Explained | What causes haze and smoke in Delhi?

Delhi is currently buried under haze and smoke. The Air Quality Index (AQI) in the national capital has drastically reduced, leaving the government scrambling for solutions. The AAP government has banned construction activities and movement of vehicles that do not comply with BS-VI norms to bring this under control for now.

But why does Delhi suffer through this every year? Let us take a look.

What causes haze and smoke in Delhi?

The main reason behind the accumulation of haze and smoke in Delhi is the burning of agricultural waste in Punjab and Haryana. The particulate matter from the burning contributes 30-40 per cent of the PM2.5 concentrations in Delhi’s air during this time. PM2.5 is an air pollutant responsible for reducing visibility and causing the air to appear hazy.

Also Read | Delhi's air quality remains 'severe'; max temperature likely to be around 31 degree Celsius

Does weather play a role in deteriorating air quality in Delhi?

Weather plays a crucial role in deteriorating Delhi’s air quality. This is indicated by the fact that agricultural waste is burnt in the neighbouring states even in May-June. But this situation doesn't arise in Delhi then, as it does in October and November.

In the summers of May and June, hot air tends to rise up and cold air takes its place. The hot air carries with it the pollutants.

However, in October and November, the air stays put in the cool weather and pollutants stay suspended in the lower belt of air, causing haze and smoke.

What is the AQI?

A single composite index, the AQI helps communicate the severity of air pollution that is measured by recording the levels of multiple pollutants in the air.

Eight individual pollutants are monitored for the calculation of AQI, namely, PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ground-level ozone, ammonia, and lead.

How is AQI calculated?

The sub-indices for the aforementioned individual pollutants at a particular monitoring station are calculated using their average concentration value over a 24-hour period (8 hours in case of carbon monoxide and ozone) and their health breakpoint concentration range.

The worst sub-index value is taken to be the AQI for that particular location.

However, in some cases, all eight pollutants may not be monitored at a particular location.

In such cases, AQI is only calculated if data is available for at least three of the eight aforementioned pollutants, with at least one of those three being PM2.5 or PM10. Further, a minimum of 16 hours' data is deemed necessary for the calculation of the values of sub-indices.

If the aforementioned criteria cannot be met, data is considered insufficient for the calculation of AQI.