**1. Introduction**

346 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

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The detailed mechanism and controlling factors of SOA formation are not fully understood yet, which leads to the lower SOA level prediction from air quality models than the ambient measurements (Volkamer et al., 2006). Using smog chamber, SOA formation process can be investigated under controlled experimental conditions. Series of smog experiments have been conducted by different research groups to investigate the effects of background seed aerosols on SOA formation (Cao and Jang, 2007, Czoschke et al., 2003, Gao et al., 2004, Jang et al., 2002, Liggio and Li, 2008). Increased SOA formation and SOA yields were observed with the presence of acid seed aerosols. The effects of acidic seeds suggest that aerosol phase reactions may play an important role on SOA formation (Jang et al., 2002). Interactions between the organic and inorganic components of aerosols are important for further understanding the SOA formation process. Most research concludes that acid-catalyzed aerosol-phase reactions generate additional aerosol mass due to the production of oligomeric products with large molecular weight and extremely low volatility (Cao and Jang, 2007, Czoschke et al., 2003, Gao et al., 2004) and, therefore, enhance SOA formation.

© 2012 Jiang 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 Jiang 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.

Uptake of semivolatile organic products to acidic sulfate aerosols was also found contributing to enhance SOA formation (Liggio and Li, 2008). In these studies, (NH4)2SO4 or H2SO4 seed aerosols were widely used to study the effect of particle acidity on SOA formation from both biogenic and aromatic hydrocarbons.

Effects of Inorganic Seeds on Secondary Organic Aerosol (SOA) Formation 349

A gas chromatograph (GC, Beifen SP-3420) equipped with a DB-5 column (30 m×0.53 mm×1.5 mm, Dikma) and flame ionization detector (FID) measured the concentration of the hydrocarbon every 15 min. NOx and O3 were monitored with an interval of 1 min by a NOx analyzer (Thermo Environmental Instruments, Model 42C) and an O3 analyzer (Thermo Environmental Instruments, Model 49C), respectively. Size distribution of particle matter (PM) was measured by a scanning mobility particle sizer (SMPS, TSI 3936) in the range of 17-1000 nm with a 6-min cycle. The volume concentration of aerosols was estimated from the measured size distribution by assuming the particles were geometrically spherical and

Due to deposition of particles on the Teflon film, the measured aerosol concentration had to be corrected. Takekawa et al. (2003) developed a particle size-dependent correction method, in which the aerosol deposition rate constant (k(dp), h-1) is a four-parameter function of

The resulting k(dp) values for different dp (40-700 nm) were determined by monitoring the particle number decay under dark conditions at low initial concentrations (<1000 particles cm-3) to avoid serious coagulation. Based on more than 500 sets of k(dp) values (dp ranges from 40 to 700 nm), the optimized values of parameter a, b, c, and d were calculated to be 6.46×10-7, 1.78, 13.2, and -0.957, respectively. It should be noted that the estimation of deposited aerosol concentrations using this method might introduce some error (Takekawa et al., 2003) because some scatter was recognized when fitting k(dp) values into equation (1). To reduce error due to wall deposition, SOA yields were calculated when the measured particle concentration reached its maximum in the experiments because deposited aerosols were a greater proportion of the aerosol concentration change in the

Several researchers have measured SOA density, providing an estimated range of 0.6-1.5 g cm-3 (Bahreini et al., 2005, Poulain et al., 2010, Qi et al., 2010, Song et al., 2007, Yu et al., 2008). In our study, we used a unit density (1.0 g cm-3) to calculate SOA mass concentrations.

The fractional SOA yield (Y), defined as the ratio of the generated organic aerosol concentration (Mo) to the reacted hydrocarbon concentration (ΔHC), was used to represent the aerosol formation potential of the hydrocarbon (Pandis et al., 1992). Odum et al. (1996) developed a gas/particle absorptive partitioning model to describe the phenomenon that Y

This follows the approach used in Takekawa et al. (2003) and Verheggen et al. (2007).

b d

p pp k(d ) = a d +c d (1)

nonporous.

**3. Results and discussion** 

reactor after that time.

**3.2. Calculation of SOA yields** 

**3.1. Estimating the generated SOA mass (Mo)** 

particle diameter (dp, nm), as shown in equation (1):

Atmospheric aerosols always have a very complex composition. Studying the effects of (NH4)2SO4 or H2SO4 seed aerosols did not draw the whole picture of the role that inorganic seed aerosols play in SOA formation. Metal-containing aerosols are important components of the atmosphere. Calcium and iron are the most abundant metal species in atmospheric aerosols and the average concentration of them in Beijing could be as high as about 1.2 μg m-3 and 1.1 μg/m3 in PM2.5 (He et al., 2001) respectively. In this study, we tested the effect of different inorganic seeds on SOA formation using a smog chamber. Two aromatic hydrocarbon precursors toluene and m-xylene are used. Effects of various inorganic seeds, including neutral inorganic seed CaSO4, acidic seed (NH4)2SO4, transition metal contained inorganic seeds FeSO4 and Fe2(SO4)3, and a mixture of (NH4)2SO4 and FeSO4, were examined during *m-*xylene or toluene photooxidation with the presence of nitrogen oxides (NOx).
