Temporal dynamics and controlling factors of CO2 and CH4 variability in the urban atmosphere of Wroclaw, Poland

Abstract

Temporal and spatial distribution of both biogenic and anthropogenic components of atmospheric carbon dioxide (CO₂) and methane (CH₄) is crucial for understanding the environmental impacts of climate change over urban areas. This research focuses on applying stable isotope source-partitioning studies to determine the interactions between biogenic and anthropogenic CO₂ and CH₄ emissions in an average-sized city environment. Study signifies the weight of instantaneous variability and diurnal averaging as compared with seasonal records of variations of the atmospheric CO₂ and CH₄ at a variety of typical urban sites in the city of Wroclaw, conducted during a one-year period from June 2017 to August 2018. The findings reveal distinct temporal variations in atmospheric CO₂ and CH₄ mole fractions and their isotopic composition. The average atmospheric CO₂ and CH₄ mole fractions during the study period were 416.4 ± 20.5 ppm, and 1.95 ± 0.09 ppm, respectively. The study highlights the high variability of driving forces, including current energy use patterns, natural carbon reservoirs, planetary boundary layer dynamics, and atmospheric transport. Additionally, the relationship between the evolution of the convective boundary layer depth and the CO₂ budget was analyzed using the CLASS model with input parameters based on field observations, resulting in insights such as an increase in the range of 25-65 ppm of CO₂ during stable nocturnal boundary layers. The observed changes in stable isotopic signatures of air samples allowed for the identification of two main source categories in the city area: fuel combustion and biogenic processes. The δ¹³C-CO₂ values of collected samples suggest that biogenic emissions dominate (up to 60 % of CO₂ excess mole fraction) during the growing season, but are reduced by plant photosynthesis during summer afternoons. In contrast, local fossil-fuel CO₂ contribution (up to 90 % of excess CO₂ mole fraction) from domestic heating, vehicle emissions, and heat and power plants predominantly influence the urban GHG budget during winter. The δ¹³C-CH₄ values indicate anthropogenic sources related to fossil fuel combustion during winter, with values ranging from -44.2 ‰ to -51.4 ‰, while slightly more depleted values, between -47.1 ‰ and -54.2 ‰, reflect a larger input of biological processes in the methane urban budget during summer. Overall, instantaneous and hourly variability of the above-mentioned readings of gas mole fraction and isotopic composition, have shown higher variability than seasonal amplitudes. Hence, respecting this granularity is the key to alignment and understanding significance of such localized atmospheric pollution studies. Additionally, the changing overprint of the system's framework, such as variability of wind and atmospheric layering patterns, weather events, provides context of sampling and data analysis at different frequencies.

Keywords: Atmospheric observations; Granularity variability; Greenhouse gases; Stable ((13)C/(12)C) isotope composition; Urbanization.