Real-Time Investigation of Chemical Compositions and Hygroscopic Properties of Aerosols Generated from NaCl and Oxalic Acid Mixture Solutions Using in Situ Raman Microspectrometry

초록

Recently, ambient sea spray aerosols (SSAs) were reported to undergo reactions with dicarboxylic acids (DCAs). The hygroscopic properties of SSAs could be altered when the chemical compositions of SSAs are modified, which in turn alters their optical properties and cloud-droplet nucleation efficiency. Among water-soluble DCAs, oxalic acid (OA) is most abundant, in atmospheric aerosols followed by succinic acid and malonic acid. In this work, the hygroscopic behavior and chemical reactivity of aerosols generated from NaCl-OA mixture solutions were studied using in situ Raman micro-spectrometry (RMS). The change in size, chemical composition, and phase as a function of the relative humidity (RH) for individual aerosols were simultaneously monitored. The hygroscopic measurement was performed first through a dehydra-tion process, followed by a humidification process. Raman spectra showed that monosodium oxalate (MSO) was formed due to the chemical reaction between NaCl and OA from solutions of NaCl:OA = 3:1 and 2:1 after particles were generated. As RH decreased, disodium oxalate (DSO) was produced during the dehydration process. The aerosols generated from the solution of NaCl:OA = 3:1 and 2:1 showed single efflorescence, where MSO and DSO were in crystalline forms. Deliquescence transition of unreacted NaCl moiety was around 76% and complete dissolution of MSO and DSO was not observed even at RH > 98%. For the aerosols from NaCl:OA = 1:1 and 1:2 solutions, MSO was formed after generation. No deliquescence transition was observed for the aerosols from NaCl:OA = 1:1 and 1:2 solutions and water absorption started at RH > 95%. The observed different hygroscopic behavior was due to the different contents of NaCl, OA, MSO and DSO in the aerosols. Real-time investigation of chemical compositions of aerosols from NaCl-OA mixture solutions was investigated for first time by in situ RMS and quantitative analysis of chemical compositions during hygroscopic measurement are under