95 research outputs found
Dry deposition of nitrogen compounds (NO 2 , HNO 3 , NH 3 ), sulfur dioxide and ozone in west and central African ecosystems using the inferential method
Abstract. This work is part of the IDAF program (IGAC-DEBITS-AFRICA) and is based on the long-term monitoring of gas concentrations (1998â2007) established at seven remote sites representative of major African ecosystems. Dry deposition fluxes were estimated by the inferential method using on the one hand surface measurements of gas concentrations (NO2, HNO3, NH3, SO2 and O3) and on the other hand modeled exchange rates. Dry deposition velocities (Vd) were calculated using the big-leaf model of Zhang et al. (2003b). The bidirectional approach is used for NH3 surfaceâatmosphere exchange (Zhang et al., 2010). Surface and meteorological conditions specific to IDAF sites have been used in the models of deposition. The seasonal and annual mean variations of gaseous dry deposition fluxes (NO2, HNO3, NH3, O3 and SO2) are analyzed. Along the latitudinal transect of ecosystems, the annual mean dry deposition fluxes of nitrogen compounds range from â0.4 to â0.8 kg N haâ1 yrâ1 for NO2, from â0.7 to â1.0 kg N haâ1 yrâ1 for HNO3 and from â0.7 to â8.3 kg N haâ1 yrâ1 for NH3 over the study period (1998â2007). The total nitrogen dry deposition flux (NO2+HNO3+NH3) is more important in forests (â10 kg N haâ1 yrâ1) than in wet and dry savannas (â1.6 to â3.9 kg N haâ1 yrâ1). The annual mean dry deposition fluxes of ozone range between â11 and â19 kg haâ1 yrâ1 in dry and wet savannas, and â11 and â13 kg haâ1 yrâ1 in forests. Lowest O3 dry deposition fluxes in forests are correlated to low measured O3 concentrations, lower by a factor of 2â3, compared to other ecosystems. Along the ecosystem transect, the annual mean of SO2 dry deposition fluxes presents low values and a small variability (â0.5 to â1 kg S haâ1 yrâ1). No specific trend in the interannual variability of these gaseous dry deposition fluxes is observed over the study period
The influence of biogenic emissions from Africa on tropical tropospheric ozone during 2006: a global modeling study
We have performed simulations using a 3-D global chemistry-transport model to investigate the influence that biogenic emissions from the African continent exert on the composition of the troposphere in the tropical region. For this purpose we have applied two recently developed biogenic emission inventories provided for use in large-scale global models (Granier et al., 2005; LathiSre et al., 2006) whose seasonality and temporal distribution for biogenic emissions of isoprene, other volatile organic compounds and NO is markedly different. The use of the 12 year average values for biogenic emissions provided by LathiSre et al. (2006) results in an increase in the amount of nitrogen sequestrated into longer lived reservoir compounds which contributes to the reduction in the tropospheric ozone burden in the tropics. The associated re-partitioning of nitrogen between PAN, HNO3 and organic nitrates also results in a similar to 5% increase in the loss of nitrogen by wet deposition. At a global scale there is a reduction in the oxidizing capacity of the model atmosphere which increases the atmospheric lifetimes of CH4 and CO by similar to 1.5% and similar to 4%, respectively. Comparisons against a range of different measurements indicate that applying the 12 year average of LathiSre et al. (2006) improves the performance of TM4_AMMA for 2006 in the tropics. By the use of sensitivity studies we show that the release of NO from soils in Africa accounts for between similar to 2-45% of tropospheric ozone in the African troposphere, similar to 10% in the upper troposphere and between similar to 5-20% of the tropical tropospheric ozone column over the tropical Atlantic Ocean. The subsequent reduction in OH over the source regions allows enhanced transport of CO out of the region. For biogenic volatile organic C1 to C3 species released from Africa, the effects on tropical tropospheric ozone are rather limited, although this source contributes to the global burden of VOC by between similar to 2-4% and has a large influence on the organic composition of the troposphere over the tropical Atlantic Ocean
the Creative Commons Attribution 3.0 License. Atmospheric Chemistry and Physics
The influence of biogenic emissions from Africa on tropical tropospheric ozone during 2006: a global modeling stud
Satellite evidence of substantial rain-induced soil emissions of ammonia across the Sahel
Atmospheric ammonia (NH3) is a precursor to fine particulate matter
formation and contributes to nitrogen (N) deposition, with potential
implications for the health of humans and ecosystems. Agricultural soils and
animal excreta are the primary source of atmospheric NH3, but natural
soils can also be an important emitter. In regions with distinct dry and wet
seasons such as the Sahel, the start of the rainy season triggers a pulse of
biogeochemical activity in surface soils known as the Birch effect, which is
often accompanied by emissions of microbially produced gases such as carbon
dioxide and nitric oxide. Field and lab studies have sometimes, but not
always, observed pulses of NH3 after the wetting of dry soils;
however, the potential regional importance of these emissions remains poorly
constrained. Here we use satellite retrievals of atmospheric NH3
using the Infrared Atmospheric Sounding Interferometer (IASI)
regridded at 0.25° resolution, in combination with satellite-based
observations of precipitation, surface soil moisture, and nitrogen dioxide
concentrations, to reveal substantial precipitation-induced pulses of
NH3 across the Sahel at the onset of the rainy season in 2008. The
highest concentrations of NH3 occur in pulses during March and April
when NH3 biomass burning emissions estimated for the region are low.
For the region of the Sahel spanning 10 to 16° N and 0 to
30° E, changes in NH3 concentrations are weakly but
significantly correlated with changes in soil moisture during the period from
mid-March through April when the peak NH3 concentrations occur
(râ=â0.28, pâ=â0.02). The correlation is also present when evaluated on an
individual pixel basis during April (râ=â0.16, pâ<â0.001). Average emissions
for the entire Sahel from a simple box model are estimated to be between 2
and 6 mg NH3 mâ2 dâ1 during peaks of the observed
pulses, depending on the assumed effective NH3 lifetime. These early
season pulses are consistent with surface observations of monthly
concentrations, which show an uptick in
NH3 concentration at the start
of the rainy season for sites in the Sahel. The NH3 concentrations in
April are also correlated with increasing tropospheric NO2
concentrations observed by the Ozone Monitoring Instrument (râ=â0.78, pâ<â0.0001), which have previously been attributed to the Birch effect. Box
model results suggest that pulses occurring over a 35-day period in March and
April are responsible for roughly one-fifth of annual emissions of
NH3-N from the Sahel. We conclude that precipitation early in the
rainy season is responsible for substantial NH3 emissions in the
Sahel, likely representing the largest instantaneous fluxes of gas-phase N
from the region during the year.</p
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
Spatial and temporal assessment of organic and black carbon at four sites in the interior of South Africa
Limited data currently exist for atmospheric organic carbon (OC) and black carbon (BC) in South Africa (SA). In this paper OC and BC measured in SA were explored in terms of spatial and temporal patterns, mass fractions of the total aerosol mass, as well as possible sources. PM10 and PM2.5 samples were collected at five sites in SA operated within the Deposition of Biogeochemical Important Trace Species-IGAC DEBITS in Africa (DEBITS-IDAF) network. OC were higher than BC concentrations at all sites in both size fractions, while most OC and BC occurred in the PM2.5 fraction. OC/BC ratios reflected the location of the different sites, as well as possible sources impacting these sites. The OC and BC mass fraction percentages of the total aerosol mass varied up to 24% and 12%, respectively. A relatively well defined seasonal pattern was observed, with higher OC and BC measured from May to October, which coincides with the dry season in the interior of SA. An inverse seasonal pattern was observed for the fractional mass contributions of OC and BC to the total aerosol mass, which indicates substantially higher aerosol load during this time of the year. The relationship between OC and BC concentrations with the distance that air mass back trajectories passed by biomass burning fires and large point sources proved that biomass burning fires contribute significantly to regional OC and BC during the burning season, while large point sources did not contribute that significantly to regional OC and BC. The results from a highly industrialised and populated site also indicated that household combustion for space heating contributed at least to local OC and BC concentrations
Trends and seasonal variability in ammonia across major biomes in western and central Africa inferred from long-term series of ground-based and satellite measurements
Ammonia (NH3) is the most abundant alkaline
component in the atmosphere. Changes in NH3 concentrations have
important implications for atmospheric chemistry, air quality, and ecosystem
integrity. We present a long-term ammonia (NH3) assessment in the
western and central African regions within the framework of the International
Network to study Deposition and Atmospheric chemistry in Africa (INDAAF)
programme. We analyse seasonal variations and trends in NH3
concentrations and total column densities along an African ecosystem
transect spanning dry savannas in Banizoumbou, Niger, and Katibougou, Mali;
wet savannas in Djougou, Benin, and Lamto, CÎte d'Ivoire; and forests in Bomassa, Republic of the Congo, and Zoétélé, Cameroon. We use a
21-year record of observations (1998â2018) from INDAAF passive samplers and an 11-year record of observations (2008â2018) of atmospheric vertical column densities from the Infrared Atmospheric Sounding Interferometer (IASI) to evaluate NH3 ground-based concentrations and total column densities,
respectively. Climatic data (air temperature, rainfall amount, and leaf area
index), as well as ammonia emission data of biomass combustion from the
fourth version of the Global Fire Emissions Database (GFED4) and
anthropogenic sources from the Community Emissions Data System (CEDS), were
compared with total NH3 concentrations and total columns over the same
periods. Annual mean ground-based NH3 concentrations are around 5.7â5.8âppb in dry savannas, 3.5â4.7âppb in wet savannas, and 3.4â5.6âppb in forests. Annual IASI NH3 total column densities are 10.0â10.7âĂâ1015âmolec.âcmâ2 in dry savanna, 16.0â20.9âĂâ1015âmolec.âcmâ2 in wet
savanna, and 12.4â13.8âĂâ1015âmolec.âcmâ2 in forest stations. Non-parametric statistical MannâKendall trend tests applied to annual data show that ground-based NH3 concentrations increase at Bomassa (+2.56â%âyrâ1) but decrease at ZoĂ©tĂ©lĂ© (â2.95â%âyrâ1) over the 21-year period. The 11-year period of IASI NH3 total column density measurements show yearly increasing trends at Katibougou (+3.46â%âyrâ1), Djougou (+2.24â%âyrâ1), and ZoĂ©tĂ©lĂ© (+3.42â%âyrâ1). From the outcome of our
investigation, we conclude that air temperature, leaf area index, and
rainfall combined with biomass burning, agricultural, and residential
activities are the key drivers of atmospheric NH3 in the INDAAF
stations. The results also show that the drivers of trends are (1)Â agriculture in the dry savanna of Katibougou; (2)Â air temperature and agriculture in the wet savanna of Djougou and Lamto; and (3)Â leaf area index, air temperature, residential, and agriculture in the forest of Bomassa.</p
Global and regional trends of atmospheric sulfur
The profound changes in global SO2 emissions over the last decades have affected atmospheric composition on a regional and global scale with large impact on air quality, atmospheric deposition and the radiative forcing of sulfate aerosols. Reproduction of historical atmospheric pollution levels based on global aerosol models and emission changes is crucial to prove that such models are able to predict future scenarios. Here, we analyze consistency of trends in observations of sulfur components in air and precipitation from major regional networks and estimates from six different global aerosol models from 1990 until 2015. There are large interregional differences in the sulfur trends consistently captured by the models and observations, especially for North America and Europe. Europe had the largest reductions in sulfur emissions in the first part of the period while the highest reduction came later in North America and East Asia. The uncertainties in both the emissions and the representativity of the observations are larger in Asia. However, emissions from East Asia clearly increased from 2000 to 2005 followed by a decrease, while in India a steady increase over the whole period has been observed and modelled. The agreement between a bottom-up approach, which uses emissions and process-based chemical transport models, with independent observations gives an improved confidence in the understanding of the atmospheric sulfur budget
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