Properties Of Biomass Burning Aerosols Research Articles

The hygroscopic properties of biomass burning aerosol from Eucalyptus and cow dung under different combustion conditions

Aerosols hygroscopic growth is of current interest due to its effect on atmospheric light scattering and extinction. Knowledge of hygroscopic properties is essential for predicting the effect of aerosol on cloud formation, regional visibility, climate impact, and lung deposition. Previous studies have used surrogate components of complex aerosol; and the application to complex multi-component aerosol like biomass burning aerosol is limited. We report aerosol hygroscopicity parameter of biomass burning aerosol derived from Eucalyptus and cow dung in a humidified smog chamber using two techniques: (1) measuring extinction hygroscopic enhancement factor f(RH) of aerosol using a cavity ring down spectrometer and deriving the hygroscopicity parameter using empirical equations and (2) directly using a Cloud Condensation Nuclei Counter (CCNC). Based on repeated measurements of the same fuel under different burning conditions, quantified by the modified combustion efficiency (MCE), we found that aerosol hygroscopicity depends on both fuel types and to some extent on burning conditions. For similar MCE, cow dung has a consistently larger value of hygroscopicity compared to Eucalyptus. Hygroscopicity measurements ranged from 0.024 to 0.577 for MCE measurements that ranged from 0.73 to 0.99. Specifically, sub-saturated hygroscopicity parameter values for cow dung are 0.024–0.131 and Eucalyptus are 0.012–0.300, while supersaturated hygroscopicity parameter values for cow dung are 0.071–0.577 and Eucalyptus are 0.021–0.494. There is a clear increase in hygroscopicity with increasing MCE for supersaturated cow dung measurements using CCNC but the dependence of kappa on MCE is generally inconclusive.

Optical properties of biomass burning aerosol during the 2021 Oregon fire season: comparison between wild and prescribed fires

The Mt. Bachelor Observatory was frequently impacted by biomass burning smoke in 2021, an extreme forest fire year in the state of Oregon.

Spring 2020 Atmospheric Aerosol Contamination over Kyiv City

Extraordinarily high aerosol contamination was observed in the atmosphere over the city of Kyiv, Ukraine, during the March–April 2020 period. The source of contamination was the large grass and forest fires in the northern part of Ukraine and the Kyiv region. The level of PM2.5 load was investigated using newly established AirVisual sensor mini-networks in five areas of the city. The aerosol data from the Kyiv AERONET sun-photometer site were analyzed for that period. Aerosol optical depth, Ångström exponent, and the aerosol particles properties (particle size distribution, single-scattering albedo, and complex refractive index) were analyzed using AERONET sun-photometer observations. The smoke particles observed at Kyiv site during the fires in general correspond to aerosol with optical properties of biomass burning aerosol. The variability of the optical properties and chemical composition indicates that the aerosol particles in the smoke plumes over Kyiv city were produced by different burning materials and phases of vegetation fires at different times. The case of enormous PM2.5 aerosol contamination in the Kyiv city reveals the need to implement strong measures for forest fire control and prevention in the Kyiv region, especially in its northwest part, where radioactive contamination from the Chernobyl disaster is still significant.

Linking the chemical composition and optical properties of biomass burning aerosols in Amazonia

The absorption efficiency of chemically resolved organic aerosols of Southwestern Amazonia showed strong spectral dependence and significant light absorption in the UV-vis wavelength range indicating the presence of organic species that act as BrC.

Impact of combustion conditions on physical and morphological properties of biomass burning aerosol

The study of biomass burning particle density provides information on aging, new particle formation, transport properties, and is an important parameter in aerosol impacts modeling. Density is used...

Study of Chemical and Optical Properties of Biomass Burning Aerosols during Long-Range Transport Events toward the Arctic in Summer 2017

Biomass burning related aerosol episodes are becoming a serious threat to the radiative balance of the Arctic region. Since early July 2017 intense wildfires were recorded between August and September in Canada and Greenland, covering an area up to 4674 km2 in size. This paper describes the impact of these biomass burning (BB) events measured over Svalbard, using an ensemble of ground-based, columnar, and vertically-resolved techniques. BB influenced the aerosol chemistry via nitrates and oxalates, which exhibited an increase in their concentrations in all of size fractions, indicating the BB origin of particles. The absorption coefficient data (530 nm) at ground reached values up to 0.6 Mm–1, highlighting the impact of these BB events when compared to average Arctic background values, which do not exceed 0.05 Mm–1. The absorption behavior is fundamental as implies a subsequent atmospheric heating. At the same time, the AERONET Aerosol Optical Depth (AOD) data showed high values at stations located close to or in Canada (AOD over 2.0). Similarly, increased values of AODs were then observed in Svalbard, e.g., in Hornsund (daily average AODs exceeded 0.14 and reached hourly values up to 0.5). Elevated values of AODs were then registered in Sodankylä and Andenes (daily average AODs exceeding 0.150) a few days after the Svalbard observation of the event highlighting the BB columnar magnitude, which is crucial for the radiative impact. All the reported data suggest to rank the summer 2017 plume of aerosols as one of the biggest atmosphere related environmental problems over Svalbard region in last 10 years.

Biomass burning aerosol characteristics for different vegetation types in different aging periods

Eighteen years of sun/sky photometer measurements at seven worldwide AErosol RObotic NETwork (AERONET) sites in typical biomass burning regions were used in this research. The AERONET measurements were analyzed with the help of Moderate-resolution Imaging Spectroradiometer (MODIS) fire products and the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The variation in the physicochemical and optical properties of biomass burning aerosols (BBAs), as well as their shortwave radiative forcing, was revealed for different vegetation types in different aging periods. The result indicated that, with aerosol aging, the BBA characteristics have a non-negligible evolution trend with obvious clustering features for different burning vegetation types. During the aging process, the volume fraction of black carbon (BC) declined (with a maximum drop of 38%) accompanied by particle size growth (with a maximum increment of 0.017 μm). Driven by the change in physicochemical properties, the Single Scattering Albedo (SSA) and the asymmetry factor increased as the aerosol aged (with maximum increments of 0.026 and 0.018 for the SSA and asymmetry factor respectively). The grass and shrub type had a higher volume fraction of BC (2.5 times higher than that in the forest and peat type) and a smaller fine mode volume median radius (with a difference of 0.037 μm from that of the forest and peat type). Such a phenomenon results in a lower SSA (with a difference of 0.103) and asymmetry factor (with a difference of 0.035) in the grass and shrub type when compared to the forest and peat type. Negative (−74 to −30 W/m2) clear-sky top of atmosphere (TOA) shortwave radiative forcing, strengthened during the aging process, was generally found for BBA. The BBA in the forest and peat region usually had stronger negative radiative forcing efficiency.

Size-resolved chemical composition, effective density, and optical properties of biomass burning particles

Abstract. Biomass burning aerosol has an important impact on the global radiative budget. A better understanding of the correlations between the mixing states of biomass burning particles and their optical properties is the goal of a number of current studies. In this work, the effective density, chemical composition, and optical properties of rice straw burning particles in the size range of 50–400 nm were measured using a suite of online methods. We found that the major components of particles produced by burning rice straw included black carbon (BC), organic carbon (OC), and potassium salts, but the mixing states of particles were strongly size dependent. Particles of 50 nm had the smallest effective density (1.16 g cm−3) due to a relatively large proportion of aggregate BC. The average effective densities of 100–400 nm particles ranged from 1.35 to 1.51 g cm−3 with OC and inorganic salts as dominant components. Both density distribution and single-particle mass spectrometry showed more complex mixing states in larger particles. Upon heating, the separation of the effective density distribution modes confirmed the external mixing state of less-volatile BC or soot and potassium salts. The size-resolved optical properties of biomass burning particles were investigated at two wavelengths (λ = 450 and 530 nm). The single-scattering albedo (SSA) showed the lowest value for 50 nm particles (0.741 ± 0.007 and 0.889 ± 0.006) because of the larger proportion of BC content. Brown carbon played an important role for the SSA of 100–400 nm particles. The Ångström absorption exponent (AAE) values for all particles were above 1.6, indicating the significant presence of brown carbon in all sizes. Concurrent measurements in our work provide a basis for discussing the physicochemical properties of biomass burning aerosol and its effects on the global climate and atmospheric environment.

Chemical and physical properties of biomass burning aerosols and their CCN activity: A case study in Beijing, China.

Biomass burning emits large amounts of both trace gases and particles into the atmosphere. It plays a profound role in regional air quality and climate change. In the present study, an intensive campaign was carried out at an urban site in Beijing, China, in June 2014, which covered the winter wheat harvest season over the North China Plain (NCP). Meanwhile, two evident biomass-burning events were observed. A clear burst in ultrafine particles (below 100nm in diameter, PM1) and subsequent particle growth took place during the events. With the growth of the ultrafine particles, the organic fraction of PM1 increased significantly. The ratio of oxygen to carbon (O:C), which had an average value of 0.23±0.04, did not show an obvious enhancement, indicating that a significant chemical aging process of the biomass-burning aerosols was not observed during the course of events. This finding might have been due to the fact that the biomass-burning events occurred in the late afternoon and grew during the nighttime, which is associated with a low atmospheric oxidation capacity. On average, organics and black carbon (BC) were dominant in the biomass-burning aerosols, accounting for 60±10% and 18±3% of PM1. The high organic and BC fractions led to a significant suppression of particle hygroscopicity. Comparisons among hygroscopicity tandem differential mobility analyzer (HTDMA)-derived, cloud condensation nuclei counter (CCNc)-derived, and aerosol mass spectrometer-based hygroscopicity parameter (κ) values were consistent. The mean κ values of biomass-burning aerosols derived from both HTDMA and CCNc measurements were approximately 0.1, regardless of the particle size, indicating that the biomass-burning aerosols were less active. The burst in particle count during the biomass-burning events resulted in an increased number of cloud condensation nuclei (CCN) at supersaturation (SS)=0.2-0.8%.

Ozonolysis of surface-adsorbed methoxyphenols: kinetics of aromatic ring cleavage vs. alkene side-chain oxidation

Abstract. Lignin pyrolysis products, which include a variety of substituted methoxyphenols, constitute a major component of organics released by biomass combustion, and may play a central role in the formation of atmospheric brown carbon. Understanding the atmospheric fate of these compounds upon exposure to trace gases is therefore critical to predicting the chemical and physical properties of biomass burning aerosol. We used diffuse reflectance infrared spectroscopy to monitor the heterogeneous ozonolysis of 4-propylguaiacol, eugenol, and isoeugenol adsorbed on NaCl and α-Al2O3 substrates. Adsorption of gaseous methoxyphenols onto these substrates produced near-monolayer surface concentrations of 3 × 1018 molecules m−2. The subsequent dark heterogeneous ozonolysis of adsorbed 4-propylguaiacol cleaved the aromatic ring between the methoxy and phenol groups with the product conclusively identified by GC-MS and 1H-NMR. Kinetic analysis of eugenol and isoeugenol dark ozonolysis also suggested the formation of ring-cleaved products, although ozonolysis of the unsaturated substituent groups forming carboxylic acids and aldehydes was an order of magnitude faster. Average uptake coefficients for NaCl-adsorbed methoxyphenols were γ = 2.3 (± 0.8) × 10−7 and 2 (± 1) × 10−6 for ozonolysis of the aromatic ring and the unsaturated side chain, respectively, and reactions on α-Al2O3 were approximately two times slower. UV–visible radiation (λ > 300 nm) enhanced eugenol ozonolysis of the aromatic ring by a factor of 4(± 1) but had no effect on ozonolysis of the alkene side chain.

Forest fire smoke layers observed in the free troposphere over Portugal with a multiwavelength Raman lidar: optical and microphysical properties.

Vertically resolved optical and microphysical properties of biomass burning aerosols, measured in 2011 with a multiwavelength Raman lidar, are presented. The transportation time, within 1-2 days (or less), pointed towards the presence of relatively fresh smoke particles over the site. Some strong layers aloft were observed with particle backscatter and extinction coefficients (at 355 nm) greater than 5 Mm−1 sr−1 and close to 300 Mm−1, respectively. The particle intensive optical properties showed features different from the ones reported for aged smoke, but rather consistent with fresh smoke. The Ångström exponents were generally high, mainly above 1.4, indicating a dominating accumulation mode. Weak depolarization values, as shown by the small depolarization ratio of 5% or lower, were measured. Furthermore, the lidar ratio presented no clear wavelength dependency. The inversion of the lidar signals provided a set of microphysical properties including particle effective radius below 0.2 μm, which is less than values previously observed for aged smoke particles. Real and imaginary parts of refractive index of about 1.5-1.6 and 0.02i, respectively, were derived. The single scattering albedo was in the range between 0.85 and 0.93; these last two quantities indicate the nonnegligible absorbing characteristics of the observed particles.

Retrieval of aerosol microphysical and optical properties above liquid clouds from POLDER/PARASOL polarization measurements

Abstract. Most of the current aerosol retrievals from passive sensors are restricted to cloud-free scenes, which strongly reduces our ability to monitor the aerosol properties at a global scale and to estimate their radiative forcing. The presence of aerosol above clouds (AAC) affects the polarized light reflected by the cloud layer, as shown by the spaceborne measurements provided by the POlarization and Directionality of Earth Reflectances (POLDER) instrument on the PARASOL satellite. In a previous work, a first retrieval method was developed for AAC scenes and evaluated for biomass-burning aerosols transported over stratocumulus clouds. The method was restricted to the use of observations acquired at forward scattering angles (90–120°) where polarized measurements are highly sensitive to fine-mode particle scattering. Non-spherical particles in the coarse mode, such as mineral dust particles, do not much polarize light and cannot be handled with this method. In this paper, we present new developments that allow retrieving also the properties of mineral dust particles above clouds. These particles do not much polarize light but strongly reduce the polarized cloud bow generated by the liquid cloud layer beneath and observed for scattering angles around 140°. The spectral attenuation can be used to qualitatively identify the nature of the particles (i.e. accumulation mode versus coarse mode, i.e. mineral dust particles versus biomass-burning aerosols), whereas the magnitude of the attenuation is related to the optical thickness of the aerosol layer. We also use the polarized measurements acquired in the cloud bow to improve the retrieval of both the biomass-burning aerosol properties and the cloud microphysical properties. We provide accurate polarized radiance calculations for AAC scenes and evaluate the contribution of the POLDER polarization measurements for the simultaneous retrieval of the aerosol and cloud properties. We investigate various scenes with mineral dust particles and biomass-burning aerosols above clouds. For clouds, our results confirm that the droplet size distribution is narrow in high-latitude ocean regions and that the droplet effective radii retrieved from both polarization measurements and from total radiance measurements are generally close for AAC scenes (departures smaller than 2 μm). We found that the magnitude of the primary cloud bow cannot be accurately estimated with a plane parallel transfer radiative code. The errors for the modeling of the polarized cloud bow are between 4 and 8% for homogenous cloudy scenes, as shown by a 3-D radiative transfer code. These effects only weakly impact the retrieval of the Aerosol Optical Thickness (AOT) performed with a mineral dust particle model for which the microphysical properties are entirely known (relative error smaller than 6%). We show that the POLDER polarization measurements allow retrieving the AOT, the fine-mode particle size, the Ångström exponent and the fraction of spherical particles. However, the complex refractive index and the coarse-mode particle size cannot be accurately retrieved with the present polarization measurements. Our complete and accurate algorithm cannot be applied to process large amounts of data, so a simpler algorithm was developed to retrieve the AOT and the Ångström exponent above clouds in an operational way. Illustrations are provided for July–August 2008 near the African coast. Large mean AOTs above clouds at 0.865 μm (>0.3) are retrieved for oceanic regions near the coasts of South Africa that correspond to biomass-burning aerosols, whereas even larger mean AOTs above clouds for mineral dust particles (>0.6) are also retrieved near the coasts of Senegal. For these regions and time period, the direct AAC radiative forcing is likely to be significant. The final aim of this work is the global monitoring of the AAC properties and the estimation of the direct aerosol radiative forcing in cloudy scenes.

Aircraft measurements of biomass burning aerosol over West Africa during DABEX

This paper investigates the properties of biomass burning aerosols over West Africa using data from the UK FAAM aircraft during the Dust and Biomass‐burning Experiment (DABEX). Aged biomass burning aerosols were widespread across the region, often at altitudes up to 4 km. Fresh biomass burning aerosols were observed at low altitudes by flying through smoke plumes from agricultural fires. The aircraft measured aerosol size distributions, optical properties, and vertical distributions. Single scattering albedo varied from 0.73 to 0.93 (at 0.55 μm) in aerosol layers dominated by biomass burning aerosol. We attribute much of this variation to the variable proportion of mineral dust and biomass burning aerosol. We estimate the single scattering albedo of aged biomass burning aerosol to be around 0.81 with an instrumental uncertainty of ±0.05. External mixing, and possibly internal mixing, between the biomass burning aerosol and mineral dust presents an additional source of uncertainty in this estimate. The size distributions of biomass burning aerosols were dominated by particles with radii smaller than 0.35 μm. A 20% increase of count mean radius was observed when contrasting fresh and aged biomass burning aerosols, accompanied by changes in the shape of the size distribution. These changes suggest growth by coagulation and condensation. Extinction coefficients, asymmetry parameters, and Angstrom exponents are calculated from Mie theory, using the lognormal fits to the measured size distributions and assumed refractive indices.

Impact of biomass burning on aerosol properties over tropical wet evergreen forests of Arunachal Pradesh, India

Large spatial extent of biomass burning occurs in northeast region of India during annual dry season for shifting cultivation purposes. Characterization of optical properties of resultant biomass burning aerosols is important for the study of atmospheric radiative process and for remote sensing of both surface and atmospheric properties in these regions. In the present study, physical and optical properties of biomass burning aerosols in Arunachal Pradesh, North Eastern Region of India have been studied for the first time using ground based measurements using a MICROTOPS-II sunphotometer, an Aethalometer, a quartz crystal microbalance impactor (QCM), SO2 analyser, and an UV meter. Aerosol size distribution suggested dominance of accumulation mode particle loading during burning days compared to normal days. The slope of data points between simultaneous measurements of AOD (500 nm) and UV ery suggested that every 0.1 increase in aerosol optical depth (AOD) causes 0.1 minimal erythemal dose (MED h − 1 ) reduction during normal day and reduction of 0.36 MED h − 1 in ground reaching UV ery during biomass burning periods. Diurnal variations of black carbon aerosol (BC) concentrations increased by a factor of ~ 2 during morning and evening hours compared to afternoon hours during biomass burning period. Daily average black carbon aerosol loading and SO 2 concentrations were found to be high during burning day compared to background values. The proportion of BC to total aerosol mass concentration was observed to be ~ 5% during normal days and ~ 14% during burning days. The changes in black carbon mass concentration values have implications for estimating radiative forcing due to aerosols over the region.

Modelling the optical properties of fresh biomass burning aerosol produced in a smoke chamber: results from the EFEU campaign

Abstract. A better characterisation of the optical properties of biomass burning aerosol as a function of the burning conditions is required in order to quantify their effects on climate and atmospheric chemistry. Controlled laboratory combustion experiments with different fuel types were carried out at the combustion facility of the Max Planck Institute for Chemistry (Mainz, Germany) as part of the "Impact of Vegetation Fires on the Composition and Circulation of the Atmosphere" (EFEU) project. The combustion conditions were monitored with concomitant CO2 and CO measurements. The mass scattering efficiencies of 8.9±0.2 m2 g−1 and 9.3±0.3 m2 g−1 obtained for aerosol particles from the combustion of savanna grass and an African hardwood (musasa), respectively, are larger than typically reported mainly due to differences in particle size distribution. The photoacoustically measured mass absorption efficiencies of 0.51±0.02 m2 g−1 and 0.50±0.02 m2 g−1 were at the lower end of the literature values. Using the measured size distributions as well as the mass scattering and absorption efficiencies, Mie calculations provided effective refractive indices of 1.60−0.010i (savanna grass) and 1.56−0.010i (musasa) (λ=0.55 μm). The apparent discrepancy between the low imaginary part of the refractive index and the high apparent elemental carbon (ECa) fractions (8 to 15%) obtained from the thermographic analysis of impactor samples can be explained by a positive bias in the elemental carbon data due to the presence of high molecular weight organic substances. Potential artefacts in optical properties due to instrument bias, non-natural burning conditions and unrealistic dilution history of the laboratory smoke cannot be ruled out and are also discussed in this study.

Sensitivity of multiangle imaging to the optical and microphysical properties of biomass burning aerosols

The treatment of biomass burning (BB) carbonaceous particles in the Multiangle Imaging SpectroRadiometer (MISR) Standard Aerosol Retrieval Algorithm is assessed, and algorithm refinements are suggested, based on a theoretical sensitivity analysis and comparisons with near‐coincident AERONET measurements at representative BB sites. Over the natural ranges of BB aerosol microphysical and optical properties observed in past field campaigns, patterns of retrieved Aerosol Optical Depth (AOD), particle size, and single scattering albedo (SSA) are evaluated. On the basis of the theoretical analysis, assuming total column AOD of 0.2, over a dark, uniform surface, MISR can distinguish two to three groups in each of size and SSA, except when the assumed atmospheric particles are significantly absorbing (mid‐visible SSA ∼0.84), or of medium sizes (mean radius ∼0.13μm); sensitivity to absorbing, medium‐large size particles increases considerably when the assumed column AOD is raised to 0.5. MISR Research Aerosol Retrievals confirm the theoretical results, based on coincident AERONET inversions under BB‐dominated conditions. When BB is externally mixed with dust in the atmosphere, dust optical model and surface reflection uncertainties, along with spatial variability, contribute to differences between the Research Retrievals and AERONET. These results suggest specific refinements to the MISR Standard Aerosol Algorithm complement of component particles and mixtures. They also highlight the importance for satellite aerosol retrievals of surface reflectance characterization, with accuracies that can be difficult to achieve with coupled surface‐aerosol algorithms in some higher AOD situations.

Case study of modeled aerosol optical properties during the SAFARI 2000 campaign

We present modeled aerosol optical properties (single scattering albedo, asymmetry parameter, and lidar ratio) in two layers with different aerosol loadings and particle sizes, observed during the Southern African Regional Science Initiative 2,000 (SAFARI 2,000) campaign. The optical properties were calculated from aerosol size distributions retrieved from aerosol layer optical thickness spectra, measured using the NASA Ames airborne tracking 14-channel sunphotometer (AATS-14) and the refractive index based on the available information on aerosol chemical composition. The study focuses on sensitivity of modeled optical properties in the 0.3-1.5 microm wavelength range to assumptions regarding the mixing scenario. We considered two models for the mixture of absorbing and nonabsorbing aerosol components commonly used to model optical properties of biomass burning aerosol: a layered sphere with absorbing core and nonabsorbing shell and the Maxwell-Garnett effective medium model. In addition, comparisons of modeled optical properties with the measurements are discussed. We also estimated the radiative effect of the difference in aerosol absorption implied by the large difference between the single scattering albedo values (approximately 0.1 at midvisible wavelengths) obtained from different measurement methods for the case with a high amount of biomass burning particles. For that purpose, the volume fraction of black carbon was varied to obtain a range of single scattering albedo values (0.81-0.91 at lambda=0.50 microm). The difference in absorption resulted in a significant difference in the instantaneous radiative forcing at the surface and the top of the atmosphere (TOA) and can result in a change of the sign of the aerosol forcing at TOA from negative to positive.

Characterization of aerosols from biomass burning––a case study from Mizoram (Northeast), India

Physical and optical properties of biomass burning aerosols in Northeastern region, India analyzed based on measurements made during February 2002. Large spatial extent of Northeastern Region moist tropical to moist sub-tropical forests in India have high frequency of burning in annual dry seasons. Characterization of resultant trace gases and aerosols from biomass burning is important for the atmospheric radiative processes. Aerosol optical depth (AOD) observed to be high during burning period compared to pre- and post-burning days. Peak period of biomass burning is highly correlated with measured AOD and total columnar water vapor. Size distribution of aerosols showed bimodal size distribution during burning day and unimodal size distribution during pre- and post-burning days. Size distribution retrievals from biomass burning aerosols show dominance of accumulation mode particles. Weighted mean radius is high (0.22 μm) during burning period. Columnar content of aerosols observed to be high during burning period in addition to the drastic reduction of visibility. During the burning day Anderson sampler measurements showed dominance of accumulation mode particles. The diurnal averaged values of surface shortwave aerosol radiative forcing of biomass burning aerosols varies from −59 to −87 W m −2 on different days. Measured and modeled solar irradiances are also discussed in the paper.

Raman lidar and sunphotometric measurements of aerosol optical properties over Thessaloniki, Greece during a biomass burning episode

Abstract The influence of biomass burning smoke on the aerosol loading in the free troposphere over Thessaloniki, Greece (40.5°N, 22.9°E) is discussed in this paper. A selected case during summer 2001 is presented, when very high aerosol optical depth values were observed, benefiting from the synergy of various remote sensing instruments. The data that were collected allow the characterization of the optical properties of the aerosols in this region, where only little information has so far been available. Four-day back trajectories indicated that air masses were advected from Bulgaria and the northern coast of the Black Sea, where strong forest fires occurred in early August 2001. In order to investigate the optical properties of biomass burning aerosols, we used a two-wavelength lidar system that combines Raman and elastic-backscatter observations, in addition to a Brewer spectrophotometer, a nephelometer and a multi-filter rotating shadowband radiometer. The lidar measurements on 9 August 2001 recorded an integrated aerosol optical depth at 355 nm of the order of 1.35 during cloud-free conditions. The estimated mean extinction–to-backscatter ratios from the Raman lidar were 60 sr for 355 nm and 50 sr for 532 nm. Estimated values of the single scattering albedo, using spectral UV measurements and modeling were of the order of 0.90, consistent with previous findings, indicating a weak contribution of absorption to the total extinction. The Angstrom exponent, calculated from the multi-filter rotating shadowband radiometer exhibited also high values around 1.78, indicating the presence of rather small particles.

Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign

The physical and optical properties of biomass burning aerosols in a savanna region in south central Africa (Zambia) were analyzed from measurements made during the Zambian International Biomass Burning Emissions Experiment (ZIBBEE) during August‐September 1997. Due to the large spatial extent of African savannas and the high frequency of occurrence of burning in the annual dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiative processes and for remote sensing of both surface and atmospheric properties in these regions. Aerosol Robotic Network Sun‐sky radiometer spectral measurements of direct Sun observations and directional sky radiances were utilized to infer spectral aerosol optical depths (τa), aerosol size distributions, and single‐scattering albedos. During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the region, there was a high correlation between the measured τaand the total column water vapor or precipitable water vapor (PWV), suggesting transport of smoke aerosol from regions with higher PWV. Size distribution retrievals of the biomass burning smoke show that the accumulation mode dominated and a comparison with smoke from Amazonia (Bolivia) shows a shift toward smaller particles for African savanna smoke. This may be the result of differences in mode of combustion (flaming versus smoldering), fuel type and moisture content, and the aging processes of the aerosol. The single‐scattering albedo (ω0) of the aerosols were retrieved using several approaches, yielding average values of ω0at ∼550 nm during ZIBBEE varying from ∼0.82 to ∼0.85, thus showing good agreement within the retrieval uncertainty of ∼0.03 of these methods. In general, ω0was relatively constant as a function of aerosol loading, with very little change occurring for τaat 440 nm ranging from 0.7 to 1.7. African savanna smoke exhibits significantly higher absorption than smoke from Amazonian forested regions and also a greater rate of decrease of ω0with increasing wavelength. Variations in the spectral change of the Angström wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions.