Hermann E. Gerber

Portable cell for simultaneously measuring the coefficients of light scattering and extinction for ambient aerosols

A portable multipass cell is described in which optical transm ittance and scattering measurements are made simultaneously on the same volume of aerosol. The cell is filled with ambient aerosolin which the dispersed phase has been concentrated centrifugally by more than 1 order of magnitude. The cell was tested on board a ship for a wide variety of sea and atmospheric conditions. Measurement errors were identified and evaluated. The mean resolution of the transmittances was +/-0.16%, and the mean resolution of the scattering coefficients was +/-5 x 10(-6) m(-1) . Their combination gives the bulk single scattering albedo, which was resolved by +/-0.1 for atmospheres with a visual range of approximately 50 km and by rapidly increasing accuracy for shorter ranges. Concentrating the dispersed phase was not essential for the scattering measurements and, with achievable improvements in the cell, would not be necessary for the transmittance measurements.

Infrared aerosol extinction from visible and near-infrared light scattering.

The capability to scale from short-wavelength scattered light measured by several hypothetical instruments to IR aerosol extinction coefficients is evaluated in this simulation. Instruments consisting of a forward-scatter meter, integrating nephelometer, and combinations thereof are exposed to a large number of maritime aerosols with size distributions weighted by their frequency of occurrence. Statistical analysis of the wavelength scaling shows that particular two-channel scattering instruments give estimates of IR extinction for wavelengths between 1.06 and 10.6 μm with an accuracy of ≤10%. A forward-scattering instrument gives an output proportional to particle volume and extinction at 10.6 μm.

Light absorption by aerosol particles: First International Workshop

The First International Workshop on light absorption by aerosol particles is introduced, and its goal-to separate the two sources of variability in light absorption measurements by establishing instrumentation errors-is reviewed.

Forward-scatter meter for estimating 10.6-μm aerosol extinction

A forward-scatter meter (FSM) measures the light scattered from aerosols irradiated by a He-Ne laser beam (0.6328 microm) over a narrow angular range in the forward direction and over a path length of 0.5 m. Comparisons are made between the FSM and visible (0.55-microm) and IR (10.6-microm) transmissometers operating in hazes and fogs containing waterlike aerosol particles with widely varying size distributions. While poor correlation is found between the FSM measurements and the visible transmissometer, the FSM measurements can be scaled to the aerosol extinction coefficient at 10.6 microm with an accuracy of +/-10% at the 90% confidence level.

Down looking lidar inversion constrained by ocean reflection and forward scatter of laser light

The laser aureole at 1.06 microm resulting from the redirection of light by sea surface reflection and forward scatter through maritime boundary layer aerosols to a sensor high above the ocean surface is modeled for profiles with typical North Atlantic aerosol size distributions. The magnitude of this laser aureole is highly correlated with the optical depth for these profiles. This optical depth, estimated from the laser aureole, is used to adjust the power of the extinction-backscatter relationship in a Bernoulli-Riccati lidar inversion. Using a lognormal marine aerosol model, 150 profiles of aerosol size distributions are selected by their probability of occurrence in the North Atlantic boundary layer. For these profiles, the lidar inversion using the estimated optical depth predicted the surface extinction 5 times better than the lidar inversions using a climatological backscatter-extinction relationship.

On the performance of the Goetz Aerosol Spectrometer

Abstract The first detailed experimental evaluation of the Goetz Aerosol Spectrometers capability to resolve size distributions of polydisperse aerosols is made. The result of this evaluation removes the lack of confidence in the instrument brought on by the disagreement of previous theoretical and experimental evaluations. Monodisperse polystyrene latex particles of different sizes are used to demonstrate that 1. (1) the performance of the instrument is dependent on its operating modes and is independent of different instruments 2. (2) the commercially available instrument overestimates the contribution of large particles in the measured size distributions 3. (3) the previous theoretical arguments are invalid since the complicated velocity distribution in the channels of the instrument was disregarded, and 4. (4) the previous inconsistent experimental evaluations are explained by properly limiting their conclusions. Furthermore, the capability of the instrument to determine accurately the size of particles ranging from 0.03 to 3.0 μm is found, after a simple modification is made to the geometry of the entrance to the instruments deposition channels. Particle Brownian diffusion, analyzing microscope field of view, and random instrument errors are unimportant for the practical application of the modified instrument.

First International Workshop on Light Absorption by Aerosol Particles: Background, Activities, and Preliminary Results, 28 July–8 August 1980 Ft. Collins, Colo.

The amount of light absorbed by aerosol particles has not been determined with certainty because errors in measurement techniques have been difficult to quantify. To improve this situation, a workshop was conducted to establish experimentally the errors for the various techniques. The workshop was held between 28 July and 8 August 1980 at the Cloud Simulation and Aerosol Laboratory at Colorado State University. Preliminary results show that, for the same well-characterized aerosol particles, substantial differences exist between results from the various techniques. These differences can explain a fraction of the variations reported for the light absorption properties of similar types of atmospheric aerosol particles.

Monte Carlo simulations of laser-generated sea surface aureole.

Monte Carlo simulations are used to study the signals generated by the scatter from aerosol in the marine boundary layer and reflection off a rough sea surface when a laser pulse at 1.06microm passes down through the atmosphere. The model estimates the probability of photons being returned to a receiver collocated with the laser which has two detectors: one with a narrow field of view (lidar detector) and another with a wide field of view where the directly reflected photons are blocked (aureole detector). The simulations are done for nine different aerosol size distributions, three different boundary layer depths, and three different wave conditions. A comparison of the boundary layer optical depth and normalized aureole signal is presented. In addition, a comparison is made between the normalized aureole signal at 1.06 microm (when the detector field of view is reduced) and boundary layer optical depths at 3.75 microm.