Temperature-Controlled Dual-Beam Optical Trap for Single Particle Studies of Organic Aerosol

An optical trapping cell that is capable of suspending particles using two counter-propagating beams in a temperature-controlled environment is reported here. With this dual-beam optical trap, we are able to hold single micron-sized droplets at temperatures down to 253 K (-20 °C) for hours at a time and in metastable (supercooled) states. As particles are trapped at the shared focal points of two intense beams, strong cavity-enhanced Raman scattering (CERS) is observed and allows for high precision measurements of physical properties. Here, the evaporation of highly oxygenated organic systems was monitored using CERS and was used to determine temperature-dependent vapor pressures and enthalpies of vaporization. The wavelength- and temperature-dependent optical properties were also simultaneously retrieved using CERS.

Low-temperature Bessel beam trap for single submicrometer aerosol particle studies

We report on a new instrument for single aerosol particle studies at low temperatures that combines an optical trap consisting of two counter-propagating Bessel beams (CPBBs) and temperature control down to 223 K (-50 °C). The apparatus is capable of capturing and stably trapping individual submicrometer- to micrometer-sized aerosol particles for up to several hours. First results from studies of hexadecane, dodecane, and water aerosols reveal that we can trap and freeze supercooled droplets ranging in size from ~450 nm to 5500 nm (radius). We have conducted homogeneous and heterogeneous freezing experiments, freezing-melting cycles, and evaporation studies. To our knowledge, this is the first reported observation of the freezing process for levitated single submicrometer-sized droplets in air using optical trapping techniques. These results show that a temperature-controlled CPBB trap is an attractive new method for studying phase transitions of individual submicrometer aerosol particles.

Crystallization of Aqueous Inorganic−Malonic Acid Particles: Nucleation Rates, Dependence on Size, and Dependence on the Ammonium-to-Sulfate Ratio

Using an electrodynamic balance, we determined the relative humidity (RH) at which aqueous inorganic-malonic acid particles crystallized, with ammonium sulfate ((NH(4))(2)SO(4)), letovicite ((NH(4))(3)H(SO(4))(2)), or ammonium bisulfate (NH(4)HSO(4)) as the inorganic component. The results for (NH(4))(2)SO(4)-malonic acid particles and (NH(4))(3)H(SO(4))(2)-malonic acid particles show that malonic acid decreases the crystallization RH of the inorganic particles by less than 7% RH when the dry malonic acid mole fraction is less than 0.25. At a dry malonic acid mole fraction of about 0.5, the presence of malonic acid can decrease the crystallization RH of the inorganic particles by up to 35% RH. For the NH(4)HSO(4)-malonic acid particles, the presence of malonic acid does not significantly modify the crystallization RH of the inorganic particles for the entire range of dry malonic acid mole fractions studied; in all cases, either the particles did not crystallize or the crystallization RH was close to 0% RH. Size dependent measurements show that the crystallization RH of aqueous (NH(4))(2)SO(4) particles is not a strong function of particle volume. However, for aqueous (NH(4))(2)SO(4)-malonic acid particles (with dry malonic acid mole fraction = 0.36), the crystallization RH is a stronger function of particle volume, with the crystallization RH decreasing by 6 +/- 3% RH when the particle volume decreases by an order of magnitude. To our knowledge, these are the first size dependent measurements of the crystallization RH of atmospherically relevant inorganic-organic particles. These results suggest that for certain organic mole fractions the particle size and observation time need to be considered when extrapolating laboratory crystallization results to atmospheric scenarios. For aqueous (NH(4))(2)SO(4) particles, the homogeneous nucleation rate data are a strong function of RH, but for aqueous (NH(4))(2)SO(4)-malonic acid particles (with dry organic mole fraction = 0.36), the rates are not as dependent on RH. The homogeneous nucleation rates for aqueous (NH(4))(2)SO(4) particles were parametrized using classical nucleation theory, and from this analysis we determined that the interfacial surface tension between the crystalline ammonium sulfate critical nucleus and an aqueous ammonium sulfate solution is between 0.053 and 0.070 J m(-2).

Fast in situ sizing technique for single levitated liquid aerosols

A fast in situ sizing technique for single levitated aerosol particles is presented. It makes use of the analysis of fast-Fourier-transformed vertically polarized Mie scattering patterns from single liquid aerosols levitated in a Paul-trap-type electrodynamic balance. The technique is shown to give reliable results for a test sphere of known physical properties. A first application to aqueous sodium chloride aerosol particles that changed size on humidity variation is presented, yielding good agreement with the full Mie theory.