A.-M. Sundström

Aerosol retrieval over land using the (A)ATSR dual-view algorithm

Aerosols play an important role in climate and air quality. They have a direct effect on climate by scattering and/or absorbing the incoming solar radiation [Haywood and Boucher, 2000]. Reflection of solar radiation increases the atmospheric albedo, causing a negative radiative effect and therefore cooling of the atmosphere. On local scales absorbing aerosols can cause net positive radiative forcing resulting in warming of the atmospheric layer. Aerosols have an indirect effect on climate through their influence on cloud microphysical properties and, as a consequence, on cloud albedo and precipitation. The aerosol net effect on the Earth’s radiative balance depends on the aerosol chemical and physical properties, the surface albedo and the altitude of the aerosol layer [Torres et al., 1998]. The uncertainty in the effect of aerosols on climate stems from the large variability of aerosol sources, i.e., their concentrations and physical, chemical and optical properties, in combination with their short atmospheric residence time of a few days. In the IPCC (2007) [Forster et al., 2007] assessment report the total direct aerosol radiative forcing as derived from models and observations is estimated to be — 0.5 [ ± 0.4] Wm−2, with a medium-low level of scientific understanding. The radiative forcing due to the effect on cloud albedo is estimated as — 0.7 [— 1.1,+ 0.4] Wm−2 with a low level of understanding. Long-lived greenhouse gases are estimated to contribute + 2.63 [ ± 0.26] Wm−2. Improved satellite measurements have contributed to the increase of the level of scientific understanding since the third IPCC assessment report in 2001. The continued improvement of aerosol retrieval from satellite-based instruments, to provide consistent information with a known level of accuracy, is important to further our understanding of climate and climate change.

On the Quantitative Low-Level Aerosol Measurements Using Ceilometer-Type Lidar

Abstract The objective of this work is to investigate whether a commercial ceilometer-type lidar can be used as a quantitative aerosol measurement instrument. To this end, lidar backscattering measurements are compared with exact theoretical calculations of backscattering, which are based on in situ–measured size distributions and account for uncertainties in particle composition and shape. The results show that the differences between simulated and measured backscattering remain nearly constant and within the uncertainties involved. The differences are most plausibly explained by an error in the overlap function of the lidar and/or errors in the calibration of either the lidar or the in situ instruments used to measure the aerosol size distribution. Occasionally, large differences occur that are obviously connected to the unrepresentativeness of the in situ and lidar measurement volumes because of insufficient atmospheric mixing. The results imply that the absolute accuracy of the instrument investigated...

Optical modeling of volcanic ash particles using ellipsoids

The single-scattering properties of volcanic ash particles are modeled here by using ellipsoidal shapes. Ellipsoids are expected to improve the accuracy of the retrieval of aerosol properties using remote sensing techniques, which are currently often based on oversimplified assumptions of spherical ash particles. Measurements of the single-scattering optical properties of ash particles from several volcanoes across the globe, including previously unpublished measurements from the Eyjafjallajokull and Puyehue volcanoes, are used to assess the performance of the ellipsoidal particle models. These comparisons between the measurements and the ellipsoidal particle model include consideration of the whole scattering matrix, as well as sensitivity studies on the point of view of the Advanced Along Track Scanning Radiometer (AATSR) instrument. AATSR, which flew on the ENVISAT satellite, offers two viewing directions but no information on polarization, so usually only the phase function is relevant for interpreting its measurements. As expected, ensembles of ellipsoids are able to reproduce the observed scattering matrix more faithfully than spheres. Performance of ellipsoid ensembles depends on the distribution of particle shapes, which we tried to optimize. No single specific shape distribution could be found that would perform superiorly in all situations, but all of the best-fit ellipsoidal distributions, as well as the additionally tested equiprobable distribution, improved greatly over the performance of spheres. We conclude that an equiprobable shape distribution of ellipsoidal model particles is a relatively good, yet enticingly simple, approach for modeling volcanic ash single-scattering optical properties.

Estimating the concentration of nucleation mode aerosol particles over South Africa using satellite remote sensing measurements

In this work satellite based observations were used to estimate the concentration of nucleation mode aerosols over South Africa. The nucleation mode aerosols can not be detected directly with satellite instruments since they are much smaller than the optically active aerosols, hence the concentrations were estimated using proxies introduced by Kulmala et al. (2011). Results showed enhanced values of both primary and regional scale nucleation proxies over the Mpumalanga Highveld industrial area, whereas over the Johannesburg-Pretoria megacity only the primary nucleation proxy showed elevated values. To estimate how well satellite based proxies work, the relation between satellite and in situ based quantities was studied in more detail. The correlation between aerosol optical depth (AOD) and condensation sink (CS) was 0.2-0.3 depending on the location. Boundary layer height affected the correlation somewhat, but there are other factors, such as the effect of dust on AOD, that are more likely to have a stron...

The use of satellite observations of fire radiative power to estimate the availabilities (activity patterns) of pyrometallurgical smelters

emission inventories (how much of what pollutant species are emitted, where and when) are poorly developed in many countries. South Africa, which is the largest industrialized economy in southern Africa, can be considered as an example. At present South African environmental legislation requires significant atmospheric emitters to supply emission data to the government, but this information is protected by some legal safeguards and this has thus far prevented a comprehensive peer-reviewed emission inventory from becoming available in the public domain. Applications can be made to the Air Pollution Control Officer to access some information on the atmospheric emission licences of the aforementioned emitters, but this process is lengthy and does not yield all the required information. Therefore, atmospheric modelling studies undertaken for this region have to a large extent depended on global emission inventory databases (e.g. Kuik et al., 2015; Lourens et al., 2016) that might not contain sufficient detail. Also, it is almost impossible for atmospheric scientists to account for fluctuations in the temporal activity patterns (if a source emits or not) of large point sources so as to provide temporally resolved emission inventories. In this paper we demonstrate how satellitederived fire data can be used to locate and reflect the activity patterns of large point sources. Open biomass burning, both natural (e.g. lighting-induced fires) and anthropogenic (e.g. intentional or accidental grassland, savannah, or forest fires), has long been recognized as one of the most significant sources of atmospheric pollutant species such as carbon dioxide (CO2), carbon monoxide (CO), particulate matter (PM), black carbon (BC), and precursors for secondary pollutants (e.g. Vakkari et al., 2014; Chiloane et al., 2017). This has led to a multitude of studies using satellite-derived fire and/or burned area detection data to improve fire emission estimates and explain pollutant observations (e.g. Mafusire et al., 2016). However, comparatively little attention has been given to alternative uses of these satellite-derived fire data-sets.

Satellite study over Europe to estimate the single scattering albedo and the aerosol optical depth

Aerosol particles have a significant effect on the Earth climate on regional and global scales by perturbing the radiation balance both directly due to scattering and absorption of solar radiation and indirectly due to their effect on cloud macroscopic and microphysical properties (IPCC 2007 [1]). One of the main contributors to the radiative effect of aerosols is the Single Scattering Albedo (SSA). One of the research topics is the uncertainty in estimating and improving the SSA value. In radiative transfer studies, single scattering albedo is the ratio of scattering optical depth and the total optical depth of the atmosphere. The SSA and the Aerosol Optical Depth (AOD) are two of the main parameters to estimate aerosol radiative forcing. In this study we show results of the SSA and the AOD at 0.555 μm retrieved from Advanced Along Track Scanning Radiometer (AATSR) data, with focus on forest fires over Europe. The retrieval results are validated using AERONET AOD level 2.0 data and the SSA is compared wi...