Data released by System of Air Quality and Weather Forecasting And Research (SAFAR) of the ministry of earth sciences showed PM2.5 concentration on January 9 to be at the lowest level since December 25. It was also lower than the level recorded on the same day in 2015 (see chart on ).

The figures released by Central Pollution Control Board also show a fall in pollution levels after January 8. While January 7 recorded PM2.5 level at 289, it was 225 on January 8 and 121 on January 9. Though the cause of the fall is yet to be fully analysed, weather is one big contributor. Since the start of odd-even scheme, PM2.5 concentration has been running higher than January 2015 levels as measured by SAFAR. That was at least partly attributed to weather conditions. The fall on January 8 and 9 too is driven mostly by weather conditions.

A study by a team from the department of public health sciences at the University of Miami shows reduction in pollution in the national capital vis-a-vis its suburbs since the onset of odd-even scheme.

Using high resolution data from MODerate resolution Imaging Spectroradiometer on board Nasa's Terra and Aqua satellites, Naresh Kumar, Associate Professor of Environmental health in Epidemiology has released two sets of comparative maps, one of NCR between December 23 and 31 and another from January 1 to 9. The maps show a significant increase in Aerosol Optical Depth (AOD) in the bordering areas and areas outside Delhi, which did not enforce any traffic control measures like odd-even.

AOD measures the concentration of aerosols or solid and liquid particles suspended in air that can arise from human activities, such as driving and construction, or natural factors like dust and water vapour.

"The important implication of the trial is that reduction in reactive gases and PM2.5 during the trial also helped reduce smog, because these gases and fine particulate matter help condensation of water vapors. A tiny particle serves as "seed" or nuclei that cause water vapors to condense, causing fog. This process is called "cloud condensation nuclei. Also these reactive gases can change to vapor phase providing bases for the formulation of smog.

Thus, reduction in reactive gases and fine particulate matter helped in reducing the overall PM2.5 concentration in the city. There was a significant increase in the levels of PM2.5 and Aerosol Optical Depth (AOD) in the bordering areas and areas outside Delhi, which did not enforce any traffic control measures. The average increase in PM2.5 levels in these areas was 32.4 g/m3," Kumar explained.

However, even after the fall, the AOD level is still up to six times higher than the safe levels prescribed by the WHO.