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Delhi’s struggle with smog every winter is a stark reminder of the city’s severe air quality crisis, a problem that extends far beyond the much-discussed winter months. While the seasonal surge in pollution levels during winter grabs headlines, it is essential to highlight the persistent challenges that plague the city year-round, particularly the compounding effects of heat and ozone during summer.
The air quality graph of the national capital paints a stark picture. “Good” air days — defined as those with ultrafine particulate matter (PM2.5) levels below 30 micrograms per cubic metre (µg/m³) — are exceedingly rare, occurring only during the monsoon months of July, August, and September.
Even then, the peak of “good” days is limited to August, with July and September offering only a handful. The respite provided by the rains is welcome, but fleeting.
In contrast, “moderate” days, with PM2.5 levels between 61 and 90 µg/m³, dominate the pre-monsoon and summer months from February to June. April emerges as the most challenging month, coinciding with Delhi's extreme heat. This period witnesses a complex interplay of rising temperatures, particulate matter and ground-level ozone, which exacerbate health risks for residents.
Year-round PM2.5 trends: From good to moderate air quality days
While winter pollution is primarily attributed to meteorological conditions, Delhi’s geographical location, and local sources such as stubble burning, summer pollution are a different beast.
The scorching heat of summer drives chemical reactions in the atmosphere, leading to the formation of ground-level ozone — a pollutant that often goes unnoticed but poses severe risks to respiratory health. Heat waves amplify these effects, creating a hazardous cocktail of high temperatures and elevated pollution levels.
Additionally, the intense summer sun contributes to the photochemical reactions that convert precursor pollutants like nitrogen oxides and volatile organic compounds into ozone. This phenomenon is particularly pronounced in urban areas like Delhi, where vehicular emissions, industrial activities, and construction dust provide ample sources of these precursors.
Heatmap calendar showing PM2.5 distribution for 2024
Delhi’s pollution doesn’t just ebb and flow — it transitions seamlessly across seasons. Each season brings its unique challenges—winter smog, summer ozone, and the transitional periods in between.
Factors like the city’s geography, increasing urbanisation, vehicular emissions, and industrial activities ensure that pollution never truly takes a backseat. Year-round, the capital grapples with the health impacts of dirty air, from respiratory illnesses to heat-induced stress.
Thus, the national capital’s air pollution isn’t just a seasonal issue, it’s a perennial reality that demands acknowledgement beyond the winters. The numbers tell a story of a city fighting to breathe, regardless of the season.