Atmospheric Chemistry and Physics 

The Tropospheric Ozone Assessment Report (TOAR-II) is an official activity of the International Global Atmospheric Chemistry Project (IGAC) (https://igacproject.org/activities/TOAR/TOAR-II). The goal of TOAR-II is to build on the success of TOAR-I (2014–2019) and produce an updated assessment of tropospheric ozone’s global distribution and trends from the surface to the tropopause. This effort will include an analysis of the impacts of ozone on human health, crop and ecosystem productivity, and climate. Original tropospheric ozone research conducted by the TOAR-II community may be submitted to one of six Copernicus journals (ACP/AMT/BG/GMD/ESSD/ASCMO), and the accepted papers will be linked through the TOAR-II Community Special Issue.

The climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses give very different results for the same diagnostics. For example, the global energy budget and hydrological cycle, the Brewer–Dobson circulation, stratospheric vortex weakening and intensification events, and large-scale wave activity at the tropical tropopause are known to differ among reanalyses.

The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) began in 2013 as a coordinated activity to compare numerous key diagnostics in reanalysis data sets. The objectives of this project were

• to understand the causes of differences among reanalyses,
• to provide guidance on the appropriate usage of various reanalysis products in scientific studies,
• to contribute to future improvements in the reanalysis products by establishing collaborative links between the reanalysis centres and the SPARC community.

Phase 1 of the S-RIP project culminated with the publication of the S-RIP report (https://www.sparc-climate.org/sparc-report-no-10/) in January 2022 and a very successful special issue (https://acp.copernicus.org/articles/special_issue829.html) in ACP and ESSD with over 50 papers. Phase 1 was very successful in achieving the above objectives, and in doing so it taught us the value and importance of continuing reanalysis intercomparisons and communications between the reanalysis centres and the SPARC community. The above objectives thus remain the primary aims of S-RIP as the project moves into Phase 2.

This special issue is being initiated in the early stages of S-RIP Phase 2. The community is continuing to produce valuable papers including both updates using new reanalyses of diagnostics studied in Phase 1 and evaluation of diagnostics for processes and atmospheric regions that were not emphasized in Phase 1. This special issue welcomes papers both during this transitional period and in the following years of Phase 2 and both updates of work on processes studied in Phase 1 and new studies focused on additional processes and/or atmospheric regions.

The S-RIP project focuses primarily on differences among reanalyses, but studies that include operational analyses and studies comparing reanalyses with observations or model outputs are encouraged. Phase 1 of S-RIP emphasized diagnostics in the upper troposphere, stratosphere, and mesosphere. This special issue will collect research relevant to S-RIP, including broadening of the scope to, for example, evaluation of new reanalyses and of chemical reanalyses; more comprehensive evaluation of processes in the upper stratosphere and mesosphere; evaluation of tropospheric processes such as blocking, jet stream variations, and temperature anomalies; and more comprehensive evaluation of links between the stratospheric, upper tropospheric, and near-surface circulation and implications for extreme weather events.

All researchers are encouraged to submit to this issue regardless of past participation in S-RIP; we further encourage researchers to participate in and help guide S-RIP Phase 2.

Rapid population growth and urbanization worldwide accelerate eco-environmental and 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 parameterizations, and adequate modelling tools and computer resources. Urban- to hyperlocal-scale (at street or city block level) air pollution, climate change, and their impacts on population exposure and human health have increasingly received attention from both researchers and policy makers around the world. Several state-of-the-science models have recently been developed for urban- to hyperlocal-scale 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 (SinG) model 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, such as CALIOPE-Urban, the Operational Street Pollution Model (OSPM) coupled with the Danish Eulerian Hemispheric Model (DEHM), and the Parallelized Large-Eddy Simulation Model (PALM). Meanwhile, air quality measurement data at hyperlocal scales have become increasingly available for model validation and improvement. Recognizing the urgent need for scientific advancement, pollution and exposure assessment, policy making, and public health protection at urban to hyperlocal scales, we launched a special issue on air quality research at street level in 2018, in which we have published 20 journal papers: https://acp.copernicus.org/articles/special_issue994.html. This special issue (Part II) will continue to advance scientific understanding of local-scale atmospheric phenomena, promote discussion on state-of-the-science urban as well as hyperlocal street- and city-block-level air quality research including measurements, emissions, and model development, and encourage application for complex interactions among urban air pollution, climate, and health. The special issue (Part II) is open for all submissions which address the following themes.

• 3-D Street-in-Grid (SinG) model development and application
• Urban canyon and network model development and its incorporation into 3-D CTMs
• Urban and 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
• Hyperlocal (street and city block scales) air quality measurement and modelling
• Urban infrastructure-induced circulation and its impact on city planning

ACP was launched in September 2001 as the first open-access journal with open peer review in its field. To celebrate its success and immense growth since then, we compile in this special issue a collection of opinion articles to reflect on the developments of the field of atmospheric chemistry and physics in the last 20 years and point to exciting directions for the future, which will stimulate the scientific debate in our field for the next 20 years. Only invited contributions to the special issue will be considered, and the review process will be overseen by the ACP executive and senior editors.

The purpose of the special issue (SI) is the collection of relevant contributions on the detection, analysis, and prediction of long-term changes and trends of trace gases and physical parameters of the atmosphere from the upper troposphere up into the ionosphere as presented at the TRENDS2022 workshop (11th International Workshop on Long-Term Changes and Trends in the Atmosphere). The SI will be open for contributions to the TRENDS2022 conference and co-located SPARC LOTUS and SPARC ATC working group meetings, as well as for other publications directly related to the scope of the SI.