Measurements of optical properties of organic aerosols using photoacoustic instrumentation

Description

Light absorption and scattering by atmospheric aerosols has a considerable effect on the earth’s direct and semi-direct radiative forcing. It is well known that light absorption by dust and black carbon aerosols (soot) has a warming effect on climate, while scattering by sulfate, nitrate, and sea salt aerosols has a cooling effect. However, there are large uncertainties associated with light absorption and scattering by various primary and secondary organic aerosols, especially in the near-UV and UV. Here, we present preliminary results from a recent laboratory study focused on measuring absorption and scattering properties of selected primary organic aerosol (POA) and secondary organic aerosol (SOA) systems using photoacoustic (PA) instruments at four wavelengths: 355, 405, 532, and 870 nm. In the POA category, we examined optical properties of lubricating oil (SAE15W-40) aerosol, black carbon aerosol (generated from India ink solution), and black carbon aerosol coated with lubricating oil. The results showed that light absorption of black carbon aerosol was significantly enhanced by the lubricating oil coating. In the SOA category, we examined light absorption properties of secondary organic aerosol (SOA) formed from the oxidation of α-pinene with ozone and nitrate radical in the presence of pre-existing neutral and acidic seed aerosols. Our results show that light absorption of α-pinene SOA is strongly dependent on the experimental conditions, such as relative humidity, oxidants, and acidity of the pre-existing inorganic seed aerosols. Strong absorption at 355 nm and 405 nm was observed for SOA from oxidation of α-pinene and nitrate (NO3) radical in the presence of strong acidic inorganic seed aerosol at low RH (<3%). No light absorption was observed when RH was higher than 12% or in the presence of weak and neutral inorganic seed aerosols or when ozone was used as oxidant. Our results also indicate that α-pinene itself could be absorbed into strong acidic seed aerosols and enhance light absorption, possibly by forming oligomeric species via aerosol-phase heterogeneous reactions.