Measurement of the coagulation rate constant for sulphuric acid particles as a function of particle size

A new method for the determination of coagulation rate constants for monodisperse, neutral particles is described. In this method, a differential mobility analyzer (DMA) is used to prepare a monodisperse aerosol and a second DMA is used to separate the coagulation products from the original monodisperse particles. The experiments are carried out under initial rate conditions so that typically 5–9% of the monomer particles undergo coagulation. Experimental results at 298±1 K for H2SO4/H2O particles with diameters of 49–127 nm and a composition of 72–73% H2SO4 by mass gave enhancement factors, relative to rate constants calculated for hard spheres, that vary from about 1.2 for the largest particles to 2.8 for the smallest particles. Fitting these results to a theoretical expression accounting for van der Waals forces gives a Hamaker constant of (6.4±2.6)×10−13 erg. We also give convenient formulas for computing coagulation enhancement factors from the Hamaker constant

Application of absolute principal component analysis to size distribution data: identification of particle origins

International audienceAbsolute principal component analysis can be applied, with suitable modifications, to atmospheric aerosol size distribution measurements. This method quickly and conveniently reduces the dimensionality of a data set. The resulting representation of the data is much simpler, but preserves virtually all the information present in the original measurements. Here we demonstrate how to combine the simplified size distribution data with trace gas measurements and meteorological data to determine the origins of the measured particulate matter using absolute principal component analysis. We have applied the analysis to four different sets of field measurements that were conducted at three sites in southern Ontario. Several common factors were observed at all the sites; these were identified as photochemically produced secondary aerosol particles, regional pollutants (including accumulation mode aerosol particles), and trace gas variations associated with boundary layer dynamics. Each site also exhibited a factor associated specifically with that site: local industrial emissions in Hamilton (urban site), processed nucleation mode particles at Simcoe (polluted rural site), and transported fine particles at Egbert (downwind from Toronto)

Laboratory studies of bromide oxidation in the presence of ozone: Evidence for glass-surface mediated reaction

The reaction of sodium bromide particles in the presence of ozone was studied in a flow system both under dark conditions and with 254 nm radiation. We found that there was significant formation of gaseous bromine (probably Br2) in the presence of ozone in the dark, and that bromide deposited to the walls of the Pyrex reaction flask was its source. The observed rate of gaseous bromine formation in these experiments was approximately 100–1000 times faster than expected based on the knownrate constant for aqueous reaction of bromide with ozone. While the mechanism responsible for this enhanced reactivity was not identified, based on previous reports we suggest that the glass surface converted ozone to more reactive species, such as hydroxyl radical, which in turn oxidized bromide. In the presence of 254 nm radiation, rates of gaseous bromine collection were further enhanced, likely as a result of increased radical production in the system, and wall-deposited bromide was also the source of the gaseous bromine. In these `light' experiments, there was a significant decline in ozone mixing ratios, consistent with bromine radical chemistry. These results suggest the possibility that ozone reacting with internally mixed silicate/sea-salt particles might be a significant mechanism for the oxidation of particulate halides, and subsequent release of photoactive halogen species, in the marine boundary layer

An inverse modeling procedure to determine particle growth and nucleation rates from measured aerosol size distributions

Classical nucleation theory is unable to explain the ubiquity of nucleation events observed in the atmosphere. This shows a need for an empirical determination of the nucleation rate. Here we present a novel inverse modeling procedure to determine particle nucleation and growth rates based on consecutive measurements of the aerosol size distribution. The particle growth rate is determined by regression analysis of the measured change in the aerosol size distribution over time, taking into account the effects of processes such as coagulation, deposition and/or dilution. This allows the growth rate to be determined with a higher time-resolution than can be deduced from inspecting contour plots ('banana-plots''). Knowing the growth rate as a function of time enables the evaluation of the time of nucleation of measured particles of a certain size. The nucleation rate is then obtained by integrating the particle losses from time of measurement to time of nucleation. The regression analysis can also be used to determine or verify the optimum value of other parameters of interest, such as the wall loss or coagulation rate constants. As an example, the method is applied to smog chamber measurements. This program offers a powerful interpretive tool to study empirical aerosol population dynamics in general, and nucleation and growth in particular

The Mount Wilson optical interferometer: The first automated instrument and the prospects for lunar interferometry

Before contemplating an optical interferometer on the Moon one must first review the accomplishments achieved by this technology in scientific applications for astronomy. This will be done by presenting the technical status of optical interferometry as achieved by the Mount Wilson Optical Interferometer. The further developments needed for a future lunar-based interferometer are discussed

Practical beam transport for PFI

The Planet Formation Imager (PFI) is a future kilometric-baseline infrared interferometer to image the complex physical processes of planet formation. Technologies that could be used to transport starlight to a central beam-combining laboratory in PFI include free-space propagation in air or vacuum, and optical fibres. This paper addresses the design and cost issues associated with free-space propagation in vacuum pipes. The signal losses due to diffraction over long differential paths are evaluated, and conceptual beam transport designs employing pupil management to ameliorate these losses are presented and discussed.This is the author accepted manuscript. The final version is available from SPIE via http://dx.doi.org/10.1117/12.223238

IRAS observations of giant molecular clouds in the Milky Way

The IRAS data base has been used to study infrared radiation from molecular clouds in our galaxy. The sample of clouds was restricted to those with reliably determined molecular masses from large area, multi-isotope CO maps. They were normalized to X(CO-13)= 2x10 to the -6. Flux densities at 60 microns and 100 microns were determined by integrating the flux density within rectangles drawn on the sky flux plates after subtracting a suitable background. The rectangles were chosen to be coextensive with the areas mapped in CO. Color corrections were made and luminosites calculated by assuming the optical depths were proportional to frequency. The flux densities were converted to dust masses with a value for 4a rho/3Q = .04 g/cm at 100 microns