Retention during freezing of raindrops – Part 2 : Investigation of ambient organics from Beijing urban aerosol samples

Abstract

The freezing of hydrometeors causes certain water-soluble organic compounds dissolved in the supercooled cloud droplets to be released into the gas phase. This may lead to the vertical redistribution of substances that become available for atmospheric processes in the upper troposphere, such as new particle formation or ozone formation. Drop-freezing experiments were performed on the Mainz acoustic levitator (M-AL) using aqueous extracts of ambient samples of Beijing urban aerosol. The retention coefficients of over 450 compounds were determined. Most nitro-aromatics and organosulfates were fully retained, along with the aliphatic amines (AAs) and higher-order amines and amides, while the observed sulfides, lipids, aromatic hydrocarbons, and long-chain compounds are among the most unretained and, incidentally, are the fewest species present. The findings here also indicate that N- and S-containing compounds, primarily nitro and sulfate components of secondary organic aerosols (SOAs) anthropogenically related to NOx and SO2 chemistry, have enhanced retention, likely due to their increased polarity. An insignificant positive correlation between polarity and freezing retention, along with a significant negative correlation with vapor pressure and freezing retention, was observed. No sigmoidal relationship with the effective Henry's law constant was observed. This differs from the parameterizations of riming retention presented in the current literature, which is justified by the lower surface-to-volume ratio of the large drop size investigated. This study greatly expands upon the available experimental measurements of retention by investigating hundreds of compounds in complex chemical conditions that are more similar to the atmosphere than in previous literature studies.