How Are Strong Acids or Strong Bases Substituted by Weak Acids or Weak Bases in Aerosols?

Due to significant influence on global climate and human health, atmospheric aerosols have attracted numerous interests from the atmospheric science community. To provide insight into the aerosol effect, it is indispensable to investigate the aerosol properties comprehensively.

Since atmospheric aerosols are surrounded by substantial gas phase and have high specific surface area, the composition partitioning between particle phase and gas phase must be considered as a key aerosol property, which is termed as volatility for volatile organic/inorganic components. Recent studies show that the aerosol volatility can also be induced by the reaction of components in addition to the volatile compositions. Herein, we summarize four types of volatility induced by reaction, namely chloride depletion, nitrate depletion, ammonia depletion and volatility induced by salt hydrolysis. For chloride depletion and nitrate depletion, these processes can be regarded as reactions that strong acids are substituted by weak acids. The high volatility of the formed HCl or HNO₃ drives the reaction continuously moving forward.

For ammonium depletion, we observed the reaction occurs between (NH₄)₂SO₄ and organic acid salts during dehydration process by ATR-FTIR. For example, when molar ratio is 1:1, significant depletion of ammonium was observed in the disodium succinate/(NH₄)₂SO₄ particles, indicating the evaporation of NH₃. Besides, the hygroscopicity of the aerosol particles decreased after the dehydration, which should be attributed to the formation of less hygroscopic succinic acid and ammonium depletion. By regarding organic acid salts as weak bases, the ammonium depletion is a reaction that strong base substituted by weak base, driving by the continuous release of NH₃. In addition to volatility induced by reactions within multi-component aerosols, we also found that the salt hydrolysis can also cause the formation of volatile product. For magnesium acetate (MgAc₂) aerosols, we found significant water loss of the aerosol particles under constant relative humidity condition, while the amount of acetate was also decreased. We infer that the acetic acid (HAc) evaporation is caused by the hydrolysis of MgAc₂, leading to the volatility and declined hygroscopicity. Two factors contribute to the volatility of MgAc₂ aerosols. One is the volatile acid donner (Ac^2-), which can lead to the formation of volatile HAc. The other is the residual ion accepter (Mg²⁺), which can combine residual OH^- after the proton is depleted by the evaporation of HAc. The formation of insoluble Mg(OH)₂ effectively maintains the aqueous pH in a suitable range, keeping the reaction moving forward. It should be noted that the co-exist of volatile acid donner and residual ion accepter is indispensable for the volatility induced by hydrolysis.

Generally, for the volatile species present in atmosphere, the aerosol volatility induced by the reaction of components can be an important pathway for their recycling processes. Due to the substantial composition modification, the hygroscopicity is also affected by such reaction. Therefore, this partitioning behavior of aerosols needs to be considered in the future atmospheric aerosol study, which may prevent the underestimate of particle volatilization or overestimate of hygroscopicity.