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.

This special issue includes papers based on recent field campaigns, including US Department of Energy Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA), NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), and the North Atlantic Climate System Integrated Study (ACSIS), and long-term measurements from observatories (e.g., the Atmospheric Radiation Measurement Eastern North Atlantic site on Graciosa Island in the Azores). Presently the responses of marine low-cloud systems to changes in atmospheric greenhouse gases and aerosols are major sources of uncertainty that limit our ability to predict future climate. Major contributions to this uncertainty derive from poor understanding of the cloud responses to aerosol changes and the natural aerosol state that is being perturbed by anthropogenic emissions. One overarching scientific objective of these recent field campaigns is to understand key processes that drive the properties and interactions of aerosol and clouds under representative meteorological and cloud conditions. Another important focus is to validate and improve ground-based retrieval algorithms using comprehensive in situ aircraft measurements. This will lead to high-quality, long-term datasets from the routine ground-based remote sensing, which allow greater statistical reliability in the observed properties and relationships among aerosols, clouds, and precipitation than is possible with the aircraft measurements alone.

With a wide variety of aerosol optical, physical, and chemical properties, China offers a test bed for the study of aerosol processes and techniques leading to a better understanding of aerosol properties. Aerosol concentrations in the southeast of China are periodically among the highest in the world, while at other times they are relatively low. The prevailing aerosol chemical composition (e.g., sulfate, nitrate, black carbon, brown carbon, particulate organic matter, sea salt) also changes significantly with the quick evolution of both the economy and society, along with the government’s strong environmental actions. The complex mechanisms of the secondary formation of aerosols and haze-fog interaction are still not well understood in this important region. In areas other than the southeast of China, topography determines the aerosol concentrations, which are consistently high. Examples are Sichuan Province or the Guanzhong Basin, i.e. large basins surrounded by high mountains. Furthermore, the large deserts in the west and north of China are strong sources of dust particles influencing the aerosol properties over a large part of the country. There are however also large areas with relatively low aerosol loading, in particular in mountainous areas and over the high plateaus such as Tibet. Aerosol chemical properties in the center and east of China still need to be clarified both geographically and temporally, e.g. distinguish spectrally absorbing components like mineral dust and brown carbon from remote sensing observations. Anthropogenic emissions of aerosols and precursor gases have a large influence on aerosol, in regards to both chemical and physical properties, and consequently optical properties, and their effects can be augmented by the transport of aerosol from elsewhere, such as carbonaceous particles from biomass burning aerosol and dust particles emitted from the deserts. The complex mechanisms and intrinsic relations of aerosol chemistry and physics, and their impacts on optical properties, need better interpretation based on comprehensive observation and analysis. Weather conditions have a large influence on the formation of haze, and haze episodes occur more and more frequently. The influence of large-scale weather systems such as the monsoon or the Siberian high may result in inter-annual variations in different regions of China. In particular the progress of the monsoon from south to north during spring and summer, and back in the autumn, results in strong variations in aerosol content.

In this special issue we focus on the application of remote sensing techniques using both ground-based and satellite measurements to study the temporal and spatial variations in aerosol properties over China on different scales. Results from campaigns in the Taklamakan Desert (Kashi), on the Tibetan Plateau, and other remote areas such as the NE of China are contrasted with those from densely populated and industrialized urban areas in the east of China. The special issue aims to bring together results from different projects such as Kashi, MOST, and regional air quality studies. Contributions are solicited on aerosol optical, physical, and chemical properties inferred from remote sensing observations and related optical techniques, and the spatial and temporal variations in these properties across China, with emphasis on processes leading to the observed differences, including the influence of meteorological conditions and large-scale weather systems.