Variation Trends of Air Quality and Pollution Characteristics in Baoji City from 2017 to 2025

Ambient air pollution remains a leading environmental risk factor globally. Over the past decade, China has achieved marked reductions in particulate matter (PM) following the 2013 Air Pollution Prevention and Control Action Plan; however, surface ozone (O 3 ) has increased in multiple urban agglomerations, signalling a transition from single-pollutant to complex multi-pollutant regimes. At the western terminus of the Fenwei Plain—a national key region designated in 2018—systematic, county-level assessments of long-term air-quality evolution are lacking for Baoji City. This study presents a nine-year (2017–2025) analysis of six criteria pollutants (SO 2 , NO 2 , CO, O 3 , PM 2 . 5 , and PM 10 ) across all 12 administrative divisions (four districts and eight counties) of Baoji, using continuous monthly data from 16 national and provincial monitoring stations (n = 108 months). Spearman rank correlation, Mann–Kendall trend tests, and Sen’s slope estimators were applied to (i) quantify the significance of inter-annual concentration trends, (ii) characterise the spatial decoupling between particulate matter and ozone at the county level, and (iii) document the timing and magnitude of the regime shift in dominant pollution types. CO, NO 2 , PM 10 , and PM 2 . 5 exhibited statistically significant downward trends at >90 % of monitoring sites (p < 0.05), with cumulative reduction rates exceeding 45 %, 46 %, 51 %, and 53 %, respectively. Annual mean SO 2 , NO 2 , and CO continuously met the National Grade-II Standard from 2020 onward, while PM 2 . 5 achieved stable compliance from 2023 to 2025. The 90th percentile of daily maximum 8-hour O 3 (O 3 -8h-90th) remained below the Grade-II limit of 160 μg/m 3 throughout the study period, despite a phased rebound during 2022–2024. Spatially, PM 2 . 5 and PM 10 followed a west-low / centre-east-high distribution with a continuously shrinking regional gradient, whereas O 3 maintained a stable east-high / south-low pattern, reflecting persistent photochemical pressure in the eastern plain counties. The proportion of O 3 as the primary pollutant rose from 15 % in 2017 to 28 % in 2025, marking a regime shift from PM-dominated to O 3 –PM 2 . 5 co-dominated air pollution. These findings provide empirical evidence for synergistic pollution–carbon governance and inform seasonally differentiated, location-specific control strategies in Baoji and analogous industrial basin cities.