182 research outputs found
Long Term Precipitation Chemistry and Wet Deposition in a Remote Dry Savanna Site in Africa (Niger)
A long-term measurement of precipitation chemistry has been carried-out in a rural area of Banizoumbou, in the Sahel (Niger), representative of the african semi-arid savanna ecosystem. A total of 305 rainfall samples, representing 90% of the total annual rainfall, were collected with an automatic wet-only rain sampler from June 1994 to September 2005. Using ionic chromatography, pH major inorganic and organic ions were analyzed. Rainwater chemistry at the site is controlled by soil dust emissions associated to a strong terrigeneous contribution represented by SO42¿, Ca2+, Carbonates, K+ and Mg2+. Calcium and carbonates represent about 40% of the total ionic charge of precipitation. The second highest contribution is nitrogenous, with annual Volume Weighed Mean (VWM) NO3¿ and NH4+, concentrations of 11.6 and 18.1 ”eq.l-1, respectively. This is thesignature of ammonia sources related to animals and NOx emissions from savannas soils rain-induced, at the beginning of the rainy season. The mean annual NH3 and NO2 air concentration are of 6 ppbv and 2.6 ppbv, respectively. The annual VWM precipitation concentration of sodium and chloride are both of 8.7 ”eq.l-1 and reflects the marine signature from the monsoon humid air masses coming from the ocean. The mean pH value, calculated from the VWM of H+, is 5.64. Acidity is neutralized by mineral dust, mainly carbonates, and/or dissolved gases such NH3. High level of organic acidity with 8 ”eq.l-1 and 5.2 ”eq.l-1 of formate and acetate were found, respectively. The analysis of monthly Black Carbon emissions and FAPAR values show that both biogenic emission from vegetation and biomass burning sources could explain the organic acidity content of the precipitation. The interannual variability of the VWM concentrations around the mean (1994¿2005) presents fluctuations between ±5% and ±30% mainly attributed to the variations of sources strength associated with rainfall spatio-temporal distribution. From 1994 to 2005, the total mean wet deposition flux in the Sahelian region is 60.1 mmol.m-2.yr-1 and fluctuates around ±25%. Finally, Banizoumbou measurements, are compared to other long-term measurements of precipitation chemistry in the wet savanna of Lamto (CÎte d'Ivoire) and in the forested zone of Zoétélé (Cameroon). The total chemical loadings presents a strong negative gradient from the dry savanna to the forest (143.7, 100.2 to 86.6 ”eq.l¿1), associated with the gradient of terrigeneous compounds sources. The wet deposition fluxes present an opposite gradient, with 60.0 mmol.m-2.yr-1 in Banizoumbou, 108.6 mmol.m-2.yr¿1 in Lamto and 162.9 mmol.m-2.yr-1 in Zoétélé, controlled by the rainfall gradient along the ecosystems transect.JRC.DDG.H.3-Global environement monitorin
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The contribution of carbonaceous aerosols to climate change
Contribution of aerosols to climate change results from two effects: clear-sky and cloudy-sky forcing. The clear-sky climate forcing by carbonaceous aerosols from biomass burning and fossil fuel burning depends on the relative contribution of scattering and absorption by the aerosols which in turn depends on the fraction of aerosol mass associated with black carbon and its size distribution. This paper reviews estimates for the emission of carbonaceous aerosols, placing these estimates in the context of estimates for the emissions of anthropogenic and natural sulfate aerosols and natural sources of organic particulate matter. The cloudy-sky forcing from carbonaceous aerosols is difficult to estimate because, among other factors, it depends on the amount of absorption by the aerosols in the cloud. It is also highly sensitive to the assumed pre-existing, natural aerosol abundance. An upper limit for this cloudy-sky forcing is -4.4 W/m{sup 2}, but may range as low as -2.4 W/m{sup 2}, depending on background aerosol concentrations. These estimates do not yet account for absorption of radiation by black carbon associated with cloud or the presence of pre-existing dust particles
Comparison of global inventories of CO emissions from biomass burning derived from remotely sensed data
We compare five global inventories of monthly CO emissions named VGT, ATSR, MODIS, GFED3 and MOPITT based on remotely sensed active fires and/or burned area products for the year 2003. The objective is to highlight similarities and differences by focusing on the geographical and temporal distribution and on the emissions for three broad land cover classes (forest, savanna/grassland and agriculture). Globally, CO emissions for the year 2003 range between 365 Tg CO (GFED3) and 1422 Tg CO (VGT). Despite the large uncertainty in the total amounts, some common spatial patterns typical of biomass burning can be identified in the boreal forests of Siberia, in agricultural areas of Eastern Europe and Russia and in savanna ecosystems of South America, Africa and Australia. Regionally, the largest difference in terms of total amounts (CV > 100%) and seasonality is observed at the northernmost latitudes, especially in North America and Siberia where VGT appears to overestimate the area affected by fires. On the contrary, Africa shows the best agreement both in terms of total annual amounts (CV = 31%) and of seasonality despite some overestimation of emissions from forest and agriculture observed in the MODIS inventory. In Africa VGT provides the most reliable seasonality. Looking at the broad land cover types, the range of contribution to the global emissions of CO is 64â74%, 23â32% and 3â4% for forest, savanna/grassland and agriculture, respectively. These results suggest that there is still large uncertainty in global estimates of emissions and it increases if the comparison is carried by out taking into account the temporal (month) and spatial (0.5° Ă 0.5° cell) dimensions. Besides the area affected by fires, also vegetation characteristics and conditions at the time of burning should also be accurately parameterized since they can greatly influence the global estimates of CO emissions
Historical (1850-2000) gridded anthropogenic and biomass burning emissions of reactive gases and aerosols: Methodology and application
We present and discuss a new dataset of gridded emissions covering the historical period (1850-2000) in decadal increments at a horizontal resolution of 0.5 degrees in latitude and longitude. The primary purpose of this inventory is to provide consistent gridded emissions of reactive gases and aerosols for use in chemistry model simulations needed by climate models for the Climate Model Intercomparison Program #5 (CMIP5) in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). Our best estimate for the year 2000 inventory represents a combination of existing regional and global inventories to capture the best information available at this point; 40 regions and 12 sectors are used to combine the various sources. The historical reconstruction of each emitted compound, for each region and sector, is then forced to agree with our 2000 estimate, ensuring continuity between past and 2000 emissions. Simulations from two chemistry-climate models is used to test the ability of the emission dataset described here to capture long-term changes in atmospheric ozone, carbon monoxide and aerosol distributions. The simulated long-term change in the Northern mid-latitudes surface and mid-troposphere ozone is not quite as rapid as observed. However, stations outside this latitude band show much better agreement in both present-day and long-term trend. The model simulations indicate that the concentration of carbon monoxide is underestimated at the Mace Head station; however, the long-term trend over the limited observational period seems to be reasonably well captured. The simulated sulfate and black carbon deposition over Greenland is in very good agreement with the ice-core observations spanning the simulation period. Finally, aerosol optical depth and additional aerosol diagnostics are shown to be in good agreement with previously published estimates and observations
Clues for a standardised thermal-optical protocol for the assessment of organic and elemental carbon within ambient air particulate matter
Along with some research networking programmes, the European Directive
2008/50/CE requires chemical speciation of fine aerosol (PM<sub>2.5</sub>),
including elemental (EC) and organic carbon (OC), at a few rural sites in
European countries. Meanwhile, the thermal-optical technique is considered by
the European and US networking agencies and normalisation bodies as a
reference method to quantify ECâOC collected on filters. Although commonly
used for many years, this technique still suffers from a lack of
information on the comparability of the different analytical protocols
(temperature protocols, type of optical correction) currently applied in the
laboratories. To better evaluate the ECâOC data set quality and related
uncertainties, the French National Reference Laboratory for Ambient Air
Quality Monitoring (LCSQA) organised an ECâOC comparison exercise for
French laboratories using different thermal-optical methods (five laboratories
only). While there is good agreement on total carbon (TC) measurements among
all participants, some differences can be observed on the EC / TC ratio, even
among laboratories using the same thermal protocol. These results led to
further tests on the influence of the optical correction: results obtained
from different European laboratories confirmed that there were higher
differences between OC<sub>TOT</sub> and OC<sub>TOR</sub> measured with
NIOSH 5040 in comparison to EUSAAR-2. Also, striking differences between
EC<sub>TOT</sub> / EC<sub>TOR</sub> ratios can be observed when comparing
results obtained for rural and urban samples, with EC<sub>TOT</sub> being
50% lower than EC<sub>TOR</sub> at rural sites whereas it is only
20% lower at urban sites. The PM chemical composition could explain
these differences but the way it influences the ECâOC measurement is not
clear and needs further investigation. Meanwhile, some additional tests seem
to indicate an influence of oven soiling on the ECâOC measurement data
quality. This highlights the necessity to follow the laser signal decrease
with time and its impact on measurements. Nevertheless, this should be
confirmed by further experiments, involving more samples and various
instruments, to enable statistical processing. All these results provide
insights to determine the quality of ECâOC analytical methods and may
contribute to the work toward establishing method standardisation
Highly controlled, reproducible measurements of aerosol emissions from combustion of a common African biofuel source
Particulate emissions from biomass burning can both alter the atmosphere's radiative
balance and cause significant harm to human health. However, due to the large
effect on emissions caused by even small alterations to the way in which
a fuel burns, it is difficult to study particulate production of biomass
combustion mechanistically and in a repeatable manner. In order to address
this gap, in this study, small wood samples sourced from CĂŽte D'Ivoire in
West Africa were burned in a highly controlled laboratory environment. The
shape and mass of samples, available airflow and surrounding thermal
environment were carefully regulated. Organic aerosol and refractory black
carbon emissions were measured in real time using an Aerosol Mass
Spectrometer and a Single Particle Soot Photometer, respectively. This
methodology produced remarkably repeatable results, allowing aerosol
emissions to be mapped directly onto different phases of combustion.
Emissions from pyrolysis were visible as a distinct phase before flaming was
established. After flaming combustion was initiated, a black-carbon-dominant
flame was observed during which very little organic aerosol was produced,
followed by a period that was dominated by organic-carbon-producing
smouldering combustion, despite the presence of residual flaming. During
pyrolysis and smouldering, the two phases producing organic aerosol, distinct
mass spectral signatures that correspond to previously reported variations in
biofuel emissions measured in the atmosphere are found. Organic aerosol
emission factors averaged over an entire combustion event were found to be
representative of the time spent in the pyrolysis and smouldering phases,
rather than reflecting a coupling between emissions and the mass loss of the
sample. Further exploration of aerosol yields from similarly carefully
controlled fires and a careful comparison with data from macroscopic fires
and real-world emissions will help to deliver greater constraints on the
variability of particulate emissions in atmospheric systems
Summary of research paper published in atmospheric chemistry and physics titled: Spatial, temporal and source contribution assessments of black carbon over the northern interior of South Africa
Non peer reviewe
Updated African biomass burning emission inventories in the framework of the AMMA-IDAF program, with an evaluation of combustion aerosols
African biomass burning emission inventories for gaseous and particulate species have been constructed at a resolution of 1 km by 1km with daily coverage for the 2000â2007 period. These inventories are higher than the GFED2 inventories, which are currently widely in use. Evaluation specifically focusing on combustion aerosol has been carried out with the ORISAM-TM4 global chemistry transport model which includes a detailed aerosol module. This paper compares modeled results with measurements of surface BC concentrations and scattering coefficients from the AMMA Enhanced Observations period, aerosol optical depths and single scattering albedo from AERONET sunphotometers, LIDAR vertical distributions of extinction coefficients as well as satellite data. Aerosol seasonal and interannual evolutions over the 2004â2007 period observed at regional scale and more specifically at the Djougou (Benin) and Banizoumbou (Niger) AMMA/IDAF sites are well reproduced by our global model, indicating that our biomass burning emission inventory appears reasonable
Cross-hemispheric transport of Central African biomass burning pollutants: implications for downwind ozone production
Pollutant plumes with enhanced concentrations of trace gases and aerosols were observed over the southern coast of West Africa during August 2006 as part of the AMMA wet season field campaign. Plumes were observed both in the mid and upper troposphere. In this study we examined the origin of these pollutant plumes, and their potential to photochemically produce ozone (O3) downwind over the Atlantic Ocean. Their possible contribution to the Atlantic O3 maximum is also discussed. Runs using the BOLAM mesoscale model including biomass burning carbon monoxide (CO) tracers were used to confirm an origin from central African biomass burning fires. The plumes measured in the mid troposphere (MT) had significantly higher pollutant concentrations over West Africa compared to the upper tropospheric (UT) plume. The mesoscale model reproduces these differences and the two different pathways for the plumes at different altitudes: transport to the north-east of the fire region, moist convective uplift and transport to West Africa for the upper tropospheric plume versus north-west transport over the Gulf of Guinea for the mid-tropospheric plume. Lower concentrations in the upper troposphere are mainly due to enhanced mixing during upward transport. Model simulations suggest that MT and UT plumes are 16 and 14 days old respectively when measured over West Africa. The ratio of tracer concentrations at 600 hPa and 250 hPa was estimated for 14â15 August in the region of the observed plumes and compares well with the same ratio derived from observed carbon dioxide (CO2) enhancements in both plumes. It is estimated that, for the period 1â15 August, the ratio of Biomass Burning (BB) tracer concentration transported in the UT to the ones transported in the MT is 0.6 over West Africa and the equatorial South Atlantic.
Runs using a photochemical trajectory model, CiTTyCAT, initialized with the observations, were used to estimate in-situ net photochemical O3 production rates in these plumes during transport downwind of West Africa. The mid-troposphere plume spreads over altitude between 1.5 and 6 km over the Atlantic Ocean. Even though the plume was old, it was still very photochemically active (mean net O3 production rates over 10 days of 2.6 ppbv/day and up to 7 ppbv/day during the first days) above 3 km especially during the first few days of transport westward. It is also shown that the impact of high aerosol loads in the MT plume on photolysis rates serves to delay the peak in modelled O3 concentrations. These results suggest that a significant fraction of enhanced O3 in mid-troposphere over the Atlantic comes from BB sources during the summer monsoon period. According to simulated occurrence of such transport, BB may be the main source for O3 enhancement in the equatorial south Atlantic MT, at least in August 2006. The upper tropospheric plume was also still photochemically active, although mean net O3 production rates were slower (1.3 ppbv/day). The results suggest that, whilst the transport of BB pollutants to the UT is variable (as shown by the mesoscale model simulations), pollution from biomass burning can make an important contribution to additional photochemical production of O3 in addition to other important sources such as nitrogen oxides (NOx) from lightning
Soot and smoke aerosol may not warm climate
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94810/1/jgrd10551.pd
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