Retention of α-pinene oxidation products and nitro-aromatic compounds during riming

Riming is an important growth process of graupel and hailstones in mixed-phase zones of clouds, during which supercooled liquid droplets freeze on the surface of ice particles by contact. Compounds dissolved in the supercooled cloud droplets can remain in the ice or be released to the gas phase during freezing, which might play an important role in the vertical redistribution of these compounds in the atmosphere by convective cloud processes. This is important for estimating the availability of these compounds in the upper troposphere, where organic matter can promote new particle formation and growth. The amount of organics remaining in the ice phase can be described by the retention coefficient. Experiments were performed in the Mainz vertical wind tunnel under dry and wet growth conditions (temperature from -12 to -3 °C and a liquid water contents (LWC) of 0.9 ± 0.2 g m-3 and 2.2 ± 0.2 g m-3) as well as different pH values (4 and 5.6) to obtain the retention coefficients of α-pinene oxidation products and nitro-aromatic compounds. For cis-pinic acid, cis-pinonic acid, and (-)-pinanediol mean retention coefficients of 0.96 ± 0.07, 0.92 ± 0.11and 0.98 ± 0.08 were obtained. 4-Nitrophenol, 4-nitrocatechol, 2‑nitrobenzoic acid, and 2‑nitrophenol showed mean retention coefficients of 1.01 ± 0.07, 1.01 ± 0.14, 0.99 ± 0.04 and 0.16 ± 0.10. Only the retention coefficient of 2-nitrophenol showed a dependence on temperature, growth regime, and pH. This is in accordance with previous studies which showed a dependence between the dimensionless effective Henry's law constant H* and the retention coefficient for inorganic and small organic molecules. Our results reveal that this correlation can also be applied to more complex organic molecules, and that retention under these conditions is negligible for molecules with H* below 103, while unity retention can be expected for compounds with H* above 108.