Glenn K. Yue

A two-dimensional model of sulfur species and aerosols

A two-dimensional model of sulfate aerosols has been developed. The model includes the sulfate precursor species H2S, CS2, DMS, OCS, and SO2. Microphysical processes simulated are homogeneous nucleation, condensation and evaporation, coagulation, and sedimentation. Tropospheric aerosols are removed by washout processes and by surface deposition. We assume that all aerosols are strictly binary water-sulfuric acid solutions without solid cores. The main source of condensation nuclei for the stratosphere is new particle formation by homogeneous nucleation in the upper tropical troposphere. A signficant finding is that the stratospheric aerosol mass may be strongly influenced by deep convection in the troposphere. This process, which could transport gas-phase sulfate precursors into the upper troposphere and lead to elevated levels of SO2 there, could potentially double the stratospheric aerosol mass relative to that due to OCS photooxidation alone. Our model is successful at reproducing the magnitude of stratospheric aerosol loading following the Mount Pinatubo eruption, but the calculated rate of decay of aerosols from the stratosphere is faster than that derived from observations.

Intercomparison of stratospheric ozone profiles obtained by Stratospheric Aerosol and Gas Experiment II, Halogen Occultation Experiment, and ozonesondes in 1994–1995

Stratospheric ozone mixing ratio profiles measured by satellite-borne Stratospheric Aerosol and Gas Experiment (SAGE II data version 5.93) and Halogen Occultation Experiment (HALOE data versions 17 and 18) are compared with each other and with balloon-borne ozonesonde profiles at Payerne, Lauder, and Macquarie Island stations during 1994-1995. The SAGE II-HALOE ozone mixing ratio profiles are compared at 13-55 km altitudes, and the satellite-ozonesonde profiles are compared at 13-33 km altitudes. It is found that HALOE data version 18 ozone values are systematically larger than HALOE data version 17, particularly in the lower stratosphere. HALOE data version 18 ozone profiles are about 2%, 5-8%, and 10-20% larger than the corresponding HALOE data version 17 at 40-55, 25-40, and 15-25 km altitudes, respectively. The new version of HALOE ozone agrees better with SAGE II and Payerne ozonesonde profiles than HALOE data version 17 does. In general, the agreements between satellite-borne sensors and ozonesondes are all within their stated level of uncertainties. Intercomparison results between SAGE II-Payerne ozonesonde and SAGE II-Lauder ozonesonde are compared with earlier comparative studies between SAGE I-Payerne ozonesonde and SAGE II-Lauder ozonesonde by Veiga et al. [1995]. Better agreement is achieved at Payerne station, but no improvement is discernible at Lauder ozonesonde station. At altitudes above 20 km the intercomparisons between SAGE II and HALOE data version 18 and between SAGE II and ozonesondes seem to indicate that aerosol has no statistically significant impact on SAGE II ozone retrieval during the period of 1994-1995.

A mechanism for hydrochloric acid production in cloud

A theoretical model describing the general interaction between atmospheric trace gases, such as S02, NH3, C02 and 02, chemical reactant gaseous product H2SO4 and hydrometeors containing NaCl is proposed to study a possible mechanism for HCl production in non-precipitating cloud and the determination of the pH value of cloud droplets.Four different cloud droplet distributions have been used to estimate the upper limit of the amount of gaseous HCl released into the atmosphere resulting from the evaporation of cloud droplets. It is shown that the acid production and the amount of HCl released depend on the following factors: (a) the temperature of the cloud; (b) the oxidation rates; (c) the ambient concentration of SO2, NH3, and H2SO4; (d) the life cycle of the cloud; and (e) the liquid content of the cloud.This proposed chemical model also predicts a pH value spectrum depending on the cloud droplet distribution. Field measurements for the dependence of pH value on particle size and spatial distribution of gaseous HCl are recommended.

A new approach to retrieval of aerosol size distributions and integral properties from SAGE II aerosol extinction spectra

A new approach to the retrieval of aerosol size distributions and integral properties from Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction spectra is proposed. This method assumes that the aerosol size distribution can be approximated by a histogram of number density as a function of particle size. The retrieved number density in each bin is expressed as a linear combination of the SAGE II aerosol extinctions at four or fewer wavelengths. The coefficients in the weighted linear combination are obtained by minimizing the retrieval error averaged for a set of testing size distributions. The same method has been applied to retrieve aerosol surface area and volume densities from SAGE II aerosol extinctions. The retrieval accuracy was studied by calculating the aerosol surface area and volume densities for six aerosol size distributions retrieved by using the proposed linear minimizing error method. In general, the retrieval error increases with the decreasing number of wavelength-dependent extinction measurements available for retrieval. Besides accuracy, the proposed method has the advantage of not requiring an initial guess or a weighting function. Furthermore, it is very simple and fast. Another advantage is that the proposed method can still be applied to the case when the number of reliable aerosol extinction is less than four. The formulas presented in this paper can be used easily by investigators to retrieve surface area and volume densities from SAGE II aerosol extinction spectra at different altitudes for a variety of purposes. The proposed new method can also be extended to retrieve properties from other remote sensing systems. The application of the proposed method for instrument design is discussed.

An empirical model study of the tropospheric meridional circulation based on SAGE II observations

This study investigates the tropospheric mean meridional circulation important to the development of opaque clouds and the measurement opportunity of the 1.02-μm channel of the Stratospheric Aerosol and Gas Experiment (SAGE) II in the troposphere. A simple empirical model is formulated to derive the mean meridional circulation from the 6-year (1985-1990) statistics of the SAGE II tropospheric measurement frequency. The vertical circulation of the model is assumed to be related to the departure field of the zonally averaged SAGE II measurement frequency from the corresponding global mean in a linear fashion. The proportional constant is calibrated with the observed upwelling circulation statistics in the tropics. The obtained model vertical circulation is then used to determine the distribution of meridional velocity according to the continuity equation. The derived model mean circulation features the influence from both the diabatic circulation and the eddy quasi-isentropic transport, with a distinct pattern of material advection into the upper troposphere from both the lower troposphere and the stratosphere. Most significantly, the model circulation is shown to be highly consistent with the observed free tropospheric aerosol and ozone distributions, particularly with their seasonal variations given the aerosol and ozone source regions. This high degree of consistency illustrates the intimate relationship between the large-scale circulation, cloudiness, and the SAGE II tropospheric measurement frequency, and the robust nature of the empirical model despite the models simplicity. The discussion in relating the model circulation to the conventional Eulerian circulation and the Lagrangian transport, based on isentropic consideration. is also provided.

Intercomparison of multiplatform stratospheric aerosol and ozone observations

Stratospheric aerosol extinction profiles from the satellite-borne Stratospheric Aerosol and Gas Experiment (SAGE II, data version 5.93) and Halogen Occultation Experiment (HALOE, data version 17) instruments are intercompared with the aerosol lidar data at Garmisch-Partenkirchen, Germany (47.5°N, 11.1°E) during January and April 1993. Both the HALOE 5.26 μm aerosol extinction and aerosol lidar 0.532 μm backscattering profiles are converted to extinction profiles at the SAGE II 0.525 μm wavelength channel using aerosol size distributions measured by ER-2 in northern midlatitudes. Within the altitude range of 18-22 km, where most of the stratospheric aerosol resides, the differences between the SAGE II and the HALOE extinction coefficients with respect to HALOE are in general within 30%. The SAGE II-lidar and HALOE-lidar differences (with respect to lidar) are generally within 50% in 15-25 km altitudes. The HALOE extinction values are systematically larger than the SAGE II and aerosol lidar values above 25 km altitude. Stratospheric ozone mixing ratio profiles measured by the SAGE II and HALOE sensors are also intercompared during the same period. The percent differences between the SAGE II and HALOE ozone mixing ratios with respect to HALOE are within 20% in 20-60 km altitudes, in general, within the separation between the compared profiles.

Lagrangian approach for Stratospheric Aerosol and Gas Experiment (SAGE) II profile intercomparisons

Trajectory calculations are employed to identify Stratospheric Aerosol and Gas Experiment (SAGE) II flights sampling the same air mass as is observed by a ground-based aerosol lidar at Garmisch-Partenkirchen, Germany (47.5°N, 11.1°E, 735 m above sea level), during the periods of January-April 1993 and January-December 1998. Intercomparisons between lidar-observed and SAGE II-derived backscatters at 532 nm are conducted. Percentage differences between trajectory-tracked SAGE II profiles and ground-based lidar observations with respect to aerosol lidar are generally within 20-40%, though localized discrepancies >50% are found for some cases. In addition, aerosol extinction, aerosol to molecular extinction ratio, and ozone mixing ratio profiles obtained from the SAGE II flights overpassing the vicinity of Garmisch-Partenkirchen during the January-April 1993 period are compared with profiles obtained from corresponding trajectory-tracked SAGE II flights. Percentage discrepancies between SAGE II ozone profiles are generally within 10-20% above the Junge layer. Data comparisons for aerosol profiles show mixed results. While some cases agree within the error bars, there are several cases where percentage discrepancies exceed 50%.

A model for identifying the aerosol‐only mode of SAGE II 1.02‐μm extinction coefficient data at altitudes below 6.5 km

A model is proposed for identifying the aerosol mode of the second Stratospheric Aerosol and Gas Experiment (SAGE II) 1.02-μm extinction coefficient measurements at altitudes below 6.5 km, which also contain cloud samples. This development allows one to extend the SAGE II satellite data analysis from the lower limit at 6.5 km of the SAGE II two-wavelength method into the lower troposphere. Thus the proposed model provides opportunities for fully utilizing the SAGE II tropospheric measurements important to the understanding of the global behavior of tropospheric aerosols, clouds, and ozone and related transports. The effectiveness of this model is examined by using the SAGE II two-wavelength technique at 6.5 km. Sample applications of the proposed model reveal encouraging results. To assess the quality of the aerosol 1.02-μm data, it is recommended that a comprehensive data comparison analysis be conducted by using tropospheric measurements from different instruments.

Retrieval of aerosol size distributions and integral properties from simulated extinction measurements at SAGE III wavelengths by the linear minimizing error method

The retrieval of aerosol size distribution and integral properties from simulated aerosol extinction measurements of the new satellite instrument, the Stratospheric Aerosol and Gas Experiment (SAGE) III, is investigated. The retrieval method used in this study is the newly proposed linear minimizing error (LME) method. This approach assumes that the aerosol size distribution can be approximated by a histogram of number density as a function of particle size. The retrieved number density in each bin and the integral properties, including surface area and volume densities, are expressed as a linear combination of the SAGE III aerosol extinction coefficients at eight or fewer wavelengths. The coefficients in the weighted linear combination are obtained by minimizing the average of retrieval errors over a set of testing size distributions. The choice of aerosol channels for retrieval was first determined by comparing the average composite errors of the integral properties retrieved from different combinations of aerosol extinctions. The accuracy of the proposed method was studied by comparing the actual and retrieved properties for six aerosol size distributions. Results of this investigation indicate that unimodal, as well as bimodal, lognormal size distributions can be retrieved with reasonable accuracy. When all eight aerosol extinctions are available for retrieval, the average composite errors, which include systematic errors and errors due to measurement uncertainties for the retrieval of surface area and volume densities, are 9.4±4.3 and 4.7±2.0%, respectively. The retrieved aerosol integral properties are more accurate than those retrieved from SAGE II. However, it is not advantageous to use all eight SAGE III wavelengths for retrieval. The proper choice of aerosol extinctions for retrieval is elucidated. The derived formulas listed in this paper can be easily used by investigators to retrieve aerosol integral properties and estimate the retrieval errors from SAGE III aerosol extinctions at different altitudes for a variety of purposes. The application of the LME method to instrumental design is discussed.

Properties of the 1979 SAM II Antarctic 1.0-μm extinction coefficients: Implications of dehydration and seasonal evolution of the Antarctic polar vortex

The present study investigates the 1.0-μm extinction coefficient measurements obtained in the Antarctic region in 1979 from the Stratospheric Aerosol Measurement (SAM) II, with particular focus on the background aerosol properties. Correlative meteorological information from the National Centers for Environmental Prediction is incorporated in this investigation. The results indicate that the data frequency distribution of the background aerosol extinction coefficient in the local summer and fall can be adequately modeled by using a single-mode normal distribution, and that a binormal distribution is needed for modeling the distribution in the local winter and spring because of the different characteristics of the aerosols inside and outside the polar vortex. In general, the vertical distribution of the aerosol mean extinction coefficient exhibits two regions of different seasonal variation. Above 16 km the extinction coefficient is the highest during the local summer, and the lowest during the local spring inside the polar vortex. Below 16 km the aerosol seasonal variation is more complex, but the winter enhancement of the aerosol extinction coefficient inside the Antarctic polar vortex is clearly evident. As the season changes from winter to spring, the results inside the Antarctic polar vortex also indicate a reduction in aerosol optical depth in the stratosphere, but no significant changes in the upper troposphere. The present study further indicates that the bottom of the winter polar vortex in Antarctica is located at an altitude as low as 8 to 9 km, which is about 4 to 5 km lower than the bottom of the Arctic polar vortex. This difference may be attributable to the different strengths of the winter polar vortex and the planetary wave activities between the two hemispheres. In summary, the properties of the Antarctic background aerosol are very consistent with the effect of polar stratospheric clouds on the aerosol vertical distribution through their formation, sedimention, and evaporation, and with the seasonal evolution of the polar vortex. Finally, the result of the present study provides valuable opportunities for fully utilizing the multiyear SAM II tropospheric and stratospheric measurements to investigate the aerosol climatology and long-term variations in the Arctic and Antarctic regions.