A comprehensive study on hygroscopic behaviour and nitrate depletion of NaNO3 and dicarboxylic acid mixtures: implications for nitrate depletion in tropospheric aerosols

The nitrate depletion and HNO3 release in internally mixed nitrate and dicarboxylic acid (DCA) particles have been widely detected in field and laboratory studies. Nevertheless, considerable discrepancies are still present among these measurements, and the influencing factors for this acid-displacement reaction have not yet been elucidated. In this work, the hygroscopic growth and chemical composition evolution of mixtures of NaNO3 and DCAs, i.e. oxalic acid (OA), malonic acid (MA), and glutaric acid (GA), were measured using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and vacuum FTIR techniques. The nitrate depletion from NaNO3-OA mixtures was observed in both the measurements, owing to the relatively higher acidity of OA compared to MA and GA. At the same time, the NaNO3 phase state was found to act as a key regulator of nitrate depletion. Amorphous NaNO3 solids at relative humidity (RH)g<5g% were inert to liquid OA. With increasing RH, the mixtures experienced three interesting stages of phase changes showing different HNO3 release rates; e.g. at around 15g% RH, the slow HNO3 release was detected by the vacuum IR spectra, potentially indicating the transformation of amorphous solids to semisolid NaNO3. In the second stage (sudden RH increase from g1/415g% to 61g%), the HNO3 release rate was increased by about an order of magnitude. When NaNO3 deliquescence occurred in the third stage, this displacement reaction proceeded due to more available NO3- ion formation. Compared to OA, MA and GA reacted with nitrate only in vacuum FTIR measurement, while in ATR-FTIR measurement, the mixtures tended to be effloresced completely without nitrate depletion. Further, the influences of ambient pressure, chemical composition, and water activity on HNO3 release rates were estimated via Maxwell steady-state diffusive mass transfer equation. The results showed that weaker acidity of MA and GA as well as relatively lower HNO3 diffusion rate in the ambient gas phase mainly contributed to the unobserved nitrate depletion in ATR-FTIR measurement. Our findings reveal that chemical component, phase state, and water activity of particles, as well as HNO3 gas-phase diffusion, play crucial roles in HNO3 release from nitrate and DCA mixtures. This work may provide a new perspective on nitrate depletion in the ageing processes during transport of tropospheric aerosols.