An AQI between zero and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", then 401 and between 500 is considered "severe".
The level of PM10 (226) and PM2.5 (122) pollutants in the air were recorded in the moderate and very poor categories, respectively with the latter being the major source of pollutant in the city.
As per the System of Air Quality and Weather Forecasting And Research's (SAFAR) estimation "Prevailing cold wave conditions in the north-west region of Delhi and winds blowing from west/northwest direction affect weather in Delhi. However, minimum and maximum temperatures show a tendency to increase slightly for December 31 and January 1 and 2. Consistently high wind speeds are likely for these three days," it said in a bulletin.
The mixing layer height continues to be 1.0 - 1.5 km. New year celebrations on December 31 night may increase emissions and worsen AQI but the net effect is likely to keep AQI within "very poor" or "higher end of the very poor" for these days, SAFAR added.
Notably, new year celebrations are banned in the national capital in wake of the spike in Covid-19 cases. Light rain as the result of a western disturbance this week led to a slight improvement in the air quality in the past two days when Delhi's AQI settled below 250.
According to the weather bulletin, the air quality is likely to remain in the poor to lower range of the very poor category till January 4 and deteriorate to reach the upper end of the very poor on January 5 and 6, but improve thereafter.