Light absorption coefficient measurement of SOA using a UV–Visible spectrometer connected with an integrating sphere

Abstract

Abstract A method for measuring an aerosol light absorption coefficient ( B a ) has been developed using a conventional UV–visible spectrometer equipped with an integrating sphere covering a wide range of wavelengths (280–800 nm). The feasibility of the proposed method was evaluated in both the transmittance mode (TUV-IS) and the reflective mode (RUV-IS) using the reference aerosol known for the cross-sectional area. The aerosol was collected on a conventional filter and measured for B a values. The resulting RUV-IS method was applied to measure light absorption of secondary organic aerosol (SOA). SOA was produced through photooxidation of different precursor hydrocarbons such as toluene, d -limonene and α -pinene in the presence of NO x (60–70 ppb) and inorganic seed aerosol using a 2-m 3 indoor Teflon film chamber. Of the three precursor hydrocarbons, the B a value of toluene SOA (0.574 m 2 g −1 at 350 nm) was the highest compared with B a values for α -pinene SOA (0.029 m 2 g −1 ) and d -limonene SOA (0.038 m 2 g −1 ). When d -limonene SOA or toluene SOA was internally mixed with neutral [(NH 4 ) 2 SO 4 ] or acidic inorganic seed (NH 4 HSO 4 :H 2 SO 4 = 1:1 by mole), the SOA showed 2–3 times greater B a values at 350 nm than the SOA with no seed. Aerosol aging with a light source for this study reduced B a values of SOA (e.g., on average 10% for toluene SOA and 30% for d -limonene SOA within 4 h). Overall, weak absorption appeared for chamber-generated SOA over wavelengths ranging from 280 to 550 nm, which fall into the sunlight spectrum.