Hygroscopic properties and compositional evolution of internally mixed sodium nitrate-amino acid aerosols

Although nitrates and amino acids are ubiquitous in the atmosphere, the interactions of amino acids with nitrates on the hygroscopicity of aerosols are still not clear. We studied the hygroscopic properties and compositional evolution of three atmospherically relevant species, sodium nitrate (NaNO3), glycine and L-alanine (alanine). The investigations on the NaNO3, glycine, alanine and their internally NaNO3/amino acid mixed particles with molar ratios of 3:1, 1:1 and 1:3 were conducted using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The mixed NaNO3/glycine droplets with molar ratios of 3:1, 1:1 and 1:3 exhibited efflorescence transition at 52.3%, 62.6% and 63.2%RH, respectively. For the NaNO3/alanine mixed systems with molar ratios of 3:1, 1:1 and 1:3, the efflorescence relative humidity (ERH) was measured to be 50.2%, 59.8% and 69.3%, respectively. These results suggested that in each of the systems studied the addition of amino acids to aerosols could promote the crystallization of mixed droplets. However, the higher deliquescence relative humidity (DRH) with increased fraction of glycine for the mixed NaNO3/glycine (3:1, 79.2%RH; 1:1, 86.3%RH; 1:3, 86.5%RH) also indicated that glycine could inhibit the deliquescence phase transition of mixed systems. In the case of NaNO3/alanine systems, the DRH for 3:1 and 1:3 mixed particles was 78.2% and 82.7% while no deliquescence transition occurred for the 1:1 mixed particles. The hygroscopic properties of mixed NaNO3/amino acids droplets were modified due to the formed amino acid-NaNO3 complex salts from the interactions of amino acids with NaNO3 in aerosols during dehydration process. The discrepancies in water content for the mixed NaNO3/amino acids with molar ratios of 1:1 and 1:3 between dehydration and hydration processes at high RH could be attributed to the crystallization of amino acid-NaNO3 complex salts during dehydration and the residual amino acids.