Atmospheric Chemistry and Physics 

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.

Unlike in traditional bulk and bin microphysical approaches, atmospheric particles like hydrometeors and aerosol particles are not described by field variables but by computational particles. Over the last years, such particle-based methods have become more and more popular in aerosol and cloud physics. The special issue is open to all submissions with a clear focus on model description or evaluation or on studies specifically exploiting the advantages of particle-based methods. Contributions of any particle-based model related to aerosol and cloud modelling are welcome. This inter-journal special issue of GMD and ACP serves as a thematic collection and intends to increase visibility and foster collaboration among researchers.

In April 2017, the Senate of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) established the Priority Programme “Polarimetric Radar Observations meet Atmospheric Modelling (PROM)” (SPP 2115). The programme is designed to run for 6 years and is now in the third funding year. The overall motivation of the SPP PROM is a better understanding and representation of cloud and precipitation processes in numerical weather prediction and climate models. Since 2015 the whole atmosphere over Germany has been monitored by 17 state-of-the-art polarimetric Doppler weather radar observations suitable to challenge the representation of cloud and precipitation processes in atmospheric models. Data assimilation merges observations and models for state estimation as a prerequisite for prediction and can be regarded as a smart interpolation between observations while exploiting the physical consistency of atmospheric models as mathematical constraints. However, considerable knowledge gaps exist both in radar polarimetry and atmospheric models, which impede the full exploitation of the triangle radar-polarimetry–atmospheric-models–data-assimilation. Objectives of SPP PROM and thus the envisioned publications within the special issue are (1) the exploitation of radar polarimetry for quantitative process detection in precipitating clouds and for model evaluation, (2) the improvement of cloud and precipitation schemes in atmospheric models based on process fingerprints detectable in polarimetric observations, (3) monitoring of the energy budget evolution due to phase changes in the cloudy, precipitating atmosphere for a better understanding of its dynamics, (4) the generation of precipitation system analyses by the assimilation of polarimetric radar observations into atmospheric models for weather forecasting, and (5) radar-based detection of the initiation of convection for the improvement of thunderstorm prediction.

The Clouds, Aerosol Monsoon Processes Philippines Experiment (CAMP2Ex) was a NASA-led study to understand the interdisciplinary relationships between aerosol lifecyle, convection and the radiation budget within the Southeast Asian boreal summer southwest monsoon system. Central to CAMP2Ex was a 25 August–5 October 2019 airborne campaign based at Clark, Philippines, including the NASA P3 and SPEC Inc. Lear 35 research aircraft outfitted with radars, microwave sounders, lidar, radiation, cloud microphysics, composition and state instrumentation, aligned with significant remote sensing data collections and informatics efforts. Operations were the pinnacle of a multi-year monitoring effort that included development of an aerosol and radiation monitoring site at the Manila Observatory, performed in partnership with the ONR-sponsored Propagation of Intraseasonal Tropical Oscillations (PISTON) R/V Sally Ride deployment and contributing to the International Years of the Maritime Continent (YMC) effort. Given its interdisciplinary nature, CAMP2Ex is generating papers on numerous classes of research. We request a Special Issue be granted to the CAMP2Ex team and its partners specifically as a venue for aerosol–cloud–radiation research.