Temperature-dependent Henry's law constants of atmospheric organics of biogenic origin

There have been growing interests in modeling studies to understand oxidation of volatile organic compounds in the gas phase and their mass transfer to the aqueous phase for their potential roles in cloud chemistry, formation of secondary organic aerosols, and fate of atmospheric organics. Temperature-dependent Henry's law constants, key parameters in the atmospheric models to account for mass transfer, are often unavailable. In the present work, we investigated gas-liquid equilibriums of isoprene, limonene, α-pinene, and linalool using a bubble column technique. These compounds, originating from biogenic sources, were selected for their implications in atmospheric cloud chemistry and secondary organic aerosol formation. We reported Henry's law constants (K_H), first order loss rates (k), and gas phase diffusion coefficients over a range of temperatures relevant to the lower atmosphere (278-298 K) for the first time. The measurement results of K_H values for isoprene, limonene, α-pinene, and linalool at 298 K were 0.036 ± 0.003; 0.048 ± 0.004; 0.029 ± 0.004; and 21.20 ± 0.30 mol L^-1 atm^-1, respectively. The fraction for these compounds in stratocumulus and cumulonimbus clouds at 278 K were also estimated in this work (isoprene, 1.0 × 10^-6, 6.8 × 10 ^-6; limonene, 1.5 × 10^-6, 1.0 × 10 ^-5; α-pinene, 4.5 × 10^-7, 3.1 × 10 ^-6; and linalool, 6.2 × 10^-4, 4.2 × 10 ^-3). Our measurements in combination with literature results indicated that noncyclic alkenes could have smaller K_H values than those of cyclic terpenes and that K_H values may increase with an increasing number of double bonds. It was also shown that estimated Henry's law constants and their temperature dependence based on model prediction can differ from experimental results considerably and that direct measurements of temperature-dependent Henry's law constants of atmospheric organics are necessary for future work.