Recent Improvements in the air quality of the megacity Delhi

Abstract:

Ambient PM_2.5 exposure affects health adversely and is identified as one of the major risk factors in India. The megacity Delhi has been dubiously called the most polluted capital city in the world due to its perennial air quality problem. Several initiatives, both at a national and local scale, are taken in recent years to improve the air quality in India with a greater attention on Delhi. The number of ground-based monitoring sites (under the Central Pollution Control Board network) has expanded substantially to provide an improved spatial coverage. Using the data from these reference-grade monitors, we show that annual ambient PM_2.5 in Delhi has decreased by 27.1% from 140 µg m^-3 in 2017 to 102 µg m^-3 in 2019. PM_2.5 concentration during the peak pollution season decreased by 30% in the last two years. Two major pollution episodes are observed in Delhi – one in mid-October to early November and the second one in late December to early January. The air pollution is found to be quite homogeneous (high correlation between monitoring stations) in the months of June and July and spatially heterogeneous in the months of August and September. Not only PM_2.5, but other major criteria pollutants (PM₁₀, SO₂, CO, and NO₂) also decreased from 2017 to 2019. Only, the O₃ level increased during this period. The number of days with PM_2.5 concentrations below the 24-hour national ambient air quality standard of 60 µg m^-3 increased from 10% in 2017 to 27% in 2019. In terms of air quality index (a measure of various air pollutants), the peak class of “very unhealthy” has shifted to “unhealthy” category in the last three years. As Delhi is leapfrogging to BS-VI fuel, the vehicular emission is expected to drop significantly in the next few years. Along with this, other efforts (e.g. strict compliance of industrial and power plant emissions and construction activities, shifting of polluting industries outside Delhi, bans of heavy-duty vehicles inside Delhi, program to curb seasonal open biomass burning, etc.) are also getting implemented. Our analysis suggests that these programs are bearing fruit and must continue more aggressively to bring the pollution level further down while Delhi continues to develop.