Atmospheric Environment. Part A. General Topics最新文献

During daytime, the redox cycling of dissolved iron compounds in atmospheric waters, and the related in-cloud transformations of photooxidants, are affected by reactions of Fe and Cu with hydroperoxy (HO₂) and superoxide (O₂⁻) radicals and the photoreduction of Fe(III)-oxalato complexes. We have investigated several of the important chemical reactions in this redox cycle, through laboratory simulation of the system, using γ-radiation to produce HO₂/O₂⁻. At concentrations comparable to those measured in atmospheric waters, the redox cycling of Fe was dramatically affected by the presence of oxalate and trace concentrations of Cu. At concentrations more than a hundred times lower than Fe, Cu consumed most of the HO₂/O₂⁻, and cycled between the Cu(II) and Cu(I) forms. Cu⁺ reacted with FeOH²⁺ to produce Fe(II) and Cu(II), with a second order rate constant of approximately 3 × 10⁷ M⁻¹s⁻¹. The presence of oxalate resulted in the formation of Fe(III)-oxalato complexes that were essentially unreactive with HO₂/O₂⁻. Only at high oxalate concentrations was the Fe(II)C₂O₄ complex also formed, and it reacted relatively rapidly with hydrogen peroxide (k = (3.1 ± 0.6) × 10⁴ M⁻¹s⁻¹). Simulations incorporating measurements for other redox mechanisms, including oxidation by ozone, indicate that, during daytime, Fe should be found mostly in the ferrous oxidation state, and that reactions of FeOH²⁺ with Cu(I) and HO₂/O₂⁻, and to a lesser degree, the photolysis of Fe(III)-oxalato complexes, are important mechanisms of Fe reduction in atmospheric waters. The catalytic effect of Cu(II)/Cu(I) and Fe(III)/Fe(II) should also significantly increase the sink function of the atmospheric liquid phase for HO₂ present in a cloud. A simple kinetic model for the reactions of Fe, Cu and HO₂/O₂⁻, accurately predicted the changes in Fe oxidation states that occurred when authentic fogwater samples were exposed to HO₂/O₂⁻.

Speciation results from earlier methods of quantitative analysis of TSP, dichotomous, and PM₁₀ filters for compounds by XRD were often marginal to unacceptable because of very light loadings and the interfering effects of substrate fibres in cases of ultrasonic extraction and concentration. A method for analysis of very lightly loaded PM₁₀ filters has been developed, using ultrasonic particle stripping, X-ray transmission of unloaded blank filters, substrate diffraction X-ray analysis, and mass absorption balance, along with the usual X-ray diffraction scanning. Analysis of a set of filters having less than 105 μg m cm⁻² sample load is used to illustrate the improved procedure.

The aerodynamics of a building can strongly influence the dispersion of pollutants released from nearby sources. Low releases may be entrained into the building's highly turbulent flow region and result in high pollutant concentrations on the building surface where building air intakes are located. High releases may result in increased concentrations at ground level downwind of the building as a result of the building's influence on the mean flow field. High releases at distances far upwind of the building can produce significant concentrations on the building surfaces if the building extends up into the elevated plume. Concentration measurements from a wind-tunnel study for numerous release locations upwind, above and downwind of each of four rectangular buildings are presented and compared with some previous measurements and calculations. Both building surface and ground-level values are presented. The concentrations are used to compute “building amplification factors”, which are defined as the ratios of the maximum concentration from a given source near the building to the maximum concentration from the same source in the absence of the building. This simple measure of the building's influence showed a significant influence of the building on concentrations from sources far upwind of the building, sources well above the building cavity and sources in the near wake of the building.

In air pollution models, semi-Lagrangian methods are often used to solve the advective part of the corresponding model equations. Interpolation is an essential part of these methods. In this paper, five different interpolation methods will be discussed and results of numerical experiments will be presented. To keep the concentration field non-negative, filtering techniques are used. A monotone interpolation method is also examined.

The size distribution of atmospheric secondary organic aerosol is simulated by a Lagrangian trajectory model that includes descriptions of gas-phase chemistry, inorganic and organic aerosol thermodynamics, condensation/evaporation of aerosol species, dry deposition and emission of primary gaseous and particulate pollutants. The model is applied to simulate the dynamics of aerosol size and composition along trajectories on 27–28 August 1987 during the Southern California Air Quality Study (SCAQS). The secondary organic aerosol material is predicted to condense almost exclusively on the submicron aerosol in agreement with the available measurements, and its size distribution for Claremont, CA, is predicted to be unimodal with a mass mean diameter of roughly 0.2 μm. The distributions of the various secondary organic aerosol species are predicted to be essentially the same. The secondary organic aerosol (SOA) size distribution is found to depend crucially on the mass and size distribution of primary aerosol on which the secondary species condense and on the surface accomodation coefficient of the condensable species. The SOA size distribution is predicted not to be significantly affected by diffusional dry deposition, sources and sinks of ammonia, emissions of VOC, and secondary aerosol yields from precursor hydrocarbons. A bimodal secondary organic aerosol size distribution is predicted only if the submicron primary dust particles reside mainly in the 0.5–1.0 μm diameter size range, or if the condensable species have a strong preference (an accomodation coefficient difference of two orders of magnitude) for the 0.5–1.0 μm diameter particles. The secondary organic aerosol distribution in Claremont is predicted to shift slightly towards the larger aerosol particles during the nighttime hours with it mass mean diameter peaking around midnight at 0.21 μm and having its minimum in early afternoon at 0.18 μm. In coastal locations of the Los Angeles basin, secondary organic material exists in relatively smaller particles (mass mean diameter 0.16 μm) but in far inland locations it condenses on the available larger particles (mass mean diameter 0.23 μm).

A coupled boundary-layer model and Lagrangian particle model are used to investigate the role of boundary-layer shear especially that produced by inertial oscillations in affecting the horizontal dispersion of pollutants on time-scales of 24–36 h. The coupled models show that the amplitude and the effective periods of the inertial oscillations are the main cause of nocturnal accelerating dispersion. The effective width of the plume in the morning is determined by whether the morning daytime mixing coincides with the phase of the inertial oscillation being at a maximum or minimum value. The phase of the oscillation is determined by latitude. Thus, latitude is shown to be an extremely important parameter in determining horizontal dispersion. An analytical model is introduced to investigate the role of external parameters such as latitude in influencing the horizontal dispersion. The analytical model is based on a simple Ekman-type model for the daytime and nighttime boundary layer. The Ekman model is used to provide initial conditions to an inertial oscillation regime between the nighttime boundary layer and the old daytime boundary layer. The analytical model was able to reproduce the magnitude and phase of the inertial oscillations reasonably well. However, the Ekman model overestimates the shear in the boundary layer causing the inertial oscillation to be too large. A semi-empirical method was used to provide more reasonable estimates of the daytime boundary-layer structure. This semi-empirical approach gave rates of the horizontal dispersion which were in general agreement with the numerical results.

The uptake of NO, NO₂ and O₃ by sunflowers (Helianthus annuus L. var. giganteus) and tobacco plants (Nicotiana tabacum L. var. Bel W3), using concentrations representative for moderately polluted air, has been determined by gas exchange experiments. Conductivities for these trace gases were measured at different light fluxes ranging from 820 μEm⁻²s⁻¹ to darkness. The conductivities to water vapor and the trace gases are highly correlated. It is concluded that the uptake of NO, NO₂ and O₃ by sunflowers and tobacco plants is linearly dependent on stomatal opening. While the uptake of NO is limited by the mesophyll resistance, the uptake of NO₂ is only by diffusion through the stomata. Loss processes by deposition to the leaf surfaces are more pronounced for O₃ than for NO and NO₂.