Formic and acetic acids in atmospheric condensate in Wilmington,North Carolina

Formic and acetic acids occurred in atmospheric condensate with concentrations similar to rainwater collected in Wilmington, North Carolina, during the sampling period from June to October of 1990. Atmospheric concentrations of these acids (calculated from the condensate concentrations) were higher in continental versus maritime air masses. Concentrations of formic and acetic acids were correlated with each other in both condensate and air. Traffic was a source of acetic acid and of bisulfite to atmospheric condensate in this study.     

An Intercomparison of Large-Eddy Simulations of the Stable Boundary Layer

Results are presented from the first intercomparison of large-eddy simulation (LES) models for the stable boundary layer (SBL), as part of the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study initiative. A moderately stable case is used, based on Arctic observations. All models produce successful simulations, in as much as they generate resolved turbulence and reflect many of the results from local scaling theory and observations. Simulations performed at 1-m and 2-m resolution show only small changes in the mean profiles compared to coarser resolutions. Also, sensitivity to subgrid models for individual models highlights their importance in SBL simulation at moderate resolution (6.25 m). Stability functions are derived from the LES using typical mixing lengths used in numerical weather prediction (NWP) and climate models. The functions have smaller values than those used in NWP. There is also support for the use of K-profile similarity in parametrizations. Thus, the results provide improved understanding and motivate future developments of the parametrization of the SBL.     

Climate stability and the development of agricultural societies

Although Modern Man had developed long before the migration from Africa began ～ 55,000 years ago no agricultural societies developed until about ～ 10,000 years ago. In the next 5,000 years agricultures developed independently in at least six regions of the world. It is virtually certain that it was not a chance occurrence that so many new agricultures appeared in the same 5,000 years. What inhibited agriculture world wide for 44,000 years and what changed ～ 10,000 years ago? Here we suggest that a major factor influencing the development of agricultural societies was climate stability. From the experience of four cultures we estimate that the development of agriculture needed ～ 2,000 years of climate free from significant climate variations on time scales of a few centuries. Using the Empirical Mode Decomposition technique specifically designed to exhibit the time history of the amplitude of variations in non-stationary time series such as climate proxy records, we find that between 50,000 years ago and the termination of the Younger Dryas ～ 11,600 years ago there was probably no time span as long as 2,000 years that was free of relatively large century scale variations. Furthermore variations on these time scales appear to have been relatively small since the Younger Dryas (YD) ended, supporting our proposition concerning the importance of climate stability in the history of human culture.     

Photobleaching of chromophoric dissolved organic matter (CDOM) in rainwater

Significant photodegradation of chromophoric dissolved organic matter (CDOM) in rainwater was observed after exposure to simulated sunlight. Fluorescence excitation emission spectra (EEMS) of precipitation revealed the presence of four major peaks all of which degraded upon photolysis with the greatest loss in the region characteristic of marine CDOM. Photobleaching of absorbance also occurred in the wavelength region between 250 and 375 nm with the greatest loss of absorbance in the upper end of the UV-A region near 275 nm. There was a strong positive correlation between absorbance loss and total integrated fluorescence loss suggesting these optical properties and the degree to which they are photobleached in rainwater are directly related. The quantum yield of CDOM photodegradation in rainwater decreased dramatically with increasing wavelength and decreasing energy of incoming radiation with the average quantum yield at 325 nm approximately an order of magnitude greater than at 460 nm. The similarity of photolytic response between rainwater and Cape Fear estuarine CDOM indicates that some fraction of the compounds that make up rainwater CDOM may be derived from surface sources and/or that the processes that produce or modify humic-like substances in the atmosphere result in similar types of compounds as non-atmospheric processes.     

Sensitivity of the regional climate model RegCM4.2 to planetary boundary layer parameterisation

This study investigates the performance of two planetary boundary layer (PBL) parameterisations in the regional climate model RegCM4.2 with specific focus on the recently implemented prognostic turbulent kinetic energy parameterisation scheme: the University of Washington (UW) scheme. When compared with the default Holtslag scheme, the UW scheme, in the 10-year experiments over the European domain, shows a substantial cooling. It reduces winter warm bias over the north-eastern Europe by 2 °C and reduces summer warm bias over central Europe by 3 °C. A part of the detected cooling is ascribed to a general reduction in lower tropospheric eddy heat diffusivity with the UW scheme. While differences in temperature tendency due to PBL schemes are mostly localized to the lower troposphere, the schemes show a much higher diversity in how vertical turbulent mixing of the water vapour mixing ratio is governed. Differences in the water vapour mixing ratio tendency due to the PBL scheme are present almost throughout the troposphere. However, they alone cannot explain the overall water vapour mixing ratio profiles, suggesting strong interaction between the PBL and other model parameterisations. An additional 18-member ensemble with the UW scheme is made, where two formulations of the master turbulent length scale in unstable conditions are tested and unconstrained parameters associated with (a) the evaporative enhancement of the cloud-top entrainment and (b) the formulation of the master turbulent length scale in stable conditions are systematically perturbed. These experiments suggest that the master turbulent length scale in the UW scheme could be further refined in the current implementation in the RegCM model. It was also found that the UW scheme is less sensitive to the variations of the other two selected unconstrained parameters, supporting the choice of these parameters in the default formulation of the UW scheme.     

Occurrence and fate of polycyclic aromatic hydrocarbons in the coastal surface microlayer

Polycyclic aromatic hydrocarbons (PAHs) were measured in the coastal sea surface microlayer (SML), the sub-surface waters (SSW) and the overlying atmosphere in order to investigate the influence of the SML on contaminant enrichment and air-sea exchange. Samples were collected at two contrasting locations of the NW Mediterranean, one urban influenced (off Barcelona, Spain) and another comparatively clean (off Banyuls-sur-Mer, France). Statistical data analysis confirmed the accumulation of PAHs in the SML with respect to the SSW (20.3+/-9.1 vs. 13.1+/-10.0 ng L(-1) in the dissolved phase; 709+/-207 vs. 158+/-111 ng g(-1) in the particulate phase). This accumulation was higher at the contaminated location (Barcelona station) compared with the more pristine one, with PAH enrichments 1.5 and 4.5 times higher for the dissolved and particulate phases, respectively, indicating that the enrichment of PAHs in the SML is dominated by particle transport processes.     

A hybrid artificial neural network-numerical model for ground water problems

Numerical models constitute the most advanced physical-based methods for modeling complex ground water systems. Spatial and/or temporal variability of aquifer parameters, boundary conditions, and initial conditions (for transient simulations) can be assigned across the numerical model domain. While this constitutes a powerful modeling advantage, it also presents the formidable challenge of overcoming parameter uncertainty, which, to date, has not been satisfactorily resolved, inevitably producing model prediction errors. In previous research, artificial neural networks (ANNs), developed with more accessible field data, have achieved excellent predictive accuracy over discrete stress periods at site-specific field locations in complex ground water systems. In an effort to combine the relative advantages of numerical models and ANNs, a new modeling paradigm is presented. The ANN models generate accurate predictions for a limited number of field locations. Appending them to a numerical model produces an overdetermined system of equations, which can be solved using a variety of mathematical techniques, potentially yielding more accurate numerical predictions. Mathematical theory and a simple two-dimensional example are presented to overview relevant mathematical and modeling issues. Two of the three methods for solving the overdetermined system achieved an overall improvement in numerical model accuracy for various levels of synthetic ANN errors using relatively few constrained head values (i.e., cells), which, while demonstrating promise, requires further research. This hybrid approach is not limited to ANN technology; it can be used with other approaches for improving numerical model predictions, such as regression or support vector machines (SVMs).