The hygroscopicity of atmospheric particulate matter can determine its optical behaviors, cloud condensation nuclei activity, and heterogeneous chemical reaction on the surface. However, the effect of secondary organic matter on the hygroscopicity of inorganic salts is still rarely studied. In this study, a 2 L gas-phase flow tube was used to stimulate the oxidation process of α-pinene. NaCl particles at the end of the flow tube served as seeds to absorb the product of α-pinene oxidation. The absorption times were 11, 15, and 20 h. Transmission electron microscopy results show that this system could produce particles with typical core-shell structure, and that the thickness of the organic shell would be thicker with longer collection time. Results from individual particle hygroscopic systems indicate that the organic shell can affect the deliquescent point of NaCl (pure NaCl deliquesces at 77%):for 11 h collection time, the NaCl core began to deliquesce at 75.5% relative humidity, and completely deliquesced at 78%; for 15 h collection time, the deliquescent range of the NaCl core was 75%-78.5%; and the deliquescent range was 76%-83% for the NaCl core with 20 h collection time. Furthermore, the organic shell could affect the hygroscopic growth of the NaCl core:the growth factor of the NaCl core with 11 h, 15 h, and 20 h collection time was 2.5, 2.3, and 1.8, respectively. These results indicate that:① the secondary organic shell could cause water absorption of the NaCl core in advance:② the organic shell could postpone the completely deliquescent point of the NaCl core, and this effect is more obvious with a thicker organic shell; ③ the organic shell could restrain the hygroscopic growth factor of the NaCl core. This study suggests that it is necessary to consider the influence of secondary organic matter on inorganic salts when studying the hygroscopic properties of atmospheric particles in climate models and field observations.
Keywords: NaCl; hygroscopicity; ozone oxidation; secondary organic aerosols; α-pinene.