Retention During Freezing of Raindrops, Part II: Investigation of Ambient Organics from Beijing Urban Aerosol Samples

Abstract. The freezing of hydrometeors incurs 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 new particle formation in the upper troposphere. 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 nitroaromatics and organosulfates were fully retained along with the aliphatic amines (AA) and higher-order amines and amides while sulfides, lipids, aromatic hydrocarbons, and long chain compounds are among the most unretained and incidentally the fewest species observed. The findings here also indicate that NO_x and SO_x chemistry, particularly anthropogenically related, enhances the retention of the resulting secondary organic aerosols (SOA). A positive correlation between polarity and freezing retention along with a negative correlation with vapor pressure and freezing retention was observed. No sigmoidal relationship with effective Henry’s law constant was observed which differs with the parameterizations of riming retention presented in current literature, which is justified by the lower surface-to-volume ratio of the large drop size investigated.