MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) has been measuring atmospheric middle-infrared limb-emission spectral radiances from the European Polar Satellite ENVISAT from 2002 to 2012. Several algorithms have been developed by the scientific community to process MIPAS data up to Level 2, which is to retrieve the vertical distribution profiles of many atmospheric minor constituents. Among these algorithms, the one developed by the European Space Agency (ESA) is of utmost importance as it delivers high-quality profiles of a large set of gases, retrieved from the measurements of the whole MIPAS mission. The papers included in the special issue deal with the most recent (and possibly the last) release of MIPAS/ENVISAT products obtained from the ESA Level 2 processor version 8. The papers deal with both the products and the algorithms employed to derive them from direct measurements. The algorithms are presented and assessed. The features of the Level 2 products are analyzed and the products themselves are validated by intercomparison with other independent co-located measurements.

Sand and dust storms (SDSs) are extreme meteorological phenomena that generate significant amounts of airborne mineral dust particles. SDSs play a significant role in different aspects of weather, climate and atmospheric chemistry. In addition, SDSs represent a severe hazard for life, health, property, the environment and the economy, which is aligned with several Sustainable Developed Goal (SDG) targets established by the United Nations (UN). Understanding, managing and mitigating SDS risks and effects requires fundamental and cross-disciplinary knowledge and filling current knowledge gaps on the dust cycle. Recent developments in Earth observation have led to big advances in the observational capabilities of desert dust particles. Research products show the possibility of exploring the wide dimensional range of desert dust particles by integrating different observational techniques on the ground, while satellite observations are currently providing aerosol description with finer and finer spatiotemporal resolution. Experts on dust observations and modelling participating in the EU-funded COST Action InDust (www.cost-indust.eu, CA16202) have identified critical scientific gaps in our current knowledge of dust aerosol measurements, modelling, processes and effects and have also identified a critical mass of scientists working towards addressing them. Such gaps include at least issues such as - the need for improvement of surface-based dust aerosol measurements (in situ, sun photometric, lasers), including the use of new technologies, improved algorithms and combined approaches; - the improvement of the global coverage on aerosol dust observation through improvement of satellite-based retrievals; - advancements in dust aerosol modelling (sources, mechanisms, deposition); - the presentation of intensive campaign results with a focus on dust aerosols; - dust mineralogy; - the assessment of the dust particle size distribution with an emphasis on giant particles; - enhancements regarding dust aerosol effects on different communities (e.g. solar energy, aviation, health, climate, air quality, agriculture, ocean life). This special issue focuses on addressing the above-mentioned gaps through individual and combined efforts from at least the COST InDust community (0000108:0000100 scientists from 52 countries). A list of papers to be submitted has been created including results related to SDS-WAS, ACTRIS/EARLINET activities, long-term measurement analysis of measured and modelled data, dust retrieval algorithm and techniques, dust effects, and foreseen results of the ESA Aeolus Cal/Val activity “ASKOS” (Cabo Verde, June - July, 2020).

Satellite limb measurements have become an essential tool to remotely sense the chemical composition of the Earth's atmosphere with a particular focus on the stratosphere. They enable combining near global sampling on a daily basis with vertical profiling capability and thus overcome some of the limitations of nadir or occultation measurements. Although the number of satellite limb instruments is currently declining, several missions are still operational (e.g., OSIRIS, SMR, ACE-FTS, MAESTRO, OMPS-LP and SAGE III) and several new missions are planned or will be launched in the near future (e.g., MATS, ICON, ALTIUS and several mini satellites). In addition, the data sets of several past limb sounders, particularly the Envisat instruments SCIAMACHY, MIPAS and GOMOS are still used for many scientific studies and new data products are developed based on measurements with these instruments.

This combined ACP/AMT special issue is open for submissions dealing with all aspects related to limb measurements (including limb scatter, limb emission and occultation) in the optical, IR and microwave spectral regions. This includes instrument development and characterization, retrieval algorithm development, validation studies, as well as scientific applications based on the retrieved data sets. We note that radio occultation measurements are outside the scope of the special issue.

The version 8 spectral data of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), released in January 2019, are supposed to be the final set. Besides the ORM reprocessed version 8 level-2 MIPAS data distributed by ESA, a research processor has been run at IMK and IAA to generate temperature and trace gas concentrations. The IMK–IAA data product includes a larger list of atmospheric constituents and covers a wider altitude range in the case of the special observation modes than the operational data. The processing is based on independent algorithms. This special issue collects articles which 1) document the retrieval strategies used to derive these distributions along with uncertainty estimates, 2) report validation studies, and 3) use the data to answer current questions in atmospheric research.

This ACP Special Issue seeks to report research advancements in the understanding of mercury sources, distribution pathways, levels, physicochemical characterization, measurement traceability, impacts on global ecosystems, and policy-relevant information presented at the ICMGP 2019 and being developed under the MercOx (Metrology of oxidized mercury) project and two ERA-PLANET initiatives – Integrative and Comprehensive Understanding on Polar Environments (iCURE) and Integrated Global Observing Systems for Persistent Pollutants (iGOSP). The Special Issue is open to all publications related to the scope of the Special Issue.