The determination of accurate size distributions and chemical composition of volatile and semivolatile ultrafine aerosol particles with sizes in the subnanometer to several tens of nanometers range is a problem that plagues many studies in aerosol research. We propose to employ sodium-doping of the aerosol particles with subsequent photoionization in the ultraviolet combined with mass spectrometric detection to solve this problem. Comparison with “soft” EUV ionization demonstrates that this technique can determine size distributions and to some extent the chemical composition of weakly bound ultrafine aerosol particles largely destruction-free. We also discuss how sodium-doping can be turned into a viable quantitative technique for the sizing of ultrafine aerosol particles.

The fragmentation of methanol, water, dimethyl ether, and acetic acid clusters upon photoionization with a single vacuum ultraviolet (VUV) photon of 10.1 eV, 13.3 eV, or 17.5 eV energy is studied with mass spectrometry. The sodium-doping method is used as an independent approximate measure of the original cluster size distribution providing information on the degree of fragmentation upon VUV ionization. The experimental results show strong fragmentation for (CH(3))(2)O and CH(3)CO(2)H clusters but minor fragmentation for H(2)O and CH(3)OH clusters. The pronounced cluster decay for (CH(3))(2)O and CH(3)CO(2)H is explained by additional intracluster chain reactions that occur after the initial fast proton transfer in the ionic clusters, i.e. the decay of (CH(3))(2)O molecules into H(2), CO, and CH(4) catalyzed by the methoxymethyl radical, and the decay of CH(3)CO(2)H molecules into CO(2) and CH(4) catalyzed by the acetyloxy radical. The absence of equivalent reaction cycles in ionic H(2)O and CH(3)OH clusters after the fast proton transfer is consistent with the much less pronounced cluster fragmentation observed upon VUV ionization. The study shows that VUV photoionization even at threshold cannot in general be considered a soft ionization method for weakly-bound clusters, largely because of potential intracluster reaction.

Waveguide-based cavity ring-down spectroscopy (CRD) can be used for quantitative measurements of chemical concentrations in small amounts of liquid, in gases or in films. The change in ring-down time can be correlated to analyte concentration when using fiber optic sensing elements that change their attenuation in dependence of either sample absorption or refractive index. Two types of fiber cavities, i.e., fiber loops and fiber strands containing reflective elements, are distinguished. Both types of cavities were coupled to a variety of chemical sensor elements, which are discussed and compared.

We report on a new instrument that allows for the investigation of weakly-bound molecular aggregates under equilibrium conditions (constant temperature and pressure). The aggregates are formed in a Laval nozzle and probed with time-of-flight mass spectrometry in the uniform postnozzle flow; i.e. in the equilibrium region of the flow. Aggregates over a very broad size range from the monomer to particle sizes of 10-20 nm can be generated and studied with this setup. Soft ionization of the aggregates is performed with single photons from a homemade vacuum ultraviolet laser. The mass spectrometric detection provides molecular-level information on the size and chemical composition of the aggregates. This new instrument is useful for a broad range of cluster studies that require well-defined conditions.

Optical feedback cavity ring-down spectroscopy (OF-CRDS) using a continuous wave distributed feedback diode laser at 1650 nm has been used to measure extinction of light by samples of monodisperse spherical aerosol particles <1 mum in diameter. The OF-CRDS method allows measurements of low levels of extinction of incident light to be made at repetition rates of 1 kHz or greater. A statistical model is proposed to describe the linear relationship between the extinction coefficient (alpha) and its variance (Var(alpha)). Application of this model to experimental measurements of Var(alpha) for a range of alpha values typically below approximately 1 x 10(-6) cm(-1) allows extinction cross-sections for the aerosol particles to be obtained without need for knowledge of the particle number density. Samples of polystyrene spheres with diameters of 400, 500, 600, and 700 nm were used to test the model by comparing extinction cross-sections determined from the experiment with the predictions of Mie theory calculations. Fitting of ring-down decay traces exhibiting amplitude noise to extract cavity ring-down times introduces additional quadratic and higher order polynomial dependencies of the variance that become significant for larger particle number densities and thus extinction coefficients (typically for alpha > 1 x 10(-6) cm(-1) under our experimental conditions). Aggregation of particles at larger number densities is suggested as a further source of variance in the measurements. Extinction cross-sections are severely underestimated if the measurements are made too rapidly to sample uncorrelated distributions of particle numbers and positions.