Delhi’s air pollution challenge has entered a new phase. According to a recent study by the Centre for Research on Energy and Clean Air (CREA), secondary pollutants now account for nearly one-third of the city’s annual PM2.5 burden. This finding marks a significant shift from earlier pollution patterns that focused largely on direct emissions such as vehicle exhaust and road dust.

Secondary Pollutants

Importantly, secondary pollutants form after gases enter the atmosphere. In particular, sulphur dioxide, nitrogen oxides and ammonia react chemically in the air. It creates fine particulate matter such as ammonium sulphate and ammonium nitrate. As a result, air pollution no longer remains confined to visible or local sources alone.

Moreover, CREA’s analysis shows that the contribution of secondary PM2.5 rises sharply during winter and post-monsoon months. During these periods, stagnant weather conditions accelerate chemical reactions, thereby intensifying smog episodes. In fact, during peak pollution days, secondary particles contribute more than 40 per cent of Delhi’s PM2.5 levels.

At the same time, the study points to coal-based power plants as a major driver of this trend. These plants emit large volumes of sulphur dioxide, which acts as a key precursor in secondary particle formation

Consequently, experts argue that traditional pollution-control measures alone will not suffice. While actions such as dust management and traffic regulation remain necessary. They must operate alongside strong controls on precursor gases across the entire National Capital Region. Without such a regional and chemical-focused approach, long-term air-quality improvement will remain elusive.

Finally, health experts underline the seriousness of the findings. PM2.5 particles—especially secondary particles—penetrate deep into the lungs and bloodstream. Over time, they increase the risk of respiratory diseases, cardiovascular disorders and premature mortality. Therefore, CREA’s study reinforces the urgent need for coordinated, science-based policies that prevent pollution before it forms rather than reacting after damage occurs.

LEARNING FROM HOME/ WITHOUT CLASSES/ BASICS

Particular Matter

PM 2.5 means particulate matter in the air. It is caused by motor exhaust or anything combustible – that is less than 2.5 micrometers.

People sometimes call airborne particles ‘particulate matter’ or ‘PM. They include dust, dirt, soot, smoke, and liquid droplets. Various sources, both natural and human-related, directly emit some particles into the air

Natural sources include bushfires, dust storms, pollens, and sea spray.

           Those related to human activity include motor vehicle emissions, industrial processes (eg electricity generation, incinerators, and stone crushing), unpaved roads, and wood heaters.

            Particles can be classified based on their size, referred to as their ‘aerodynamic diameter’. ‘Coarse particles’ are those between 10 and 2.5 micrometres (µm) in diameter; ‘fine particles’ are smaller than 2.5 µm; and ‘ultrafine particles’ are smaller than 0.1 µm. Studies have linked exposure to particle pollution to several health problems including
respiratory illnesses (such as asthma and bronchitis) and cardiovascular disease.

INDIA’S STANDARD FOR LEVEL OF POLLUTION

India has set standards for what it thinks are appropriate warnings for a particular level of pollutant. Air Quality Index (AQI) helps in comparing pollution levels at a glance with a color code and a numerical value. In India, AQIs use the concentrations of pollutants like PM2.5 (fine, respirable particles), sulfur dioxide (SO2), nitrogen dioxide (NO2), and carbon monoxide (CO) to determine air quality.

 There are six AQI categories, namely: Good, Satisfactory, Moderately polluted, Poor, Very poor and Severe.

The Index centers around five chief pollutants.

      They are Particulate Matter with a diameter of less than 10 micro meters (PM10).

Particulate Matter with a diameter of less than 2.5 micrometers (PM2.5), ozone  (O3), Nitrogen Dioxide (NO2), and Carbon Monoxide (CO).