Urban Air Quality Trends and Meteorological Influences Across Five Global Megacities, 2018–2023

Urban air pollution remains a major challenge in rapidly growing metropolitan regions, but pollutant behavior and meteorological controls differ substantially among cities. This study compares air-quality trends in Shanghai, Delhi, Paris, Los Angeles, and São Paulo during 2018–2023 using observations of PM2.5, PM10, O3, NO2, and SO2 together with temperature, sea-level pressure, and wind speed. Annual and seasonal means, exceedance frequencies, and pollutant–meteorology relationships were evaluated. PM2.5 declined in Shanghai from 43.52 µg/m3 in 2018 to 27.02 µg/m3 in 2022 before increasing to 30.28 µg/m3 in 2023. Delhi remained the most persistently polluted city, with PM2.5 remaining above 87 µg/m3 throughout the period, whereas Los Angeles maintained substantially lower PM2.5 concentrations. Paris showed declining PM2.5 and NO2, while São Paulo exhibited increasing PM2.5 by 2023. The analysis of the study concerning exceedances showed that Delhi and Shanghai had 2025 and 1881 PM2.5 exceedance days, respectively, as compared to 366 in Los Angeles. For O3, Los Angeles had the highest number of exceedance days, 1973, followed by Shanghai, 1801, then Paris, 1347, and São Paulo, 1196, whereas Delhi had the lowest figure, 571. Moreover, when the influence of meteorological factors on PM2.5 and O3 is examined, it is observed that temperature was negatively correlated with PM2.5 in Delhi (r = −0.66), in Shanghai (r = −0.25), and in Paris (r = −0.21), while Los Angeles (r = 0.16) and São Paulo (r = 0.09) showed a weak positive relationship. Sea-level pressure was most strongly associated with PM2.5 in Delhi (r = 0.65), and wind speed was negatively correlated with PM2.5 in all five cities. Temperature–ozone relationships were strongest in Paris (r = 0.71) and Los Angeles (r = 0.66). These results emphasize strong city-specific differences in pollutant trends and meteorological sensitivity and support locally tailored air-quality management strategies.