Abstract.A joint Mexican, Russian, and American research effort has been initiated to develop new methods to remotely sense atmospheric parameters using ground-based, aircraft, and satellite observations.As a first step in this program, ground-based spectrophotometric measurements of the direct solar radiation have been obtained for the extremely polluted Mexico City atmosphere for the period of April-June 1992. These observations were made at more than 1300 channels in the spectral range of 0.35-0.95 _tm. In the ultraviolet (UV) portions of the spectrum (e.g., 0.35 txm), aerosol optical thicknesses were found to range between 0.6 and 1.2; in the visible portion of the spectrum ( e.g., 0.5 Inn) they ranged from 0.5 to 0.8; and in the near-infrared (NIR) spectra (e.g., 0.85 Inn), values of 0.3 -0.5 were found. Applying a spectral optical depth (SOD) model of x(_,) = C + A _,-oc, values of 1.55 < oc < 1.85 were obtained for polluted, cloudless days, with values of 1.25 < o: < 1.60 on days with haze. The aerosol particles in the polluted Mexico City atmosphere were found to be strongly absorbing, with a single-scattering albedo of 0.7 -0.9 in the UV, 0.6 -0.8 in the visible portion of the spectrum, and 0.4 -0.7 in the NIR. These values are possibly consistent with a high soot concentration, contributed both by vehicular traffic and heavy industry.Analysis of the measured aerosol SOD using the optical parameters of an urban aerosol model permits the concentration of aerosol particles to be estimated in the vertical column; a maximum value of 3 x 10 9 cm "2 was found. This concentration of aerosol particles exceeds that found in most other regions of the globe by at least an order of magnitude. Near the ground the aerosol size distributions measured using an optical particle counter were found to be strongly multimodal.

During the GATE observational program, aircraft radiative flux measurements were taken at several altitudes in the Saharan dust. On the basis of these flux measurements at 28 wavelengths in the 0.4–0.9 μm wavelength region, the corresponding vertical particle size spectra have been inferred. These particle size distributions, calculated by using spherical Mie theory, have been represented by quadramodal combinations of gamma functions. The calculated quadramodal size distributions were characterized by mode radii at 0.1, 0.2, 1.0, and 2.0 μm, with a relatively narrow spread of particle sizes centered about each mode radius. The number density of the smallest particle mode was found to increase with increasing height, while that of the largest particle mode decreased with increasing height. Comparison of deduced particle sizes of a ‘dusty’ day (September 4) and a ‘clear’ day (August 13) showed that the dusty day was characterized far more by the presence of large particles than by an increase in small particle concentration. Heating rates as large as 0.4°C/hour μm were found for September 4, 1974.

A spectroradiometer with wedge interference filters (SWIF) (the filters were produced by Carl Zeiss, Jena, Germany) and a CCD matrix (which was of Russian production) that functions as the sensor has been designed and built for use in ground-based optical sensing of the atmosphere and the Earth's surface in the spectral range of 0.35-1.15 µm. Absolute calibration of this instrument was performed through a series of observations of direct solar radiation at Mauna Loa Observatory (MLO) in Hawaii in May and June 1993. Spectral optical depth (SOD) measurements that were made during these field experiments provided detailed spectral information about both aerosol extinction (scattering plus absorption) and molecular absorption in the atmosphere above the site at MLO. The aerosol-SOD measurements were compared with narrow-band radiometer measurements at wavelengths of 380, 500, and 778 nm The SWIF and narrow-band radiometer measurements are in agreement to within the experimental error. At a wavelength of 500 nm, the aerosol SOD was found to be approximately 0.045. Adescription of the SWIF instrument, its absolute calibration, and the determination of atmospheric SOD's at MLO are presented.

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This report presents a summary of a joint Soviet/American exchange program to compare calculations with observations in the real atmosphere, in order to determine and study the diabatic processes that are important for tropospheric energetics. The primary purpose of the present investigation was to determine the effect of aerosols upon radiation characteristics in the real atmosphere. Spectral measurements of the radiation field as a function of height have been made in 0.01 pm increments from 0.4 pm to 0.9 pm along with aerosol optical properties. Using these data, radiative transfer calculations have been performed using the 4-term expansion of the spherical harmonics method in order to provide an independent check on the accuracy of the previous measurements as well as provide recommendations for improved observations.The effect of a small amount of highly absorbing material, such as hematite, has been demonstrated. Extensive data are presented giving spectral shortwave radiation flux and flux divergence values as a function of height in the atmosphere. Because no "best fit" overall spectral regions were obtained, further studies were performed to test the sensitivity of the theoretical results to small variations in the input parameters. The results indicate that the major reason for discrepancies between measured and calculated radiation fields is difficulties with the radiative and aerosol measurements, rather than shortcomings of the calculational scheme.

This report presents a summary of a joint Soviet/American exchange program to compare calculations with observations in the real atmosphere, in order to determine and study the diabatic processes that are important for tropospheric energetics. The primary purpose of the present investigation was to determine the effect of aerosols upon radiation characteristics in the real atmosphere. Spectral measurements of the radiation field as a function of height have been made in 0.01 pm increments from 0.4 pm to 0.9 pm along with aerosol optical properties. Using these data, radiative transfer calculations have been performed using the 4-term expansion of the spherical harmonics method in order to provide an independent check on the accuracy of the previous measurements as well as provide recommendations for improved observations. The effect of a small amount of highly absorbing material, such as hematite, has been demonstrated. Extensive data are presented giving spectral shortwave radiation flux and flux divergence values as a function of height in the atmosphere. Because no "best fit" overall spectral regions were obtained, further studies were performed to test the sensitivity of the theoretical results to small variations in the input parameters. The results indicate that the major reason for discrepancies between measured and calculated radiation fields is difficulties with the radiative and aerosol measurements, rather than shortcomings of the calculational scheme.

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