By using 193 nm laser photolysis and cavity ring-down spectroscopy to produce and monitor the propargyl radical (CH2CCH), the self-reaction and oxygen termolecular association rate coefficients for the propargyl radical were measured at 295 K between total pressures of 300 Pa and 13300 Pa (2.25 and 100 Torr) in Ar, He, and N2 buffer gases. The rate coefficients obtained by simple second-order fits to the decay data were observed to vary with the photolytic precursors: allene, propargyl chloride, and propargyl bromide. By using a numerical fitting routine and more comprehensive mechanisms, a distinct rate coefficient for the self-reaction was determined, k ∞(C3H3+C3H3) = (4.3 ± 0.6) × 10-11 cm3 molecule-1 s-1 at 295 K. This rate coefficient, which is a factor of 2.8 times slower than reported previously, was independent of total pressure and buffer choice over the entire pressure range. Other rate coefficients derived during the modeling included k(C3H3+H 665 Pa He) = (2.5 ± 1.1) × 10-10 cm3 molecule-1 s-1, k(C3H3+C3H3Cl2) = (7 ± 4) × 10-11 cm3 molecule-1 s-1, and k(C3H3+C3H3Br2) = (2.4 ± 2) × 10-11 cm3 molecule-1 s-1. The association reaction C3H3 + O2 was found to lie in the falloff region between linear and saturated pressure dependence for each buffer gas (Ar, He, and N2) between 300 Pa and 13300 Pa. A fit of these data derived the high-pressure limiting rate coefficient k ∞(C3H3+O2) = (2.3 ± 0.5) × 10-13 cm3 molecule-1 s-1. Three measurements of the propargyl radical absorption cross-section obtained σ332.5 = (413 ± 60) × 10-20 cm2 molecule-1 at 332.5 nm. Stated uncertainties are two standard deviations and include the uncertainty of the absorption cross section, where appropriate.
The Reno Aerosol Optics Study (RAOS) was designed and conducted to compare the performance of many existing and new instruments for the in situ measurement of aerosol optical properties with a focus on the determination of aerosol light absorption. For this study, simple test aerosols of black and white particles were generated and combined in external mixtures under low relative humidity conditions and delivered to each measurement system. The aerosol mixing and delivery system was constantly monitored using particle counters and nephelometers to ensure that the same aerosol number concentration and amount reached the different instruments. The aerosol light-scattering measurements of four different nephelometers were compared, while the measurements of seven light-absorption instruments (5 filter based, 2 photoacoustic) were evaluated. Four methods for determining the aerosol lightextinction coefficient (3 cavity ring-down instruments and 1 foldedpath optical extinction cell) were also included in the comparisons. An emphasis was placed on determining the representativeness of the filter-based light absorption methods, since these are used Address correspondence to Patrick J. Sheridan, Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, CO 80305, USA. E-mail: patrick.sheridan@noaa.gov widely and because major corrections to the raw attenuation measurements are known to be required. The extinction measurement from the optical extinction cell was compared with the scattering measurement from a high-sensitivity integrating nephelometer on fine, nonabsorbing ammonium sulfate aerosols, and the two were found to agree closely (within 1% for blue and green wavelengths and 2% for red). The wavelength dependence of light absorption for small kerosene and diesel soot particles was found to be very near λ −1 , the theoretical small-particle limit. Larger, irregularly shaped graphite particles showed widely variable wavelength dependencies over several graphite runs. The light-absorption efficiency at a wavelength of 530 nm for pure kerosene soot with a number size distribution peak near 0.3 µm diameter was found to be 7.5 ± 1.2 m 2 g −1 . The two most fundamental independent absorption methods used in this study were photoacoustic absorption and the difference between suspended-state light extinction and scattering, and these showed excellent agreement (typically within a few percent) on mixed black/white aerosols, with the photoacoustic measurement generally slightly lower. Excellent agreement was also observed between some filter-based light-absorption measurements and the RAOS reference absorption method. For atmospherically relevant levels of the aerosol light-absorption coefficient (<25 Mm −1 ), the particle soot absorption photometer (PSAP) absorption measurement at mid-visible wavelengths agreed with the reference absorption measurement to within ∼11% for experiment tests on externally mixed kerosene soot and ammonium sulfate. At higher absorption ...
Abstract. Measurements of the optical properties (absorption, scattering and extinction) of PM1, PM2.5 and PM10 made at two sites around Sacramento, CA, during the June 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) are reported. These observations are used to establish relationships between various intensive optical properties and to derive information about the dependence of the optical properties on photochemical aging and sources. Supermicron particles contributed substantially to the total light scattering at both sites, about 50 % on average. A strong, linear relationship is observed between the scattering Ångström exponent for PM10 and the fraction of the scattering that is contributed by submicron particles (fsca, PM1) at both sites and with similar slopes and intercepts (for a given pair of wavelengths), suggesting that the derived relationship may be generally applicable for understanding variations in particle size distributions from remote sensing measurements. At the more urban T0 site, the fsca, PM1 increased with photochemical age, whereas at the downwind, more rural T1 site the fsca, PM1 decreased slightly with photochemical age. This difference in behavior reflects differences in transport, local production and local emission of supermicron particles between the sites. Light absorption is dominated by submicron particles, but there is some absorption by supermicron particles ( ∼ 15 % of the total). The supermicron absorption derives from a combination of black carbon that has penetrated into the supermicron mode and from dust, and there is a clear increase in the mass absorption coefficient of just the supermicron particles with increasing average particle size. The mass scattering coefficient (MSC) for the supermicron particles was directly observed to vary inversely with the average particle size, demonstrating that MSC cannot always be treated as a constant in estimating mass concentrations from scattering measurements, or vice versa. The total particle backscatter fraction exhibited some dependence upon the relative abundance of sub- versus supermicron particles; however this was modulated by variations in the median size of particles within a given size range; variations in the submicron size distribution had a particularly large influence on the observed backscatter efficiency and an approximate method to account for this variability is introduced. The relationship between the absorption and scattering Ångström exponents is examined and used to update a previously suggested particle classification scheme. Differences in composition led to differences in the sensitivity of PM2.5 to heating in a thermodenuder to the average particle size, with more extensive evaporation (observed as a larger decrease in the PM2.5 extinction coefficient) corresponding to smaller particles; i.e., submicron particles were generally more susceptible to heating than the supermicron particles. The influence of heating on the particle hygroscopicity varied with the effective particle size, with larger changes observed when the PM2.5 distribution was dominated by smaller particles.
A general formalism is presented for the production, characterization, and use of uniform supersonic flows for the study of structural and collisional molecular properties. These flows make possible the generation of thermally equilibrated gaseous environments at temperatures generally extending from near 10 to above 300 K at pressures between 0.1 and 10 Torr. In addition, the invariance of flow conditions for distances of many nozzle diameters (5–30) beyond the exit makes the flows ideal for collisional or temporal studies. A concise outline for the design of the Laval nozzles necessary to produce specified density and temperature conditions is presented. Pulsed operation of these expansions is demonstrated, and a variety of useful characterization tools providing information on flow uniformity and thermal characteristics is considered. Recent applications of these flows for the study of low-temperature chemical reactions are reviewed.
A small portable system is described which is used to directly determine the optical extinction of the atmospheric aerosol. The requisite highly sensitive measurement of the optical extinction is accomplished simultaneously at two wavelengths in the near-infrared (1064 nm) and visible (532 nm), using the pulsed cavity ring-down (CRD) approach. The measurement at the two wavelengths can aid in separating the scattering and absorption components of the optical extinction. Rayleigh equivalent optical extinction of approximately 10 x 10(-6) m(-1) from particulate matter in the atmospherically important 0.1-2.5 pm diameter size range (fine particle accumulation mode) can be readily observed with short (<5 s) integration times. Optical extinction is inversely related to the visual range, and so the instrument provides a direct measurement of this particulate-related air quality indicator. The instrument can also provide particle size range-selected multiwavelength optical property measurements, which can be inverted to provide valuable information about the extant airborne particulate distribution.
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