Measurement report: Temporal variability of vertical profiles of CO2 and CH4 over an urban environment

Abstract

Improving estimates of city emissions requires a better understanding of pollutant transport dynamics within the urban boundary layer (UBL), particularly for greenhouse gases (GHGs), given the importance of cities in mitigation efforts. Due to heterogeneous emission sources and complex atmospheric processes, diverse observational datasets are needed to constrain urban transport and support inversion frameworks to estimate emissions. In this context, vertical profiles of GHGs provide critical information. This study presents results of 11 vertical profiling campaigns conducted in Krakow, southern Poland, over one year (March 2021–February 2022). The aim was to obtain high-resolution measurements of CO2 and CH4 mole fractions, together with key meteorological variables, within the urban boundary layer of a mid-latitude city with a population of close to one million. Such datasets of vertical profiles up to 280 m above ground level (a.g.l.) are rare, particularly within city limits, and are intended to support the development of a city-scale inversion system. Measurements were carried out using a Picarro G2311-f cavity ring-down spectrometer deployed on two platforms: (i) a tethered tourist balloon in the city centre, reaching up to 280 m a.g.l., and (ii) an uncrewed aerial vehicle (UAV) operating up to 100 m a.g.l. For UAV measurements, the analyser remained on the ground and sampled air through a 200 m tube. To our knowledge, this is the first application of this approach for vertical GHG profiling. Meteorological variables, including temperature, relative humidity, pressure, and wind, were measured using calibrated low-cost sensors mounted on both platforms. The observations enabled a detailed analysis of the structure and evolution of the UBL under varying meteorological conditions throughout the year. The altitude range of the platforms allowed frequent detection of stable nocturnal boundary layers, as well as elevated CO2 and CH4 plumes above them during the night and morning periods, likely associated with strong emission sources outside the city. In general, the dataset provides a valuable record of high-resolution vertical GHG distributions and meteorological conditions in an urban environment. It helps fill a key observational gap in the lower troposphere over cities and offers important insights for studying urban boundary layer processes and improving emission estimates.