Absorption and scattering characterization of airborne microparticulates by a cavity ringdown technique

Determining the size and refractive index of homogeneous spherical aerosol particles using Mie resonance spectroscopy

Methods for determining the size and refractive index of single, homogeneous, micrometer-sized aerosol particles using Mie resonance spectroscopy are studied using measurements from optically trapped particles and light-scattering calculations based on Mie theory. We consider both single-particle broadband light scattering and cavity-enhanced Raman scattering (CERS) and demonstrate that, when resonances observed in either type of spectroscopy are fitted using Mie theory, the accuracy of the best fits are similar. However, broadband measurements can yield more resonances than CERS, thus reducing the uncertainty in the retrieved parameters of best fit and increasing the range of particles that can be characterized. Resonance fitting methods are also compared to methods that fit the entire Mie scattering spectrum. Through calculations, it is shown that measured scattering spectra are sensitive to small changes in how light is collected, while Mie resonance positions are much less sensitive. This means that additional parameters are required to accurately fit entire light-scattering spectra using Mie theory, but these parameters are not needed to accurately determine Mie resonance positions.

Characterization of aerosol optical properties, chemical composition and mixing states in the winter season in Shanghai, China

Physical and chemical properties of ambient aerosols at the single particle level were studied in Shanghai from December 22 to 28, 2009. A Cavity-Ring-Down Aerosol Extinction Spectrometer (CRD-AES) and a nephelometer were deployed to measure aerosol light extinction and scattering properties, respectively. An Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) was used to detect single particle sizes and chemical composition. Seven particle types were detected. Air parcels arrived at the sampling site from the vicinity of Shanghai until mid-day of December 25, when they started to originate from North China. The aerosol extinction, scattering, and absorption coefficients all dropped sharply when this cold, clean air arrived. Aerosol particles changed from a highly aged type before this meteorological shift to a relatively fresh type afterwards. The aerosol optical properties were dependent on the wind direction. Aerosols with high extinction coefficient and scattering Ångström exponent (SAE) were observed when the wind blew from the west and northwest, indicating that they were predominantly fine particles. Nitrate and ammonium correlated most strongly with the change in aerosol optical properties. In the elemental carbon/organic carbon (ECOC) particle type, the diurnal trends of single scattering albedo (SSA) and elemental carbon (EC) signal intensity had a negative correlation. We also found a negative correlation (r=-0.87) between high mass-OC particle number fraction and the SSA in a relatively clean period, suggesting that particulate aromatic components might play an important role in light absorption in urban areas.

Simultaneous retrieval of the complex refractive indices of the core and shell of coated aerosol particles from extinction measurements using simulated annealing

Simultaneously retrieving the complex refractive indices of the core and shell of coated aerosol particles given the measured extinction efficiency as a function of particle dimensions (core diameter and coated diameter) is much more difficult than retrieving the complex refractive index of homogeneous aerosol particles. Not only must the minimization be performed over a four-parameter space, making it less efficient, but in addition the absolute value of the difference between the measured extinction and the calculated extinction does not have an easily distinguished global minimum. Rather, there are a number of local minima to which almost all conventional retrieval algorithms converge. In this work, we develop a new (to our knowledge) retrieval algorithm that employs the numerical method known as simulated annealing with an innovative "temperature" schedule. This study is limited only to spherical particles with a concentric shell and to cases in which the diameter of both the core and the coated particle are known. We find that when the top ranking particle sizes according to their information content are combined from separate experiments to make up the particle size distribution, the simulated annealing retrieval algorithm is quite robust and by far superior to a greedy random perturbation approach often used.

Optical-feedback cavity ring-down spectroscopy measurements of extinction by aerosol particles

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.

Cavity ring-down spectroscopy measurements of single aerosol particle extinction. I. The effect of position of a particle within the laser beam on extinction

A continuous wave distributed feedback diode laser operating in the near infrared at wavelengths close to 1650 nm has been used to measure the extinction of light by single aerosol particles. The technique of optical feedback cavity ring-down spectroscopy (CRDS) was used for measurement of CRDS events at a repetition rate of 1.25 kHz. This very high repetition rate enabled multiple measurements of the extinction of light by single aerosol particles for the first time and demonstrated the dependence of light scattering on the position of a particle within the laser beam. A model is proposed to explain quantitatively this phenomenon. The minimum detectable dimensionless extinction coefficient epsilonmin was determined to be 3x10(-6). Extinction values obtained for single spherical polymer beads from a monodisperse sample of particles of diameter of 4 microm are in near-quantitative agreement with the values calculated by the Mie scattering theory. The deviations from the Mie theory expected for measurement of extinction by CRDS using a continuous wave laser are discussed in the companion paper.