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.

This special issue will include papers that guide the path forward in making reliable predictions of aerosol effects on the heat budget of the South East Asian subcontinent by improving our knowledge of aerosol processes and their influence on the Indian monsoon. Several recent major studies have aimed to provide a detailed determination of aerosol physical and chemical properties across India prior to and during the Indian monsoon, and results from these studies will be welcomed. Most recently the South West Asian Monsoon Interactions project used a combination of UK and Indian research aircraft and a network of ground-based measurement sites. The analysis of data will enable assessment of aerosol composition and mixing state, provide source characterization, and help quantify aerosol optical properties such as extinction, absorption, and single scattering albedo that drive regional climate. Results that use this detailed characterization to test representations of aerosol properties in regional and global climate models will be included. The special issue will report on evaluations of and improvements in model representations of aerosol properties with data over India.

The purpose of this special issue is the compilation of modelling and observational studies in connection with five international field deployments (AEROCLO-sA, CLARIFY, LASIC, ORACLES, and NaFoLiCA) that focus on the interactions of natural and anthropogenic aerosols with radiation, clouds, and regional climate in the South Atlantic Ocean and the southern African region. These deployments, based in Namibia, Ascension Island, and São Tomé, took place between 2016 and 2018 and support a significant number of investigations extending beyond just the individual science teams. The airborne and ground-based observations, as well as the related satellite measurements and climate modelling studies, address all aspects of aerosol–cloud–climate interactions, including the link of aerosol properties to meteorological fields and dynamical processes that influence aerosol emission and transport. The projects also target the advancement of remote sensing of aerosols for complex scenes over land, sea, and clouds. The special issue will be open to all submissions, with complementary goals to the five mentioned deployments, so as to encourage the exchange of ideas from inside and outside the science teams of all projects.

We conducted the Fifth International Workshop on Ice Nucleation (FIN) to (1) understand the microphysics of how particles nucleate ice, (2) determine the number of ice forming particles as a function of atmospheric properties such as temperature and relative humidity, (3) measure the atmospheric distribution of ice forming particles and (4) ascertain the role of anthropogenic activities in producing or changing the behaviour of ice forming particles. To accomplish these goals we held three distinct workshops on the topic of atmospheric ice nucleation. The first was an intercomparison of instruments to determine the composition of ice forming particles in a controlled laboratory setting. This took place in autumn 2014 at the location of the last ice nucleation instrument intercomparison: the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber located at the Karlsruhe Institute of Technology. The second was an intercomparison of instruments used to determine cloud formation conditions. This activity also took place at AIDA and was conducted in spring 2015. Because ice nucleation predominantly takes place at the low temperatures found at high altitude, a critical requirement for the third workshop was a facility that offers access to free-tropospheric air masses with minimal local particle sources. We used the Desert Research Institute’s recently renovated Storm Peak Laboratory for this workshop in autumn 2015.