Atmospheric Chemistry and Physics 

Geostationary Environment Monitoring Spectrometer (GEMS) is the first instrument to observe air quality from a geostationary Earth orbit (GEO) successfully launched on 19 February 2020 and in initial operation after in-orbit tests (IOTs). GEMS provides hourly air quality information on both aerosols and gases at unprecedented spatial resolution of 7 km × 8 km. GEMS is a scanning UV–visible spectrometer measuring the hyperspectral spectrum in the ultraviolet and visible, which allows for the observation of key atmospheric constituents including O₃, NO₂, CO, SO₂, CH₂O, CHOCHO, aerosols, clouds, and UV indices. The mission opened a new era of air quality monitoring from space and will be joined by NASA’s TEMPO and ESA’s Sentinel-4 to form the GEO Air Quality Constellation in ~3 years to cover the most polluted region in the Northern Hemisphere. In this ACP–AMT special issue, the first results will be presented including instruments, IOT results, initial products, algorithm, calibration, validation from the recent GMAP field campaign, applications in modelling, etc.

Atmospheric ozone is one of the most important trace gases in the Earth's atmosphere due to its multiple roles: filtering harmful levels of ultraviolet (UV) solar radiation and shortwave absorption, being a greenhouse gas in the Earth's climate system, and being a powerful oxidant in the troposphere. Also, surface level ozone produces harmful effects on human health, ecosystems, and agricultural production. The latest findings and emerging research on ozone-related topics were presented at the 2021 Quadrennial Ozone Symposium (QOS) held online on October 3–9 2021. This special issue covers studies presented at the QOS and other relevant studies that are broadly related to the following subject areas:

1. stratospheric ozone science;
2. tropospheric ozone science;
3. ozone-depleting substances sources, sinks, and budget;
4. ozone, climate, and meteorology;
5. ozone monitoring and measurement techniques;
6. environmental and human health effects of atmospheric ozone and UV radiation.

This issue is open to all submissions within its scope.

Clouds over the ocean and their radiative properties are partially influenced by the biology, biogeochemistry and physics of the surface ocean. For example, the production of sea spray via bubble bursting associated with breaking waves can be influenced by biogenic material present in surface seawater, and biogenic material carried in sea spray aerosol can also influence ice-nucleating properties in air overlying the ocean. Furthermore, phytoplankton produce a range of compounds, including gases and particles, which when emitted to the atmosphere participate in the atmospheric chemistry. Our recent ship campaign Sea2Cloud had a primary focus of investigating the link between marine particle emissions and the biogeochemical properties of the seawater in subantarctic and subtropical waters, examining new particle formation processes and the relationship between surface ocean biogeochemistry and sea spray aerosol in dedicated ship-borne experiments. In parallel, ambient air aerosol, gas and cloud properties were measured along the ship track, which will allow for testing of these relationships in modelling exercises. Along with the ship campaign, longer-term measurements of aerosol properties were monitored at the Baring Head GAW research station (NZ) as well as continuously ongoing onboard the R/V Marion Dufresne over the years 2021–2022. These measurements will be used for the inspection of the representativeness of the ship campaign's process-orientated results. The Sea2Cloud project is running from July 2018 to July 2023, with potential additional laboratory and ship-borne measurements in the Southern Ocean by January 2023.

ACTRIS statements of the purpose of the special issue on stay-at-home policies in response to the COVID-19 pandemic have resulted in an unprecedented decrease in pollutant emissions and in a well-publicized improvement of air quality in many cities in Asia, Europe and America. While the impact of lockdown on air quality was unambiguously detected in the urban areas through both in situ and space remote-sensing observations and its spatial and temporal extents, the specific role of meteorology and the cascade responses from indirect and non-linear effects are far from being fully evaluated. Throughout the very specific year 2020, ACTRIS and its partner institutions engaged in a pan-European effort to document the impact of governmental policies on atmospheric composition. ACTRIS maintained its operations at fully nominal standards, even increasing its sampling capacity in some cases. The ACTRIS data collection containing measurements of aerosol, cloud and trace gas properties across Europe measured during the year 2020 has been compiled and made available by the ACTRIS Data Centre units. Data from the year 2021 will soon follow. A community of scientists started evaluating the impact of the repeated lockdowns over the European regions. Because concentration and properties of short-lived atmospheric species are highly variable in space and time, evaluating the impact of reduced emissions is not straightforward. The special issue “Quantifying the impacts of stay-at-home policies on atmospheric composition and properties of aerosol and clouds over the European regions using ACTRIS related observations” will gather a series of scientific papers dealing with the measurable effects of lockdown measures over Europe. The special issue will particularly be dealing with • quantifying the spatial and temporal extent of stay-at-home policies on the European atmosphere, at both local and regional scales, • evaluating the impact of lockdown measures on the formation of secondary pollutants, • documenting the impact of reduced emissions (including air-traffic emissions) on cloud properties and occurrence, and • estimating the “missing” emissions using observation–model approaches. The outcome from the special issue aims to provide an in-depth analysis of the perturbation induced by the repeated lockdowns on the complex atmospheric system. The special issue is open for all submissions within its scope.

During spring 2020 a German research consortium performed the airborne BLUESKY measurement campaign, using the HALO and DLR Falcon aircraft, to investigate the atmospheric composition changes associated with the COVID-19 lockdown. Comprehensive atmospheric chemistry, aerosol, and cloud measurements were performed with the two aircraft, from the boundary layer to the lower stratosphere during 20 scientific flights over Europe and the North Atlantic. Many groups participated in the campaign (DLR Oberpfaffenhofen, three departments of the MPI for Chemistry, Mainz, University of Frankfurt, KIT Karlsruhe, FZ Jülich). The measurement data are unique and are supplemented by satellite remote sensing data and numerical model simulations. The data analysis also involves comparison with measurement data from many pre-lockdown aircraft campaigns and long-term projects (e.g. CARIBIC). A subject of special interest is the impact of aircraft emissions on atmospheric composition, as the lockdown was associated with a 70–90% reduction of air traffic.