The creation of this dataset was supported by the European Commission under contract "Deployment of a lower-cost ambient air quality sensor system in three cities: Antwerp, Oslo, Zagreb" (07027747/2019/812686/SER/ENV.C.3), with funding from the European Parliament (Pilot Project "Integrating smart sensors and modelling for air quality monitoring in cities"). The opinions expressed in this document are those of the authors only and should not be considered as representative of the European Commission's official position. ©2022 European Union. All rights reserved. We would like to thank the city of Antwerp for their support in selecting sensor locations and practical arrangements. We are also thankful to all contacts at the various locations for providing support and access to their infrastructure for sensor installation. The Municipality of Oslo is gratefully acknowledged for providing access to the Kirkeveien reference monitoring station for the co-location study. We would like to thank Urban Infrastructure Partner for making the Oslo City Bike infrastructure available to the project. We thank all VITO, VMM, NILU and IMI co-workers that were involved in test site selection, sensor installation and field operation.

We provide a unique curated dataset of urban air quality measurements acquired using dense networks of low-cost sensor systems in three European cities for the years 2020 and 2021. The dataset includes the raw sensor data of quality-controlled sensor networks along with co-located reference data sets. Sensor data are collected using the AirSensEUR sensor system, including sensors to monitor NO, NO2, O3, CO, PM2.5, PM10, PM1, CO2, and meteorological parameters. In total, 85 sensor systems were deployed throughout the years 2020 and 2021 in three European cities (Antwerp , Oslo and Zagreb), resulting in a dataset comprising different meteorological and ambient conditions. The main data collection included two co-location campaigns in different seasons at an air quality monitoring station in each city and a deployment at different locations in each city (including also locations at other air quality monitoring stations). The dataset consists of data files with sensor and reference data, and metadata files with description of locations, deployment dates and description of sensors and reference instruments.