Global distributions of UV-absorbing aerosols are obtained using measured differences between the 340 and the 380 nm radiances from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) for the years 1979–1993. Time series are shown for major sources of biomass burning and desert dust giving the frequency of occurrence and areal coverage over land and oceans. Minor sources of UV-absorbing aerosols in the atmosphere are also discussed (volcanic ash and oil fires). Relative values of year-to-year variability of UV-absorbing aerosol amounts are shown for major aerosol source regions: (1) central South America (Brazil) near 10°S latitude; (2) Africa near 0°–20°S and 0° to 10°N latitude; (3) Saharan Desert and sub-Saharan region (Sahel), Arabian Peninsula, and the northern border region of India; (4) agricultural burning in Indonesia, Eastern China, and Indochina, and near the mouth of the Amazon River; and (5) coal burning and dust in northeastern China. The first three of these regions dominate the injection of UV-absorbing aerosols into the atmosphere each year and cover areas far outside of their source regions from advection of UV-absorbing particulates by atmospheric wind systems. During the peak months, smoke and dust from these sources are transported at altitudes above 1 km with an optical depth of at least 0.1 and can cover about 10% of the Earth's surface. Boundary layer absorbing aerosols are not readily seen by TOMS because the small amount of underlying Rayleigh scattering leads to a small signal. Significant portions of the observed dust originate from agricultural regions frequently within arid areas, such as in the Sahel region of Africa, especially from the dry lake-bed near Lake Chad (13.5°N, 14°E), and intermittently dry drainage areas and streams. In addition to drought cycle effects, this suggests there may be an anthropogenic component to the amount of dust injected into the atmosphere each year. Detection of absorbing aerosols and calculation of optical depths are affected by the presence of large-scale and subpixel clouds in the TOMS field of view.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JD03680

Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data

. Airborne lidar and in-situ measurements of aerosols and trace gases were performed in volcanic ash plumes over Europe between Southern Germany and Iceland with the Falcon aircraft during the eruption period of the Eyjafjalla volcano between 19 April and 18 May 2010. Flight planning and measurement analyses were supported by a refined Meteosat ash product and trajectory model analysis. The volcanic ash plume was observed with lidar directly over the volcano and up to a distance of 2700 km downwind, and up to 120 h plume ages. Aged ash layers were between a few 100 m to 3 km deep, occurred between 1 and 7 km altitude, and were typically 100 to 300 km wide. Particles collected by impactors had diameters up to 20 μm diameter, with size and age dependent composition. Ash mass concentrations were derived from optical particle spectrometers for a particle density of 2.6 g cm−3 and various values of the refractive index (RI, real part: 1.59; 3 values for the imaginary part: 0, 0.004 and 0.008). The mass concentrations, effective diameters and related optical properties were compared with ground-based lidar observations. Theoretical considerations of particle sedimentation constrain the particle diameters to those obtained for the lower RI values. The ash mass concentration results have an uncertainty of a factor of two. The maximum ash mass concentration encountered during the 17 flights with 34 ash plume penetrations was below 1 mg m−3. The Falcon flew in ash clouds up to about 0.8 mg m−3 for a few minutes and in an ash cloud with approximately 0.2 mg m−3 mean-concentration for about one hour without engine damage. The ash plumes were rather dry and correlated with considerable CO and SO2 increases and O3 decreases. To first order, ash concentration and SO2 mixing ratio in the plumes decreased by a factor of two within less than a day. In fresh plumes, the SO2 and CO concentration increases were correlated with the ash mass concentration. The ash plumes were often visible slantwise as faint dark layers, even for concentrations below 0.1 mg m−3. The large abundance of volatile Aitken mode particles suggests previous nucleation of sulfuric acid droplets. The effective diameters range between 0.2 and 3 μm with considerable surface and volume contributions from the Aitken and coarse mode aerosol, respectively. The distal ash mass flux on 2 May was of the order of 500 (240–1600) kg s−1. The volcano induced about 10 (2.5–50) Tg of distal ash mass and about 3 (0.6–23) Tg of SO2 during the whole eruption period. The results of the Falcon flights were used to support the responsible agencies in their decisions concerning air traffic in the presence of volcanic ash.

/pdf/airborne-observations-of-the-eyjafjalla-volcano-ash-cloud-3dz485lnbg.pdf

Airborne observations of the Eyjafjalla volcano ash cloud over Europe during air space closure in April and May 2010

The total ozone mapping spectrometer (TOMS), first flown on the Nimbus 7 satellite, has delivered an unanticipated set of unique information about volcanic plumes because of its contiguous spatial mapping and use of UV wavelengths. The accuracies of TOMS sulfur dioxide retrievals, volcanic plume masses, and eruption totals under low-latitude conditions are evaluated using radiative transfer simulations and error analysis. The retrieval algorithm is a simultaneous solution of the absorption optical depth equations including ozone and sulfur dioxide at the four shortest TOMS wavelengths and an empirical correction based on background condition residuals. The retrieval algorithm reproduces model stratospheric sulfur dioxide plume amounts within ±10% over most central scan angles and moderate solar zenith angles if no aerosols or ash are present. The errors grow to 30% under large solar zenith angle conditions. Volcanic ash and sulfate aerosols in the plume in moderate optical depths (0.3) produce an overestimation of the sulfur dioxide by 15–25% depending on particle size and composition. Retrievals of tropospheric volcanic plumes are affected by the reflectivity of the underlying surface or clouds. The precision of individual TOMS SO2 soundings is limited by data quantization to ±6 Dobson units. The accuracy is independent of most instrument calibration errors but depends linearly on relative SO2 absorption cross-section errors at the TOMS wavelengths. Volcanic plume mass estimates are dependent on correction of background offsets integrated over the plume area. The errors vary with plume mass and area, thus are highly individual. In general, they are least for moderate size, compact plumes. Estimates of the total mass of explosively erupted sulfur dioxide depend on extrapolation of a series of daily plume masses backward to the time of the eruption. Errors of 15–30% are not unusual. Effusive eruption total mass estimates are more uncertain due to difficulties in separating new from old sulfur dioxide in daily observations.

/pdf/volcanic-sulfur-dioxide-measurements-from-the-total-ozone-533c3lfwp5.pdf

Volcanic sulfur dioxide measurements from the total ozone mapping spectrometer instruments

. Volcanic ash plumes, emitted by the Eyjafjallajokull volcano (Iceland) in spring 2010, were observed by the lidar systems MULIS and POLIS in Maisach (near Munich, Germany), and by a CIMEL Sun photometer and a JenOptik ceilometer in Munich. We retrieve mass concentrations of volcanic ash from the lidar measurements; spectral optical properties, i.e. extinction coefficients, backscatter coefficients, and linear depolarization ratios, are used as input for an inversion. The inversion algorithm searches for model aerosol ensembles with optical properties that agree with the measured values within their uncertainty ranges. The non-sphericity of ash particles is considered by assuming spheroids. Optical particle properties are calculated using the T-matrix method supplemented by the geometric optics approach. The lidar inversion is applied to observations of the pure volcanic ash plume in the morning of 17 April 2010. We find 1.45 g m −2 for the ratio between the mass concentration and the extinction coefficient at λ = 532 nm, assuming an ash density of 2.6 g cm −3 . The uncertainty range for this ratio is from 0.87 g m −2 to 2.32 g m −2 . At the peak of the ash concentration over Maisach the extinction coefficient at λ = 532 nm was 0.75 km −1 (1-h-average), which corresponds to a maximum mass concentration of 1.1 mg m −3 (0.65 to 1.8 mg m −3 ). Model calculations show that particle backscatter at our lidar wavelengths (λ ≤ 1064 nm), and thus the lidar retrieval, is hardly sensitive to large particles ( r ≳ 3 μm); large particles, however, may contain significant amounts of mass. Therefore, as an independent cross check of the lidar retrieval and to investigate the presence of large particles in more detail, we model ratios of sky radiances in the aureole of the Sun and compare them to measurements of the CIMEL. These ratios are sensitive to particles up to r ≈ 10 μm. This approach confirms the mass concentrations from the lidar retrieval. We conclude that synergistic utilization of high quality lidar and Sun photometer data, in combination with realistic aerosol models, is recommended for improving ash mass concentration retrievals.

https://acp.copernicus.org/articles/11/2209/2011/acp-11-2209-2011.pdf

Volcanic ash from Iceland over Munich: mass concentration retrieved from ground-based remote sensing measurements

[1] During April–May 2010 the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft flew 12 flights targeting volcanic ash clouds around the UK. The aircraft observed ash layers between altitudes of 2–8 km with peak mass concentrations typically between 200–2000μg/m3, as estimated from a Cloud and Aerosol Spectrometer (CAS). A peak value of 2000–5000 μg/m3 was observed over Scotland on 14 May 2010, although with considerable uncertainty due to the possible contamination by ice. Aerosol size distributions within ash clouds showed a fine mode (0.1–0.6 μm) associated with sulphuric acid and/or sulphate, and a coarse mode (0.6–35 μm) associated with ash. The ash mass was dominated by particles in the size range 1–10 μm (volume-equivalent diameter), with a peak typically around 3–5μm. Electron-microscope images and scattering patterns from the SID-2H (Small Ice Detector) probe showed the highly irregular shape of the ash particles. Ash clouds were also accompanied by elevated levels of SO2 (10–100 ppbv), strong aerosol scattering (50–500 × 10−6 m−1), and low Angstrom exponents (−0.5 to 0.4) from the 3-wavelength nephelometer. Coarse-mode mass specific aerosol extinction coefficients (kext), based on the CAS size distribution varied from 0.45–1.06 m2/g. A representative value of 0.6 m2/g is suggested for distal ash clouds (∼1000 km downwind) from this eruption.

/pdf/in-situ-observations-of-volcanic-ash-clouds-from-the-faam-jqyg6eqy6t.pdf

In situ observations of volcanic ash clouds from the FAAM aircraft during the eruption of Eyjafjallajökull in 2010

The visible wavelength optical properties of the ash from the 1982 El Chichon eruptions are measured using ash samples collected at three surface sites at distances between 12 and 80 km from the volcano. The most distant sample is assumed to be the most representative of the silicate ash injected into the stratosphere. The measured optical properties are presented as a complex refractive index n, while the aerosol absorption is expressed as the imaginary component of the refractive index, n sub IM. Results show that each of these samples exhibited low values of absorption, with n sub IM at 500 nm ranging from 0.0015 for the 12 km sample to 0.001 for the 80 km sample. Based on these measurements, it is estimated that n for the stratospheric silicate ash is given by n = 1.53 - 0.001i.

E. M. Patterson

Papers

Optical properties of the ash from El Chichon volcano