Volatile organic compounds at a rural site in western Senegal

The objectives of this study were to identify species and levels of volatile organic compounds (VOCs), and determine their oxidation capacity in the rural atmosphere of western Senegal. A field study was conducted to obtain air samples during September 14 and September 15, 2006 for analyses of VOCs. Methanol, acetone, and acetaldehyde were the most abundant detected chemical species and their maximum mixing ratios reached 6 parts per billion on a volume basis (ppbv). Local emission sources such as firewood and charcoal burning strongly influenced VOC concentrations. The VOC concentrations exhibited little temporal variations due to the low reactivity with hydroxyl radicals, with reactivity values ranging from 0.001 to 2.6 s⁻¹. The conditions in this rural site were rather clean. Low ambient NO _x levels limited ozone production. Nitrogen oxide (NO _x ) levels reached values less than 2 ppbv and maximum VOC/NO _x ratios reached 60 ppbvC/ppbv, with an overall average of 2.4 ± 4.5 ppbvC/ppbv. This indicates that the rural western Senegal region is NO _x limited in terms of oxidant formation potential. Therefore, during the study period photochemical ozone production became limited due to low ambient NO _x levels. The estimated ozone formation reactivity for VOCs was low and ranged between −5.5 mol of ozone/mol of benzaldehyde to 0.6 mol/mol of anthropogenic dienes.     

Controls on Boundary-Layer Thermodynamics and Dynamics in Coastal West Africa During the Rainy Season of 2006

We investigate dominant processes modulating the coastal West African atmospheric boundary layer during August and September 2006. We evaluated boundary-layer attributes using upper air soundings, tower-based observations, and information from the European Centre for Medium-Range Weather Forecasts reanalyses. Boundary-layer thermodynamics exhibited continental and maritime attributes in response to influences from regional onshore (sea to land) flows and local land–atmosphere exchanges of energy and moisture. Onshore flows transported maritime air inland and gave rise to deep (>1 km) nighttime mixed layers whose heat and moisture content resulted in maximum virtual potential temperatures of 306 K and specific humidities up to 20 g kg⁻¹. The presence of the Saharan Air Layer corresponded with capping inversions greater than 4 K and lapse rates exceeding 7 K km⁻¹ above the mixed layer. Mixed layers at these times became deeper than expected (≈1 km) because dust layer events were often concurrent with strong onshore flows. Despite diurnally variable land–atmosphere fluxes of sensible and latent heat that reached maximum values of 200 and 400 W m⁻², respectively, the mixed-layer depth exhibited little diurnal variation due to the influences of onshore flows. Daytime heating of the land, the upward transport of moisture, and onshore flows produced boundary layers with high convective available potential energy that often exceeded 3,000 J kg⁻¹. These results demonstrate that the atmospheric boundary-layer thermodynamics in western Senegal can be favorable for storm development during both day and night. Mesoscale and regional models applied in this region should include several processes controlling the boundary-layer attributes to realistically estimate the energy available for storm development.     

A Hybrid Algorithm for Spectral Analysis

In this paper we present a method that combines evolution strategies (ES) and standard optimization algorithms to solve the problem of fitting line profiles of stellar spectra. This method provides a reliable decomposition and a reduction in computing time over conventional algorithms. Using a stellar spectrum as input, we implemented an evolution strategy to find an approximation of the continuum spectrum and spectral lines. After a few generations, the parameters found by ES are given as starting search point to a standard optimization algorithm, which then finds the correct spectral decomposition. We used Gaussian functions to fit spectral lines and the Planck function to represent the continuum spectrum. Our experimental results present the application of this method to real spectra, showing that they can be approximated very accurately. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lateral Variations of Lg Coda Q in Southern Mexico

—Broad band digital three-component data recorded at UNM, a GEOSCOPE station, were used to estimate Lg coda Q for 34 medium size (3.9 ≤m _b ≤ 6.3) earthquakes with travel paths laying in different geological provinces of southern Mexico in an effort to establish the possible existence of geological structures acting as wave guides and/or travel paths of low attenuation between the Pacific coast and the Valley of Mexico. The stacked spectral ratio method proposed by XIE and NUTTLI (1988) was chosen for computing the coda Q. The variation range of Q ₀ (Q at 1?Hz) and the frequency dependence parameter η estimates averaged on the frequency interval of 0.5 to 2?Hz for the regions and the three components considered are: i) Guerrero region 173 ≤Q¯ ₀≤ 182 and 0.6 ≤Q¯ ₀ ≤ 0.7, ii) Oaxaca region 183 ≤Q¯ ₀ ≤ 198 and 0.6 ≤Q¯ ₀ ≤ 0.8, iii) Michoacan-Jalisco region 187 ≤Q¯ ₀ ≤ 204 and 0.7 ≤Q¯ ₀ ≤ 0.8 and iv) eastern portion of the Transmexican Volcanic Belt (TMVB) 313 ≤Q ₀≤ 335 and η = 0.9. ¶The results show a very high coda Q for the TMVB as compared to other regions of southern Mexico. This unexpected result is difficult to reconcile with the geophysical characteristics of the TMVB, e.g., low seismicity, high volcanic activity and high heat flow typical of a highly attenuating (low Q) region. Visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the TMVB, the amplitude decay of Lg coda is anomalously slow as compared to other earthquakes in southern Mexico. Thus, it seems that the high Q value found does not entirely reflect the attenuation characteristics of the TMVB but it is probably contaminated by a wave-guide effect. This phenomenon produces an enhancement in the time duration of the Lg wave trains travelling along this geological structure. This result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the September 19, 1985 Michoacan earthquake. ¶The overall spatial distribution of coda Q values indicates that events with focus in the Michoacan-Jalisco and Oaxaca regions yield slightly higher values than those from Guerrero. This feature is more pronounced for the horizontal component of coda Q. A slight dependence of average coda Q ^?1 on earthquake focal depth is observed in the frequency range of 0.2 to 1.0?Hz approximately on the horizontal component. Deeper (h > 50?km) events yield lower values of Q ^?1 than shallower events. For frequencies higher than 1.0?Hz no clear dependence of Q ^?1 on focal depth is observed. However, due to the estimates uncertainties this result is not clearly established.     

The complementary use of H NMR, C NMR, FTIR and size exclusion chromatography to investigate the principal structural changes associated with composting of organic materials with diverse origin

The aim of this work is to study the structural changes involved in humification processes. Total humic extracts (THE) obtained from five composted materials of diverse origin (solid wastes of wineries, solid mill olive wastes, domestic wastes, ovine manures plus straw, and a mixture of animal manures), and their corresponding initial raw fresh organic mixtures were studied using ¹³C nuclear magnetic resonance (NMR) using the cross-polarization magic angle spinning technique (CPMAS), ¹H NMR, Fourier transform infrared spectroscopy (FTIR) and high pressure size exclusion chromatography (HPSEC). One group of three humic acids extracted from soils, and a second group consisting of two reference humic acids and two reference fulvic acids (1S104H, 1R103H, 1R101F and 1R107F) obtained from the International Humic Substances Society were also characterized using these techniques, in order to compare the features of reference humic and fulvic acids with those of composted materials. Likewise, the results were compared with those obtained in previous studies, in which UV–Visible and fluorescence spectroscopies were employed to characterize the humification degree of the molecular systems.The results obtained by ¹³C CPMAS NMR, ¹H NMR and FTIR indicate that, in general, humification seems to be associated with an increase in the aromatic character of the systems, with the presence of phenol groups as principal substituents and a reduction in oxygen containing functional groups, principally carboxylic or carbonylic groups, as well as the development of molecular fractions with larger size. These results also support the suitability of UV–Visible and fluorescence spectroscopies in the assessment of the humification course of humic extracts in composting processes.     

Downward transport and modification of tropospheric ozone through moist convection

This study estimated the largely unstudied downward transport and modification of tropospheric ozone associated with tropical moist convection using a coupled meteorology-chemistry model. High-resolution cloud resolving model simulations were conducted for deep moist convection events over West Africa during August 2006 to estimate vertical transport of ozone due to convection. Model simulations realistically reproduced the characteristics of deep convection as revealed by the estimated spatial distribution of temperature, moisture, cloud reflectivity, and vertical profiles of temperature and moisture. Also, results indicated that vertical transport reduced ozone by 50% (50 parts per billion by volume, ppbv) in the upper atmosphere (12–15 km) and enhanced ozone by 39% (10 ppbv) in the lower atmosphere (<2 km). Field observations confirmed model results and indicated that surface ozone levels abruptly increased by 10–30 ppbv in the area impacted by convection due to transport by downdrafts from the upper troposphere. Once in the lower troposphere, the lifetime of ozone decreased due to enhanced dry deposition and chemical sinks. Ozone removal via dry deposition increased by 100% compared to non-convective conditions. The redistribution of tropospheric ozone substantially changed hydroxyl radical formation in the continental tropical boundary layer. Therefore, an important conclusion of this study is that the redistribution of tropospheric ozone, due to deep convection in non-polluted tropical regions, can simultaneously reduce the atmospheric loading of ozone and substantially impact the oxidation capacity of the lower atmosphere via the enhanced formation of hydroxyl radicals.     

Vertical attributes of precipitation systems in West Africa and adjacent Atlantic Ocean

Summary This study reports the findings of TRMM (Tropical Rainfall Measuring Mission) satellite data analyses undertaken to investigate differences in intensity and depth of precipitating systems in the transition region from continental to maritime environments in West Africa during the rainy season of June to September in 1998–2004. The results of this study are interpreted in the context of regional thermodynamic variables such as equivalent potential temperature and equivalent convective available potential energy to discern the processes governing storm development. Over continental West Africa, convective-type precipitating storms exhibit a substantially larger vertical extent compared to the ones over the eastern Atlantic Ocean. In contrast, the stratiform precipitating systems show similar vertical reflectivity patterns, depth and intensity over both land and adjacent ocean in West Africa. The differences in the attributes of storms, as they move from the continent to the ocean, can be partly explained in terms of the surface-atmosphere interactions that provide the necessary transports of energy and water vapor from the surface to the cloud layer. Authors’ addresses: Jose D. Fuentes, Teferi Dejene, Paolo D’Odorico, Department of Environmental Sciences, University of Virginia, 291 McCormick Road, Clark Hall, Charlottesville, VA 22904, USA; Bart Geerts, Department of Atmospheric Science, University of Wyoming, Laramie, WY 82071, USA; Everette Joseph, Department of Physics, Howard University, Washington, DC 20059, USA.