On efficient numerical solution of one-dimensional convection–diffusion equations in modelling atmospheric processes

A new approach is proposed to the numerical solution of one-dimensional convection–diffusion equations that arise in modelling atmospheric processes and air pollution modelling. The technique is based on upstream-type difference approximations for first-order derivatives and non-standard difference approximations for second-order derivatives of convection–diffusion equations. This approach leads to the significant qualitative improvements in the numerical solutions behaviour. The relative contribution of convection and diffusion is directly incorporated into the corresponding numerical scheme in such a way that large spatial grids can be taken without affecting solution stability. The method is compared with the contemporary computational schemes for solving problems with severe internal and boundary gradients and is shown to be stable and computationally efficient. The results of a numerical experiment are given.     

NOy speciation with a combined wet effluent diffusion denuder – aerosol collector coupled to ion chromatography

A wet effluent denuder - aerosol collector (WEDD/AC) system coupled to ion chromatography for the measurement of atmospheric HONO, HNO₃ and particulate nitrite, nitrate and sulfate is described. Several experiments were performed to outline its performance. The main features are low detection limits and a fast response to concentration changes which enables measurements with high time resolution. In contrast to highly soluble gases, the collection efficiency of less soluble gases is shown to depend on the Henry’s law constant rather than on the uptake kinetics. To improve the collection efficiency for HONO under simultaneous presence of acidifying gases, NaHCO₃ was added to the effluent solution. The system was tested in a field campaign in the suburban area of Zürich, Switzerland. Elevated concentrations of nitrous acid up to 3.2 ppb were detected during the measurement campaign. The diurnal variation of the HONO to NO₂ ratio clearly points to a fast and persistent process producing HONO in the atmosphere. The correlation with NO_x and black carbon suggests a heterogeneous formation of HONO, and is consistent with a reaction on soot aerosol particle surfaces postulated from previous laboratory results.     

A validated 2-D diffusion–advection model for prediction of drift from ground boom sprayers

Correct field drift prediction is a key element in environmental risk assessment of spraying applications. A reduced order drift prediction model based on the diffusion–advection equation is presented. It allows fast assessment of the drift potential of specific ground boom applications under specific environmental wind conditions that obey the logarithmic wind profile. The model was calibrated based on simulations with a validated Computational Fluid Dynamics (CFD) model. Validation of both models against 38 carefully conducted field experiments is successfully performed for distances up to 20 m from the field edge, for spraying on flat pasture land. The reduced order model succeeded in correct drift predictions for different nozzle types, wind velocities, boom heights and spray pressures. It used 4 parameters representing the physical aspects of the drift cloud; the height of the cloud at the field edge, the mass flux crossing the field edge, the settling velocity of the droplets and the turbulence. For the parameter set and range considered, it is demonstrated for the first time that the effect of the droplet diameter distribution of the different nozzle types on the amount of deposition spray drift can be evaluated by a single parameter, i.e., the volume fraction of droplets with a diameter smaller than 191 μm. The reduced order model can be solved more than 4 orders of magnitude faster than the comprehensive CFD model.     

Degrading a mixture of three textile dyes using photo-assisted electrochemical process with BDD anode and O2–diffusion cathode

In this paper, degradation of a mixture of three azo dyes was studied by the photo-assisted electrochemical process using an O₂-diffusion cathode containing carbon nanotubes and boron-doped diamond (BDD) anode. The concentration of three textile dyes (C.I. Acid Orange 8 (AO8), C.I. Acid Orange 10 (AO10), and C.I. Acid Orange 12 (AO12)) was determined simultaneously despite the severe overlap of their spectra. For this purpose, partial least square (PLS), as a multivariate calibration method, was utilized based on recording UV–Vis spectra during the decolorization process. Moreover, the central composite design was used for the modeling of photo-assisted electrochemical decolorization of the aqueous solutions containing three dyes. The investigated parameters were the initial concentration of three dyes, applied current and reaction time. Analysis of variance (ANOVA) revealed that the obtained regression models match the experimental results well with R (Khataee et al. 2010, Clean-Soil Air Water 38 (1):96–103, 2010) of 0.972, 0.971, and 0.957 for AO8, AO10, and AO12, respectively. Three-dimensional surface and contour plots were applied to describe the relation between experimental conditions and the observed response. The results of TOC analysis confirmed good ability of proposed photo-assisted electrochemical process for degradation and mineralization of textile industry wastewater.