As designed in the 1940s by Beuttell and Brewer, the integrating nephelometer offers a direct method of measuring light scattering by airborne particles without assumptions about particle ...composition, shape, or physical state. A large number of such instruments have been deployed; however, only a limited number of validation experiments have been attempted. This paper reports a set of closure experiments in which a gas-calibrated nephelometer is used to measure the scattering coefficient of laboratory-generated particles of known size and refractive index. Specifically, it evaluates the performance of a high-sensitivity, three-wavelength, total scatter/backscatter integrating nephelometer (TSI, Inc., model 3563). Sources of uncertainty associated with the gas-calibration procedure, with photon-counting statistics, and with nonidealities in wavelength and angular sensitivity are investigated. Tests with particle-free gases indicate that noise levels are well predicted by photon-counting statistics and that the nephelometer response is linear over a wide range of scattering coefficients. Tests with particles show average discrepancies between measured and predicted scattering of 4%-7%. Error analysis indicates that these discrepancies are within experimental uncertainty, which was dominated by particle generation uncertainty. The simulation of nephelometer response, which is validated by these tests, is used to show that errors arising from nephelometer nonidealities are less than 10% for accumulation-mode or smaller particles (i.e., size distributions for which the volume mean diameter is 0.4 mu m or less) and that significant differences exist between the total scatter and backscatter uncertainties. Based on these findings, appropriate applications of the model 3563 nephelometer are discussed.
Aerosol measurements were made on three cruises in the mid‐Pacific along longitude 140°W from 55°N to 70°S for a total of about 90 days in 1992 and 1993. The three data sets document the aerosol ...concentration and general features of its number‐size distribution in the marine boundary layer (MBL) and their variation with latitude and meteorological conditions. Mean concentration varied from 300 cm−3 in the tropics to 500 cm−3 in the midlatitudes outside of continental air masses. Infrequent short‐term spikes in concentration ranged up to 2000 cm−3. Two dominant modes were observed, the Aitken and accumulation, with mean diameters of 25 to 60 nm and 150 to 200 nm, respectively. An intermittent ultrafine mode was noted at diameters less than 25 nm. The concentration and dominance of one mode over another depended on the relative strength of the entrainment of ultrafine and Aitken particles from the free troposphere (FT) into the MBL compared to the rate of growth of Aitken mode into accumulation mode particles and removal rate of the accumulation mode. In general, aging times were shorter in the subtropics, longer in the tropics, and variable in the midlatitudes. The rate of new particle formation within the MBL itself was either low and did not contribute significantly to the observed number concentration or, if the rate was high, it occurred infrequently and was not observed in these experiments.
Aerosol scattering coefficients (sigma(sub sp)) have been measured over the ocean at different relative humidities (RH) as a function of altitude in the region surrounding the Canary Islands during ...the Second Aerosol Characterization Experiment (ACE-2) in June and July 1997. The data were collected by the University of Washington passive humidigraph (UWPH) mounted on the Pelican research aircraft. Concurrently, particle size distributions, absorption coefficients and aerosol optical depth were measured throughout 17 flights. A parameterization of sigma(sub sp) as a function of RH was utilized to assess the impact of aerosol hydration on the upwelling radiance (normalized to the solar constant and cosine of zenith angle). The top of the atmosphere radiance signal was simulated at wavelengths corresponding to visible and near-infrared bands of the EOS (Earth Observing System) AM-1 (Terra) detectors, MODIS (Moderate Resolution Imaging Spectroradiometer) and MISR (Multi-angle Imaging Spectroradiometer). The UWPH measured sigma(sub sp) at two RHs, one below and the other above ambient conditions. Ambient sigma(sub sp) was obtained by interpolation of these two measurements. The data were stratified in terms of three types of aerosols: Saharan dust, clean marine (marine boundary layer background) and polluted marine aerosols (i.e., two- or one-day old polluted aerosols advected from Europe). An empirical relation for the dependence of sigma(sub sp) on RH, defined by sigma(sub sp)(RH) = k.(1 - RH/100)(sup gamma), was used with the hygroscopic exponent gamma derived from the data. The following gamma values were obtained for the 3 aerosol types: gamma(dust) = 0.23 +/- 0.05, gamma(clean marine) = 0.69 +/- 0.06 and gamma(polluted marine) = 0.57 +/- 0.06. Based on the measured gammas, the above equation was utilized to derive aerosol models with different hygroscopicities. The satellite simulation signal code 6S was used to compute the upwelling radiance corresponding to each of those aerosol models at several ambient humidities. For the prelaunch estimated precision of the sensors and the assumed viewing geometry of the instrument, the simulations suggest that the spectral and angular dependence of the reflectance measured by MISR is not sufficient to distinguish aerosol models with various different combinations of values for dry composition, gamma and ambient RH. A similar behavior is observed for MODIS at visible wavelengths. However, the 2100 nm band of MODIS appears to be able to differentiate between at least same aerosol models with different aerosol hygroscopicity given the MODIS calibration error requirements. This result suggests the possibility of retrieval of aerosol hygroscopicity by MODIS.
Intensive measurements of chemical and physical properties of the atmospheric aerosol have been performed at two sites in central Europe during the Melpitz‐Intensive (MINT) in November 1997 and the ...Lindenberg Aerosol Characterization Experiment 1998 (LACE 98) in July and August 1998. Number‐size distributions, hygroscopic particle growth, size‐segregated gravimetric mass, and size‐segregated chemical masses of water‐soluble ions and organic and elemental carbon of aerosol particles have been measured. To obtain information on the quality of the different methods, the number‐derived, gravimetric, and chemically derived mass distributions are compared. Gravimetric mass of fine particles is attributed completely to chemical composition by carbonaceous material and ions, including an estimate of the water content due to hygroscopic compounds. For the characterization of coarse particles, which contribute less to the total mass concentration, insoluble material has to be included in the mass balance. Mass concentrations calculated from the number‐size distributions are well correlated with the gravimetric mass concentration; however, the calculated mass is larger, especially for the Aitken and accumulation modes. The number‐derived mass concentration is most sensitive to the sizing uncertainty of the measured number‐size distribution. Moreover, the impactor cutoffs and the limited knowledge about the density of the particles (especially with high carbon content) account for a major part of the uncertainties. The overall uncertainty of the calculated mass, determined as the standard deviation of the average value in a Monte Carlo approach, is found to be about 10%. Lognormal parameters for the number‐size and volume‐size distributions as well as gravimetric mass‐size distribution and corresponding chemical composition are presented for different air mass types. Most of the modal parameters do not differ significantly between the air mass types. Higher mass concentrations are mostly due to an increase in size (of Aitken and accumulation mode) rather than an increase in the number of particles in a given mode. Generally, the mass percent carbon content increases with decreasing particle size. The most pronounced difference with season is an increase of carbon content from summer to winter as well as an increase in nitrate content, resulting in a decrease of sulfate. For nitrate a strong dependence on air mass direction is observed. Sulfate and nitrate are predominantly neutralized by ammonium. With the results of the two experiments, quality‐controlled mode parameters and corresponding chemical composition of atmospheric aerosol particles in central Europe are now available for application in models.
Ground‐based aerosol size distribution measurements at the Gosan, Korea, sampling site during ACE‐Asia showed occasionally elevated concentrations of nucleation mode aerosol particles, with ...subsequent growth to the Aitken mode. Similar results from aircraft and ship‐based measurements at around the same time indicated that at least one event exhibited a broad spatial extent. One of the most pronounced events, with total number concentrations of particles as high as 105 cm−3, occurred on 12 April at the same time as a large dust event, when the total aerosol surface area of particles with diameters less than 10 μm was increase by a factor of 1.7 compared to the average value for the study. Aircraft data showed layers of enlarged SO2 concentration coinciding with an increased aerosol total number concentration, with the highest detected SO2 concentration being 12 ppbv and aerosol number concentration reaching 8 × 104 cm−3 at 800 m altitude. Aircraft data showed that newly formed particles and dust were stratified in different layers in marine environment and transported to the surface close to the island. Atmospheric thermodynamic data suggested that particles were formed above the marine boundary layer and transported by vertical mixing toward the surface as a result of a 3 K north‐to‐south increase in the sea surface temperature. This temperature increase changed atmosphere stratification from stable to neutral/slightly unstable and deepened the internal mixed layer from below 300 m to above 800 m altitude. Measured hygroscopic properties of nucleation mode aerosol particles were consistent with those of ammonium sulfate particles. Here we reported a novel case study of tropospheric aerosol formation identifying particle chemical composition of nucleation mode aerosols in real time by measuring their water uptake properties. Then we demonstrated that particles formed in a layer aloft and were transported to the ground site by growth of the mixed boundary layer while nucleation was chemically induced and did not require turbulent mixing.
Changes in the elemental ratios of Cl/Na and S/Na in sea‐salt particles are expected from the atmospheric reactions of sulfuric and nitric acids with these particles. Chloride depletion is expected ...to occur upon the liberation of HCl to the gas phase, with the particles remaining enriched in sulfate or nitrate. The elemental ratios of Ca/Na, Mg/Na and K/Na should remain constant during this process. Analysis of chloride depletion and sulfur enrichment was obtained for individual sodium‐containing particles from the remote marine Pacific atmosphere in both the accumulation mode (0.06 ≤ Dp ≤ 1.0 μm, where Dp is the particle diameter) and the coarse mode (Dp > 1.0 μm) size range. Sodium‐containing particles comprised close to 100% of the coarse mode and 11 to 31% of the accumulation mode by number. Aerosols were collected with a low‐perssure impactor and examined with a transmission electron microscope (TEM) coupled with an energy‐dispersive X ray (EDX) detector. The elemental ratios obtained from the atmospheric particles were determined by comparison with values obtained from laboratory‐generated sea‐salt, sodium chloride, and sodium sulfate particles of known size and chemical composition, which served as a calibration set. The elemental ratios of Ca/Na, Mg/Na, and K/Na were found to remain fairly constant between individual sea‐salt particles of various sizes for more than 85% of the particles examined. Deviations in the ratio of Cl/Na and S/Na from that of reference seawater values were observed most commonly for the submicrometer sea‐salt aerosol. The Cl/Na ratio was significantly (Student's t test, 99.9%) lower than that of reference seawater for 89% of the particles examined, while the S/Na ratios were higher for 100% of the particles. The Cl/Na ratio measured in 48% of the coarse sea‐salt particles (1.0 < Dp ≤ 2.5 μm) reflected the ratio in bulk seawater, while the remaining particles had statistically lower ratios and qualitatively different morphologies. All but 3% of these coarse particles had enhanced S/Na ratios over that of bulk seawater. Estimates of non‐sea‐salt (nss) sulfate mass ranged from 216 to 1422 fg for particles of 0.50 μm in diameter to 861 and 5235 fg for particles of 0.80 μm in diameter, corresponding to 74 to 96% of the sea‐salt particle mass. These values are compared with the recent measurements of Mouri and Okada 1993 as well as predictions from the atmospheric chemistry models of in‐cloud sulfate production of Hegg et al., 1992 and estimations of S(IV) oxidation in sea‐salt aerosol water by Chameides and Stelson 1992.
Airborne measurements of size‐resolved aerosol hygroscopicity are presented using an optical particle counting and sizing technique. The measurement range of 0.25 to 3.5 μm is significantly greater, ...and extends to larger sizes, than previous in situ techniques. Preliminary results reveal a peak in aerosol hygroscopicity in the 0.5 –1.5 μm diameter size range in both marine and polluted aerosols. Geometric growth factors range from 1.3 to 1.5 and 1.1 to 1.3 for the sub and super‐micron particles, respectively.
Aerosol samples were collected by aircraft during the summer of 2004 in the Northeastern Pacific and compared to measurements of aerosol hygroscopicity. Chemical speciation analyses of the samples ...revealed that a significant portion of the marine aerosols was organic, and on average 8% of the total aerosol mass was insoluble organic material, tentatively attributed to natural marine emissions. Two chemical models were explored in an attempt to achieve closure between the marine aerosol chemical and physical properties through reproduction of the observed aerosol growth, both in the subsaturated and supersaturated regimes. Results suggest that at subsaturated relative humidities, the nonideal behavior of water activity with respect to aerosol chemistry has an important effect on aerosol growth. At supersaturations, the underprediction of critical supersaturations by all models suggests the hypothesis that formation of a complete monolayer by the insoluble organics may inhibit the activation of aerosols to form cloud droplets.
The extinction‐to‐backscatter ratio S is a crucial parameter for quantitative interpretation of lidar data, yet empirical knowledge of S for tropospheric aerosols is extremely limited. Here we review ...that knowledge and extend it using a recently developed in situ technique that employs a 180° backscatter nephelometer. This technique allows robust quantification of measurement uncertainties and permits correlations with other aerosol and meteorological properties to be explored. During 4 weeks of nearly continuous measurements in central Illinois, S was found to vary over a wide range, confirming previous indications that geographical location by itself is not necessarily a good predictor. The data suggest a modest dependence of S on relative humidity, but this explains only a small portion of the variation. Most variation was associated with changes between two dominant air mass types: rapid transport from the northwest and regional stagnation. The latter category displayed much higher aerosol concentrations and a systematically higher and more tightly constrained range of S. Averages and standard deviations were 64±4 sr for the stagnant category and 40±9 sr for the rapid transport category. Considering the 95% confidence precision uncertainty of the measurements, the difference between these averages is at least 13 sr and could be as large as 35 sr. The wavelength dependence of light scattering, as measured by a conventional nephelometer, is shown to have some discriminatory power with respect to S.
New particle formation in the marine boundary layer Covert, David S.; Kapustin, Vladimir N.; Quinn, Patricia K. ...
Journal of Geophysical Research,
20 December 1992, Letnik:
97, Številka:
D18
Journal Article
Recenzirano
Aerosol measurements were made in the marine boundary layer along the coast of Washington State during the Pacific Stratus Sulfur Investigation. On April 22 the particle concentration increased to ...levels much higher than usual for the clean marine boundary layer. The total particulate number concentration greater than 3 nm diameter increased rapidly from about 250 cm−3 to 3200 cm−3, remained near that level for 7 hours, and then decreased over the next 2 hours to less than 400 cm−3. The change could not be attributed to either local or distant contamination. Immediately before the increase particulate surface area concentration dropped from 25 μm2 cm−3 to less than 5 μm2 cm−3. The SO2 concentration increased from about 20 pptv to 40–60 pptv just before the increase in particle concentration. While these measurements cannot distinguish between changes in number concentration caused by particle nucleation versus advection or vertical mixing, clearly there was recent or continuing particle production on a mesoscale in the air mass. Related aircraft measurements and model results support the hypothesis of new particle formation. These data provide evidence that at times high concentrations of new, ultrafine particles are formed at low SO2 concentrations under marine conditions. This homogeneous nucleation, as opposed to heterogeneous condensation on existing particles, is strongly and inversely dependent on the concentration of existing particles.
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