Photochemical formation rates of organic aerosols through time‐resolved in situ laboratory measurements

Abstract

[1] The fundamental kinetics of photochemical particle formation and the mechanism(s) for polymerization of hydrocarbons to form condensable species under anoxic conditions have yet to be determined experimentally. Thus these processes remain highly parameterized in models of planetary atmospheres. We have developed instrumentation for simultaneously measuring the net production rates of complex gas-phase hydrocarbons and of organic aerosols formed from irradiating mixtures of simple precursor gases through online quadrupole mass spectrometry measurements of stable gas-phase species and in situ optical scattering detection of the particulates at 633 nm. The new technique and results for the generation of hydrocarbon aerosol from the irradiation of pure, gasphase CH 4 at a pressure of 70 Torr with 8.8 ± 0.8 Â 1015 photons s À1 of vacuum ultraviolet light (120-300nm) are reported. The net production rates for the following gaseous species are measured: H 2 = 9.9 ± 2.2 Á 10 11 , C 2 H 2 = 2.8 ± 0.5 Á 10 10 , C 2 H 4 = 5.5 ± 9.4 Á 10 9 , C 2 H 6 = 8.6 ± 2.5 Á 10 10 , C 3 H 4 = 2.5 ± 1.2 Á 10 9 , C 4 H 2 = 6.6 ± 5.0 Á 10 8 , C 4 H 10 = 1.3 ± 0.5 Á 10 10 cm À3 s À1. The production of hydrocarbon particulates is detected by optical scattering, and the condensed phase C-C bond formation rate is inferred to be 7.5 ± 3.1 Á 10 11 cm À3 s À1 . Applications of this technique to the atmospheres of Titan and terrestrial-like planets, such as the early Earth before the rise of molecular oxygen, are discussed. Citation: Á dámkovics, M., and K. A. Boering, Photochemical formation rates of organic aerosols through time-resolved in situ laboratory measurements,

Nitrogen Incorporation in CH4-N2 Photochemical Aerosol Produced by Far Ultraviolet Irradiation

“…The low particle yield in Ar is consistent with previous results reported by Bar-Nun and Podolak (1979), in which no condensed phase was observed after hours of irradiation of the same gas mixture by the same FUV wavelength range. Adamkovics and Boering (2003) saw aerosol production from CH 4 -only irradiation with the same FUV source, but the CH 4 pressure, static chamber, and particle detection methods used were dissimilar enough from our experiments that it is not possible to compare their aerosol production rates to ours either with or without N 2 . From Fig.…”

Dissociative recombination of C3H4+: preferential formation of the C3H3 radical

“…in dark and translucent interstellar clouds [7]. Moreover, it has been unambiguously identified in the atmosphere of both Jupiter and Titan [8,9], while, in laboratory studies, it has also been readily produced through the UV irradiation of pure methane, which constitutes an important component in the atmosphere of giant planets [10].…”

Role of photoionization in the formation of complex organic molecules in Titan's upper atmosphere

“…Limitations in those numerical models are imperfect treatments of the formation of neutral species via dissociative recombination of ion species and the UV irradiation wavelength dependence of dissociation/ionization mechanisms of neutral species. Despite the importance of N 2 /CH 4 photolysis by EUV‐VUV light, laboratory simulation of such photochemical processes has been largely limited to CH 4 photolysis at wavelengths longer than ∼110 nm [e.g., Ádámkovics and Boering , 2003; Trainer et al , 2006]. The roles of EUV‐VUV light shorter than 110 nm and the role of N 2 molecules in the photochemical process remain to be answered.…”