ABSTRACT
The condensation and evaporation processes of semi-volatile organic compounds (SVOCs) in atmospheric aerosols can induce significant evolutions of their chemical and physical properties. Hence, for interpreting and predicting composition changes of atmospheric aerosols, it is indispensable to provide insight into the partitioning behaviors of SVOCs between condensed and gas phases. In this research, optical tweezers coupled with cavity-enhanced Raman spectroscopy were employed to observe the volatility of internally mixed citric acid (CA)/(NH4)2SO4 (AS) particles, and the effect of AS on the gas/particle partitioning behaviors of atmospheric organic acids was investigated. The radii and refractive indexes of the levitated droplets were determined in real time from the wavelength positions of simulated Raman spectra and the effective vapor pressures of CA at different relative humidities (RHs) were obtained according to Maxwell equation. For the CA/AS particle with organic to inorganic mole ratio (OIR) of 1:1, the effective vapor pressure of CA decreased with the decreasing of RH. When the RH decreased from 67% to 8.2%, the effective vapor pressure of CA decreased from (1.35±0.508)×10-4Pa to (3.0±1.0)×10-6Pa. Meanwhile, the CA/AS particles with OIR of 3:1, 1:3 were also studied, and the results show the same phenomenon compared to the particles with OIR of 1:1. When under constant RHs, the effective vapor pressures of CA decreased with the increasing of AS contents, suggesting that the presence of AS suppressed the partitioning of CA to aqueous particles. In addition, the mass transfer processes of water in CA and CA/AS/H2O systems were further studied. The characteristic time ratio between the droplet radius and RH was used to describe the water mass transfer difference dependent on RH. Compared to the characteristic time ratio of pure CA, the characteristic time ratio of CA/AS particles apparently increased. For CA/AS particles under the same RH steps, the characteristic time ratio increased with the AS content increase. According to the differential isotherm, the diffusion coefficients of citric acid and citric acid/ammonium sulfate at low RHs (RH ≈7%-1%, RH≈1%-7%) were calculated respectively. Generally, the key aspect of the current work was to deeply explore the relationship between the evaporation rates of SVOCs and water transport process.
ABSTRACT
Hygroscopicity and volatility of single magnesium acetate (MgAc2) aerosol particles at various relative humidities (RHs) are studied by a single-beam optical tweezers, and refractive indices (RIs) and morphology are characterized by cavity enhanced Raman spectroscopy. Gel formation and volatilization of acetate acid (HAc) in MgAc2 droplets are observed. Due to the formation of amorphous gel structure, water transposition in droplets at RHâ¯<â¯50% is significantly impeded on a time scale of 140,000â¯s. Different phase transition at RHâ¯<â¯10% is proposed to explain the distinct water loss after the gel formation. To compare volatilization of HAc in different systems, MgAc2 and sodium acetate (NaAc) droplets are maintained at several different stable RHs during up to 86,000â¯s. At RHâ¯≈â¯74%, magnesium hydroxide (Mg(OH)2) inclusions are formed in MgAc2 droplets due to the volatilization of HAc, and whispering gallery modes (WGMs) of MgAc2 droplets in the Raman spectrum quench after 50,000â¯s. In sharp contrast, after 86,000â¯s at RHâ¯≈â¯70%, NaAc droplets are in well-mixed liquid states, containing soluble sodium hydroxide (NaOH). At this state, the RI of NaAc droplet is increased, and the quenching of WGMs is not observable.
