Identification of surface ozone sensitivity for NO₂ and secondary HCHO in Sweden

Abstract Background Ozone a short-lived pollutant contributing to the warming of the lower atmosphere. Increased level of ozone can lead to local warming and may impose adverse effect on human and ecosystem health. Although Sweden is not a major source of ozone precursors, its surface ozone levels can be influenced by long-range transport, climate-driven feedbacks, and changes in local anthropogenic and biogenic emissions. Over past decade Sweden has experienced a significant rise in temperature, consistent with the broader Arctic warming trends. Therefore, aim of the present study is to investigate long term trends of surface ozone in Sweden using CAMS reanalysis dataset from 2004 to 2023. Methods Present study utilized the CAMS Formaldehyde (HCHO) and Nitrogen dioxide (NO 2 ) at surface to analyze the ozone sensitivity. The ratio of secondary HCHO to NO 2 used as the tracer to analyze O 3 -NO x -VOC sensitivities in Swedish region. HCHO/NO₂ ratio acts as a diagnostic tool because it reflects the relative abundance of VOC-derived radicals versus NOx. This ratio enables identification of ozone formation regimes NO x -limited, VOC-limited, or transitional. Results From results a statistically significant increasing trend is observed for the surface ozone with a magnitude greater than 0.3 ppbv per year across the Northern Sweden. Decreasing trend for NO 2 and increasing trend for HCHO across the Northern Sweden is not statistically significant at 5% significance level. HCHO/NO 2 ratio less than 1 across the southern and central part of Sweden indicates that the surface ozone production is more sensitive to VOCs levels, whereas along the Norland region ratio lies between 1 and 2 that suggests that the ozone sensitivity along the Northern Sweden region lies in a transitional regime. Conclusions Present study findings suggest that surface ozone production in majority of the urban areas in Sweden is sensitive to VOC-limited regime, only rural areas of Northern Sweden lies in transitional regime shift from VOC-limited to NOx-limited. Increasing level of surface ozone results from increased anthropogenic and biogenic emissions that indicates production of surface ozone in Sweden becoming more sensitive to VOCs levels. This study offers information corresponding to ozone production sensitivities to support the strategic management decisions.