New Delhi has experienced severe drop in air quality in the first week of November. The air was said to be ‘unbreathable’. Air Quality is defined as the combined pollution in the ambient atmosphere in the form of solid particles and chemical pollutants. The main parameters used to identify air quality are PM2.5 and PM10, NO₂, CO, SO₂, O₃, NH₃ and Pb (source – https://app.cpcbccr.com/ccr_docs/About_AQI.pdf). The Air Quality Index (AQI) is the combination of all these pollutants using a mathematical formula. This AQI is used to identify the pollution levels in a given place. The increase or decrease in these parameters causes the changes in air quality. The main contributors from the above parameters that majorly affect Air Quality are PM2.5, PM10, NO₂ and CO. These parameters can be studied from space-based measurements. PM2.5 and PM10 can be studied using Aerosol Optical Depth (AOD) available from satellite platforms.

Figure-1 : Smoke observed over New Delhi and surrounding regions on 3^rd Nov 2023 (left) and 5^th Nov 2023 (right) using EOS-06 data.

Figure-2 : AOD values using MODIS Terra and Aqua combined product (a, b), NO₂ (c, d) and CO values (e, f) (in molecules/cm2) using Sentinel 5-P data over New Delhi and surrounding regions in the third week of October and first week of November.

It has been observed that AOD values increased between 2-2.5 times, NO₂ concentrations by about 2 times and CO concentrations by more than 2 times (Figure 2) using MODIS and Sentinel 5P data. This increase as observed from satellite data clearly explains the decrease in air quality over New Delhi region. Certain pollutants, when present in sufficient quantities, undergo chemical reactions in the atmosphere, leading to the formation of smog. It is interesting to note that the EOS-06 images (Figure 1) depict transit of a smoke plume over the Punjab, Haryana and Delhi regions. These data indicate that a combination of these events have resulted into a smogy environment in the first week of November 2023.

Date : 15^th Nov 2023