ALBERT

Description: With increasing NO2 concentration in the troposphere, the importance of NO2 reaction with mineral dust in the atmosphere needs to be evaluated. Until now, little is known about the reaction of NO2 with CaCO3. In this study, the heterogeneous reaction of NO2 on the surface of CaCO3 particles was investigated at 296 K and NO2 concentrations between 4.58×1015 molecules cm−3 to 1.68×1016 molecules cm−3, using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), under wet and dry conditions. Nitrate formation was observed under both conditions, while nitrite was observed under wet conditions, indicating the reaction of NO2 on the CaCO3 surface produced nitrate and probably nitrous acid (HONO). Relative humidity (RH) influences both the initial uptake coefficient and the reaction mechanism. At low RH, surface −OH is formed through dissociation of the surface adsorbed water via oxygen vacancy, thus determining the reaction order. As RH increases, water starts to condense on the surface and the gas-liquid reaction of NO2 with the condensed water begins. With high enough RH (〉52% in our experiment), the gas-liquid reaction of NO2 with condensed water becomes dominant, forming HNO3 and HONO. The initial uptake coefficient γ0 was determined to be (4.25±1.18)×10−9 under dry conditions and up to (6.56±0.34)×10−8 under wet conditions. These results suggest that the reaction of NO2 on CaCO3 particle is unable to compete with that of HNO3 in the atmosphere. Further studies at lower NO2 concentrations and with a more accurate assessment of the surface area for calculating the uptake coefficient of the reaction of NO2 on CaCO3 particle and to examine its importance as a source of HONO in the atmosphere are needed.

Description: The Penman-Monteith (P-M) model has been applied to estimate evapotranspiration in terrestrial ecosystem throughout the world. As shown in many studies, bulk canopy resistance is an especially important factor in the application of the P-M model. In this study, the authors used the Noilhan and Planton (N-P) approach and the Jacobs and De Bruin (J-D) approach to express the bulk canopy resistance. The P-M model was applied to a maize field using the two approaches in an arid area of northwest China and evaluated on the basis of measured half-hourly values from the eddy covariance system. The results indicate that the N-P approach slightly underestimates the bulk canopy resistance, while the J-D approach overestimates it. Over the entire maize growing season, the N-P approach yielded a more consistent estimate of bulk canopy resistance than did the J-D approach. Correspondingly, the P-M model with J-D bulk canopy resistance slightly underestimated the latent heat flux throughout the maize growing season, but overestimated the latent heat flux during the dry season as compared to the N-P approach results. The good fit between the simulated latent heat flux estimated by the P-M model using the N-P approach and the data measured at half-hour time steps demonstrates that the application of this approach is reasonable in relatively homogenous maize fields that are not drought-stressed. Further research to improve the performance of P-M model to simulate evapotranspiration in the cropped fields is discussed.

Description: Digital Elevation Models (DEMs) have been successfully used in a large range of environmental issues. Several methods such as digital contour interpolation and remote sensing have allowed the generation of DEMs, some of which are now freely available for almost the entire globe. The Soil and Water Assessment Tool (SWAT) is a widely used semi-distributed model operating at the watershed level and has previously been shown to be very sensitive to the quality of the input topographic information. The objective of this study was to evaluate the impact of DEMs generated from different data sources, respectively DLG5m (local Digital Line Graph, 5 m interval), ASTER30m (1 arc-s ASTER Global DEM Version 1, approximately 30 m resolution), and SRTM90m (3 arc-s SRTM Version 4, approximately 90 m resolution), on SWAT predictions for runoff, sediment, total phosphor (TP) and total nitrogen (TN). Eleven resolutions, from 5 m to 140 m, were considered in this study. Results indicate that the predictions of TPs and TNs decreased substantially with coarser resampled resolution. Slightly decreased trends could be found in the predicted sediments when DEMs were resampled to coarser resolutions. Predicted runoffs were not sensitive to resampled resolutions. The predicted outputs based on DLG5m were more sensitive to resampled resolutions than those based on ASTER30m and SRTM90m. At original resolutions, the predicted outputs based on ASTER30m and SRTM90m were similar, but the predicted TNs and TPs based on ASTER30m and SRTM90m were much lower than the one based on DLG5m. For the predicted TNs and TPs, which were substantially sensitive to DEM resolutions, the output accuracies of SWAT derived from ASTER30m and SRTM90m could be improved by down-scaled resampling, but they could not improve on finer DEM (DLG5m) at the same resolution. This study helps GIS environmental model users to understand the sensitivities of SWAT to DEM resolution, and choose feasible DEM data for environmental models.

Description: The Penman-Monteith (P-M) model has been applied to estimate evapotranspiration in terrestrial ecosystem widely in the world. As shown in many studies, bulk canopy resistance is an especially important factor in the application of P-M model. In this study, the authors used the Noilhan and Planton (N-P) approach and Jacobs and De Bruin (J-D) approach to express the bulk canopy resistance. The application of P-M mode to a maize field with two approaches in the arid area of Northwest China was evaluated by the measured half-hourly values from the eddy covariance system. The results indicate that the N-P approach underestimates slightly the bulk canopy resistance, while the J-D approach overestimates that. The estimation of bulk canopy resistance with N-P approach was then better and more consistent than that with J-D approach during the entire maize growing season. Correspondingly, the P-M model with J-D bulk canopy resistance slightly underestimated the latent heat flux throughout the maize growing season, but overestimated the latent heat flux during the dry period of the soil as compared to that with N-P approach. The good fitness of the simulated latent heat flux by the P-M model with N-P bulk canopy resistance approach to the measured one at a half-hour time step demonstrates the application of the approach is reasonable in the relative homogenous and not drought-stressed maize fields of the arid areas during the entire growing season. Further researches are discussed on enhancing the field observation, taking the correction for atmospheric stability into estimating aerodynamic resistance, to improve the performance of P-M model to simulate evapotranspiration in the cropped fields.

Description: Hydrological processes in the karst basin are controlled by the permeable multi-media consisting of soil pores, epikarst fractures and underground conduits. Distributed modeling of hydrological dynamics in such heterogeneously hydrogeological conditions is a challenging task. Based on the multi-layer structure of the distributed hydrology-soil-vegetation model (DHSVM), a distributed hydrological model for a karst basin was developed by integrating mathematical routings of porous Darcy flow, fissure flow and underground channel flow. Specifically, infiltration and saturated flow movement within epikarst fractures are expressed by the "cubic law" equation which is associated with fractural width, direction and spacing. A typical karst basin located in Guizhou province of southwest China was selected for this hydrological simulation. The basin has detailed meteorological, soil moisture content and underground flow discharges. In addition, in situ measurements of soil properties and hydraulic conductivities were also available. Distribution of epikarst fractures was statistically generated based on fractural features data collected during field investigations. Hydraulic conductivities of epikarst fractures are estimated based on observed flow discharges as initial values. These parameters together with others are further calibrated through marching the observed and simulated soil moisture contents and underground flow discharges from the basin outlet. The results show that the new model was able to capture the sharp increase and decrease of underground streamflow hydrograph, and as such can be used to investigate hydrological effects in such rock features.