Atmospheric Environment. Part A. General Topics最新文献

Simple sulfur chemistry is incorporated with a diagnostic cloud module to furnish an Eulerian model for slowly moving cold frontal systems. Chemical species include sulfur dioxide and sulfate in air, cloud and rain water. The diagnostic method to determine cloud water content is rather crude at present, but the primary results, including a three-dimensionmal test, seem reasonable. Two-dimensional applications of the model are made for a precipitation event presented in a frontal process in March 1988, Guagdong Province, southern China. The cross-section is selected to be representative of major industrial areas of Guangdong. The analysis of the modeling results displays the effects of the lifting and shearing of the wind field onto the distribution patterns of pollutant distributions. Clouds are shown widely polluted with the lifting and entrainment of the polluted air from lower layers, so that acid rain was generated in the precipitation area following the front, covering the most part of Guangdong Province. The pllutants are transported northerly with the front surface are generally transported southerly, but in-cloud pollutants are transported northerly with a wider extension. The results qualitatively show that the transport of acidic precursors in such a weather system is of medium scale and the acidic deposition in Guangdong might be contributed mainly by the sources of Guangdong Province itself.

A dust aerosol optical model is proposed as a result of analysis of the data obtained during the U.S.S.R.-U.S. experiment in Tadzhikistan, 1989. The model consists of spectral dependencies of the single scattering albedo, ω, the asymmetry parameter of the scattering phase function, g, and of possible values of the aerosol optical thickness, τ, for background conditions and for dust storms. This model and available published optical models of dust aerosols were used to perform simulations of radiative forcing in the dusty atmosphere. Results obtained are compared with corresponding results for Saharan aerosol. It is shown that atmospheric dust strongly decreases the total radiative balance of the underlying surface and at the same time induces general warming of the underlying surface-atmosphere system due to a decrease in the system albedo over the arid zones. The calculated heating/cooling rates in the atmosphere are analysed together with data from aerological soundings and radiation measurements.

In September 1989 a joint U.S.S.R.-U.S. study of the causes and effects of desert dust on the environment was conducted in the Tadzhik S.S.R. in Soviet Central Asia. The objectives of the study included measurements of optical absorption by suspended material, both windblown dust and aerosol “black” carbon. This latter material is a combustion effluent, prevalent in emissions from poorly controlled burning, with a long atmospheric lifetime and a large cross-section for the absorption of visible radiation. The measurements obtained from the analysis of filter samples indicate that only during periods of active dust production was there a significant contribution of dust to total absorption. At other times, the presence of black carbon from local and regional sources accounted for approximately 90° of the total aerosol optical absorption. The conclusions are that fuel combustion may produce a greater optical impact on the atmosphere in less-developed areas of the world than that arising from the effects of desert dust production.

Isoprene, a biogenic hydrocarbon emitted from vegetation, has been detected at sub-ppb levels, using a commercially available reduction gas detector (RGD) with an isothermal, portable gas chromatograph. For 1 ml samples, a detection limit of 300–500 parts per trillion (over an order of magnitude less than the traditional gas chromatography-flame ionization detection technique [GC-FID]) was achieved. The detector response was linear up to 500 ppb. This technique (GC-RGD) is compared with GC-FID measurements in studies of ambient isoprene mixing ratios within and above an oak forest in northeastern U.S.A. Applications of laboratory studies of the mechanism of isoprene production at the leaf level are also discussed.

Data were successfully obtained from an experiment during dust storms on 16 and 20 September 1989, in the Kafirnigan Valley, Tadzhik S.S.R. (Tadzhikistan). The principal purposes of the experiment were to provide data sets for modeling radiation transfer, as modified by desert dust, and to specify transport, modification, and deposition of a desert dust system. The experiment provided data for investigations of four problem areas concerning climatic effects of desert dust: (1) speciication of dust-producing source areas and meteorology, (2) specification of dust microphysics, (3) description of optical and climate effects of the desert dust, including local meteorological conditions, and (4) description of chemical composition of the desert dust.

The results of studies on dust aerosol optical properties obtained during the U.S.S.R.-U.S. experiment are discussed. The ground-based and aircraft measurements carried out during the experiment allow the estimation of characteristic values of aerosol optical depth, aerosol light-scattering coefficients, the degree of linear polarization, and aureole phase functions for different atmospheric conditions in Central Soviet Asia. Two dust storms were observed for which the recorded aerosol optical depth at λ = 0.55 μm {τ_a(0.55)} reached 1.5 on 16 and 17 September 1989, and 3.5 on 20 and 21 September 1989. The optical characteristics (spectral dependence of the optical depth, degree of linear polarization) were similar for two dust episodes.

Size distribution data obtained during the U.S.S.R.-U.S. dust experiment make it possible to propose a general conception about the size distribution of dust aerosols within the size range 0.005–100 μm. The microstructure for the optically active fraction of arid aerosol is approximated in the form of a log-normal distribution with parameters D = 3.5−6 μm, δ² = 0.5−0.8, which can be used when estimating radiative calculations.

A model of deposition and transport was constructed for the Kafirnigan Valley, in Soviet Central Asia. Data, consisting of deposition measurements at Shaartuz, atmospheric columnar mass, aerosol concentrations, wind speed, optical scattering, and movement of soil, were collected for the dust storms of 16 and 20 September 1989. Results from the model were compared with measurements of total atmospheric columnar mass loading for the dust storm of 16 September. Although sensitivity of the model to dust layer height does not recommend the model for general use, the model has some merit in predicting transport and deposition for dust contained in a river valley.

The spatial and temporal variability of the daily 1-h maximum O₃ concentrations over non-urban areas of the eastern United States of America was examined for the period 1985–1990 using principal component analysis. Utilization of Kaiser's Varimax orthogonal rotation led to the delineation of six contiguous subregions or “influence regimes” which together accounted for 64.02% of the total variance. Each subregion displayed statistically unique O₃ characteristics and corresponded well with the path and frequency of anticyclones. When compared to the entire domain, the mid-Atlantic and south subregions observe higher mean daily 1-h maximum concentrations. Concentrations are near the domain average for the northeast and southwest subregions and are lowest in the Great Lakes and Florida subregions. The percentage of observations exceeding 120 ppb were greates in the mid-Atlantic and southwest subregions, near the domain average in the northeast and south subregions, and lowest in the Great Lakes and Florida subregions.

Examination of the time series of the principal component scores associated with the subregions indicated that Great Lakes and mid-Atlantic subregions tend to observe a stronger seasonal cycle, with maximum concentrations occurring during the last week in June and first week in July, respectively. The strength of this seasonality is weakened for the northeast and south subregions and its timing delayed, until the end of July and the first of August, respectively. The southwest subregion experiences a greatly diminished seasonality, with maximum concentrations delayed until the middle of August. The seasonality found in the Florida subregion is unique in both its strength and timing, as the highest concentrations consistently occur during the months of April and May. The time series were then deseasonalized and autocorrelations and spectral density estimates calculated, revealing that persistence is much more prevalent in the Florida (autocorrelation significant to a lag of 4 days), south (3 days) and southwest (3 days) subregions. Conversely, autocorrelations are only significant to a lag of one day in the northeast and two days for the Great Lakes and mid-Atlantic subregions.

Two fast integration methods for chemical kinetics are tested. One is the Quasi-steady State Approximation (QSSA) method and the other is a new Euler Backward Iterative (EBI) method. The EBI method is based on iterative solution of the Euler backward approximation of a coupled system of nonlinear ordinary differential equations of chemical kinetics. The efficiency of the iteration process is increased by using analytical solutions for groups of species which are strongly coupled. The accuracy of both integration methods is evaluated by comparing the results with solutions obtained by a Gear method, the Livermore Solver for Ordinary Differential Equations (LSODE). The chemical scheme used is the Carbon-bond Mechanism IV (CBM-IV). The numerical methods are tested of three chemical scenarios: two scenarios without emissions and with constant reaction rates and one scenario with variable emissions and photodissociation rates. Using a short time step (50 s), both EBI and QSSA perform very well, even under extreme chemical conditions. For larger time steps the EBI method performs better than QSSA. In the case of more realistic chemical conditions, both methods perform well even with a time step of 900 s. The accuracy of QSSA depends highly on the iteration procedure. Without iterations the QSSA method performs poorly.

The great advantage of the EBI method is that concentrations are computed using linear operators only. Because of this, the method is mass conserving and can be used in air pollution transport models where higher moments of concentration distributions also need to be evaluated.

Both the QSSA and the EBI methods can be recommended for use in atmospheric transport-chemistry models, where accuracy as well as computational efficiency is important. In general, the new EBI method is, however, more efficient than QSSA with a constant number of iterations.