Aerosol chemistry, transport, and climatic implications during extreme biomass burning emissions over the Indo-Gangetic Plain

The large-scale emissions of airborne particulates from burning of agricultural residues particularly over the upper Indo-Gangetic Plain (IGP) have often been associated with frequent formation of haze, adverse health impacts, and modification in aerosol climatology and thereby aerosol impact on regional climate. In this study, short-term variations in aerosol climatology during extreme biomass burning emissions over the IGP were investigated. Size-segregated particulate concentration was initially measured and submicron particles (PM1.1) were found to dominate particulate mass within the fine mode (PM2.1). Particulate-bound water-soluble ions were mainly secondary in nature and primarily composed of sulfate and nitrate. There was evidence of gaseous NH3 dominating neutralization of acidic aerosol species (SO42−) in submicron particles, in contrast to crustal-dominating neutralization in coarser particulates. Diurnal variation in black carbon (BC) mass ratio was primarily influenced by regional meteorology, while gradual increase in BC concentration was consistent with the increase in Delta-C, referring to biomass burning emissions. The influence of biomass burning emissions was established using specific organic (levoglucosan), inorganic (K+ and NH4+), and satellite-based (UV aerosol index, UVAI) tracers. Levoglucosan was the most abundant species within submicron particles (649±177&thinsp;ng&thinsp;m−3), with a very high ratio (&gt;&thinsp;50) to other anhydrosugars, indicating exclusive emissions from burning of agriculture residues. Spatiotemporal distribution of aerosol and a few trace gases (CO and NO2) was evaluated using both spaceborne active and passive sensors. A significant increase in columnar aerosol loading (aerosol optical depth, AOD: 0.98) was evident, with the presence of absorbing aerosols (UVAI&thinsp;&gt;&thinsp;1.5) having low aerosol layer height ( ∼  1.5&thinsp;km). A strong intraseasonality in the aerosol cross-sectional altitudinal profile was even noted from CALIPSO, referring to the dominance of smoke and polluted continental aerosols across the IGP. A possible transport mechanism of biomass smoke was established using cluster analysis and concentration-weighted air mass back trajectories. Short-wave aerosol radiative forcing (ARF) was further simulated considering intraseasonality in aerosol properties, which resulted in a considerable increase in atmospheric ARF (135&thinsp;W&thinsp;m−2) and heating rate (4.3&thinsp;K&thinsp;day−1) during extreme biomass burning emissions compared to the non-dominating period (56&thinsp;W&thinsp;m−2, 1.8&thinsp;K&thinsp;day−1). Our analysis will be useful to improve understanding of short-term variation in aerosol chemistry over the IGP and to reduce uncertainties in regional aerosol–climate models.</p

The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence

Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.publishedVersio

Mineral dust and carbonaceous aerosols in West Africa : source assessment and characterization

International audienceAs part of the AMMA (African Monsoon Multidisciplinary Analysis) international research project, an intensive field campaign called "Special Observing Period" (SOP 0) was carried out in WestAfrica during the dry season (February 2006), near M'Bour, Senegal. The aim of the ground-based sampling experiment was to determine the chemical composition of dust and carbonaceousaerosols in the surface layer, then to investigate the main source areas influencing the chemical composition of the particles. Major elements (Al, Ca and Fe), total (TC) and black carbon (BC), and water soluble ion (Na+, K+, Mg2+, NH4+, Cl−, NO3−, and ) concentrations were measured. Total mass, number concentration and aethalometer measurements of PM-10 were also obtained. Mineraldust in the surface layer is principally present in the coarse fraction representing 75-90% of the collected mass (wt.%). Dust, suggested by backward trajectories of the air masses and supported by the variations of Ca/Al ratios, originates mainly from Northern Sahara. Particulate organic matter (POM) concentrations are more variable, but POM is mainly present in the fine fraction (up to 77 wt.%). Its presence is due to local sources such as domestic fire emissions rather than to remote sources as open-field vegetation fires in the Sahelian zone. Comparisons of Black Carbon (BC) concentrations measured with an aethalometer in the Ultra-violet and the Near-infrared wavelengths, show that POM originating from the adjacent Western African coast contains less aromatics than POM transported from the main biomass burning areas of the Sahelian zone. Thus, smouldering, the main combustion process for locally emitted carbon aerosols appears to generate less aromatic compounds than burning of vegetation. -------------------------------------------------------------------------------

Fast evolution of tropospheric Pb- and Zn-rich particles in the vicinity of a lead smelter

Dusts collected on air filters at a Pb–Zn refinery located in northern France were sampled in 1997, 1999 and 2002. The low temporal variability in major elements (Pb, Zn and S) abundances suggested chemical composition of particulate emissions was stable over time. In July 2001 and March 2002, atmospheric aerosols were sampled in the vicinity of the Pb–Zn refinery upwind and downwind from the smelters. Bulk concentrations of major elements (Al, Fe, Pb and Zn) and hydrosoluble ions (Na+, NH4+, Mg2+, K+, Ca2+, Cl– NO3– and SO42–) were, respectively, determined by atomic absorption spectrometry and ion chromatography. Elemental and molecular individual particle analyses were, respectively, performed by automated SEM–EDX and Raman microspectrometry. Continental air masses (campaign 2001) were characterized by low Na+ and high SO42–, NO3– and NH4+ contents upwind from the smelters. Individual particle analysis of Pb- and Zn-rich airborne particles collected downwind from the refinery indicated elemental associations and molecular speciation were similar to those obtained at the emission: Pb compounds were mainly identified as oxides, sulfates and oxy-sulfates whereas Zn compounds were identified as sulfides. Marine air masses (campaign 2002) were characterized by high Na+ contents upwind from the smelters. Individual particle analysis of Pb- and Zn-rich particles collected downwind from the refinery pointed out a systematic association with Na, not emitted by the refinery, suggesting internal mixing of marine aerosols with heavy-metals dusts emitted by the refinery. Such fast evolution of airborne particles chemical composition in the vicinity of the refinery was further proven by SEM–EDX and Raman microspectrometry mappings showing physical evolution by aggregation or coagulation of Zn- and Pb-rich particles with aged sea-salts

Mixing state of aerosols and direct observation of carbonaceous and marine coatings on African dust by individual particle analysis

The mixing state of aerosols collected at M'Bour, Senegal, during the Special Observing Period conducted in January–February 2006 (SOP-0) of the African Monsoon Multidisciplinary Analysis project (AMMA), was studied by individual particle analysis. The sampling location on the Atlantic coast is particularly adapted for studying the mixing state of tropospheric aerosols since it is (1) located on the path of Saharan dust plumes transported westward over the northern tropical Atlantic, (2) influenced by biomass burning events particularly frequent from December to March, and (3) strongly influenced by anthropogenic emissions from polluted African cities. Particle size, morphology, and chemical composition were determined for 12,672 particles using scanning electron microscopy (automated SEM-EDX). Complementary analyses were performed using transmission electron microscopy combined with electron energy loss spectrometry (TEM-EELS) and Raman microspectrometry. Mineral dust and carbonaceous and marine compounds were predominantly found externally mixed, i.e., not present together in the same particles. Binary internally mixed particles, i.e., dust/carbonaceous, carbonaceous/marine, and dust/marine mixtures, accounted for a significant fraction of analyzed particles (from 10.5% to 46.5%). Western Sahara was identified as the main source of mineral dust. Two major types of carbonaceous particles were identified: “tar balls” probably coming from biomass burning emissions and soot from anthropogenic emissions. Regarding binary internally mixed particles, marine and carbonaceous compounds generally formed a coating on mineral dust particles. The carbonaceous coating observed at the particle scale on African dust was evidenced by the combined use of elemental and molecular microanalysis techniques, with the identification of an amorphous rather than crystallized carbon structure