Headline: Calibrating low-cost sensors to measure vertical and horizontal gradients of NO2 and O3 pollution in three street canyons in Berlin

Despite improvements in air quality over the last several decades, air pollution will continue to be a leading cause of harmful health effects in European cities as urban populations continue to grow. In recent years, the technology of low-cost sensors (LCS) has been adapted for use in expanding air pollution measurements at higher spatial resolution in cities across the globe. In a novel application, this exploratory study deploys metal oxide (MOS) and electrochemical (EC) low-cost sensors housed in EarthSense Zephyr sensor systems to measure nitrogen dioxide (NO2) and ozone (O3) concentrations in three street canyons in Berlin in winter, spring, and summer from 2017 to 2020. After calibration with reference instrumentation using the seven-step methodology outlined by Schmitz et al. (2021), we compare the measured concentrations with reference and urban background concentrations and investigate relationships with meteorology. We find that, following proper calibration, LCS capture expected patterns of urban pollution in association with diurnal chemistry and meteorology well. Additionally, EC sensors outperform MOS and allow for greater insights into local patterns of pollution. Furthermore, we measure concentrations of NO2 and O3 in street canyons that match expectations from modelling studies, indicating that high spatial resolution deployment of LCS could successfully yield new insights in urban microenvironments and inform model development. While LCS have a wider range of uncertainty than reference instruments, these results suggest that they can be reliably used for several new applications, such as validating urban street canyon models or measuring air pollution alongside changes to urban infrastructure.