Optical properties of humic-like substances (HULIS) in biomass-burning aerosols (original) (raw)
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Atmospheric Chemistry and Physics, 2006
Spectral aerosol light absorption is an important parameter for the assessment of the radiation budget of the atmosphere. Although on-line measurement techniques for aerosol light absorption, such as the Aethalometer and the Particle Soot Absorption Photometer (PSAP), have been available for two decades, they are limited in accuracy and spectral resolution because of the need to deposit the aerosol on a filter substrate before measurement. Recently, a 7-wavelength (λ) Aethalometer became commercially available, which covers the visible (VIS) to near-infrared (NIR) spectral range (λ=450-950 nm), and laboratory calibration studies improved the degree of confidence in these measurement techniques. However, the applicability of the laboratory calibration factors to ambient conditions has not been investigated thoroughly yet. As part of the LBA-SMOCC (Large scale Biosphere atmosphere experiment in Amazonia-SMOke aerosols, Clouds, rainfall and Climate) campaign from September to November 2002 in the Amazon basin we performed an extensive field calibration of a 1-λ PSAP and a 7-λ Aethalometer utilizing a photoacoustic spectrometer (PAS, 532 nm) as reference device. Especially during the dry period of the campaign, the aerosol population was dominated by pyrogenic emissions. The most pronounced artifact of integrating-plate type attenuation techniques (e.g. Aethalometer, PSAP) is due to multiple scattering effects within the filter matrix. For
Scientific Reports
Atmospheric organic aerosol (OA) are considered as a significant contributor to the light absorption of OA, but its relationship with abundance, composition and sources are not understood well. In this study, the abundance, chemical structural characteristics, and light absorption property of HULIS and other low-to-high polar organics in PM0.95 collected in Tomakomai Experimental Forest (TOEF) were investigated with consideration of their possible sources. HULIS were the most abundant (51%), and correlation analysis revealed that biogenic secondary organic aerosols significantly contribute to HULIS. The mass spectra obtained using a high-resolution aerosol mass spectrometer (HR-AMS) showed that HULIS and highly polar water-soluble organic matter (HP-WSOM) were substantially oxygenated organic aerosol fractions, whereas water-insoluble organic matter (WISOM) had a low O/C ratio and more hydrocarbon-like structures. The WISOM fraction was the predominant light-absorbing organics. HULI...
Atmospheric Chemistry and Physics, 2006
As part of the Large Scale Biosphere-Atmosphere Experiment in Amazonia-Smoke, Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC) campaign, detailed surface and airborne aerosol measurements were performed over the Amazon Basin during the dry to wet season from 16 September to 14 November 2002. Optical and physical properties of aerosols at the surface, and in the boundary layer (BL) and free troposphere (FT) during the dry season are discussed in this article. Carbon monoxide (CO) is used as a tracer for biomass burning emissions. At the surface, good correlation among the light scattering coefficient (σ s at 545 nm), PM2.5, and CO indicates that biomass burning is the main source of aerosols. Accumulation of haze during some of the large-scale biomass burning events led to high PM2.5 (225 µg m −3), σ s (1435 Mm −1), aerosol optical depth at 500 nm (3.0), and CO (3000 ppb). A few rainy episodes reduced the PM2.5, number concentration (CN) and CO concentration by two orders of magnitude. The correlation analysis between σ s and aerosol optical thickness shows that most of the optically active aerosols are confined to a layer with a scale height of 1617 m during the burning season. This is confirmed by aircraft profiles. The average mass scattering and absorption efficiencies (545 nm) for small particles (diameter D p <1.5 µm) at surface level are found to be 5.0 and 0.33 m 2 g −1 , respectively, when relating the aerosol optical properties to PM2.5 aerosols. The observed mean single scattering albedo (ω o at 545 nm) for submicron aerosols at the surface is 0.92±0.02. The light scattering by particles
Modeled spectral optical properties for smoke aerosols in Amazonia
Geophysical Research Letters, 2003
The optical properties of aerosols from biomass burning in Brazil were derived from measurements by the AERONET sun photometer network in Amazonia. A dynamical aerosol model was constructed from Mie Theory calculations, using as input a constant complex refractive index and the average size distribution of aerosols for 12 ranges of aerosol optical thickness. The aerosol optical properties were described for 24 wavelengths in the spectral range of 0.20-3.00 mm. The model was used to simulate radiances and irradiances and the results agreed well with the measurements. The derived optical model is immediately applicable to estimate aerosol forcing by biomass burning aerosol emitted on the development frontier in Amazonia.
Atmospheric Environment, 2013
The mass specific optical absorption coefficient (MAC) of Humic-Like Substances (HULIS), isolated from a fine aerosol fraction (PM1) collected at a typical Central European rural background site (Kpuszta) was measured in the re-dispersed aerosol phase at 1064, 532, 355 and 266 nm wavelengths by our recently developed four wavelength photoacoustic spectrometer. It is found to be practically negligible in the visible (0.03 m 2 g À1 @532 nm), while in the ultraviolet (4.9 m 2 g À1 @266 nm) it becomes comparable with that of black carbon (BC), a major absorbing fraction of the ambient aerosol. This type of wavelength dependency was already hypothesized for HULIS aerosol, but it was proved previously only by indirect measurements on HULIS samples dissolved in the aqueous phase. On the other hand, the other generally accepted hypothesis, that this wavelength dependency can be described by a single, wavelength independent absorption Angström-exponent (AAE) is not justified by the presented measurements.
Refractive index of aerosol particles over the Amazon tropical forest during LBA-EUSTACH 1999
Journal of Aerosol Science, 2003
Optical properties of aerosol particles were characterized during two field campaigns at a remote rainforest site in Rondônia, Brazil, as part of the project European Studies on Trace Gases and Atmospheric Chemistry, a contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). The measurements included background (wet season), biomass burning (dry season), and transition period conditions. Optical measurements of light scattering and absorption were combined with data on number/size distributions in a new iterative method, which retrieves the effective imaginary refractive index of the particles at a wavelength of . For ambient relative humidities lower than 80%, background aerosols exhibited an average refractive index of 1.42−0.006i. Biomass burning aerosols displayed a much larger imaginary part, with an average refractive index of 1.41−0.013i. Other climate-relevant parameters were estimated from Mie calculations. These include single-scattering albedos of 0.93±0.03 and 0.90±0.03 (at ambient humidity), asymmetry parameters of 0.63±0.02 and 0.70±0.03, and backscatter ratios of 0.12±0.01 and 0.08±0.01 for background and biomass burning aerosols, respectively.
Spectral dependence of aerosol light absorption over the Amazon Basin
Atmospheric Chemistry and Physics, 2011
In this study, we examine the spectral dependence of aerosol absorption at different sites and seasons in the Amazon Basin. The analysis is based on measurements performed during three intensive field experiments at a pasture site (Fazenda Nossa Senhora, Rondônia) and at a primary forest site (Cuieiras Reserve, Amazonas), from 1999 to 2004. Aerosol absorption spectra were measured using two Aethalometers: a 7-wavelength Aethalometer (AE30) that covers the visible (VIS) to near-infrared (NIR) spectral range, and a 2-wavelength Aethalometer (AE20) that measures absorption in the UV and in the NIR. As a consequence of biomass burning emissions, about 10 times greater absorption values were observed in the dry season in comparison to the wet season. Power law expressions were fitted to the measurements in order to derive the absorptionÅngström exponent, defined as the negative slope of absorption versus wavelength in a log-log plot. At the pasture site, about 70 % of the absorptionÅngström exponents fell between 1.5 and 2.5 during the dry season, indicating that biomass burning aerosols have a stronger spectral dependence than soot carbon particles.Ångström exponents decreased from the dry to the wet season, in agreement with the shift from biomass burning aerosols, predominant in the fine mode, to biogenic and dust aerosols, predominant in the coarse mode. The lowest absorptionÅngström exponents (90 % of data below 1.5) were observed at the forest site during the dry season. Also, results indicate that low absorption coefficients were associated with lowÅngström exponents. This finding suggests
Environmental Science & Technology, 2007
The chemical composition of organic atmospheric aerosols is only poorly understood. Although a significant fraction of organic aerosols consists of humic-like substances (HULIS), only little is known about this class of compound, and accurate quantification remains difficult, partly due to the lack of appropriate standards. Here, evaporative light-scattering detection (ELSD) was applied for the first time to quantify water-soluble HULIS in aerosol particles smaller than 1 µm. This detection method was shown to be suitable for the quantification of compounds with unknown structures and lacking appropriate quantification standards. As compared to organic carbon determination of isolated HULIS, no organic carbon/organic mass (OC/OM) conversion factor needs to be applied with ELSD and therefore eliminates this significant uncertainty factor of the OC/OM method, which is frequently used to quantify HULIS. Solid-phase extraction and size-exclusion chromatography were applied to separate inorganic ions and low molecular weight compounds from HULIS before ELSD quantification. The ELSD itself provides an additional separation step where low volatility HULIS are separated from high volatility, small compounds. Electrospray ionization mass spectrometry was used to identify the molecular weight range of the compounds quantified with ELSD. The most intensive peaks were in the range of m/z 200-500, with some masses up to m/z 800. We showed that UV detection using fulvic acid as surrogate quantification standard underestimates the HULIS concentration by a factor of 1.1 to 2.5, which is in agreement with earlier studies. During a 6 week winter 2005-2006 campaign at a suburban site near Zurich, Switzerland, an average of 1.1 µg/m 3 HULIS was found, which is about 4-6% of the total particle mass smaller than 1 µm (PM1) and 10-35% of the organic matter in PM1.