Formation and Transformation of Metastable Double Salts from the Crystallization of Mixed Ammonium Nitrate and Ammonium Sulfate Particles

Ammonium nitrate (AN) and ammonium sulfate (AS) are ubiquitous components of atmospheric aerosols. Thermodynamic models predict formation of pure (AN and AS) and double salts (3AN·AS and 2AN·AS) for the AN/AS system. Because of the high supersaturation at which a droplet crystallizes, metastable crystal formation is possible. In this study, the identity of the crystals formed from the crystallization of equimolar AN/AS mixed droplets was investigated in an electrodynamic balance coupled with a Raman spectroscopic system. Raman spectra of bulk AN/AS double salts possibly formed in this system are first reported for comparison with the single particle Raman results. The double-salt 3AN·AS, not predicted from thermodynamics, was observed in the freshly crystallized single particles. The degree of metastability can be different among several crystallization processes of the same particles. The metastable salt 3AN·AS gradually transformed into stable 2AN·AS, and the rate of such transformation increased with increasing relative humidity. This study illustrates the possibility of occurrence of metastable salts in atmospheric aerosols

Measurement of Retention Efficiency of Filters against Nanoparticles in Liquids using an Aerosolization Technique

An aerosolization technique has been developed to measure liquid-borne nanoparticles down to 30 nm and applied to evaluate retention efficiencies of liquid filters. This technique involves dispersing nanoparticle suspensions into air-borne form and measuring the size and concentration by a differential mobility analyzer coupled to a condensation particle counter. Polystyrene latex particles larger and smaller than 70 nm in diameter were dispersed by a constant output atomizer, COA, and an electrospray aerosol generator, ES, respectively, to avoid the interference from residue particles. With the ES, residue particles can be controlled to be less than 10 nm, allowing latex particles as small as 30 nm to be clearly distinguished from the size distribution measurements. Calibrations with 30, 50, 125, and 200 nm latex particles showed that liquid-borne and air-borne particle concentrations are proportionally related. This provides an effective way to quantify liquid-borne particles as small as 30 nm, which cannot be analyzed by state-of-the-art liquid particle counters. An application of this technique is to evaluate the nanoparticle retention performance of liquid filters. Both 200 and 400 nm rated Nuclepore filters were challenged with latex particles of different sizes, and retention efficiency as a function of particle size was determined by comparing the particle concentrations upstream and downstream of the tested filters. The results are comparable with the nominal pore size stated by the manufacturer if sieving is the dominant filtration mechanism and demonstrate the feasibility of using the aerosolization technique to evaluate the retention efficiency of filters against nanoparticles in liquids