In this special issue papers resulting from the project “Marine biological production, organic aerosol particles and marine clouds: a Process Chain – MarParCloud” and closely related projects shall be aggregated. MarParCloud is a highly interdisciplinary project funded by the German Leibniz Society (https://www.leibniz-gemeinschaft.de/en/about-us/leibniz-competition/projekte-2016/funding-line-2/), which aims to improve our knowledge on the nature, formation, transfer and effects of organic matter (OM) in the marine environment. To this end the biological formation of OM in the ocean, its accumulation in the sea surface microlayer (SML), the export to marine aerosol particles and, finally, the importance of OM for the formation of cloud droplets and ice crystals are investigated. Within MarParCloud several field and tank studies were performed to investigate ambient seawater, the SML, aerosol particles and cloud water as well as water and air samples obtained during artificial bubble experiments. Closely related campaigns comprise the EU project MARSU (“Advanced analytical and mass spectrometric techniques to study ocean-atmosphere interactions”) and the MILAN study (“sea-surface MIcroLAyer at Night”), focusing on diel dynamics of the uppermost layer of the ocean, as well as the cruise EMB184 “Aerosol” performed with the R/V Elisabeth Mann Borgese. In addition to results from these projects and campaigns, this special issue is open for all submissions concerning the cycling of OM in the marine environment with potential atmospheric implications.

This inter-journal special issue of AMT, ACP, and GMD serves as a thematic collection for manuscripts arising from the work of Horizon 2020 integrated infrastructure project Eurochamp-2020; cf. https://www.eurochamp.org/ . As Eurochamp-2020 has project parts dealing with new instrumentation, with atmospheric chemistry and physics studies as well as with modelling, contributions are expected for all three journals as part of the inter-journal special issue.

The Sentinel-5 Precursor mission is a single-payload satellite in a low-Earth orbit successfully launched on 13 October 2017. The mission provides daily global information at an unprecedented spatial resolution (~3.5x7km²) on trace gases and aerosols, important for air quality, climate forcing, and the ozone layer. The single payload of the mission is the TROPOspheric Monitoring Instrument (TROPOMI), which is a nadir-viewing push-broom spectrometer measuring in the ultraviolet, visible, near infrared, and shortwave infrared. The selected wavelength range allows for the observation of key atmospheric constituents, including O₃, NO₂, CO, SO₂, CH₄, CH₂O, aerosols, and clouds. The mission flies in loose formation with Suomi NPP to obtain detailed information on clouds and for further scientific exploitation of combined data products. In this combined ACP/AMT special issue the first results will be presented, including validation of operational products.

Rapid population growth and urbanisation worldwide accelerate eco-environmental/socio-economic stress as well as adverse climatic and health impacts on urban dwellers. Atmospheric modelling research has largely been performed on a horizontal grid spacing of kilometres or larger due to a lack of understanding of the local-scale phenomena, appropriate parameterisations, and adequate modelling tools, and computer resources. Urban/local street-level air pollution, climate change, and their impacts on population exposure and human health have increasingly received attentions by both researchers and policymakers around the world. Several state-of-the-science models have been recently developed for urban/local street-level air pollution modelling, including the street-network model, the Model of Urban Network of Intersecting Canyons and Highways (MUNICH) that incorporates detailed representations of gas-phase chemistry and secondary aerosol formation pathways, and the Street-in-Grid model (SinG) that dynamically combines a 3-D Eulerian chemical-transport model (CTM), Polair3D, with MUNICH. There have been increasing numbers of developers and users for MUNICH, SinG, and other similar coupled 3-D CTMs and urban canyon models for street-level air pollution modelling worldwide. Recognising the urgent need for scientific advancement, pollution/exposure assessment, policymaking, and public health protection at urban/local-street scales, we launch this special issue to advance scientific understanding of local-scale atmospheric phenomena and promote research and discussion on state-of-the-science urban/local-street-level air quality model development and application for complex interactions among urban air pollution, climate, and health. The special issue is open for all submissions which address the following themes:

• 3-D Street-in-Grid (SinG) model development and application;
• urban canyon/network model development and its incorporation into 3-D CTMs;
• urban/street-level air quality modelling in support of human exposure assessment;
• impact of urban traffic emissions on air quality and human health at a street level.

The research foci in the MICS-Asia III study include the following aspects: development of reliable emission inventories in Asia; ability and uncertainty of current air quality models in Asia in predicting ozone, nitrogen oxides, VOC, sulfur oxides, PM_2.5, etc.; ability of online coupled models to simulate aerosol radiative forcing and ozone and aerosol–weather/climate interaction through aerosol–radiation–cloud feedback; and prediction of future air pollution and climate. The special issue is open to all publications that inform about these aspects and provide answers to the MICS-Asia III scientific questions.

• What is the ability of models to reproduce pollutant concentrations of photochemical smog under highly polluted conditions (regional haze) and to model a sharp PM_2.5 increase such as the episodes between January 2010 and March 2018?
• What are the uncertainties of process (physical and chemical processes) and model resolutions (horizontal and vertical) in air quality modelling? In particular, uncertainties in key boundary layer parameters, which have both direct and indirect impacts on modelling results by transporting meteorological fields and providing parameters in physical/chemical modules, respectively, need to be addressed.
• What are the air quality responses to specific emission perturbations in a common case?
• What are the impacts of anthropogenic emissions on radiation and climate change over East and Southeast Asia?
We especially welcome the publications related to MICS-Asia Phase III studies to inform the research community about the recent findings and progress of air quality and regional climate change studies over East and Southeast Asia. The findings from these publications will also help to better understand capability, uncertainty, and future direction within the current CTMs.