Wavelength Dependence of Refractive Index of Secondary Organic Aerosols Generated during the Ozonolysis and Photooxidation of α-Pinene

Abstract

Atmospheric aerosol particles play an important role in climate change by scattering and absorbing solar radiation. To quantify the direct influence of aerosols on the atmospheric radiation balance, a detailed understanding of the complex refractive index (RI) for a variety of aerosols is required. The wavelength dependence of the RI values for secondary organic aerosols (SOAs) generated during the ozonolysis and photooxidation of α-pinene, which is a major biogenic volatile organic compound (BVOC), was investigated using a photoacoustic soot spectrometer (PASS) and a cavity ring-down spectrometer (CRDS). The real part of the RI values for SOAs generated from both ozonolysis and photooxidation was found to lie between 1.4 and 1.5 at 405, 532, and 781 nm, and to slightly increase with decreasing wavelength. Additionally, the imaginary part of the RI values for these SOAs was found to be negligible (< 0.003) at all of the wavelengths studied (405, 532, and 781 nm). The obtained complex RI values were also compared with values reported in the literature and the chemical properties of the SOAs, which were measured using a high resolution time-of-flight aerosol mass spectrometer (H-ToF-AMS).

Relating hygroscopicity and optical properties to chemical composition and structure of secondary organic aerosol particles generated from the ozonolysis of α-pinene

“…Figure 9 shows that the real part of the CRI decreases substantially as the O : C of the bulk SOA decreases. This trend is in agreement with the positive correlation between the extinction coefficient at 532 nm wavelength and O : C observed by for the heterogeneous OH oxidation of squalane and aze- (Nakayama et al, 2012) -pinene (Lambe et al 2013) toluene (Nakayama et al, 2013) 1 naphtalene (Lambe et al 2013) guaiacol (Lambe et al 2013) tricyclo[5.2.1.0 2,6 ]decane (Lambe et al 2013) squalane azelaic acid Photooxidation Ozonolysis OH-oxidation laic acid particles. Nakayama et al (2013) reported an increasing real part of the CRI at 532 nm with increasing O : C for SOA generated from the photooxidation of toluene.…”

“…Recently, Flores et al (2014) showed that the real part of the CRI at 405 nm is positively correlated to the O : C for a mixture of biogenic VOC (α-pinene and limonene) and biogenic VOC mixture with subsequent addition of an anthropogenic VOC (p-xylene-d 10 ). On the other hand, an opposite trend was observed by Nakayama et al (2012) with a decrease of the real part of the CRI at 532 nm wavelength with increasing O : C for SOA produced from the ozonolysis and photooxidation of α-pinene. Lambe et al (2013) studied SOA formed by OH oxidation of various gas-phase precursors (naphthalene, tricyclo[5.2.1.0 2,6 ]decane, guaiacol and α-pinene) and retrieved a decreasing real part of the CRI with increasing oxidation.…”

“…Schnaiter et al (2003) and Nakayama et al (2010) measured no significant difference between scattering and extinction coefficient in the visible spectrum. Nakayama et al (2012) showed that the imaginary part of the CRI values was negligible (< 0.003) in the 405-781 nm wavelengths. Recently, Liu et al (2013) also reported an imaginary part of the CRI below 10 −4 in the 355-781 nm wavelengths range.…”

Light extinction by secondary organic aerosol: an intercomparison of three broadband cavity spectrometers

“…The β-pinene mixing ratios (15 ppb) and the resulting aerosol mass concentrations (30 to 40 µg m −3 ) in this work were low compared to other monoterpene SOA refractive index studies (Kim et al, 2010;Nakayama et al, 2010Nakayama et al, , 2012Lambe et al, 2013). Kim and Paulsen (2013) note that high-aerosol mass loadings are likely to result in proportionally more high-volatility species in the particle phase, and are thus less representative of ambient aerosols which have lower volatility constituents.…”

Assessment of direct radiative forcing due to secondary organic aerosol over China with a regional climate model

“…a-Pinene SOA has little absorbing effect in all wavelengths and a refractive index of 1.44 at 532 nm is recommended by Kim and Paulson (2013). Wavelength dependence of RI is studied by Nakayama et al (2012) and Yu et al (2008), and both results come to a conclusion that RI will decrease with increasing wavelength. Linear polynomial fitting is done for the data listed in Table 2, and 1.48 and 1.42 are recommended for 350 and 700 nm BSOA refractive index, respectively.…”