Measurements of Aerosol Physical Properties (See General Comment #1) 9




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НазваниеMeasurements of Aerosol Physical Properties (See General Comment #1) 9
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Aerosol Volatility


This section could be deleted with no loss to the paper. How applicable is this method to urban/urban-rural environments? What about NH4NO3, organics, and aerosols that are mixed??)

Aerosol volatilization has been used as an indirect method of inferring particle composition. For example, Brock et al. (Brock, Hamill et al. 1995) sampled in parallel with two CNCs, one of which was equipped with an inlet heated to 192 C. Laboratory measurements showed that the heated inlet volatilized 90% of the particles that were smaller than 0.04 µm and contained only H2SO4 and H2O. Field measurements showed that in some locations significant differences were found between the two CNCs, while in other locations the measurements were similar. By coupling these observations with other information that was known about the measured aerosol, they were able to conclude that ultrafine particles containing primarily H2SO4 and H2O were present at some locations. Clarke (Clarke 1993) has used similar systems in aircraft to measure the effect of volatilization on measured number concentrations and size distributions.


It would obviously be far preferable to have direct measurements of aerosol composition rather than the indirect information that is provided by volatilization. However, volatilization measurements can be done in real time (it has a time response of seconds) with instruments that are compact and convenient to deploy in the field. Fast time response is especially important for aircraft sampling.

Particle Density



Particle density is needed to convert aerodynamic sizes to geometric (Stokes) sizes and to establish the relationship between aerosol mass and volume concentrations. In practice, density is usually calculated from measured particle composition. However, uncertainties in aerosol composition and the thermodynamic properties of mixtures lead to uncertainties (can you quantify the uncertainty) in calculated densities.


Several approaches have been used to measure densities of submicron atmospheric aerosols. (what about larger size particles <10 or <2.5. Can density be measured on those or is the method limited to submicron sizes?) Hnel (HŠnel and Thudium 1977) measured the mass of bulk samples with an electronic balance, and measured the volume of the same sample using a specially designed pycnometer (how does this work? As I am ignorant about this method and perhaps others are as well). The reported accuracy for these density measurements was 2%. Because bulk samples were used, information about variations with size or among particles of a given size was not obtained. Measurements of aerosol chemical composition were not reported, so it was not possible to compare measured to expected values.


Stein et al. (Stein, Turpin et al. 1994) measured the density of atmospheric particles in the 0.06 to 0.18ʵm diameter range using the DMA-impactor technique. A DMA was used to deliver monodisperse particles of a known electromobility equivalent size (which does not depend on density and which equals the geometric size for spherical particles) to an inertial impactor. The particlesÕ aerodynamic size, which depends on density, was measured with the impactor. Particle densities were determined to within ~4% from the measured geometric and aerodynamic sizes. Densities calculated from measured aerosol composition were found to be ~20% lower than measured values; the reason for this discrepancy was not resolved.


In summary, only a few efforts have been made to measure the density of submicron particles. Additional work is required to ensure that density can be calculated from measured particle composition. Closure studies of this type are an essential ingredient in the development of experimentally verified thermodynamic models of aerosol properties.
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