Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation

Three-Dimensional Distribution of Biomass Burning Aerosols from Australian Wildfires Observed by TROPOMI Satellite Observations

We present a novel passive satellite remote sensing approach for observing the three-dimensional distribution of aerosols emitted from wildfires. This method, called AEROS5P, retrieves vertical profiles of aerosol extinction from cloud-free measurements of the TROPOMI satellite sensor onboard the Sentinel 5 Precursor mission. It uses a Tikhonov–Phillips regularization, which iteratively fits near-infrared and visible selected reflectances to simultaneously adjust the vertical distribution and abundance of aerosols. The information on the altitude of the aerosol layers is provided by TROPOMI measurements of the reflectance spectra at the oxygen A-band near 760 nm. In the present paper, we use this new approach for observing the daily evolution of the three-dimensional distribution of biomass burning aerosols emitted by Australian wildfires on 20–24 December 2019. Aerosol optical depths (AOD) derived by vertical integration of the aerosol extinction profiles retrieved by AEROS5P are compared with MODIS, VIIRS and AERONET coincident observations. They show a good agreement in the horizontal distribution of biomass burning aerosols, with a correlation coefficient of 0.87 and a mean absolute error of 0.2 with respect to VIIRS. Moderately lower correlations (0.63) were found between AODs from AEROS5P and MODIS, while the range of values for this comparison was less than half of that with respect to VIIRS. A fair agreement was found between coincident transects of vertical profiles of biomass burning aerosols derived from AEROS5P and from the CALIOP spaceborne lidar. The mean altitudes of these aerosols derived from these two measurements showed a good agreement, with a small mean bias (185 m) and a correlation coefficient of 0.83. Moreover, AEROS5P observations reveal the height of injection of the biomass burning aerosols in 3D. The highest injection heights during the period of analysis were coincident with the largest fire radiative power derived from MODIS. Consistency was also found with respect to the vertical stability of the atmosphere. The AEROS5P approach provides retrievals for cloud-free scenes over several regions, although currently limited to situations with a dominating presence of smoke particles. Future developments will also aim at observing other aerosol species.

Amplification of light within aerosol particles accelerates in-particle photochemistry

Optical confinement (OC) structures the optical field and amplifies light intensity inside atmospheric aerosol particles, with major consequences for sunlight-driven aerosol chemistry. Although theorized, the OC-induced spatial structuring has so far defied experimental observation. Here, x-ray spectromicroscopic imaging complemented by modeling provides direct evidence for OC-induced patterning inside photoactive particles. Single iron(III)-citrate particles were probed using the iron oxidation state as a photochemical marker. Based on these results, we predict an overall acceleration of photochemical reactions by a factor of two to three for most classes of atmospheric aerosol particles. Rotation of free aerosol particles and intraparticle molecular transport generally accelerate the photochemistry. Given the prevalence of OC effects, their influence on aerosol particle photochemistry should be considered by atmospheric models.

Study on particle radiative properties of lignite, hard coal and biomass fly ashes in the infrared wavelength range

Fly ash, which is a by-product of combustion in furnaces or boilers, is used in certain materials as an additive for waste recycling. The optical properties of fly ash and the effects on the heat transfer phenomena of materials used in engineering applications differ and depend on the kind of solid fuel and the combustion conditions. The morphological and the radiative properties of fly ash samples of Turkish lignite, biomass, and hard coal that are burned in different thermal power plants were studied herein. The samples, which were collected from cyclones, were morphologically examined using scanning electron microscopy (SEM) images, and their chemical compositions were analysed by energy dispersive x-ray spectroscopy (EDX). Absorbance measurements were made in the wavelength range from 2.5 to 25 μm, and the discrete dipole approximation (DDA) was applied for a numerical assessment of the radiative properties of the samples. The measured absorbance values of all samples for particle diameters of 25 and 75 μm displayed significant differences in the related wavelength range. The most prominent change was observed in the biomass sample, and the hard coal fly ash had the lowest absorbance values in the related spectrum range. Although the particle shapes of lignite and biomass fly ashes are not same, the changes in the measured absorbance values were similar. The effects of the k index of the complex refractive index (CRI) on the radiative properties were examined for values of 0.01, 0.1, 0.3, 0.5 and 1.0. According to the measured absorption values and the calculated absorption efficiency results, the k index may be between 0.3 and 1.0 at the related wavelength range. It was concluded that the effects of the particle size and absorption index of fly ash on the heat transfer properties are important in the specified wavelength range.

Optical Characterization of Fresh and Photochemically Aged Aerosols Emitted from Laboratory Siberian Peat Burning

Carbonaceous aerosols emitted from biomass burning influence radiative forcing and climate change. Of particular interest are emissions from high-latitude peat burning because amplified climate change makes the large carbon mass stored in these peatlands more susceptible to wildfires and their emission can affect cryosphere albedo and air quality after undergoing transport. We combusted Siberian peat in a laboratory biomass-burning facility and characterized the optical properties of freshly emitted combustion aerosols and those photochemically aged in an oxidation flow reactor (OFR) with a three-wavelength photoacoustic instrument. Total particle count increased with aging by a factor of 6 to 11 while the total particle volume either changed little (<8%) for 19 and 44 days of equivalent aging and increased by 88% for 61 days of equivalent aging. The aerosol single-scattering albedo (SSA) of both fresh and aged aerosol increased with the increasing wavelength. The largest changes in SSA due to OFR aging were observed at the shortest of the three wavelengths (i.e., at 405 nm) where SSA increased by less than ~2.4% for 19 and 44 days of aging. These changes were due to a decrease in the absorption coefficients by ~45%, with the effect on SSA somewhat reduced by a concurrent decrease in the scattering coefficients by 20 to 25%. For 61 days of aging, we observed very little change in SSA, namely an increase of 0.31% that was caused a ~56% increase in the absorption coefficients that was more than balanced by a somewhat larger (~71%) increase in the scattering coefficients. These large increases in the absorption and scattering coefficients for aging at 7 V are at least qualitatively consistent with the large increase in the particle volume (~88%). Overall, aging shifted the absorption toward longer wavelengths and decreased the absorption Ångström exponents, which ranged from ~5 to 9. Complex refractive index retrieval yielded real and imaginary parts that increased and decreased, respectively, with the increasing wavelength. The 405 nm real parts first increased and then decreased and imaginary parts decreased during aging, with little change at other wavelengths.

The Optical Properties of Aerosols at the Summit of Mount Tai in May and June and the Retrieval of the Complex Refractive Index

To study the optical properties of background atmospheric aerosols in East China, we carried out observations of the physical, chemical and optical properties of atmospheric aerosols at the summit of Mount Tai (Mt. Tai, 1533.7 m above sea level) from 13 May to 25 June 2017. The results show that the average scattering coefficient ( σ sca , 550 ) at 550 nm of the aerosols at the summit of Mt. Tai is 40.3 Mm−1, and the average absorption coefficient ( σ abs , 550 ) at 550 nm is 16.0 Mm−1. The complex refractive index of aerosols is a key parameter for aerosol retrieval and modeling. There are few studies on the equivalent complex refractive index of aerosol in the Taishan area. We calculated the aerosol equivalent complex refractive index using the observed aerosol scattering coefficients, absorption coefficients and particle size distribution data, providing more data support for future modeling in this region. The real part (n) of the complex refractive index at 550 nm of aerosol ranges from 1.31 to 1.98 (mostly under 1.50), with an average value of 1.38, while the imaginary part (k) ranges from 0.014 to 0.251 (less than 0.10 for over 95% samples), with an average value of 0.040. The analysis of the n and k of the aerosol average complex refractive index shows that the scattering properties of the aerosols at the summit of Mt. Tai are relatively weak and the absorption properties are relatively strong when compared with those of other kinds of aerosols. The k of the aerosol complex refractive index at the summit of Mt. Tai has strong correlations with the wind speed, temperature, as revealed by the correlation analysis.

Spectrally dependent linear depolarization and lidar ratios for nonspherical smoke aerosols

We use the numerically exact T-matrix method to model light scattering and absorption by aged smoke aerosols at lidar wavelengths ranging from 355 to 1064 nm assuming the aerosols to be smooth spheroids or Chebyshev particles. We show that the unique spectral dependence of the linear depolarization ratio (LDR) and extinction-to-backscatter ratio (or lidar ratio, LR) measured recently for stratospheric Canadian wildfire smoke can be reproduced by a range of model morphologies, a range of spectrally dependent particle refractive indices, and a range of particle sizes. For these particles, the imaginary part of the refractive index is always less than (or close to) 0.035, and the corresponding real part always falls in the range [1.35, 1.65]. The measured spectral LDRs and LRs could be produced by nearly-spherical oblate spheroids or Chebyshev particles whose shapes resemble those of oblate spheroids. Their volume-equivalent effective radii should be large enough (r _eff = 0.3 μm or greater) to produce the observed enhanced LDRs. Our study demonstrates the usefulness of triple-wavelength LDR measurements as providing additional size information for a more definitive characterization of the particle morphology and composition. Non-zero LDR values indicate the presence of nonspherical aerosols and are highly sensitive to particle shapes and sizes. On the other hand, the LR is a strong function of absorption and is very responsive to changes in the particle refractive index.