Ozone in Nehru Nagar, Delhi: Five Years of Patterns and Data Quality (2019–2023)

Abstract

We studied ground-level ozone at Nehru Nagar, a residential area in south Delhi. We used hourly CPCB data from 2019 to 2023. This is the first published description of the daily ozone cycle at this station.
We have two main findings.
First, data quality. In post-monsoon (October–November) and winter (December–February), the ozone values were too high to be real. The 21:00 IST means were 299.3 µg m⁻³ and 348.9 µg m⁻³. These are about 4–8 times and 6–17 times the ranges seen at other Delhi stations (40–80 and 20–60 µg m⁻³). We think the UV ozone monitor made an error. We left these two seasons out of the main analysis.
Second, daily patterns. In summer, ozone peaked at 09:00 IST. The mean was 255.6 ± 5.7 µg m⁻³ (95% CI [244.4, 266.7]). This is four to six hours earlier than at other Delhi stations. Across 333 summer days, the peak hour changed a lot: 36% of days peaked in the morning (06:00–11:00 IST) and 38% at night (18:00–23:00 IST). The single most common peak hour was 21:00 IST (56 of 333 days, 17%). Morning ozone was higher than afternoon ozone, and this difference passed strict significance tests.
The monsoon looked normal. It peaked at 21:00 IST (119.1 µg m⁻³), which matches ozone-rich air coming down from above at night.
During the 2020 COVID lockdown, summer morning ozone fell by 89.9 µg m⁻³ (38%). That summer was 2.6 °C cooler, which may explain 10–15 µg m⁻³ of the drop. So most of the fall is not explained by temperature.