Theory and Practice of Aerosol Science

THE EFFECT OF METAL IONS ON CHEMICAL REACTION BETWEEN DICARBOXYLIC ACID AND NITRATE WITHIN AEROSOLS DROPLETS

S. F. PANG, Y. H. ZHANG and S. XU

School of Chemistryand Chemical Engineering, Beijing Institute of Technology. Beijing 100081, People’s Republic of China.

Keywords: reactivity, malonic acid, nitrate, IR spectroscopy.

INTRODUCTION

Aerosols processing within organic acid/inorganic salt mixtures has been shown to significant alter aerosols properties such as hygroscopicity and volatility (Drozd et al, 2014). Previous studies reported the reaction between dicarboxylic acid and nitrate, and thought thatthe reaction driven force is release of volatile HNO3 and its partitioning between condensed and gas phases (Laskin et al, 2014). In order to understand the reaction, various organic acids have been chosen to react with sodium nitrate.By comparing the effect of acid dissociation constants and henry’s law constant,the release degree of gas HNO3was estimated. Herein, the reaction between malonic acid (MA) and various nitrate is investigated by vacuum FTIR technology and the results show that the metal ions of nitrate also play the key role to the reaction.

INTRODUCTION

The vacuum FTIR spectrometer (Bruker VERTEX 80v) is equipped with a SiC MIR/FIR source (10000−20 cm−1), a high precision 21° Michelson interferometer (Ultra Scan), 16 selectable mirror velocities (0.1−10 cms−1 opd) scanner, KBr on a gel beam splitter (range: 8000−350 cm−1) and liquid nitrogen cooled mercury−cadmium−telluride (MCT) detector. In experiment, the ambient RH is provided by pure water vapor and determined by the measurements of water vapor absorbance from IR spectra.Therefore, water content within aerosols, feature of compounds and water vapor amount of the aerosol canall be obtained from a same IR spectrum.

RESULTS

Figure 1 gives the FTIR spectra of mixture of MA/NaNO3, MA/Mg(NO3)2 and MA/Ca(NO3)2 when they keep at 50% for 80 min. The band at 1723 cm-1 comes from ν-COOH of MA, and the 1595 cm-1 (1595 cm-1 or 1588 cm-1) peak is assigned to ν-COO-of malonate. For MA/NaNO3 droplets, the band at 1595 cm-1 appears and the band at 1723cm-1 becomes slightly weak, implying the monosodimun malonate formation. In MA/Ca(NO3)2aerosols, there are more monocalcium malonate formation and less MAleft, because the

Figure 1 FTIR spectra of MA/NaNO3, MA/ Mg(NO3)2 and MA/Ca(NO3)2 droplets with 1:1 mole ratio when the chemical reaction is over at 4%RH

intensity of MA feature decrease more than that in MA/NaNO3. Comparing them, there is least malonic acid left for 80min in MA/Mg(NO3)2droplets by almost disappearance of thepeakat 1723cm-1. So it can be concluded that most malonic acid is confused and magnesium malonate is formed. Hence, we can know that the reaction betweenMg(NO3)2 and MA is most easy and complete.

In addition, the integration area of composition IR features can estimate the reaction process. Figure 2 present the malonic acid, malonate and nitrate varying with the time at 50%RH.It implies that the malonate increases with the reaction time, together with the decrease of MA and nitrate.After 80min, the malonate is most in MA/Mg(NO3)2 mixture and MA and nitrate lest is least among these three mixtures. So these quantitative feature area change assure that the reaction degree is dependent upon metal ions.

Figure 2The ν-COOH、ν-COO-and ν1-NO3-bandchanges with the residence time in FTIR spectra of MA/NaNO3, MA/ Mg(NO3)2 and MA/Ca(NO3)2 droplets (black open square: ν-COO-, orange open square: ν-COOH and open pink circle: ν1-NO3-)

CONCLUSIONS

By FTIR technology, the capacity of nitrate and malonic acid has been studied. The feature bands character the reaction process. At constant RH, the reaction in MA/Mg(NO3)2 is most complete and that in MA/NaNO3 is least. So it shows that the reaction between malonic acid and nitrate is dependent upon the metal ions.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (91644101, 41175119, 21473009, and 21373026)

REFERENCES

Drozd, G., Woo, J., Häkkinen, S. A. K., Nenes, A., McNeil, V. F. (2014) Inorganic salts interact with oxalic acid in submicron particles to form material with low hygroscopicity and volatility, Atoms. Chem. Phys. 14, 5205.

Wang, B. and Lakin, A. (2014) Reactions between water-soluble organic acids and nitrates in atmospheric aerosols: Recycling of nitric acid and formation of organic salts, J. Geophys. Res.:Atmos.119, 3335.