Climate forcing by anthropogenic aerosols.

Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.

Increase in the stratospheric background sulfuric Acid aerosol mass in the past 10 years.

Data obtained from measurements of the stratospheric aerosol at Laramie, Wyoming (41 degrees N), indicate that the background or nonvolcanic stratospheric sulfuric acid aerosol mass at northern mid-latitudes has increased by about 5 +/- 2 percent per year during the past 10 years. Whether this increase is natural or anthropogenic could not be determined at this time because of inadequate information on sulfur sources, in particular, carbonyl sulfide, which is thought to be the dominant nonvolcanic source of stratospheric sulfuric acid vapor. An increase in stratospheric sulfate levels has important climatic implications as well as heterogeneous chemical effects that may alter the concentration of stratospheric ozone.

SAGE II aerosol data validation based on retrieved aerosol model size distribution from SAGE II aerosol measurements.

This paper describes an investigation of the comprehensive aerosol correlative measurement experiments conducted between November 1984 and July 1986 for satellite measurement program of the Stratospheric Aerosol and Gas Experiment (SAGE II). The correlative sensors involved in the experiments consist of the NASA Ames Research Center impactor/laser probe, the University of Wyoming dustsonde, and the NASA Langley Research Center airborne 14-inch (36 cm) lidar system. The approach of the analysis is to compare the primary aerosol quantities measured by the ground-based instruments with the calculated ones based on the aerosol size distributions retrieved from the SAGE II aerosol extinction measurements. The analysis shows that the aerosol size distributions derived from the SAGE II observations agree qualitatively with the in situ measurements made by the impactor/laser probe. The SAGE II-derived vertical distributions of the ratio N0.15/N0.25 (where Nr is the cumulative aerosol concentration for particle radii greater than r, in micrometers) and the aerosol backscatter profiles at 0.532- and 0.6943-micrometer lidar wavelengths are shown to agree with the dustsonde and the 14-inch (36-cm) lidar observations, with the differences being within the respective uncertainties of the SAGE II and the other instruments.

Optical modeling of stratospheric aerosols: present status.

A stratospheric aerosol optical model is developed which is based on a size distribution conforming to direct measurements. Additional constraints are consistent with large data sets of independently measured macroscopic aerosol properties such as mass and backscatter. The period under study covers background as well as highly disturbed volcanic conditions and an altitude interval ranging from the tropopause to approximately 30 km. The predictions of the model are used to form a basis for interpreting and intercomparing several diverse types of stratospheric aerosol measurement.

Sulfuric Acid droplet formation and growth in the stratosphere after the 1982 eruption of el chichon.

The eruption of El Chichón Volcano in March and April 1982 resulted in the nucleation of large numbers of new sulfuric acid droplets and an increase by nearly an order of magnitude in the size of the preexisting particles in the stratosphere. Nearly 10(7) metric tons of sulfuric acid remained in the stratosphere by the end of 1982, about 40 times as much as was deposited by Mount St. Helens in 1980.

Stratospheric sulfuric Acid layer: evidence for an anthropogenic component.

Recent measurements of small aerosol particles in the stratosphere over Laramie, Wyoming, indicate low-concentration background conditions. A comparison of measurements made some 20 years ago with the present background concentration reveals the possibility of an increase of 9 percent per year. Since the aerosol particles are predominantly sulfuric acid droplets which form in the stratosphere from tropospheric sulfur-containing gases, such an increase may be related to man-made sulfur emissions.

Polarized light scattered from monodisperse randomly oriented nonspherical aerosol particles: measurements.

Measurements of polarized light scattered from monodisperse nonspherical randomly oriented aerosol particles are presented along with Mie theoretical results for spheres of approximately the same cross sectional area. For slightly nonspherical particles of sodium chloride and potassium sulfate with size parameter (defined as the ratio of the particle circumference to the wavelength) greater than about five, the intensity of light scattered is generally more than as predicted by Mie theory in the forward scattering lobe, but less at nonforward angles. For particles with size parameter less than five, the Mie results more closely match the measurements. Measured angular scattering patterns for randomly oriented particles are smoother than the Mie theoretical results and are nearly the same for salt and potassium sulfate particles of the same size. Measurements of particle depolarization are nearly independent of scattering angle.

Dustsonde and lidar measurements of stratospheric aerosols: a comparison.

On two nights in mid-September 1972, comparative measurements of stratospheric aerosol profiles, utilizing backscattered ruby laser light and direct in situ sampling were conducted over Laramie, Wyoming. The lidar backscattering and the particle number density profiles correlated well when the measured atmospheric molecular density profile was used to calculate the Rayleigh profile used in the lidar data reduction. The backscattered signal at 20 km was approximately 18% above Rayleigh and corresponded to a measured concentration of about one particle per cm(3) with diameters greater than 0.30 microm. Based on these initial comparative experiments, the ground-based lidar coupled with temperature soundings appears to be a possible method for determining the relative aerosol profile under present stratospheric loading conditions.

Measured light-scattering properties of individual aerosol particles compared to mie scattering theory.

Monodispersed spherical aerosols of 0.26-2-micro diameter with approximate range of indexes of refraction of atmospheric aerosols have been produced in the laboratory by atomization of liquids with a vibrating capillary. Integrated light scattered 8 through 38 degrees from the direction of forward scattering has been measured with a photoelectric particle counter and compared to Mie theory calculations for particles with complex indexes of refraction 1.4033-0i, 1.592-0i, 1.67-0.26i, and 1.65-0.069i. The agreement is good. The calculations take into account the particle counter white light illumination with color temperature 3300 K, the optical system geometry, and the phototube spectral sensitivity. It is shown that for aerosol particles of unknown index of refraction the particle counter size resolution is poor for particle size greater than 0.5micro, but good for particles in the 0.26-0.5-micro size range.

Efficiency of light-scattering aerosol particle counters.

The counting efficiency as a function of particle size has been calculated for a particular forward scattering aerosol particle counter that uses white light illumination for spherical particles with indexes of refraction 1.33-0i, 1.40-0i, 1.50-0i, 1.592-0i, 1.50-0.05i, 1.50-0.1i, 1.50-0.5i, and 1.95-0.66i. The counting efficiency differs from 100% because of statistical effects: photoelectron statistical broadening, variation in aerosol speed through the illuminated volume, nonuniform illumination of the sampled aerosol, and nonuniformity of the photomultiplier tube photo cathode surface. These spectra broadening effects have been determined for the particle counter by measurement of monodisperse aerosols, and the efficiency calculation is based on these measurements. For the particle counter studied, the efficiencies vary from 70%; to slightly over 100% in the size ranges where the counter response is single valued. The efficiency for materials for which the response is not single-valued fluctuates wildly in those size ranges from about 20% to more than 10,000%. The efficiency as a function of particle size discriminator level setting is sensitive to particle refractive index and to the aerosol size distribution locally about that particle size. The method for calculation of the efficiency can be applied to light-scattering aerosol counters available commercially.