Timo H. Virtanen

The ADV/ASV AATSR aerosol retrieval algorithm: current status and presentation of a full-mission AOD dataset

ABSTRACT An advanced along-track scanning radiometer (AATSR) global multi-year aerosol retrieval algorithm is described. Over land, the AATSR dual-view (ADV) algorithm utilizes the measured top of the atmosphere (TOA) reflectance in both the nadir and forward views to decouple the contributions of the atmosphere and the surface to retrieve aerosol properties. Over ocean, the AATSR single-view (ASV) algorithm minimizes the discrepancy between the measured and modelled TOA reflectances in one of the views to retrieve the aerosol information using an ocean reflectance model. Necessary steps to process global, multi-year aerosol information are presented. These include cloud screening, the averaging of measured reflectance, and post-processing. Limitations of the algorithms are also discussed. The main product of the aerosol retrieval is the aerosol optical depth (AOD) at visible/near-infrared wavelengths. The retrieved AOD is validated using data from the surface-based AERONET and maritime aerosol network (MAN) sun photometer networks as references. The validation shows good agreement with the reference (r = 0.85, RMSE = 0.09 over land; r = 0.83, RMSE = 0.09 at coasts and r = 0.96, RMSE = 0.06 over open ocean). The results of the aerosol retrievals are presented for the full AATSR mission years 2002–2012 with seasonally averaged time series for selected regions.

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the single-scattering optical properties of ash particles from several volcanoes across the globe, including previously unpublished measurements from the Eyjafjallajokull and Puyehue volcanoes, are used to assess the performance of the ellipsoidal particle models.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.

Characteristics of Sulphur Dioxide Monitors: Intercomparison

The performance characteristics of five commercial sulphur dioxide monitors were studied. The experiments included both laboratory tests and field monitoring in an urban environment. According to the laboratory tests, the most sensitive concentration region was between 100 and 500 ppb [1 ppb(SO2) = 2.86 μgm−3 at NTP]. The lower detection limits were 2 to 5 ppb, and the precision varied from 1 to 26 ppb in the concentration range up to 500 ppb. The field data were analyzed with orthogonal regression and principal component analysis, and the results were less favourable than in the laboratory tests.

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...