Decadal Shifts in Surface Ozone and NOx-CO Precursors Over Delhi NCR Reveal Persistent VOC Limited Chemistry and Unintended O₃ Increases Under NOx Controls (2019-2024)

Surface ozone (O3) is a secondary pollutant that forms when NOx, CO, and VOCs react with sunlight. It is a major threat to human health and air quality in the Delhi-NCR megacity region. This study analyzes hourly data from 57 continuous monitoring stations in Delhi-NCR (Delhi, Noida, Greater Noida, Ghaziabad, Gurugram, Faridabad) for the duration of 2019 to 2024, covering pre-COVID, COVID-lockdown and post-COVID periods. The algorithm employs R (v4.2.2) and the openair package to calculate daily, monthly, seasonal, and annual metrics for O3, NOx, and CO. The analysis shows that the trends are opposite from year to year: NOx and CO levels were highest before COVID (about 50-60 ppb NOx and 1.3 ppm CO) and fell during lockdowns (about 36% for NOx). O3 levels were highest of 42 ppb was found in during 2019 to 2024, as NOx lower in VOC-limited regime. Seasonal patterns show that O3 levels are highest in the summer (April-May, up to 50 ppb in peri-urban NCR) and lowest in the winter (NOx/CO levels are highest in the winter). Diurnal cycles show that O3 levels are highest in the afternoon (95 ppb before monsoon) and lowest in the morning and evening, whereas the NOx/CO levels found highest in the morning and evening. These changes show that photochemistry is limited by VOCs across Delhi-NCR. Interventions that focus on NOx (like GRAP traffic curbs) accidentally raise O3 levels, so integrated NOx-VOC controls like solvent reforms and vapor recovery are needed to lower O3 levels by 25-35%.