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- Author or Editor: W. R. Leaitch x
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Abstract
Comparisons are drawn between the aerosol cloud microphysical theory implicit in the modeling of Kaufman et al. and the cloud droplet and cloud water sulfate concentrations of Leaitch et al. for the purpose of helping to understand the effect of sulfate particles on climate through cloud modification. In terms of the range of possibilities and prospects for future climate given by Kaufman et al. for the effect of sulfur on cloud albedo, the data favor the possibility of stronger cooling. Scatter in the data makes it impossible to constrain model parameters., however, the comparisons suggest that there may not be a universal relationship, and that the uncertainties involved in trying to model this proem are large.
Abstract
Comparisons are drawn between the aerosol cloud microphysical theory implicit in the modeling of Kaufman et al. and the cloud droplet and cloud water sulfate concentrations of Leaitch et al. for the purpose of helping to understand the effect of sulfate particles on climate through cloud modification. In terms of the range of possibilities and prospects for future climate given by Kaufman et al. for the effect of sulfur on cloud albedo, the data favor the possibility of stronger cooling. Scatter in the data makes it impossible to constrain model parameters., however, the comparisons suggest that there may not be a universal relationship, and that the uncertainties involved in trying to model this proem are large.
Abstract
Cloud droplets were measured from an aircraft with an FSSP in summer cloud and in winter cloud. For 70 cloud penetrations, three cloud droplet volume spectra were derived using three different calibration schemes. The spectra were parameterized by the mean volume diameter, the effective diameter, the effective dispersion, the skewness and the kurtosis. These parameters were statistically intercompared using the Mann-Whitney test. The results show that, for the clouds studied, the mean volume diameter and the effective diameter are not significantly affected by the choice of the calibration scheme. However, the dispersion of the volume distribution, the skewness and the kurtosis are quite sensitive to the details of the calibration scheme and possibly to oscillations in the intensity of the scattered light signal, as described by Mie theory.
Abstract
Cloud droplets were measured from an aircraft with an FSSP in summer cloud and in winter cloud. For 70 cloud penetrations, three cloud droplet volume spectra were derived using three different calibration schemes. The spectra were parameterized by the mean volume diameter, the effective diameter, the effective dispersion, the skewness and the kurtosis. These parameters were statistically intercompared using the Mann-Whitney test. The results show that, for the clouds studied, the mean volume diameter and the effective diameter are not significantly affected by the choice of the calibration scheme. However, the dispersion of the volume distribution, the skewness and the kurtosis are quite sensitive to the details of the calibration scheme and possibly to oscillations in the intensity of the scattered light signal, as described by Mie theory.
Abstract
Comparisons of particle-size distributions measured by Particle Measuring Systems FSSP-300 and PCASP-100X probes through a range of relative humidities reveal that the deiced PCASP-100X probe dries hydrated submicron aerosols before measurement. The FSSP-300 appears to measure the particles in their hydrated state and detects the expected growth in the particle spectrum with increasing relative humidity. Calibration changes fox refractive-index changes with hydration are not applicable to the deiced PCASP-100X probe but are for the FSSP-300. The combined use of the two probes with their differing responses to hydrated aerosols may provide information related to the chemical composition of the aerosol.
Abstract
Comparisons of particle-size distributions measured by Particle Measuring Systems FSSP-300 and PCASP-100X probes through a range of relative humidities reveal that the deiced PCASP-100X probe dries hydrated submicron aerosols before measurement. The FSSP-300 appears to measure the particles in their hydrated state and detects the expected growth in the particle spectrum with increasing relative humidity. Calibration changes fox refractive-index changes with hydration are not applicable to the deiced PCASP-100X probe but are for the FSSP-300. The combined use of the two probes with their differing responses to hydrated aerosols may provide information related to the chemical composition of the aerosol.
Abstract
Measurements spanning much of the particle size spectrum were made on the surface aerosol arriving at Igloolik, Northwest Territories, Canada during late February 1982. Vertical profiles of aerosol particle concentration were obtained during one day of the study period. Concentrations of Aitken nuclei and cloud condensation nuclei as well as the aerosol light-scattering coefficient were measured instrumentally several times a day. Inertial impaction systems were used to separate and collect particles for microscopic sizing and chemical analysis. Suspended and precipitating ice crystals were inertially collected on microscope slides. The aerosol observations were accompanied by observations of temperature, wind speed and direction, visibility and cloud type. An upper-air station at Hall Beach, <100 km from Igloolik, provided radiosonde data.
Diurnal variations in the Aitken nucleus concentrations were observed on several clear days. The concentrations were frequently seen to follow the diurnal temperature variation, reaching a maximum near midday. Vertical profiles of Aitken nucleus concentrations obtained prior to and during one of these diurnal peaks suggests that this pattern was limited to the near-surface layer. Throughout the study, virtually all of the aerosol particle mass lay between 0.2 and 4.0 μm diameter. There was little indication of any diurnal change in the particle concentration in this size range. A clear difference in the quality of the air reaching Igloolik was detected on 23 February. Associated with this was a doubling of the particle concentration while the apparent particulate mass increased from ∼6 to ∼11 μg m−3. Impacted aerosol particles were found to be composed of 15–50% water soluble compounds before 23 February and 40-100% after this date. Sulfate was the dominant ionic species in all cases. Vertical profiles of the large aerosol particles, obtained with an airborne nephelometer, suggested a slightly enhanced concentration at the surface and a uniform concentration in the lower troposphere. Profiles of Aitken nucleus concentrations pointed to a surface source of small nuclei which diffused vertically and independently of the larger particles.
Suspended ice crystals may have accounted for a significant portion of the degradation in visibility observed throughout the study.
Abstract
Measurements spanning much of the particle size spectrum were made on the surface aerosol arriving at Igloolik, Northwest Territories, Canada during late February 1982. Vertical profiles of aerosol particle concentration were obtained during one day of the study period. Concentrations of Aitken nuclei and cloud condensation nuclei as well as the aerosol light-scattering coefficient were measured instrumentally several times a day. Inertial impaction systems were used to separate and collect particles for microscopic sizing and chemical analysis. Suspended and precipitating ice crystals were inertially collected on microscope slides. The aerosol observations were accompanied by observations of temperature, wind speed and direction, visibility and cloud type. An upper-air station at Hall Beach, <100 km from Igloolik, provided radiosonde data.
Diurnal variations in the Aitken nucleus concentrations were observed on several clear days. The concentrations were frequently seen to follow the diurnal temperature variation, reaching a maximum near midday. Vertical profiles of Aitken nucleus concentrations obtained prior to and during one of these diurnal peaks suggests that this pattern was limited to the near-surface layer. Throughout the study, virtually all of the aerosol particle mass lay between 0.2 and 4.0 μm diameter. There was little indication of any diurnal change in the particle concentration in this size range. A clear difference in the quality of the air reaching Igloolik was detected on 23 February. Associated with this was a doubling of the particle concentration while the apparent particulate mass increased from ∼6 to ∼11 μg m−3. Impacted aerosol particles were found to be composed of 15–50% water soluble compounds before 23 February and 40-100% after this date. Sulfate was the dominant ionic species in all cases. Vertical profiles of the large aerosol particles, obtained with an airborne nephelometer, suggested a slightly enhanced concentration at the surface and a uniform concentration in the lower troposphere. Profiles of Aitken nucleus concentrations pointed to a surface source of small nuclei which diffused vertically and independently of the larger particles.
Suspended ice crystals may have accounted for a significant portion of the degradation in visibility observed throughout the study.
Abstract
Airborne observations conducted in marine stratus over the cast coast of Canada during the North Atlantic Regional Experiment in the summer of 1993 are used to develop cloud microphysical parameterization schemes for general circulation models. Observations of cloud droplet number concentration (N d ), interstitial aerosol number concentration, temperature, vertical air velocity (w), and liquid water content (LWC) are considered, as well as determination of the effective radius (r eff) and total particle concentration (interstitial aerosol + cloud droplet). Statistical techniques are used to obtain regression equations among the above parameters. For individual clouds, an inverse relationship between the interstitial aerosol concentration and droplet concentration is always observed. In general, variations in r eff are determined by N d as much as by LWC. The regression equations are compared with current parameterizations for GCMS. Results showed that multiple relationships are present among N d , N t , and w; and r eff, LWC, and N d .
Abstract
Airborne observations conducted in marine stratus over the cast coast of Canada during the North Atlantic Regional Experiment in the summer of 1993 are used to develop cloud microphysical parameterization schemes for general circulation models. Observations of cloud droplet number concentration (N d ), interstitial aerosol number concentration, temperature, vertical air velocity (w), and liquid water content (LWC) are considered, as well as determination of the effective radius (r eff) and total particle concentration (interstitial aerosol + cloud droplet). Statistical techniques are used to obtain regression equations among the above parameters. For individual clouds, an inverse relationship between the interstitial aerosol concentration and droplet concentration is always observed. In general, variations in r eff are determined by N d as much as by LWC. The regression equations are compared with current parameterizations for GCMS. Results showed that multiple relationships are present among N d , N t , and w; and r eff, LWC, and N d .
Abstract
The largest uncertainty in the historical radiative forcing of climate is caused by changes in aerosol particles due to anthropogenic activity. Sophisticated aerosol microphysics processes have been included in many climate models in an effort to reduce the uncertainty. However, the models are very challenging to evaluate and constrain because they require extensive in situ measurements of the particle size distribution, number concentration, and chemical composition that are not available from global satellite observations. The Global Aerosol Synthesis and Science Project (GASSP) aims to improve the robustness of global aerosol models by combining new methodologies for quantifying model uncertainty, to create an extensive global dataset of aerosol in situ microphysical and chemical measurements, and to develop new ways to assess the uncertainty associated with comparing sparse point measurements with low-resolution models. GASSP has assembled over 45,000 hours of measurements from ships and aircraft as well as data from over 350 ground stations. The measurements have been harmonized into a standardized format that is easily used by modelers and nonspecialist users. Available measurements are extensive, but they are biased to polluted regions of the Northern Hemisphere, leaving large pristine regions and many continental areas poorly sampled. The aerosol radiative forcing uncertainty can be reduced using a rigorous model–data synthesis approach. Nevertheless, our research highlights significant remaining challenges because of the difficulty of constraining many interwoven model uncertainties simultaneously. Although the physical realism of global aerosol models still needs to be improved, the uncertainty in aerosol radiative forcing will be reduced most effectively by systematically and rigorously constraining the models using extensive syntheses of measurements.
Abstract
The largest uncertainty in the historical radiative forcing of climate is caused by changes in aerosol particles due to anthropogenic activity. Sophisticated aerosol microphysics processes have been included in many climate models in an effort to reduce the uncertainty. However, the models are very challenging to evaluate and constrain because they require extensive in situ measurements of the particle size distribution, number concentration, and chemical composition that are not available from global satellite observations. The Global Aerosol Synthesis and Science Project (GASSP) aims to improve the robustness of global aerosol models by combining new methodologies for quantifying model uncertainty, to create an extensive global dataset of aerosol in situ microphysical and chemical measurements, and to develop new ways to assess the uncertainty associated with comparing sparse point measurements with low-resolution models. GASSP has assembled over 45,000 hours of measurements from ships and aircraft as well as data from over 350 ground stations. The measurements have been harmonized into a standardized format that is easily used by modelers and nonspecialist users. Available measurements are extensive, but they are biased to polluted regions of the Northern Hemisphere, leaving large pristine regions and many continental areas poorly sampled. The aerosol radiative forcing uncertainty can be reduced using a rigorous model–data synthesis approach. Nevertheless, our research highlights significant remaining challenges because of the difficulty of constraining many interwoven model uncertainties simultaneously. Although the physical realism of global aerosol models still needs to be improved, the uncertainty in aerosol radiative forcing will be reduced most effectively by systematically and rigorously constraining the models using extensive syntheses of measurements.
