Analysing O3 levels and formation conditions via TROPOMI NO2 and HCHO retrievals for the Aegean Region

Tropospheric Ozone (O₃) is a secondary pollutant formed via non-linear chemistry in the atmosphere with significant impacts on human health, ecosystem and crop production. The spatial distributions of HCHO and NO₂ are strongly correlated with emissions of VOCs and NO_x, O₃ precursors, therefore, a proxy ratio of HCHO/NO₂ (FNR) can be an indicator to understand the O₃ formation where VOC/NO_x ratios are not available. Changes in meteorological conditions, such as wind speed and direction, temperature, and relative humidity can affect the spatial and temporal patterns of O₃ and its precursors. The Aegean Region of Turkey with its geographical location, urbanization and industry, and vegetation and agriculture, experiences high O₃ levels, given its high number of sunny days throughout the years. However, a comprehensive study in this regard has not been conducted in the region previously. The aim of this study is to investigate the spatial and temporal changes of O₃ and to understand the O₃ production regime and the relationship with its precursors in the southern Aegean Region.

Within the study scope, ground-based NO₂ and O₃ measurements from the national monitoring network were used for understanding O₃ pollution from 2019 to 2023. O₃ and NO_x (NO, NO₂) are measured together at 33 air quality monitoring stations (AQMSs) in the Aegean Clean Air Region of Türkiye. 18% of the AQMSs exceeded EU limit for MDA8 O₃ (maximum daily average 8-hr O₃ >120 µg/m³, more than average of 25 days exceedance over recent 3 years) while almost 70% of the AQMSs exceeded World Health Organization (WHO) guidelines (>100 µg/m³ as MDA8 O₃) in 2023. Manisa-Alasehir, Aydın-Efeler, and Izmir-Seferihisar AQMSs have consistently recorded the highest number of days, exceeding 100 days per year, according to the WHO guidelines. However, NO₂ exceedances are limited in the AQMSs with O₃ exceedances. Time series and pollution roses of O₃ and NO₂ were prepared to understand temporal changes along with meteorological parameters for stations with significant O₃ problem. TROPOMI NO₂ and HCHO retrievals within 6 km of AQMSs were processed, and FNR values were calculated for the same time period. O₃ concentrations were examined by FNR values in order to explain O₃ formation regimes. FNR values widely ranged in the region and the ozone season (May-September) average FNR values were estimated as 4.0<FNR<7.2 with maximum in Manisa-Alasehir, followed by Izmir-Seferihisar, and Aydın-Efeler, caused by high HCHO levels. In addition, correlation analysis of TROPOMI retrievals, ground-based measurements and meteorological parameters were performed.

The results of this study will contribute greatly to understand the ozone formation regime in the Aegean Region, to identify the areas with high O₃ formation potential and finally to determine appropriate mitigation strategies and policies for air quality management for the control O₃ and its precursors.

Keywords: Tropospheric O₃, NO₂, HCHO, FNR, the Aegean Region