Weather, Not Just Emissions, Decides How India’s Air Pollution Rises and Falls

A sweeping eight-year analysis of ground-level air quality across India has revealed a strikingly uneven picture: while fine particulate pollution has declined measurably across much of the Indo-Gangetic Plain, surface ozone has been quietly rising in several regions, and the weather itself—boundary layer height, humidity, wind, and pressure—appears to be the hidden hand steering both trends. The study, published in Environmental Monitoring and Assessment, draws on in situ measurements from India’s Central Pollution Control Board monitoring network combined with ERA5 meteorological reanalysis data, covering the period from 2017 to 2024.

The research team, led by Neelam Chaudhary and Sunita Verma of Banaras Hindu University, together with colleagues from the Space Applications Centre, Birla Institute of Technology Mesra, and Poornima College of Engineering, examined five key pollutants: particulate matter with diameters of 10 micrometres or less (PM10) and 2.5 micrometres or less (PM2.5), ground-level ozone (O3), nitrogen dioxide (NO2), and sulphur dioxide (SO2). These were paired with six meteorological variables: planetary boundary layer height, relative humidity, temperature, precipitation, wind patterns, and surface pressure. The goal was to disentangle how much of India’s air quality story is written by emissions and how much by the atmosphere’s capacity to dilute or concentrate them.

To detect long-term trends in noisy monthly data, the team applied the Mann–Kendall statistical test, a non-parametric method widely used in environmental science because it makes no assumptions about the underlying data distribution and is robust against outliers and gaps. Trend magnitudes were quantified with Sen’s slope, a median-based estimator of the rate of change that resists distortion by individual extreme months. Correlation analysis then linked pollutant behaviour to meteorological conditions, providing a statistical portrait of how weather modulates pollution across India’s diverse climate zones.

The headline finding concerns particulate matter. Across numerous monitoring sites in the Indo-Gangetic Plain—the densely populated arc stretching from Punjab through Delhi and Uttar Pradesh to West Bengal—PM concentrations showed statistically significant declines. Sen’s slope values ranged from −0.893 to −0.097 micrograms per cubic metre per month for PM2.5 and from −1.320 to −0.169 micrograms per cubic metre per month for PM10. Over the full study period, these rates compound into substantial reductions, a signal the authors associate with the tightening of emission controls under initiatives such as the National Clean Air Programme, which targets air quality improvement in 131 Indian cities.

Yet the improvement is far from uniform. Monitoring sites in Punjab bucked the regional trend and showed increasing particulate pollution, which the researchers attribute to localised emission sources—most plausibly the intensive agricultural activity and crop residue burning for which the state is known. Post-monsoon stubble fires in Punjab and Haryana have long been identified as a dominant driver of severe winter smog episodes across northern India, and the new analysis suggests that in some locations these local sources continue to overwhelm any broader cleanup. A few southern sites also displayed region-specific behaviour that diverged from national patterns, underscoring how varied India’s pollution regimes are.

Perhaps the most consequential result is the rise of ozone. Surface ozone exhibited increasing trends in several regions, with Sen’s slope values between 0.041 and 0.529 micrograms per cubic metre per month. Ozone at ground level is not emitted directly; it forms photochemically when sunlight drives reactions between nitrogen oxides and volatile organic compounds. The study highlights two mechanisms behind the increase: enhanced precursor concentrations that feed ozone production, and aerosol inhibition—the counterintuitive process by which particulate pollution actually suppresses ozone formation. Dense aerosol loads scatter and absorb solar radiation, reducing the photolysis rates that drive ozone chemistry, and they also alter the uptake of hydroperyl radicals onto particle surfaces. As particulate levels fall, more sunlight reaches the surface and more radicals survive, accelerating ozone production. Recent modelling work by other Indian researchers has warned that continued PM reductions could drive significant rises in surface ozone, and the observational trends in this study appear consistent with that prediction.

Correlation analysis reinforced this chemical tug-of-war. Particulate matter showed positive associations with NO2 and SO2, with correlation coefficients of roughly 0.5 to 0.6, reflecting their shared combustion origins in traffic, industry, and residential energy use. Ozone, by contrast, was inversely correlated with particulate matter, with coefficients ranging from about −0.2 to −0.8, a spread that captures the interplay of aerosol attenuation of sunlight, photochemical interactions, and region-specific precursor chemistry in regulating surface ozone levels. The strength of the inverse relationship varied by location, indicating that the ozone response to particulate cleanup will differ markedly from one airshed to another.

The meteorological analysis revealed a stark north–south divide in the atmosphere’s ventilating capacity. Over northern and eastern India—Delhi being the notable exception—planetary boundary layer height and wind speed declined over the study period, while relative humidity and surface pressure increased. A shallow boundary layer acts like a lid, trapping emissions within a shallow volume of air near the surface, and weak winds prevent that air from being flushed out. Higher humidity favours the formation of secondary aerosols and the hygroscopic growth of existing particles, while elevated surface pressure is typically associated with stagnant, subsiding air masses. Together, these shifts created conditions favouring pollutant accumulation precisely where much of India’s population and industry are concentrated, helping explain why winter pollution episodes remain so severe even as long-term averages decline.

Mumbai and Bangalore tell the opposite story. These coastal and peninsular cities experienced higher boundary layer heights, stronger winds, and greater precipitation, all of which aid pollutant dispersion. Rainfall scavenges particles directly from the atmosphere, while a tall, well-mixed boundary layer dilutes emissions through a much larger volume of air. The contrast illustrates a fundamental asymmetry in Indian air quality management: cities with favourable ventilation meteorology can achieve cleaner air with comparatively modest emission reductions, while cities in the stagnant Indo-Gangetic Plain face a much steeper challenge, because even unchanged emissions translate into worsening concentrations when the atmosphere’s mixing depth shrinks.

NO2 and SO2 showed heterogeneous trends across the country, with notable decreases in some regions—likely reflecting stricter emission regulations on power plants and industry—and relative stability elsewhere. The authors emphasise that their findings carry a clear policy message: because meteorological modulation can amplify or mask emission trends, mitigation strategies must be tailored to regional conditions rather than applied uniformly. They also stress the need for continuous monitoring, since ozone’s stealthy rise in a cleaning atmosphere could offset health gains from particulate reductions. Ground-level ozone is a respiratory irritant linked to cardiovascular injury and premature mortality, and it damages crops, meaning that India’s air quality success story may be incomplete until the photochemical side of the ledger is addressed as rigorously as the particulate one.

Subject of Research: Meteorological modulation of ground-level air pollutant trends across India from 2017 to 2024

Article Title: Meteorological modulation of ground-level air quality in India based on in situ measurements

Article References: Chaudhary, N., Mishra, M. K., Payra, S., Prakash, D., & Verma, S. (2026). Meteorological modulation of ground-level air quality in India based on in situ measurements. Environmental Monitoring and Assessment, 198(11), Article 1176. https://doi.org/10.1007/s10661-026-15915-9

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15915-9

Keywords: air pollution, India, PM2.5, PM10, surface ozone, nitrogen dioxide, sulphur dioxide, planetary boundary layer, meteorology, Mann-Kendall test, Sen's slope, Indo-Gangetic Plain