major emission source components: domestic, mobile, industrial, agricultural and natural. The relative importance of these sources changes depending on the place or regulatory views; however, in the urban environment with heavy traffic, mobile source, that is vehicle exhaust is the main contributor to the atmospheric PAHs (e.g., [3-6]). Thus, health risk of the dense urban population by the exposure to those

Currently, PM10 or finer particles are major air pollutants in many urban areas. In the atmosphere, PAHs are partitioned between gaseous and particulate phases as explained earlier. Especially, PAHs of higher molecular weight species, which are often of higher carcinogenic potential, are mostly associated with fine particulate matter (e.g., [2, 7]). However, atmospheric behavior of particulate matter is known to be highly complicated in terms of its chemical compositions, size distributions, physical behavior, reactions, and so on. Accordingly, atmospheric behavior of PAHs associated with particulate matter is subjected to uncertainties and still poorly explained, including their temporal and spatial

PAHs has been of concern both in developed and developing countries.

Tomomi Hoshiko, Kazuo Yamamoto,

http://dx.doi.org/10.5772/48432

**1. Introduction** 

variations.

Fumiyuki Nakajima and Tassanee Prueksasit

Additional information is available at the end of the chapter

