Size measurement, liquid droplet aerosol

Laser Doppler velocimetry has been combined with acoustic excitation to allow the derivation of the relaxation time for particles, from which the aerodynamic diameter can be calculated [132-136], The particle relaxation time is derived from the velocity amplitude of the aerosol particle and that of the medium while the aerosol is subjected to acoustic excitation of a known frequency. A differential laser Doppler velocimeter is used to measure the velocity amplitude of the particle, and a microphone is used to measure the velocity amplitude of the medium. The aerodynamic diameter of the particle can be derived from the relaxation time and the known particle density. The method can be applied to real-time in situ measurement of the size distribution of an aerosol containing both solid and liquid droplets in the diameter range of 0.1 -10 pm. 

A low thermal capacity to warm the cooled nebulizer output air-aerosol mixture, which has been reported with other cascade im-pactors to further evaporate liquid aerosol and confound interpretation of droplet size measurements (32). 

Sprays of fine droplets can be generated by first mixing a liquid with liquefied gas under pressure and then expanding the mixture through a nozzle. This technique, referred to ssliquefied gas atomization, has been used in many applications such as commercial aerosol cans. The mean droplet size generated with this technique is very small. In very few systematic studies, the measured droplet size distribution was found rather widely spread.[881 It is not clear, however, how the liquid amount, pressure, and nozzle design affect the mean droplet size and size distribution. 

From the sample solution to be analyzed, small droplets are formed by the nebulization of the solution using an appropriate concentric or cross-flow pneumatic nebulizer/spray chamber system. Quite different solution introduction systems have been created for the appropriate generation of an aerosol from a liquid sample and for separation of large size droplets. Such an arrangement provides an efficiency of the analyte introduction in the plasma of 1-3 % only.6 The rest (97 % to 99%) goes down in the drain.7 Beside the conventional Meinhard nebulizer, together with cooled or non-cooled Scott spray chamber or conical spray chamber, several types of micronebulizers together with cyclonic spray chambers are employed for routine measurements in ICP-MS laboratories. The solvent evaporated from each droplet forms a particle which is vaporized into atoms and molecules... 

High efficiency denuders that concentrate atmospheric S02 were coupled to an ion chromatograph to yield detection limits on the order of 0.5 ppt (106). A newer approach has been introduced for the quantitative collection of aerosol particles to the submicrometer size (107). When interfaced to an inexpensive ion chromatograph for downstream analysis, the detection limit of the overall system for particulate sulfate, nitrite, and nitrate are 2.2,0.6, and 5.1 ng/m3, respectively, for an 8-min sample. A two-stage membrane sampling system coupled with an ion trap spectrometer has been utilized for the direct analysis of volatile compounds in air, with quantitation limits to low ppt levels (108). Toluene, carbon tetrachloride, tricholoroethane, and benzene were used in these studies. The measurement of nitrogen dioxide at ppb level in a liquid film droplet has been described (109) (see Air pollution). A number of elements in environmental samples have been determined by thermal ionization ms (Table 6). The detection limit for Pu was as low as 4 fg. 

Several areas in which chemical measurement technologies have become available and/or refined for airborne applications have been reviewed in this paper. It is a selective review and many important meteorological and cloud physics measurement capabilities of relevance to atmospheric chemistry and acid deposition (e.g., measurement of cloud liquid water content) have been ignored. In particular, we have not discussed particle size spectra measurements for various atmospheric condensed phases (aerosols, cloud droplets and precipitation). Further improvements in chemical measurement technologies can be anticipated especially in the areas of free radicals, oxidants, organics, and S02 and N02 at very low levels. Nevertheless, major incremental improvements in the understanding of acid deposition processes can be anticipated from the continuing airborne application of the techniques described in this review. 

The droplets are then delivered to the reactor and dried by heating in the furnace. Dry aerosol particles are collected in an electrostatic precipitator kept at approximately 150°C to avoid water condensation, while the gases are dried by passing them through a liquid nitrogen cold trap before being evacuated by a vacuum pump. The volume mean droplet size produced with this system was recently measured using a spray particle analyzer and found to be about 4-6 pm [10]. 

Aerosol droplet size di.stributions can be determined by stray-light measurements [87], [88]. Data obtained from sampling with cascade impac-tors may suffer from evaporation of the droplets on their way through the device, especially as sampling may require the application of underpressure [89]. Data for different nebulizers are available [90]. Particle size distributions depend on the nebulizer and its working conditions, on the liquid nebulized, and on the nebulization chamber. These relations have been studied in depth for organic solutions [91], [92], for which, at low gas flow, pneumatic nebulizers often... 

From lidar measurements there is clear evidence for both solid and liquid PSCs in the polar stratospheres. The volume depolarization of backscat-tered light in aerosol-free air is 1.4 % (due to the polarizability of the air molecules), and data below this value indicate liquid type-lb clouds. Ternary solution droplets in such clouds yield substantial backscatter, but only in the parallel polarization plane. In contrast, type-la clouds have large depolarization ratios but small backscatter. Toon et al, referring to lidar measurements of Browell et al, were the first to describe these characteristics. They showed that type-la clouds consist of only a few very large aspherical particles, but the identification of type-lb as ternary liquids was made only later when the composition and size of stratospheric droplets were calculated. 

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