Time Series Analysis of Air Pollutant Gas Concentrations: A Case Study in Isfahan's Urban Area

Abstract. Monitoring, documenting, and analysing atmospheric pollutant levels in urban areas are vital for understanding and mitigating their impacts on human health and ecosystems. Studying the temporal patterns of ozone (O₃) and nitrogen oxides (NO, NO₂, NOx)—the primary air pollutants—is especially important in air quality research. This study investigates the dominant variations of ozone and nitrogen oxides concentrations in Isfahan city. Data collected from air pollution stations from 2018 to 2024 were analyzed, focusing on O₃, NO, NO₂, and NOx levels. Least Squares Harmonic Estimation (LS-HE) analysis of the time series confirmed a significant annual harmonic pattern for O₃, NO, and NOx gases at a 99% confidence level. Additionally, the daily 24-hour cycle of the time series was identified. Lastly, the Pearson correlation coefficient was computed to assess the linear relationships among the studied variables. Results reveal an inverse annual relationship: peak O₃ levels occur in summer, while NO and NOx concentrations are at their lowest during this period. Daily patterns show O₃ peaking between 4:00 PM and 8:00 PM, coinciding with the lowest NO and NOx levels. Pearson correlation analysis indicates a weak, inverse linear relationship between NO₂ and O₃ concentrations. Additionally, some months in 2023 exhibited inconsistent daily patterns, highlighting complex dynamics in air pollution behaviors.