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The Hunga Tonga - Hunga Ha’apai (HTHH) eruption of 15 January 2022 was an exceptional event by the period, magnitude, and duration of propagation of the atmospheric waves it generated, circling the globe multiple times. Even though several volcanic eruptions in the past 150 years era of scientific instrumentation generated notable barometric disturbances, the HTHH eruption is comparable only to the Krakatoa eruption of 1883 by the magnitude of the atmospheric pressure waves that it generated. For instance, the very energetic Mt Pinatubo eruption of 1991 did not produce pressure waves of the same period and magnitude as the Krakatoa or HTHH eruptions. An analysis of the timing of the multiple passes at barometric stations is reported in Symons (1888) for the Krakatoa. Since the HTHH event also gave rise to a pressure wave that circled the Earth’s atmosphere multiple times in the last 134 years, it is of interest to perform similar timing statistics on the multiple passages at stations that recorded them. A review of the Krakatoa analysis and a comparison with the HTHH are presented, with possible implications for the changes in the global state of the atmosphere during the interval between the two events. Symons, G.J. (ed) The Eruption of Krakatoa and Subsequent Phenomena (Report of the Krakatoa Committee of the Royal Society). London, 1888. Internet Archive. 1888.&nbsp, The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

An algorithm for the retrieval of the aerosol optical thickness over land and over water from SCIAMACHY (SCanning Imaging Absorption SpectroMeter for Atmospheric ChartographY) is presented. Because calibrated data are not yet available for the SCIAMACHY channels used by the algorithm, the concepts were tested with GOME (Global Ozone Monitoring Experiment) data. The cloud fraction in the GOME pixels has been determined using the FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A Band) algorithm. Surface contributions to the TOA reflectance are determined from the GOME surface reflectance database [1]. The aerosol retrieval algorithm uses Look Up Tables (LUT) that were created using the radiative transfer model 6S. The algorithm allows for the retrieval of aerosol types characterized by Angstrom coefficients in the range from -0.1 to 2.8. Comparison of the results with AERONET sun photometer data for 12 sites in Europe and Africa shows very good agreement.

Algorithms for the retrieval of aerosol properties over land and over sea have been developed by the TNO Physics and Electronics Laboratory (TNO-FEL) for several instruments, such as AVHRR (Veefkind et al., 1998a), GOME, ATSR-2 (Veelkind et al. 1998a, b; 1999) and OMI (Torres et al. 2002). OMI will be launched in 2004. An algorithm for SCIAMACHY is developed for application over water and over land (Kusmierzcyk-Michulec and De Leeuw 2002a). In these algorithms, different techniques are applied, depending on the instrument properties. The current emphasis is on the Along Track Scanning Radiometer (ATSR-2) on board the ESA Earth Remote Sensing satellite ERS-2. ATSR-2 offers two views, nadir and forward. The nadir view is used over water, where simple assumptions can be used to separate the surface reflectance from the aerosol effects. Over land both views are used. Initially, the single and dual view algorithms have been applied over the US east coast and Europe, with a relative simple description of the aerosol composition in terms of sea spray and ` continental' aerosol. However, over other areas the situation can be more complicated due to the occurrence of other aerosol types with different optical properties, such as black carbon, dust and biomass aerosol. In particular when none of these aerosol types dominates it is difficult to determine the influence of each of them on the aerosol optical depth (AOD). This challenging problem has been studied using data from the INDOEX (India and Indian Ocean, see Figure 3.7.1) experiments in February and March 2000 and the SAFARI experiments over South Africa in September 2000. In the INDOEX area the aerosol consists of a complicated mixture of five or more aerosol types. An algorithm has been developed with look up tables (LUTs) that describe the influence of these aerosols on the radiation received by the ATSR-2. A result from this algorithm is presented in Fig. 3.7.1, which is a composite of retrieved AOD for the duration of the INDOEX experiment. Over land the dual view algorithm has been used, over water the single view algorithm. The algorithm also provides the aerosol type, which over land is anthropogenic which changes gradually to maritime as the air mass is advected over sea. The Angstrom coefficients have been used to determine the spatial distribution of BC in the area using the method described in Kusmierzcyk-Michulec et al. (2002b). Results from these retrievals compare favourably with ground-based data, as described in Robles Gonzalez et al. (2003a). The algorithm is now further developed for biomass aerosol, using data from the SAFARI experiments. Initial results are encouraging (Schmid et al., 2003). Apart from the INDOEX and SAFARI experiments, ATSR-2 retrieved AOD maps over Europe are available for October 1995, August 1997, and April, May and October 1999. The ATSR-2 algorithm is being converted to a user-friendly algorithm that can be used to handle large data sets to retrieve aerosols over extended areas and long periods. It will be applied for AATSR on ENVISAT. Results will be made available on the TEMIS website ( http://www.temis.nl).

To retrieve aerosol properties from the radiance measured at the top of the atmosphere in clear sky conditions, the contributions of the surface and the various atmospheric constituents need to be separated. This is easiest done over dark surfaces, i.e. with a negligible reflectance that can relatively easily be accounted for. This principle has been used for the retrieval of aerosol optical depth (AOD) over water from satellite observations in the near-infrared. The AOD at wavelengths in the UV can be determined both over water and over land, using the same principle of a dark surface. For longer wavelengths, the dual view provided by ATSR-2 is used to separate the aerosol reflectance from the surface contribution. Single arid dual-view algorithms have been developed by TNO-FEL and tested for the US east coast and over Europe. Currently the algorithms are extended with other aerosol types and tested versus data over the Indian Ocean (INDOEX area) and South Africa (SAFARI experiment). The initial results indicate that the AOD can be retrieved within reasonable limits. Apart from the ATSR-2, algorithms aimed at the determination of aerosol optical depth and composition are developed for AATSR and SCIAMACHY (ENVISAT) and OMI (EOS-TERRA).