The optical and chemical properties of discharge generated organic haze using in-situ real-time techniques

Organic hazes formed from methane are present in many planetary and satellite atmospheres and influence surface and atmospheric processes. Here we examine the compositional and optical properties of laboratory generated hazes, or tholins, formed with varying amounts of methane using spark discharge excitation. By studying the optical and chemical properties together as a function of methane precursor concentration, the radiative impact of haze can be better understood. To determine the complex refractive index of tholin, we combine two spectroscopic techniques: photoacoustic spectroscopy and cavity ring-down spectroscopy (PASCaRD). The refractive indices are retrieved at λ = 405 and 532 nm. Quadrupole aerosol mass spectrometry is used along with a technique that utilizes isotopically labeled and unlabeled methane gas to quantify elemental composition. Tholin optical and compositional measurements are performed within a flow system, eliminating the need for tholin collection on a substrate and possible post-collection changes. We observe n values within the range of n values from most previous studies. However, the observed k values, like most others from recent studies, are significantly lower than the values from Khare et al. (1984) (Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W. [1984]. Icarus 60, 127–137) that are commonly used in data retrieval programs and models. In addition, comparing the tholin k values to their approximate nitrogen and aromatic content suggests both chemical constituents are important factors for increased aerosol absorption.