Catherine Liousse

Physical, chemical, and optical properties of regional hazes dominated by smoke in Brazil

Gas and particle measurements are described for optically thick regional hazes, dominated by aged smoke from biomass burning, in the cerrado and rain forested regions of Brazil. The hazes tended to be evenly mixed from the surface to the trade wind inversion at 3–4 km in altitude. The properties of aged gases and particles in the regional hazes were significantly different from those of young smoke (<4 min old). As the smoke aged, the total amount of carbon in non-methane hydrocarbon species (C<11) was depleted by about one third due to transformations into CO2, CO, and reactive molecules, and removed by dry deposition and/or by conversion to particulate matter. As the smoke particles aged, their sizes increased significantly due to coagulation and mass growth by secondary species (e.g., ammonium, organic acids and sulfate). During aging, condensation and gas-to-particle conversion of inorganic and organic vapors increased the aerosol mass by ∼20–40%. One third to one half of this mass growth likely occurred in the first few hours of aging due to the condensation of large organic molecules. The remaining mass growth was probably associated with photochemical and cloud-processing mechanisms operating over several days. Changes in particle sizes and compositions during aging had a large impact on the optical properties of the aerosol. Over a 2 to 4 day period, the fine particle mass-scattering efficiency and single-scattering albedo increased by 1 m2 g−1, and ∼0.06, respectively. Conversely, the Angstrom coefficient, backscatter ratio, and mass absorption efficiency decreased significantly with age.

Emission factors of hydrocarbons, halocarbons, trace gases and particles from biomass burning in Brazil

Airborne measurements of the emissions of gases and particles from 19 individual forest, cerrado, and pasture fires in Brazil were obtained during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) study in August-September 1995. Emission factors were determined for a number of major and minor gaseous and particulate species, including carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, methane, nonmethane hydrocarbons, halocarbons, particulate (black and organic) carbon, and particulate ionic species. The magnitude of the emission factors for gaseous species were determined primarily by the relative amounts of flaming and smoldering combustion, rather than differences in vegetation type. Hydrocarbons and halocarbons were well correlated with CO, which is indicative of emissions primarily associated with smoldering combustion. Although there was large variability between fires, higher emission factors for SO2 and NOχ were associated with an increased ratio of flaming to smoldering combustion; this could be due to variations in the amounts of sulfur and nitrogen in the fuels. Emission factors for particles were not so clearly associated with smoldering combustion as those for hydrocarbons. The emission factors measured in this study are similar to those measured previously in Brazil and Africa. However, particle emission factors from fires in Brazil appear to be roughly 20 to 40% lower than those from North American boreal forest fires.

Effects of black carbon content, particle size, and mixing on light absorption by aerosols from biomass burning in Brazil

Black carbon mass absorption efficiencies of smoke particles were measured for various types of biomass fires during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment using thermal evolution measurements for black carbon and optical absorption methods. The obtained results range between 5.2 and 19.3 m2 g−1 with an average value of 12.1±4.0 m2 g−1. Particle size distributions and optical properties were also measured to provide a full set of physical parameters for modeling calculations. Mie theory was used to model the optical properties of the particles assuming both internal and external mixtures coupling the modeling calculations with the experimental results obtained during the campaign. For internal mixing, a particle model with a layered structure consisting of an absorbing black carbon core, surrounded by a nonabsorbing shell, was assumed. Also, for internal mixing, a discrete dipole approximation code was used to simulate packed soot clusters commonly found in electron microscopy photographs of filters collected during the experiment. The modeled results for layered spheres and packed clusters explain black carbon mass absorption coefficients up to values of about 25 m2 g−1, but measurements show even higher values which were correlated with the chemical composition and characteristics of the structure of the particles. Unrealistic high values of black carbon absorption efficiencies were linked to high concentrations of K, which influence the volatilization of black carbon (BC) at lower temperatures than usual, possibly causing artifacts in the determination of BC by thermal technique. The modeling results are compared with nephelometer and light absorption measurements.

Modeling of carbonaceous particles emitted by boreal and temperate wildfires at northern latitudes

For the first time, a spatial and monthly inventory has been constructed for carbonaceous particles emitted by boreal and temperate wildfires in forests, shrublands, and grasslands, with burned area data statistics, fuel load maps, fire characteristics, and particle emission factors. The time period considered is 1960–1997, and an important year-to-year variability was observed. On average, boreal and temperate vegetation fires represent 4% of global biomass burning, but during extreme years, their contribution may reach 12%, producing 9% and 20% of black carbon (BC) and particulate organic matter (POM), respectively, emitted by worldwide fires. The North American component of the boreal forest fires (Canada and Alaska) represents 4 to 122 Gg C yr-1 of BC and 0.07 to 2.4 Tg yr-1 of POM emitted, whereas the Eurasiatic component (Russia and northern Mongolia) may vary in the 16 to 474 Gg C yr-1 range for BC and between 0.3 and 9.4 Tg yr-1 for POM, with however great uncertainty. Temperate forests in conterminous United States and Europe have a much lower contribution with an average of 11 Gg C yr-1 of BC and 0.2 Tg yr-1 of POM. Grassland fires in Mongolia represent significant BC and POM sources which may reach 62 Gg C and 0.4 Tg, respectively. Finally, an annual average of BC emissions for shrubland fires in both the Mediterranean region and California is 20 Gg C yr-1, with average POM emissions of 0.1 Tg yr-1. These source maps obtained with a high spatial resolution (lox lo) can now be added to previous ones developed for other global carbonaceous aerosol sources (fossil fuel combustion, tropical biomass burning, agricultural and domestic fires) in order to provide global maps of particulate carbon emissions. Taking into account particle injection height in relation with each type of fire, our source map is a useful tool for studying the atmospheric transport and the impact of carbonaceous aerosols in three-dimensional transport and climate models.

Particulate content of savanna fire emissions

As part of the FOS-DECAFE experiment at Lamto (Ivory Coast) in January 1991, various aerosol samples were collected at ground level near prescribed fires or under local background conditions, to characterize the emissions of particulate matter from the burning of savanna vegetation. This paper deals with total aerosol (TPM) and carbon measurements. Detailed trace element and polycyclic hydrocarbon data are discussed in other papers presented in this issue.Near the fire plumes, the aerosols from biomass burning are primarily of a carbonaceous nature (C%∼70% of the aerosol mass) and consist predominantly of submicron particles (more than 90% in mass.) They are characterized by their organic nature (black to total carbon ratio Cb/Ct in the range 3–20%) and their high potassium content (K/Cb∼0.6). These aerosols undergo aging during their first minutes in the atmosphere causing slight alterations in their size distribution and chemical composition. However, they remain enriched in potassium (K/Cb=0.21) and pyrene, a polycyclic aromatic hydrocarbon, such that both of these species may be used as tracers of savanna burning aerosols. We show that during this period of the year, the background atmosphere experiences severe pollution from both terrigenous sources and regional biomass burning (44% of the aerosol). Daynight variations of the background carbon concentrations suggest that fire ignition and spreading occur primarily during the day. Simultaneous TPM and CO2 real-time measurements point to a temporal and spatial heterogeneity of the burning so that the ratio of the above background concentrations (ΔTPM/ΔCO2) varies from 2 to 400 g/kg C. Smoldering processes are intense sources of particles but particulate emissions may also be important during the rapidly spreading heading fires in connection with the generation of heavy brown smoke. We propose emission factor values (EF) for aerosols from the savanna biomass burning aerosols: EF (TPM)=11.4±4.6 and 69±25 g/kg Cdry plant and EF(Ct)=7.4±3.4 and 56±16 g C/kg Cdry plant for flaming and smoldering processes respectively. In these estimates, the range of uncertainty is mostly due to the intra-fire variability. These values are significantly lower than those reported in the literature for the combustion of other types of vegetation. But due to the large amounts of vegetation biomass being burnt in African savannas, the annual flux of particulate carbon into the atmosphere is estimated to be of the order of 8 Tg C, which rivals particulate carbon emissions from anthropogenic activities in temperate regions.

Comparisons of techniques for measuring shortwave absorption and black carbon content of aerosols from biomass burning in Brazil

Six methods for measuring the shortwave absorption and/or black carbon (BC) content of aerosols from biomass burning were compared during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment. The methods were the optical extinction cell (OEC), integrating plate (IP), optical reflectance (OR), particle soot/absorption photometer (PSAP), thermal evolution (TE), and remote sensing (RS). Comparisons were made for individual smoke plumes and for regional hazes dominated by smoke. Taking the OEC as a primary standard, measurements of the absorption coefficient (σa) showed that the OR method had the lowest uncertainty (17%) in σa. The other optical methods had uncertainties ranging from 20 to 40%. However, with sufficient sample size, the values of σa derived from the optical methods converged to within 20% of each other. For biomass burning aerosols in regional hazes over Brazil, this led to systematic differences of ±0.02 in the values of the single-scattering albedo derived from the various in situ techniques. It was found also that the BC content of the aerosol and σa were poorly correlated. This is likely due to a large uncertainty in the BC content of the aerosol measured by TE, and/or a high variability in the mass absorption efficiency of BC in biomass burning aerosol. Hence there is a high uncertainty in inferring σa from the BC content of smoke aerosol.

Aging of savanna biomass burning aerosols: Consequences on their optical properties

During the FOS-DECAFE experiment at Lamto, Ivory Coast, in January 1991, various ground studies were undertaken simultaneously in order to investigate the physical and chemical characteristics of smoke emitted by savanna biomass burning. Here we present sunphotometer ground-based results which allow the measurements of the spectral optical depth between 450 and 850 nm, the atmospheric water vapour content and the particle size distribution spectrum. The carbonaceous content of the savanna biomass burning aerosols is also investigated. This is the first time that the physical characteristics of particles emitted by savanna plumes are obtained from ground-field studies. All the results suggest that a rapid aging of the smoke occurs first hundred metres from the savanna fire èmission source. They show a relationship between the optical properties of smoke and the chemical aging of the aerosols primarily due to particle growth and a loss of organic material relative to the black carbon content.

Contribution of the different aerosol species to the aerosol mass load and optical depth over the northeastern tropical Atlantic

Simultaneous ground-based measurements of chemical composition, size distribution, and column optical thickness at 670 nm of atmospheric aerosols have been performed at Sal Island (Cape Verde) during the winter season when African dust is transported in the lower troposphere. Mineral dust and, occasionally, sea salt dominate the aerosol mass load, whereas the excess sulfates plus the carbonaceous aerosol (particulate organic matter and black carbon) contributions to the mass load remain lower than 5% on average. We compute the total aerosol optical depth (AOD) by combining optical properties derived from measured size distributions and vertical concentration profile of each aerosol type estimated from surface elemental concentrations and meteorological observations. Results are very consistent with direct Sun photometer measurements. This allows us to derive the chemical apportionment of AOD in this region: mineral dust from Africa controls the total AOD and generally dominates AOD even in the absence of dust outbreak; on average, sea salt, excess sulfate, and carbonaceous aerosols all together only contribute to an averaged background AOD of 0.04 at 670 nm.

Deriving Global Quantitative Estimates for Spatial and Temporal Distributions of Biomass Burning Emissions

Since the 1980’s biomass burning has been recognized as a major source of global air pollution (Seiler and Crutzen, 1980; Andreae et al., 1988; Crutzen and Andreae, 1990). The majority of the emissions occur in the Tropics, due to the conjunction of anthropogenic pressure, level of development, climate, and availability of fuel. In these regions, biomass burning remains the main source for energy supply even if the contribution of fossil fuel which used to be relatively low in many countries (figure 1), has been increasing since the 1980’s (for example from 1980 to 1995 fossil fuel consumption in South Africa has doubled). Because of the intensity of photochemistry and convection in tropical latitudes, biomass burning emissions in this region have an important atmospheric chemical and radiative impact. This was pointed out by numerous studies on the tropospheric ozone budget (Andreae et al., 1988; Chatfield et al., 1996; Thompson et al., 1996; Chandra et al. 2002), on the CO2 sources and sinks (Prentice et al., 2002), and on regional and global radiation budgets (Kaufman et al., 1991; Penner et al., 1991; Cox et al. 2000; Jacobson, 2002). Recently, Wotawa and Trainer (2000) found that emissions from fires in temperate and boreal fires in the northern hemisphere may occasionally have a regional and long-range impact comparable to the emissions from fossil fuel combustion.

Emissions of Polycyclic aromatic hydrocarbons by savanna fires

Although Polycyclic aromatic hydrocarbons (PAH) are known as anthropogenic compounds arising from the combustion or the pyrolysis of fossil fuels, they may be also emitted by the combustion of vegetation. A field study was carried out in January 1991 at Lamto (Ivory Coast) as part of the FOS DECAFE experiment (Fire Of Savanna). Some ground samplings were devoted to the qualitative and quantitative characterization of atmospheric emissions by savanna fires during prescribed burns and under background conditions. Specific collections for gaseous and particulate PAHs have shown that the African practice of burning the savanna biomass during the winter months is an important source of PAHs. These compounds are emitted mainly in gaseous form but a significant fraction, essentially heavy PAHs, is associated with fine carbonaceous particles and can therefore represent a hazard for human health, since some of these compounds are mutagenic and carcinogenic. Twelve compounds were identified during the fire episodes and in the atmospheric background. The total concentration in the fires is of the order of 10 ng m−3 for the gas phase and from 0.1 to 1 ng m−3 in the aerosols. In the atmospheric background the mean concentrations are regular, 0.15 ng m−3 and 2 pg m−3, respectively. These concentrations are comparable with what is observed in European rural zones. The particulate emissions of PAHs by the savanna fires are distinguished by the abundance of some compounds which can be considered as tracers, although they are also slightly emitted by fossil fuel sources. These compounds are essentially pyrene, chrysene and coronene. In the gas phase, although no individual PAH may be considered as specific of the biomass combustion emissions, the relative abundances of the main PAHs are characteristic of the biomass burning. The concentrations of pyrene and fluorene are always predominant; these compounds could be considered as characteristic emission products of smoldering and flaming episodes, respectively. In the background the PAH composition shows that in a tropical region the air consists of a mixture coming from the various sources, but the biomass combustion is by far the most important source.The fluxes of total PAH emitted by savanna biomass burning in Africa were estimated to be of the order of 17 and 600 ton yr−1, respectively, for the particulate PAHs and the gaseous PAHs, respectively.