'Very poor' quality air reigned in Delhi on the second day of 'odd-even', even as authorities released conflicting reports on the impact of the vehicular restrictions on January 1 primarily due to different comparative parameters.

The Delhi government issued a statement saying that level of pollutants showed a "marked decrease" as compared to previous year, while The Energy and Resources Institute (TERI) said pollution levels were more than January 31 and "violated" the prescribed limits. "The 24-hourly averaged concentrations at four locations (Mandir Marg, RK Puram, Punjabi Bagh and Anand Vihar) were observed and PM 10, PM 2.5 and NOx concentrations were found to be 5.4, 5.1, and 1.5 times higher than safe standards respectively yesterday," TERI said.

The government said that PM 2.5 (particulate matters less than 2.5 microns in size) measured in 24 locations across the city ranged between 121-226 micrograms per cubic metre. On an average, PM 2.5 hovered around 250 across the national capital in previous years on the same date, it said.

Unlike yesterday, SAFAR's hourly average graph of mass concentration of pollutants did not show any upward trend after 2 PM. It ranged around 190 micrograms per cubic metre till 6 PM.

Indian authorities consider air quality to be 'very poor' when particulate matters range between 120 and 250 and anything above that fall in the 'severe' category.

TERI went one step ahead and said that PM 2.5 concentrations were found to be "highest on January 1" in comparison to the last week, ostensibly due to adverse meteorological conditions such as lesser wind speeds.

However, the green body said that although the pollutant levels showed an increase, "it will be too early" to correlate it with the implementation of the odd-even scheme. "In comparison to previous day (31st December 2015), the concentrations of PM10 and PM2.5 increased by 2-39 per cent and 10-50 per cent respectively at all locations, except at Mandir Marg which showed a small decrease," it said.

Experts maintained that Monday would be a "big test" for the effectiveness of the scheme as traffic flow is expected to swell due to reopening of colleges and rejoining of office goers after Christmas and New Year festivities.