Photochemical formation rates of organic aerosols through time-resolved in situ laboratory measurements -: art. no. 5092

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, gas-phase CH4 at a pressure of 70 Torr with 8.8+/-0.8x10(15) photons s(-1) of vacuum ultraviolet light (120-300nm) are reported. The net production rates for the following gaseous species are measured: H-2=9.9+/-2.2.10(11), C2H2=2.8+/-0.5.10(10), C2H4=5.5+/-9.4.10(9), C2H6=8.6+/-2.5.10(10), C3H4=2.5+/-1.2.10(9), C4H2=6.6+/-5.0.10(8), C4H10=1.3+/-0.5.10(10) cm(-3) s(-1). 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+/-3.1.10(11) cm(-3) s(-1). 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.