ACTRIS-2 addresses the scope of integrating state-of-the-art European ground-based stations for long term observations of aerosols, clouds and short lived gases capitalizing work of FP7-ACTRIS. ACTRIS-2 aims to achieve the construction of a user-oriented RI, unique in the EU-RI landscape. <br/>ACTRIS-2 provides 4-D integrated high-quality data from near-surface to high altitude (vertical profiles and total-column), relevant to climate and air-quality research. ACTRIS-2 develops and implements, in a large network of stations in Europe and beyond, observational protocols that permit harmonization of collected data and their dissemination. ACTRIS-2 offers networking expertise, upgraded calibration services, training of users, trans-national access to observatories and calibration facilities, virtual access to high-quality data products. Through joint research activities, ACTRIS-2 develops new integration tools that will produce scientific or technical progresses reusable in infrastructures, thus shaping future observation strategies. <br/>Innovation in instrumentation is one of the fundamental building blocks of ACTRIS-2. Associated partnership with SMEs stimulates development of joint-ventures addressing new technologies for use in atmospheric observations. <br/>Target user-groups in ACTRIS-2 comprise a wide range of communities worldwide. End-users are institutions involved in climate and air quality research, space agencies, industries, air quality agencies. <br/>ACTRIS-2 will improve systematic and timely collection, processing and distribution of data and results for use in modelling, in particular towards implementation of atmospheric and climate services. ACTRIS-2 invests substantial efforts to ensure long-term sustainability beyond the term of the project by positioning the project in both the GEO and the on-going ESFRI contexts, and by developing synergies with national initiatives.