Light absorption by Secondary Organic Aerosol from Photooxidation of Alpha-pinene, D-limonene and Toluene

MIN ZHONG (1), Myoseon Jang (1)

(1) University of Florida, Gainesville

     Abstract Number: 344
     Last modified: May 9, 2010

Abstract
Up to date, understanding of the effect of secondary organic aerosols (SOA) on climate forcing is very limited in both fundamental optical property of SOA and the predictive model. Thus, quantifying the SOA optical property such as absorption coefficient is essential for evaluating the significance of SOA on climate change. In this study, a variety of SOA were produced from photooxidation of different precursor hydrocarbons such as alpha-pinene, d-limonene and toluene in the presence to NO$_x (60-70 ppb) varying preexisting submicron inorganic salts (no seed, (NH$_4)$_2SO$_4 seed and 1:1 mixture of NH$_4HSO$_4 and H$_2SO$_4) using a 2 m$^3 indoor Teflon film chamber. The resulting SOA were analyzed with an UV/visible spectrometer equipped with an integrating sphere for measuring the absorption coefficient of SOA. Our study shows that the light absorption of SOA was influenced by the molecular structure of the precursor oxidation products, the internal mixed seeds, and the UV irradiation. All SOA showed strong UV light absorbance in the range of 200-380 nm. For example, normalized absorption coefficients of alpha-pinene, d-limonene and toluene SOA were 1.14×10$^(-4), 5.39×10$^(-4) and 1.21×10$^(-3) m$^(-1)mg$^(-1) at 350 nm, respectively. Only d-limonene SOA and toluene SOA absorbed visible light in the 380-500nm wavelength range. The normalized absorption coefficient for d-limonene SOA was 1.58×10$^(-4) m$^(-1)mg$^(-1) at 450 nm and that of toluene SOA, 2.50×10$^(-4) m$^(-1)mg$^(-1). In the presence of inorganic seed aerosols, 2 to 3 times higher absorption coefficients were observed for the toluene and d-limonene SOA, while no significant seed aerosol effect appeared in alpha-pinene SOA. UV irradiation has reduced SOA light absorption property with different extents for three SOA systems. The observation of this study suggests that the light absorption of SOA cannot be neglected and potentially would affect radiative forcing effects of aerosols, especially in the wavelength range from 280 to 500 nm. In future studies, the investigation of SOA optical properties in the longer period using the actual sunlight is necessary.