Near-ultraviolet and visible optical properties of secondary organic aerosols and gas phase species

The interaction of light and gas and particle phase species plays an important role in climate change. Incoherent Broadband Cavity-Enhanced Absorbed Spectroscopy (IBBCEAS) was used to measure the gas phase absorption cross section of a number of atmospherically-relevant species and to investigate the optical properties. The investigated spectral region extended from 325 to 420 nm. Gas phase absorption cross section spectra of biacetyl, 1-nitronaphthalene, 2- nitrophenol, acetaldehyde and acenaphthylene were measured. The gas phase spectra of 1-nitronaphthalene and acenaphthylene have not previously been reported, while the 2-nitrophenol and acetaldehyde absorption cross sections agreed closely with previous literature values. In addition, the absorption of water in the near-UV region was also investigated. In contrast to a recently report, no water vapour absorption was seen; however, upper limits were established for the water absorption cross section at a resolution of 0.5 nm. The optical properties of secondary organic aerosols (SOA) formed by ozonolysis of α-pinene were studied. The refractive index values for α-pinene/O3 SOA ranged from 1.40 at 410 nm to 1.43 at 320 nm, and are broadly consistent with previously reported values. The VOC precursor concentration, relative humidity, and the extent of oxidation were all found to affect the refractive index values. SOA formed from photochemical reaction of 2-NP was investigated. Nitro-aromatic SOA has been proposed to have a large impact on the UV irradiance in urban areas. The 2-NP loss rate followed second order kinetics which indicated that photolysis was not the only loss mechanism; this finding provided evidence for a proposed mechanism. Addition of acetaldehyde to the reaction suppressed particle formation. Off-line UV/Vis absorption measurements showed that SOA absorption increased towards shorter wavelengths, as is typical of brown carbon. The mass absorption coefficient and imaginary part of the complex refractive index indicate that these SOA species are relatively weakly absorbing