The Influence of Gas-phase Chemistry on Organic Haze Formation

Although photochemically produced organic hazes are common in planetary atmospheres, there have been few experimental investigations of the gas-phase chemistry leading to organic haze formation. We report a laboratory study of the gas-phase compounds formed by far-ultraviolet irradiation of a CH4/N2 mixture. Using high-resolution chemical ionization mass spectrometry (CIMS) we made in situ measurements of gas-phase products up to m/z 400. Organic nitrogen species dominate the mass spectra with smaller contributions from unsaturated hydrocarbons. Using a structural group method to estimate vapor pressures, we calculate that for compounds detected at m/z > 320, ≥50% of the total compound loading (gas + condensed phase) would be present in the condensed (aerosol) phase at 300 K. Using approximations for changes in vapor pressure with temperature, we estimate that ≥50% of the total loading for species with m/z > 110 would be in the aerosol-phase at 150 K, suggesting that the measured compounds may be important for aerosol composition for a range of temperatures. Finally, we leverage the sensitivity and fast time response of the CIMS measurements to investigate how the gas-phase chemistry evolved over the course of the experiment. This analysis shows that hydrocarbons were the initial products followed by the formation of unsaturated organic nitrogen compounds. Higher molecular weight species and alkylamines appear late in the experiment. Comparison of our measurements to chemical mechanisms used to model planetary haze formation suggests that larger molecular weight compounds need to be considered in haze formation chemistry.