The numbers tell a striking story of transformation. During the 2010 to 2015 period, winter daytime ozone concentrations in Bhubaneswar reached approximately 75 parts per billion at their peak. In the 2020 to 2024 record, those winter daytime peaks have fallen to around 51 parts per billion, a change that might superficially suggest improvement. But the monsoon season tells a different and more troubling tale. Twenty-four-hour mean ozone concentrations during the monsoon have climbed to roughly 25 parts per billion, up from just 12 to 15 parts per billion a decade earlier. This doubling of the monsoon baseline means that ozone is no longer a seasonal visitor that retreats with the rains; it has become a resident pollutant sustained throughout the year.

Understanding why this shift matters requires a brief dive into ozone chemistry. Surface ozone is not emitted directly by any source. Instead, it is a secondary pollutant, cooked up in the atmosphere through photochemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight. The relationship between these precursors and ozone production is famously nonlinear. In what chemists call a VOC-limited regime, ozone production is suppressed by abundant nitrogen oxides, which quench radical chains, so reducing VOCs is the effective control strategy. In a NOx-limited regime, the opposite holds: ozone scales with nitrogen oxide availability, and cutting NOx yields direct ozone benefits. Misdiagnosing the regime can make pollution worse, not better, which is why the Bhubaneswar findings carry real policy weight.

The study’s precursor sensitivity analysis, built on satellite-derived formaldehyde-to-nitrogen-dioxide column ratios, reveals a city whose chemistry changes costume with the seasons. In winter, Bhubaneswar operates in a VOC-limited regime, with about 35 percent of conditions falling into this category. As the pre-monsoon and post-monsoon seasons arrive, the chemistry flips toward NOx limitation, accounting for roughly 34 percent and 29 percent of the respective periods. The monsoon season presents a more ambiguous mixed regime, with about 28 percent of conditions in that category. This seasonal oscillation means that a single, one-size-fits-all emission control strategy would be chemically blind to the city’s actual needs, potentially delivering ozone increases in some seasons while reducing it in others.

The health implications are quantified through the maximum daily 8-hour average, or MDA8, a standard metric for assessing human ozone exposure. The study found frequent exceedances of the 50 parts per billion threshold, particularly during winter and the pre-monsoon season. Chronic exposure at these levels is associated with respiratory inflammation, aggravated asthma, reduced lung function, and increased cardio-respiratory mortality risk, burdens documented extensively in global epidemiological literature. For a city whose population is expanding rapidly alongside its vehicle fleet and industrial base, the persistence of these exceedances across multiple seasons signals a sustained public health challenge rather than an episodic one.

Agriculture emerges as perhaps the most economically consequential casualty. The researchers deployed a battery of established ozone exposure indices, including AOT40, which accumulates hourly ozone above 40 parts per billion during daylight, along with M7, M12, and SUM60 metrics, to estimate crop damage. The results point to substantial yield losses in paddy and rabi crops, with the damage concentrated in the high-ozone seasons. This finding aligns with a growing body of national and regional assessments showing that ozone is among the most damaging air pollutants for Indian agriculture, with prior studies attributing significant reductions in wheat and rice yields across the subcontinent. Bhubaneswar’s surrounding peri-urban farmland, sitting in close proximity to the urban emission core, sits squarely in the exposure zone.

Paradoxically, the study found that despite the moderation of seasonal peaks, the frequency of episodic ozone exceedances and the estimated crop yield losses have both increased over the study period. This counterintuitive outcome underscores a critical lesson in atmospheric science: peak concentrations are a poor proxy for total exposure. A pollutant that spends more hours above harmful thresholds, even at moderate levels, can inflict more cumulative biological damage than one that spikes briefly and then vanishes. The elevated monsoon baseline means that crops, which grow actively through the monsoon kharif season, are now marinating in ozone during their most vulnerable developmental windows.

To contextualize their findings, the researchers compared Bhubaneswar with Delhi using 2024 data from the Central Pollution Control Board. The contrast is instructive. Delhi, the megacity of the Indo-Gangetic Plain, remains largely locked in a VOC-limited regime due to persistent nitrogen oxide saturation from its enormous traffic and industrial emissions. Bhubaneswar, by contrast, cycles between regimes with the seasons. This comparison demonstrates that Indian cities cannot simply copy one another’s air quality playbooks. Delhi’s chemistry demands aggressive VOC control, while Bhubaneswar requires a seasonally adaptive strategy that tightens VOC emissions in winter and shifts toward NOx management during the transitional monsoon periods.

The study’s methodology reflects the modern toolkit of atmospheric monitoring. Continuous in situ measurements were collected at the CSIR-Institute of Minerals and Materials Technology monitoring site, located near National Highway 16, a major transport corridor whose vehicular emissions feed directly into the local photochemical soup. Satellite retrievals from instruments such as OMI and TROPOMI provided the column-level precursor data needed to diagnose chemical regimes, while fire count data from NASA FIRMS helped track biomass burning influences. The work was supported by the Indian Space Research Organisation’s Geosphere-Biosphere Programme, and the authors emphasize that the datasets are available from the corresponding author upon reasonable request.

Looking forward, the researchers argue that eastern India’s urbanizing cities need three things: season- and region-specific emission controls informed by regime diagnostics, enhanced monitoring of volatile organic compounds, which remain poorly characterized across much of the country, and artificial intelligence-driven forecasting systems capable of anticipating ozone episodes before they unfold. As Bhubaneswar continues its transformation into a smart city, the study serves as a reminder that the atmosphere above it is running its own experiments, and that the chemistry of a warming, motorizing, growing urban India will keep shifting beneath our assumptions unless the monitoring and the policy keep pace with it.

Subject of Research: Seasonal surface ozone dynamics, precursor sensitivity, and health and agricultural impacts in Bhubaneswar, India

Article Title: Seasonal dynamics and impacts of surface ozone pollution over Bhubaneswar

Article References: Mishra, M., Mallik, C., Ramasamy, B., & Das, T. (2026). Seasonal dynamics and impacts of surface ozone pollution over Bhubaneswar. Environmental Monitoring and Assessment, 198(11), Article 1172. https://doi.org/10.1007/s10661-026-15989-5

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15989-5

Keywords: surface ozone, Bhubaneswar, air pollution, VOC-limited regime, NOx-limited regime, MDA8, AOT40, crop yield loss, photochemistry, satellite retrievals, seasonal variability, eastern India