The atmospheric nitrogen budget depends on emission and deposition fluxes both as reduced and oxidized nitrogen compounds. In this study, a first attempt at estimating the Sahel nitrogen budget for ...the year 2006 is made, through measurements and simulations at three stations from the IDAF network situated in dry savanna ecosystems. Dry deposition fluxes are estimated from measurements of NO2, HNO3 and NH3 gaseous concentrations and from simulated dry deposition velocities, and wet deposition fluxes are calculated from NH4+ and NO3− concentrations in samples of rain. Emission fluxes are estimated including biogenic emission of NO from soils (an Artificial Neural Network module has been inserted into the ISBA-SURFEX surface model), emission of NOx and NH3 from domestic fires and biomass burning, and volatilization of NH3 from animal excreta. Uncertainties are calculated for each contribution of the budget. This study uses original and unique data from remote and hardly-ever-explored regions.The monthly evolution of oxidized N compounds shows that emission and deposition increase at the beginning of the rainy season because of large emissions of biogenic NO (pulse events). Emission of oxidized compounds is dominated by biogenic emission from soils (domestic fires and biomass burning of oxidized compounds account for 0 to 13% at the most at the annual scale, depending on the station), whereas emission of NH3 is dominated by the process of volatilization from soils. At the annual scale, the average gaseous dry deposition accounts for 47% of the total estimated deposition flux, for both oxidized and reduced compounds. The average estimated wet plus dry deposition flux in dry savanna ecosystems is 7.5±1.8 kgN ha−1 yr−1, with approximately 30% attributed to oxidized compounds, and the rest attributed to NHx. The average estimated emission flux ranges from 8.4(±3.8) to 12.4(±5.9) kgN ha−1 yr−1, dominated by NH3 volatilization (72–82%) and biogenic emission from soils (11–17%), depending on the applied volatilization rate of NH3. While larger, emission fluxes are on the same order of magnitude as deposition fluxes. The main uncertainties are linked to the NH3 emission from volatilization. When scaled up from the 3 measurement sites to the Sahelian region (12° N:18° N, 15° W:10° E), the estimated total emission ranges from 2(±0.9) to 3(±1.4) TgN yr−1, depending on the applied volatilization rate of NH3 and estimated total deposition is 1.8(±0.4) TgN yr−1. The dry savanna ecosystems of the Sahel contribute around 2% to the global (biogenic + anthropogenic) nitrogen budget.
This paper reports the first record of extreme ozone measurements in Africa. As part of the AMMA program, the ozone vertical profile recorded on 20 December over Cotonou presents exceptionally high ...ozone concentrations with up to 295 ppb at 1 km altitude. Retroplumes from the Flexpart model show that the air masses sampled at 1 km over Cotonou on this day came from the burning area situated north-east of Cotonou and passed over Lagos, Nigeria, which is highly impacted by urban pollution. We used the Master Mechanism box model to simulate the chemical composition of the plume during its transit. We find that neither the biomass burning emissions of ozone precursors nor additional urban emissions from Lagos are high enough to simulate more than 120–150 ppb of ozone. The only way to reach almost 300 ppb of ozone within a few hours is to feed the air mass with large amounts of reactive VOCs as those recorded in the vicinity of petrochemical area. Sensitivity tests show that 250–600 ppb of VOCs combined with 35–80 ppb of NOx allow the ozone concentrations to be higher than 250 ppb. Nigeria is the first African country with gas extraction and petrochemical industries, and petrochemical explosions frequently happen in the vicinity of Lagos. The hypothesis of a petrochemical explosion in this area is the most likely scenario which could explain the 295 ppb ozone maximum measured over Cotonou, downwind of Lagos.
The Gourma site in Mali is one of the three instrumented meso-scale sites deployed in West-Africa as part of the African Monsoon Multi-disciplinary Analysis (AMMA) project. Located both in the ...Sahelian zone
sensu stricto, and in the Saharo–Sahelian transition zone, the Gourma meso-scale window is the northernmost site of the AMMA-CATCH observatory reached by the West African Monsoon.
The experimental strategy includes deployment of a variety of instruments, from local to meso-scale, dedicated to monitoring and documentation of the major variables characterizing the climate forcing, and the spatio-temporal variability of surface processes and state variables such as vegetation mass, leaf area index (LAI), soil moisture and surface fluxes. This paper describes the Gourma site, its associated instrumental network and the research activities that have been carried out since 1984. In the AMMA project, emphasis is put on the relations between climate, vegetation and surface fluxes. However, the Gourma site is also important for development and validation of satellite products, mainly due to the existence of large and relatively homogeneous surfaces. The social dimension of the water resource uses and governance is also briefly analyzed, relying on field enquiry and interviews.
The climate of the Gourma region is semi-arid, daytime air temperatures are always high and annual rainfall amounts exhibit strong inter-annual and seasonal variations. Measurements sites organized along a north–south transect reveal sharp gradients in surface albedo, net radiation, vegetation production, and distribution of plant functional types. However, at any point along the gradient, surface energy budget, soil moisture and vegetation growth contrast between two main types of soil surfaces and hydrologic systems. On the one hand, sandy soils with high water infiltration rates and limited run-off support almost continuous herbaceous vegetation with scattered woody plants. On the other hand, water infiltration is poor on shallow soils, and vegetation is sparse and discontinuous, with more concentrated run-off that ends in pools or low lands within structured endorheic watersheds.
Land surface in the Gourma is characterized by rapid response to climate variability, strong intra-seasonal, seasonal and inter-annual variations in vegetation growth, soil moisture and energy balance. Despite the multi-decadal drought, which still persists, ponds and lakes have increased, the grass cover has largely recovered, and there are signs of increased tree cover at least in the low lands.
This study deals with the problem of identifying atmospheric data influenced by local emissions that can result in spikes in time series of greenhouse gases and long-lived tracer measurements. We ...considered three spike detection methods known as coefficient of variation (COV), robust extraction of baseline signal (REBS) and standard deviation of the background (SD) to detect and filter positive spikes in continuous greenhouse gas time series from four monitoring stations representative of the European ICOS (Integrated Carbon Observation System) Research Infrastructure network. The results of the different methods are compared to each other and against a manual detection performed by station managers. Four stations were selected as test cases to apply the spike detection methods: a continental rural tower of 100 m height in eastern France (OPE), a high-mountain observatory in the south-west of France (PDM), a regional marine background site in Crete (FKL) and a marine clean-air background site in the Southern Hemisphere on Amsterdam Island (AMS). This selection allows us to address spike detection problems in time series with different variability. Two years of continuous measurements of CO2, CH4 and CO were analysed. All methods were found to be able to detect short-term spikes (lasting from a few seconds to a few minutes) in the time series. Analysis of the results of each method leads us to exclude the COV method due to the requirement to arbitrarily specify an a priori percentage of rejected data in the time series, which may over- or underestimate the actual number of spikes. The two other methods freely determine the number of spikes for a given set of parameters, and the values of these parameters were calibrated to provide the best match with spikes known to reflect local emissions episodes that are well documented by the station managers. More than 96 % of the spikes manually identified by station managers were successfully detected both in the SD and the REBS methods after the best adjustment of parameter values. At PDM, measurements made by two analyzers located 200 m from each other allow us to confirm that the CH4 spikes identified in one of the time series but not in the other correspond to a local source from a sewage treatment facility in one of the observatory buildings. From this experiment, we also found that the REBS method underestimates the number of positive anomalies in the CH4 data caused by local sewage emissions. As a conclusion, we recommend the use of the SD method, which also appears to be the easiest one to implement in automatic data processing, used for the operational filtering of spikes in greenhouse gases time series at global and regional monitoring stations of networks like that of the ICOS atmosphere network.
The biogenic volatile organic compound (BVOC) composition of ambient air at a rural field site near Djougou, Benin has been studied as part of the AMMA (African Monsoon Multidisciplinary Analysis) ...project. Ambient air was sampled during day and night during the period 2 June 2006 to 13 June 2006. Gas samples from within the forest canopy and from branch and cuvette enclosure systems for four vegetation species were also obtained and emissions flux estimates made. All samples were analysed for the presence of isoprene, monoterpenes and sesquiterpenes by either gas chromatography-time of flight mass spectrometry (GC-TOF/MS) or comprehensive gas chromatography-time of flight mass spectrometry (GCxGC-TOF/MS). Concentrations of isoprene ranged from a few tens of pptV to in excess of 3000 pptV. Similar concentration ranges for certain monoterpenes were also observed. Limonene was seen at a maximum concentration in ambient air of 5000 pptV. The combination of leaf-level observations and direct analysis of dried vegetation samples suggests that emissions of terpene species from indigenous species are unlikely to account for the unexpectedly high ambient concentrations of monoterpenes. Leaf scale emission measurements and biological sample analysis indicated that \\textit{Anacardium occidentale}, a non-native crop species found throughout the tropics, was the dominant source of monoterpenes at this location. These preliminary findings suggest that activities involving species replacement have potential implications for the chemistry of the African troposphere that have not been widely considered previously.
Freshwater reservoirs are a significant source of CO2 to the atmosphere.
CO2 is known to be emitted at the reservoir surface by diffusion at the
air–water interface and downstream of dams or ...powerhouses by degassing and
along the river course. In this study, we quantified total CO2 emissions
from the Nam Theun 2 Reservoir (Lao PDR) in the Mekong River watershed. The
study started in May 2009, less than a year after flooding and just a few
months after the maximum level was first reached and lasted until the end of
2013. We tested the hypothesis that soils from the drawdown area would be a
significant contributor to the total CO2 emissions. Total inorganic carbon, dissolved and particulate organic carbon and
CO2 concentrations were measured in 4 pristine rivers of the Nam Theun
watershed, at 9 stations in the reservoir (vertical profiles) and at 16
stations downstream of the monomictic reservoir on a weekly to monthly
basis. CO2 bubbling was estimated during five field campaigns between
2009 and 2011 and on a weekly monitoring, covering water depths ranging from
0.4 to 16 m and various types of flooded ecosystems in 2012 and 2013. Three
field campaigns in 2010, 2011 and 2013 were dedicated to the soils
description in 21 plots and the quantification of soil CO2 emissions
from the drawdown area. On this basis, we calculated total CO2
emissions from the reservoir and carbon inputs from the tributaries. We
confirm the importance of the flooded stock of organic matter as a source of
carbon (C) fuelling emissions. We show that the drawdown area contributes, depending
on the year, from 40 to 75 % of total annual gross emissions in this
flat and shallow reservoir. Since the CO2 emissions from the drawdown
zone are almost constant throughout the years, the large interannual
variations result from the significant decrease in diffusive fluxes and
downstream emissions between 2010 and 2013. This overlooked pathway in terms
of gross emissions would require an in-depth evaluation for the soil organic matter and
vegetation dynamics to evaluate the actual contribution of this area in
terms of net modification of gas exchange in the footprint of the reservoir,
and how it could evolve in the future.
Methane (CH4) emissions from hydroelectric reservoirs could represent a significant fraction of global CH4 emissions from inland waters and wetlands. Although CH4 emissions downstream of ...hydroelectric reservoirs are known to be potentially significant, these emissions are poorly documented in recent studies. We report the first quantification of emissions downstream of a subtropical monomictic reservoir. The Nam Theun 2 Reservoir (NT2R), located in the Lao People's Democratic Republic, was flooded in 2008 and commissioned in April 2010. This reservoir is a trans-basin diversion reservoir which releases water into two downstream streams: the Nam Theun River below the dam and an artificial channel downstream of the powerhouse and a regulating pond that diverts the water from the Nam Theun watershed to the Xe Bangfai watershed. We quantified downstream emissions during the first 4 years after impoundment (2009–2012) on the basis of a high temporal (weekly to fortnightly) and spatial (23 stations) resolution of the monitoring of CH4 concentration. Before the commissioning of NT2R, downstream emissions were dominated by a very significant degassing at the dam site resulting from the occasional spillway discharge for controlling the water level in the reservoir. After the commissioning, downstream emissions were dominated by degassing which occurred mostly below the powerhouse. Overall, downstream emissions decreased from 10 GgCH4 yr−1 after the commissioning to 2 GgCH4 yr−1 4 years after impoundment. The downstream emissions contributed only 10 to 30 % of total CH4 emissions from the reservoir during the study. Most of the downstream emissions (80 %) occurred within 2–4 months during the transition between the warm dry season (WD) and the warm wet season (WW) when the CH4 concentration in hypolimnic water is maximum (up to 1000 µmol L−1) and downstream emissions are negligible for the rest of the year. Emissions downstream of NT2R are also lower than expected because of the design of the water intake. A significant fraction of the CH4 that should have been transferred and emitted downstream of the powerhouse is emitted at the reservoir surface because of the artificial turbulence generated around the water intake. The positive counterpart of this artificial mixing is that it allows O2 diffusion down to the bottom of the water column, enhancing aerobic methane oxidation, and it subsequently lowered downstream emissions by at least 40 %.
Surface and remote-sensing instruments deployed during ESCOMPTE experiment over the Marseille area, along the Mediterranean coast, were used to investigate the fine structure of the atmospheric ...boundary layer (ABL) during sea-breeze circulation in relation to pollutant transport and diffusion. Six sea-breeze events are analyzed with a particular focus on 25 June 2001.
Advection of cool and humid marine air over land has a profound influence on the daytime ABL characteristics. This impact decreases rapidly with the inland distance from the sea. Nearby the coast (3 km inland), the mixing height
Zi rises up to 750 m and falls down after 15:00 (UT) when the breeze flow reaches its maximum intensity. A more classical evolution of the ABL is observed at only 11-km inland where
Zi culminates in the morning and stabilizes in the afternoon at about 1000 m height.
Fine inspection of the data revealed an oscillation of the sea-breeze with a period about 2 h 47 min. This feature, clearly discernable for 3 days at least, is present in several atmospheric variables such as wind, temperature, not only at the ground but also aloft in the ABL as observed by sodar/RASS and UHF wind profilers. In particular, the mixing height
Zi deduced from UHF profilers observations is affected also by the same periodicity. This pulsated sea-breeze is observed principally above Marseille and, at the northern and eastern shores of the Berre pond.
In summary, the periodic intrusion over land of cool marine air modifies the structure of the ABL in the vicinity of the coast from the point of view of stability, turbulent motions and pollutants concentration. An explanation of the source of this pulsated sea-breeze is suggested.
Inland waters in general and freshwater reservoirs specifically are recognized as a source of CH4 into the atmosphere. Although the diffusion at the air–water interface is the most studied pathway, ...its spatial and temporal variations are poorly documented.We measured temperature and O2 and CH4 concentrations every 2 weeks for 3.5 years at nine stations in a subtropical monomictic reservoir which was flooded in 2008 (Nam Theun 2 Reservoir, Lao PDR). Based on these results, we quantified CH4 storage in the water column and diffusive fluxes from June 2009 to December 2012. We compared diffusive emissions with ebullition from Deshmukh et al. (2014) and aerobic methane oxidation and downstream emissions from Deshmukh et al. (2016).In this monomictic reservoir, the seasonal variations of CH4 concentration and storage were highly dependent on the thermal stratification. Hypolimnic CH4 concentration and CH4 storage reached their maximum in the warm dry season (WD) when the reservoir was stratified. Concentration and storage decreased during the warm wet (WW) season and reached its minimum after the reservoir overturned in the cool dry (CD) season. The sharp decreases in CH4 storage were concomitant with extreme diffusive fluxes (up to 200 mmol m-2 d-1). These sporadic emissions occurred mostly in the inflow region in the WW season and during overturn in the CD season in the area of the reservoir that has the highest CH4 storage. Although they corresponded to less than 10 % of the observations, these extreme CH4 emissions (> 5 mmol m-2 d-1) contributed up to 50 % of total annual emissions by diffusion.During the transition between the WD and WW seasons, a new emission hotspot was identified upstream of the water intake where diffusive fluxes peaked at 600 mmol m-2 d-1 in 2010 down to 200 mmol m-2 d-1 in 2012. The hotspot was attributed to the mixing induced by the water intakes (artificial mixing). Emissions from this area contributed 15–25 % to total annual emissions, although they occur in a surface area representative of less than 1 % of the total reservoir surface. We highly recommend measurements of diffusive fluxes around water intakes in order to evaluate whether such results can be generalized.
Arable soils are a significant source of nitric oxide (NO), most of which is derived from nitrogen fertilizers. Accurate estimates of NO emissions from these soils are essential to devise strategies ...to mitigate the impact of agriculture on tropospheric ozone production and destruction. This paper presents the implementation of a soil NO emissions submodel within the environmentally-orientated soil-crop model, CERES-EGC. The submodel simulates NO production via the nitrification pathway, as modulated by soil environmental drivers. The resulting model was tested with data from 4 field experiments on wheat- and maize-cropped soils representative of two agricultural regions of France, over three years, and encompassing various climatic conditions. Overall, the model provided accurate predictions of NO emissions, but shortcomings arose from an inadequate vertical distribution of N fertilizer in the soil surface. Inclusion of a 2-cm thick topsoil layer in a 'micro-layer' version of CERES-EGC gave more realistic simulations of NO emissions and under-lying microbiological process. From a statistical point of view, both versions of the model achieved a similar fit to the experimental data, with respectively a MD and a RMSE ranging from 1.8 to 6.2 g N-NO ha-¹d-¹, and from 22.8 to 25.2 g N-NO ha-¹d-¹ across the 4 experiments. The cumulative NO losses represented 1-2% of NH ₄ ⁺ fertilizer applied in the case of maize crops, and about 1% in the case of wheat crops. The 'micro-layer' version may be used for spatialized inventories of NO emissions to improve air quality prediction.
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